Urinary Concentration – The Medullary Osmotic Gradient

June 22, 2016 Anatomy, Critical Care, Hemodynamics, Histology, Nephrology, Physiology and Pathophysiology No comments , , , , , , , , , , , , , ,

The kidneys can produce a range of urine osmolalities depending on the levels of ADH. The production of hypo-osmotic urine is an understandable process: The tubules (particularly the thick ascending limb of Henle's loop) reabsorb relatively more solute than water, and the dilute fluid that remains in the lumen is excreted. The production of hyperosmotic urine is also straightfoward in that reabsorption of water from the lumen into a hyperosmotic interstitium concentrates that luminal fluid, leaving concentrated urine to be excreted.

The Mechanism to Generate Medullary Osmotic Gradient

There is a gradient of osmolality (hyperosmotic), increasing from a nearly iso-osmotic value at the corticomedullary border to a maximum of greater than 1000 mOsm/kg at the papilla. The peak osmolality is variable depending on dehydration or overhydration, where highest during periods of dehydration and lowest (approximately half of that during excess hydration) during excess hydration.

In the steady state there must be mass balance, that is, every substance that enters the medulla via tubule or blood vessel must leave the medulla via tubule or blood vessels. However, during development of the gradient there are transient accumulations of solute, and during washout of the gradient there are losses. To develop the osmotic gradient in the medullary interstitium, there must be deposition of solute in excess of water. It is reabsorption of sodium and chloride by the thick ascending limb in excess of water reabsorbed in the thin descending limbs that accomplishes this task. At the junction between the inner and outer medulla, the ascending limbs of all loops of Henle, whether long or short, turn into thick regions and remain thick all the way back until they reach the original Bowman's capsules. As they reabsorb solute without water and dilute the luminal fluid, they simultaneously add solute without water to the surrounding interstitium. This action of the thick ascending limb is absolutely essential and is the key to everything else that happens. If transport in the thick ascending limb is innhibited, the lumen is not diluted and the interstitium is not concentrated, and the urine becomes iso-osmotic.


  • For thick ascending limbs in the cortex, reabsorbed solute is taken up by abundant cortex blood flow so intersititum osmolality in the cortex approximately equal plasma
  • High concentratiion sodium level in outer medulla interstitium makes them diffuse into DVR and AVR
  • Hyperosmotic sodium in AVR can diffuse into nearby DVR – the countercurrent exchange

For those portions of the thick ascending limbs in the cortex, the reabsorbed solute simply mixes with material reabsorbed by the nearby proximal convoluted tubules. Because the cortex contains abundant peritubular capillaries and a hight blood flow, the reabsorbed material immediately moves into the vasculature and returns to the general circulation. However, in the medulla, the vascular anatomy is arranged differently and total blood flow is much lower. Solute that is reabsorbed and deposited in the outer medullary interstitium during the establishment of the osmotic gradient is not immediately removed, that is, it accumulates. The degree of accumulation before a steady state is reached is a function of the arrangement of the vasa recta, their permeability properties and the volume of blood flowing within them.

Imagine first a hypothetical situation of no blood flow. Sodium would accumulate in the outer medulla without limit, because there would be no way to remove it. But, of course the outer medulla is perfused with blood, as are all tissues. Blood enters and leaves the outer medulla through parallel bundles of descending and ascending vasa recta (DVR and AVR). These vessels are permeable to sodium. Therefore sodium enters the vasa recta driven by the rise in concentration in the surrounding interstitium. Sodium entering the ascending vessels returns to the general circulation, but sodium in the descending vessels is distributed deeper into the medulla, where it diffuses out across the endothelia of the vassa recta and the interbundle capillaries that they feed, thereby raising the sodim content throughout the medulla.

Later, the interbundle capillaries drain into ascending vasa recta that lie near descending vasa recta. The walls of the ascending vasa recta are fenestrated, allowing movment of water and small solutes between plasma and interstitium. As the sodium concentration of the medullary interstitium rises, blood in the ascending vessels also takes on an increasingly higher sodium concentration. However, blood entering the medulla always has a normal sodium concentration (approximtely 140 mEq/L). Accordingly, some of the sodium begins to re-circulate, diffusing out of ascending vessels and reentering nearbydescending vessels that contain less sodium (countercurrent exchange). So sodium is entering descending vasa recta from 2 sources – re-circulated sodium from the ascending vasa recta, and new sodium from the thick ascending limbs. Over time, everything reaches a steady state in which the amount of new sodium entering the interstitium from thick ascending limbs matches the amount of sodium leaving the interstitium in ascending vasa recta. At its peak, the concentration of sodium in the medulla may reach 300 mEq/L, more than double its value in the general circulation. Since sodium is accompanied by an anion, mostly chloride, the contribution of salt to the medullary osmolality is approximately 600 mOsm/kg.


  • While solute can accumulate without a major effect on renal volume, the amount of water in the medullary interstitium must remain nearly constant; otherwise the medulla would undergo significiant swelling or shrinking.
  • Because water is always being reabsorbed from the medullary tubules into the interstitium (from descending thin limbs and medullary colelcting ducts), that water movement must be matched by equal water movement from the interstitium to the vasculature.
  • Blood entering the medulla has passed through glomeruli, thereby concentrating the plasma proteins. While the overall osmotic content (osmolality) of this blood is essentially isosmotic with systemic plasma, its oncotic pressure is considerably higher.

The challenge for the kidneys is to prevent dilution of the hyperosmotic interstitium by water reabsorbed from the tubules and by water diffusing out of the iso-osmotic blood entering the medulla. The endotheial cells of descending vasa recta contain aquaporins. So water is drawn osmotically into the outer medullary interstitium by the high salt content in a manner similar to water being drawn out of tubular elements. At first glance it seems that this allows the undesired diluting effect to actually take place. But, of course, solute is also constantly being added from the nearby thick ascending limbs. The loss of water from descending vasa recta in the outer medulla serves the useful purpose of raising the osmolality of blood penetrating the inner medulla and decreasing its volume, thereby reducing the tendency to dilute the inner medullary interstitium. The ascending vasa recta have a fenestrated endothelium, allowing free movement of water and small solutes. Since the oncotic pressure is high, water entering the interstitium of the outer medulla from descending vasa recta is taken up by ascending vasa recta and removed from the medulla. In addition, water reabsorbed from tubular elements (descending thin limbs and collecting ducts) is also taken up by ascending vasa recta and removed, thereby preserving constancy of total medullary water content.

The magnitude of blood flow in the vasa recta is a crucial variable. The peak osmolality in the interstitium depends on the ratio of sodium pumping by the thick ascending limbs to blood flow in the vasa recta. If this ratio is high (meaning low blood flow), water from the isosmotic plasma entering the medulla in descending vasa recta does not dilute the hyperosmotic interstitium. In effect the "salt wins" and osmolality remains at a maximum. But in conditions of water excess, this ratio is very low (high blood flow) and the diluting effect of water diffusing out of descending vasa recta is considerable. In part the tendency to diulte is controlled by ADH (due to its vasoconstriction effect to limit the blood flow of DVR).


The peak osmolality in the renal papilla reaches over 1000 mOsm/kg. Approximately half of this is accounted for by sodium and chloride, and most of the rest is (500-600 mOsm/kg) accounted for by urea. Urea is a very special substance for the kidney. It is an end product of protein metabolism, waste to be excreted, and also an important component for the regulation of water excretion. For the handle of urea in the kidneys: 

1.There are no membrane transport mechanisms in the proximal tubule; instead, it easily permeates the tight junctions of the proximal tubule where it is reabsorbed paracellularly.

2.Tubular elements beyond the proximal tubule express urea transporters and handle urea in a complex, regulated manner.

The gist of the renal handling of urea is the following: it is freely filtered. About half is reabsorbed passively in the proximal tubule. Then an amount equal to that reabsorbed is secreted back into the loop of Henle. Finally, about half is reabsorbed a second time in the medullary collecting duct. The net result is that about half the filtered load is excreted.

Urea does not permeate lipid layer because of its highly polar nature, but a set of uniporters transport urea in various places beyond the proximal tubule and in other sites within the body. Because urea is freely filtered, the filtrate contains urea at a concentration identical to that in plasma. In the proximal tubule when water is reabsorbed, the urea concentation rises well above the plasma urea concentration, driving diffusion through the leaky tight junctions. Roughly, half the filtered load is reabsorbed in the proximal tubule by the by the paracellular route. As the tubular fluid enters the loop of Henle, about half the filtered urea remains, but the urea concentration has increased somewhat above its level in the filtrate because proportionally, more water than urea was reabsorbed. At this point, the process becomes fairly complicated.

The interstitium of the medulla has a considerably higher urea concentration than does plasma. The concentration increases from the outer to the inner medulla. Since the medullary interstitial urea concentration is greater than that in the tubular fluid entering the loop of Henle, there is a concentration gradient favoring secretion into the lumen. The tight junctions in the loop of Henle are no longer permeable, but the epithelial membranes of the thin regions of the Henle's loops express urea uniporters, members of the UT family. This permits secretion of urea into the tubule. In fact, the urea secreted from the medullary interstitium into the thin regions of the loop of Henle replaces the urea previously reabsorbed in the proximal tubule. Thus, when tubular fluid enters the thick ascending limb, the amount of urea in the lumen is at least as large as the filtered load. However, because about 80% of the filtered water has now been reabsorbed, the luminal urea concentration is now several times greater than in the plasma. Beginning with the thick ascending limb and continuing all the way to the inner medullary collecting ducts (through the distal tubule and cortical collecting ducts), the apical membrane urea permeability (and the tight junction permeability) is essentially zero. Therefore, an amount of urea roughly equal to the filtered load remains within the tubular lumen and flows from the cortical into the medullary collecting ducts.

During the transit through the cortical collecting ducts variable amounts of water are reabsorbed, significantly concentrating the urea.We indicated eariler that the urea concentration in the medullary interstitium is much greater than in plasma, but the luminal concentration in the medullary collecting ducts is even higher (up to 50 times its plasma value), so in the inner medulla the gradient now favors reabsorption and urea is reabsorbed a second time via another isoform of UT urea uniporter. It is this urea reabsorbed in the inner medulla that leads to the high medullary interstitial concentration, driving urea secretion into the thin regions of the loop of Henle.

[Histology] The Blood Tissue

May 27, 2016 Hematology, Histology No comments , , , , , , , , , , , , , , ,

Major Componments of Blood

Blood is a specialized connective tissue in which cells are suspended in fluid extracellular material called plasma (ECM/extracellular matrix). When blood leaves the circulatory system, either in a test tube or in the ECM surrounding blood vessels, plasma proteins (coagulation factors) react with one another to produce a clot, which includes formed elements and a pale yellow liquid called serum. Serum contains growth factors and other proteins released from platelets during clot formation, which confer biological properties very different from those of plasma.

Collected blood in which clotting is prevented by the addition of anticoagulants can be separated by centrifugation into layers that reflect its heterogeneity. Erythrocytes make up the sedimented material and their volume, normally about 45% of the total blood volume in healthy adults, is called hematocrit. The straw-colored, translucent, slightly viscous supernatant comprising 55% at the top half of the centrifugation tube is the plasma. A thin gray-white layer called the buffy coat between the plasma and the hematocrit, about 1% of the volume, consists of leukocytes and platelets, both less dense than erythrocytes.

Function of the Blood Tissue

  • O2 is bound mainly to hemoglobin in erythrocytes and is much more abundant in arterial than venous blood, while CO2 is carried in solution as CO2 or HCO3, in addition to being hemoglobin-bound.
  • Nutrients are distributed from their sites of synthesis or absorption in the gut, while metabolic residues are collected from cells all over the body and removed from the blood by the excretory organs.
  • Hormone distribution in blood permits the exchange of chemical messages between distant organs regulating normal organ function.
  • Blood also participates in heat distribution, and the regulation of body temperature.
  • Blood maintain the acid-base and osmotic balance.
  • Blood can form clotting when bleeding happens, the hemostasis.
  • Leukocytes, complements, and antibodies have diversified functions and are one of the body's chief defenses against infection.


Plasma is an aqueous solution, pH 7.35~7.45 (in normal conditions), containing substances of low or high molecular weight that make up 7% of its volume. The dissolved components are mostly plasma proteins, but they also include nutrients, respiratory gases, nitrogenous waste products, hormones, and inorganic ions (electrolytes). Through the capillary walls, the low-molecular-weight components of plasma (e.g., most drugs) are in equilibrium with the interstitial fluid of the tissues. Thus, the composition of plasma is usually an indicator of the mean composition of the extracellular fluids in tissues.

Major plasma proteins

  • Albumin, the most abundant plasma protein, is made in the liver and serves primarily to maintain the osmotic pressure of the blood.
  • alpha-Globulins and beta-globulins, made by liver and other cells, include transferrin and other transport factors; fibronectin; prothrombin and other coagulation factors; lipoproteins and other proteins entering blood from tissues.
  • gamma-Globulins, which are immunoglobulins (antibodies) secreted by plasma cells in many locations.
  • Fibrinogen, the largest plasma protein, also made in the liver, which, during clotting, polymerizes as insoluble, cross-link fibers of fibrin that block blood loss from small vessels.
  • Complement proteins, a system of factors important in inflammation and destruction of microorganisms.


  • Human erythrocytes normally survive in the circulation for about 120 days. After 120 days, defects in the membrane's cytoskeletal lattice or ion transport systems begin to produce swelling or other shape abnormalities, as well as changes in the cells' surface oligosaccharide complexes. These RBCs are removed from the circulation mainly by macrophages ot the spleen, liver, and bone marrow.
  • Erythrocyte differentiation includes loss of the nucleus and organelles, shortly before the cells are released bone marrow into the circulation. Lacking mitochondria, erythrocytes rely on anaerobic glycolysis for their minimal energy needs.
  • The combination of hemolgobin with carbon monoxide (CO) is irreversible, howevver, reducing the cells' capacity to ransport O2 and CO2.


Leukocytes leave the blood and migrate to the tissues where they become functional and perform various activities related to immunity. According to the type of cytoplasmic granules and their nuclear morphology, leukocytes are divided into two groups: granulocytes and agranulocytes. All granulocytes are terminally differentiated cells with a life span of only a few days. Their Golgi complexes and rough ER are poorly developed. They have few mitochondria and depend largely on glycolysis for their low energy needs. Granulocytes normally die by apoptosis in the connective tissue. The resulting cellular debris is removed by macrophages and, like all apoptotic cell death, does not itself elicit an inflammatory response.

  • Granulocytes: neutrophils, esoinophils, and basophils
  • Agranulocytes: lymphocytes and monocytes


Mature neutrophils consitute 54% to 62% of circulating leukocytes; circulating immature forms raise this value by 3% to 5%. In females, the inactive X chromosome may appear as a drumstick-like appendage on one of the lobes of the nucleus although this characteristic is not obviious in every neutrophil. Neutrophils are inactive and spherical while circulating but become actively amoeboid during diapedesis and upon adhering to solid substrates such as collagen in the ECM.

Neutrophils are active phagocytes of bacteria and other small particles and are usually the first leukocytes to arrive at sites of infection, where they actively pursue bacterial cells using chemotaxis. The cytoplasmic granules of neutrophils provide the cells' functional activities and are of two main types (primary granules and secondary granules). Azurophilic primary granules resemble lysosomes as large, dense vesicles and have a major role in both killing and degrading engulfed microorganisms. They contain proteases and antibacterial proteins, including: 1.myeloperoxidase/MPO, which generates hypochlorite and other agents toxic to bacteria; 2.lysozyme, which degrades components of bacterial cell walls; and 3.defensins, small cysteine-rich proteins that bind and disrupt the cell membranes of many types of bacteria and other microorganisms. Specific secondary granules are smaller, less dense, and have diverse functions, including secretion of various ECM-degrading enzymes such as collagenases, delivery of additional bactericidal proteins to the phagolysosomes, and insertion of new cell components.

Activated neutrophils at infected or injured sites also have important roles in the inflammatory response, including the release of chemokines that attract other leukocytes; cytokines that direct activites of these and local cells of the tissue; and the release of lipid mediators of inflammation.

Neutrophils are short-lived cells with a half-life of 6 to 8 hours in blood and a life span of 1 to 4 days in connective tissues before dying by apooptosis.


Eosinophils are far less numerous than neutrophils, constituting only 1% to 3% of leukocytes. The main identifying characteristic is the abundance of large, acidophilic specific granules typically staining pink or red. Ultrastructurally the eosinophilic specific granules are seen to be oval in shape, with flattened crystalloid cores containing major basic protein/MBP, an arginine-rich factor that act to kill parasitic worms or helminths. Eosinophils also modulate inflammatory responses and allergies.


The specific granules in basophils contain heparin and other sulfated GAGs, much histamine and various other mediators of inflammation. By migrating into connective tissues, basophils appear to supplement the function of mast cells. Like mast cells, basophils secretion these granules in response to certain antigens and allergens.


By far the most numerous type of agranulocyte in normal blood smears of CBCs, lymphocytes constitute a family of leukocytes with spherical nuclei. Although they are morphologically similar, lymphocytes can be subdivided into functional groups by distinctive surface molecules (called "cluster of differentiation" or CD markers) that can be distinguished using antibodies wtih immunocytochemistry or flow cytometry. Major classes include B lymphocytes, helper and cytotoxic T lymphocytes (CD4+ and CD8+, respectively), and natural killer (NK) cells.

Lymphocytes vary in life span according to their specific functions; some live only a few days and others survive in the circulating blood or other tissues for many years.


Monocytes are agranulocytes that are precursor cells of macrophages, osteoclasts, microglia, and other cells of the mononuclear phagocyte system in connective tissue. All monocyte-derived cells are antigen-presenting cells and have important roles in immune defense of tissues.


Blood platelets are very small non-nucleated, membrane-bound cell fragments only 2 to 4 um in diameter. Platelets originate by separation from the ends of cytoplasmic processes extending from giant polyploid bone marrow cells called megakaryocytes. Platelets promote blood clotting and help repair minor tears or leaks in the wall of small blood vessels, preventing loss of blood from the microvasculature. Circulating platelets have a life span of about 10 days.

A sparse glycocalyx surrounding the platelet plasmalemma is involved in adhesion and activation during blood coagulation. The role of platelets in controlling blood loss and in wound healing can be summarized as follows:

  • Primary aggregation: Disruptions in the microvascular endothelium, which are very common, allow the platelet glycocalyx to adhere to collagen. Thus, a platelet plug is formed as a first step to stop bleeding.
  • Secondary aggregation: Platelets in the plug release a specific adhesive glycoprotein and ADP, which induce further platelet aggregation and increase the size of the platelet plug.
  • Blood coagulation: During platelet aggregation, fibrinogen from plasma, von Willebrand factor and other protein released from the damaged endothelium, and platelet factor 4 from platelet granules promote the sequential interaction of plasma proteins, giving rise to a fibrin polymer that forms a three-dimensional network of fibers trapping red blood cells and more platelets to form a blood clot, or thrombus. Platelet factor 4 is a chemokine for monocytes, neutrophils, and fibroblasts and proliferation of the fibroblasts is stimulated by PDGF.
  • Clot retraction: The clot that initially bulges into the blood vessels lumen contracts slightly because of the interaction of platelet actin and myosin.
  • Clot removal: Protected by the clot, the endothelium and surrounding tunic are restored by new tissue, and the clot is then removed, mainly dissolved by the proteolytic enzyme plasmin, formed continuously through the local action of plasminogen activators from the endothelium on plasminogen from plasma.

[Histology] Basic Four Types of Tissues – Connective Tissues

May 23, 2016 Anatomy, Histology No comments , , , , , , , , , , , , , , , , ,

The different types of connective tissue maintain the form of organs throughout the body. Differences in composition and amount of the cells, fibers, and ground substance make up variety of connective tissue types. Connective tissues provide a matrix that supports and physically connects other tissues and cells together in organs. The interstitial fluid of connective tissue gives metabolic support to cells as the medium for diffusion of nutrients and waste products. Unlike other tissue types, which consist mainly of cells, the major constituent of connective tissue is the extracellular matrix/ECM, which consists of different combinations of protein fibers (e.g., collagens and elastic fibers) and ground substance (a complex of anionic, hydrophilic proteoglycans, glycosaminoglycans/GAGs, and multiadhesive glycoproteins). The hydrated nature of connective tissue ground substance provides the medium for the exchange of nutrients and metabolic wastes between cells and the blood supply.


Fibroblasts and certain other cells are typically present in connective tissue proper. Fibroblasts originate locally from mesenchymal cells and are permanent residents of connective tissue; other cells found here, such as macrophages, plasma cells, and mast cells, originate from hematopoietic stem cells in bone marrow, circulate in the blood, and then move into connective tissue where they function. Leukocytes are transient cells of most connective tissues; they also originate in the bone marrow and move to the connective tissue where they function for a few days, the die by apoptosis.


Fibroblasts, the most common cells in connective tissue, produce and maintain most of the tissue's extracellular components. Fibroblasts synthesize and secrete collagen (the most abundant protein of the body) and elastin, which form large fibers, as well as the GAGs, proteoglycans, and multiadhesive glycoproteins that comprise the ground substance. After being secreted, most of the ECM components undergo further modification outside the cell before assembling as a matrix.

Two levels of fibroblast activity can be observed histologically. Cells with intense synthetic activity are morphologically distinct from the quiescent fibroblasts that are scattered within the matrix they have already synthesized.

Fibroblasts are targets of many families of proteins called growth factors that influence cell growth and differentiation. In adults, connective tissue fibroblasts rarely undergo division. However, stimulated by locally released growth factors, cell cycling and mitotic activity resume when the tissue requires additional fibroblasts.


Adipocytes are found in connective tissue of many organs. These cells are specialized for cytoplasmic storage of lipid as neutral fats, or less commonly for the production of heat.


Macrophages are characterized by their well-developed phagocytic ability and specialize in turnover of protein fibers and removal of dead cells, tissue debris, or other particulate material. Macrophages play an important role in the early stages of repair after tissue damage, and under such conditions of inflammation these cells accumulate in connective tissue by local proliferation of macrophages in addition to monocyte recruitment from the blood. In addition to debris removal, these cells are highly important for the uptake, processing, and presentation of antigens for lymphocyte activation.

They derive from bone marrow precursor cells that divide, producing monocytes that circulate in the blood. These monocytes cross the epithelial wall of venules to penetrate connective tissue, where they differentiate further, mature, and acquire the morphologic features of phagocytic cells. Therefore, monocytes and macrophages are the same cell at different stages of maturation.

The macrophage-like cells have been given different names in different organs, for example, Kupffer cells in the liver, microglial cells in the central nervous system, Langerhans cells in the skin, and osteoclasts in bone tissue. However, all are derived from monocytes and all are long-living cells and may survive for months in the tissues.

Mast cells

The cytoplasm of mast cells is filled with basophilic secretory granules many of which have the bioactive role in the local inflammatory response. A partial list of important molecules released from the secretory granules include:

  • Heparin
  • Histamine
  • Serine proteases
  • Eosinophil and neutrophil chemotactic factors
  • Cytokines
  • Phospholipid

Occurring in connective tissue of many organs, mast cells are especially numerous near small blood vessels in skin and mesenteries (perivascular mast cells) and in the tissue that lines digestive and respiratory tracts (mucosal mast cells); the granule content of the two populations di ers some what. These major locations suggest that mast cells place themselves strategically to function as sentinels detecting invasion by microorganisms.

Plasma cells

Plasma cells are B-lymphocyte-derived, antibody-producing cells. There are at least a few plasma cells in most connective tissues. Their average lifespan is only 10-20 days.


Besides macrophages and plasma cells, connective tissue normally contains other leukocytes derived from cells circulating in the blood. Leukocytes make up a population of wandering cells in connective tissue. They leave blood by migrating between the endothelial cells lining venules to enter connective tissue by a process called diapedesis. Most leukocytes function for a few hours or days in connective tissue and then die. However, some lymphocytes normally leave the interstitial fluid of connective tissue, enter blood or lymph, and move to selected lymphoid organs.


The fibrous components of connective tissue are elongated structures formed from protein that polymerize after secretion from fibroblasts. The three main types of fibers include collagen, reticular, and elastic fibers. Collagen and reticular fibers are both formed by proteins of the colagen family, and elastic fibers are composed mainly of the protein elastin. These fibers are distributed unequally among the different types of connective tissue, with the predominant fiber type usually responsible for conferring specific tissue properties.


The collagens constitute a family of proteins selected during evolution for their ability to form a variety of extracellular structures. The various fibers, sheets, and networks made of collagens are all extremely strong and resistant to normal shearing and tearing forces. Collagen is a key element of all connective tissues, as well as epithelial basement membranes and the external laminae of muscle and nerve cells.

Collagen is the most abundant protein in the human body, representing 30% of its dry weight. An unusually large number of posttranslational processing steps are required to prepare collagen for its final assembly in the ECM. They are many types of collagen and we will take a look at them in future.

Reticular fibers

Found in delicate connective tissue of many organs, reticular fibers consist mainly of collagen type III. This collagen forms an extensive network (reticulum) of extremely thin, heavily glycosylated fibers. Reticular fibers produced by fibroblasts occur in the reticular lamina of basement membranes and typically also surround adipocytes, smooth muscle and nerve fibers, and small blood vessels. Delicate reticular networks serve as the supportive stroma for the parenchymal secretory cells and rich microvasculature of the liver and endocrine glands. Abundant reticular fibers also characterize the stroma of hemopoietic tissue (bone marrow) and some lymphoid organs (eg, spleen and lymph nodes) where they support rapidly changing populations of proliferating cells and phagocytic cells.

Elastic fibers

Elastic fibers form sparse networks interspersed with collagen bundles in many organs,  particularly those subject to much bending or stretching. Elastic fibers have physical properties similar to those of rubber, allowing tissues to be stretched or distended and return to their original shape. Elastic fibers are a composite of fibrillin microfibrils embedded in a larger mass of cross-linked elastin.

Ground Substance

The ground substance of the ECM is a highly hydrated (with much bound water), transparent, complex mixture of macromolecules, principally of three classes: glycosaminoglycans/GAGs, proteoglycans, and multiadhesive glycoproteins. It fills the space between cells and fibers in connective tissue and, because it is viscous, acts as both a lubricant and a barrier to the penetration of invaders. GAGs bind large amounts of water, which causes the polyaminos to swell and occupy a large space in the tissue.

Types of Conenctive Tissue

Screen Shot 2016-05-23 at 10.43.00 PMLoose connective tissue is very common and generally supports epithelial tissue. It comprises a thick layer (lamina propria) beneath the epithelial lining of the digestive system and fills the spaces between muscle and nerve fibers. Usually well-vascularized whatever their location, thin layers of loose connective tissue surround most small blood vessels of the body. Also called areolar tissue, the loose connective tissue typically contain cells, fibers, and ground substance in roughly equal parts. The most numerous cells are fibroblasts, but the other types of connective tissue cells are also present, along with nerves and blood vessels. Collagen fibers predominate, but elastic and reticular fibers are also present. With a moderate amount of ground substance, loose connective tissue has a delicate consistency; it is flexible and not very resistant to stress.

Dense connective tissue is adapted to offer stress resistance and protection. It has the same components found in loose connective tissue, but with fewer cells and a clear predominance of collagen fibers over ground substance. Dense connective tissue is less flexible and far more resistant to stress than loose connective tissue. In dense irregular connective tissue bundles of collagen fibers appear randomly interwoven, with no definite orientation. The collagen fibers form a tough three-dimensional network, providing resistance to stress form all directions. Dense irregular connective tissue is often found closely associated with loose connective tissue, with the two types frequently grading into each other and making distinctions between them somewhat arbitrary. Collagen bundles of dense regular connective tissue are arranged according to a definite pattern, with fibers and fibroblasts aligned in parallel for resistance to prolonged or repeated stresses exerted in the same direction.

In reticular tissue fibers of type III collagen form a delicate 3D network that supports various types of cells. The fibrous network of this specialized connective tissue is produced by modified fibroblasts called reticular cells that remain associated with and partially covering the fibers. The loose disposition of glycosylated reticular fibers provides a framework with specialized microenvironments for cells in hemopoietic tissue and some lymphoid organs (bone marrow, lymph nodes, and spleen). The resulting cell-lined system creates a meshwork for the passage of lymphocytes and lymph. Macrophages and other cells of the mononuclear phagocyte system are also dispersed within these reticular tissues to monitor cells formed there or passing through and to remove debris.

[Histology] Basic Four Types of Tissues – Epithelia Tissues

May 18, 2016 Histology No comments , , , , , , , , , , , , , , , , , ,

Cells and ECM

Tissues have two interacting components: cells and extracellular matrix (ECM). The ECM consists of many kinds of macromolecules, most of which form complex structures, such as collagen fibrils and basement membranes. The ECM supports the cells and the fluid that transports nutrients to the cells, and carries away their catabolites and secretory products. The cells produce the ECM and are also influenced and sometimes controlled by matrix molecules. Cells and matrix interact entensively, with many components of the matrix recognized by and attaching to cell surface receptors. Many of these protein receptors span the cell membranes and connect to structural components inside the cells. Thus, cells and ECM form a continuum that functions together and reacts to stimuli and inhibitors together.

Types of Tissues

The fundamental tissues of the body are each formed by several types of cell-specific associations between cells and ECM. Organs are formed by an orderly combination of several tissues, and the precise combination of these tissues allows the functioning of each organ and of the organism as a whole. Despite its complexity, the human body is composed of only four basic types of tissue, including: epithelia, connective tissue, nervous tissue, and muscle. These tissue, which all contain cells and molecules of the extracellular matrix (ECM), exist in association with one another and in variable proportions and morphologies, forming the different organs of the body. Main characteristics of the four basic types of tissue are list in Table 4-1 below.

Screen Shot 2016-05-17 at 4.03.18 PM

Epithelial Tissue

Epithelial tissues are composed of closely aggregated polyhedral cells with strong adhesion to one another and attached to a thin layer of ECM. Epithelia are cellular sheets that line the cavities of organ and cover the body surface. The principal functions of epithelial tissues include (but not limited to) the following: 1.covering, lining, and protecting surfaces; 2.absorption; 3.secretion.


The shapes and dimensions of epithelial cells are quite variable, ranging from tall columnar to cuboidal to low squamous cells, which are generally dictated by their function. Most epithelial rest on connective tissue that contains the microvasculature bringing nutrients and O2 to both tissues. The area of contact between the epithelium and connective tissue may be increased by irregularities at the interface in the form of small evaginations called papillae which occur most frequently in epithelial tissues subject to friction.

Epithelial cells generally show polarity, with organelles and membrane protein distributed unevenly within the cell. The region of the cell contacting the connective tissue is called the basal pole and the opposite end, usually facing a space, is the apical pole. The two poles of epithelial cells differ in both structure and function. Regions of cuboidal or columnar cells that adjoin the neighboring cells are the lateral surfaces; cell membranes here often have numerous infoldings to increase the area of that surface, increasing its functional capacity.

ECM (basement membranes)

The primary ECM of epithelial tissue is the basement membranes. All epithelial cells in contact with subjacent connective tissue have at their basal surfaces a specialized, feltlike sheet of extracellular material referred to as the basement membrane. The basement membrane may be resolved into two structures. Nearest the epithelial basal poles is an electron-dense layer, 20-100 nm thick, consisting of a network of fine fibrils that comprise the basal lamina. Beneath this layer is often a more diffuse and fibrous reticular lamina. The macromolecules (laminin, type IV collagen, adhesive glycoprotein [entactin/nidogen, and perlecan]) of the basal lamina are secreted at the basal poles of the epithelial cells and form three-dimensional arrays.

Other cells besides those of epithelia (muscle cells, adipocytes, cells supporting peripheral neurons) also produce components of basal laminae but which are called external lamina. Surrounding these cells, this external lamina binds factors important for interactions with other cells and serves as semipermeable barrier further regulating macromolecular exchange between the enclosed cells and connective tissue.

PS: The term "basement membrane" and "basal lamina" are often used indiscriminately, causing confusion. Most authors use "basal lamina" to denote the extracellular epithelial layer seen ultrastructurally and "basement membrane" for the entire structure below an epithelium visible with the light microscope.

Specializations of The Apical Cell Surface

The apical ends of many tall or cuboidal epithelial cells face an organ's lumen and often have specialized projecting structures. These function either to increase the apical surface area for absorption or to move substances along the epithelial.


In epithelial cells specialized for absorption, the apical surfaces present an array of projections called microvilli. The average microvillus is about 1 um long and 0.1 um wide, but with hundreds or thousands present on the end of each absorptive cell, the total surface area can be increased by 20- or 30-fold. Glycocalyx covering intestinal microvilli is thick and includes enzymes for digestion of certain macromolecules.


Stereocilia are a much less common type of apical process, restricted to absorptive epithelial cells lining the epididymis and the proximal part of ductus deferens in the male reproductive system. Stereocilia increase the cell's surface area, facilitating absorption. More specialized stereocilia with a motion-detecting function are important components of inner ear sensory cells.


Cillia are long projecting structures, larger than microvilli, which contain internal arrays of microtubules. Most (if not all) cell types have at least one cilium of variable length, usually called a primary cilium, which is not motile but is enriched with receptors and signal transduction complexes for detection of light, odors, motion, and flow of liquid past the cells. Primary cilia are also important in the early embryo.

Motile cilia are found only in epithelia, where they are abundant on the apical domains of many cuboidal or columnar cells. Typical cilia are 5-10 um long and 0.2 um in diameter. Epithelial cilia exhibit rapid beating patterns of movement that propel a current of fluid and suspended matter in one direction over the epithelium.

Two Tyoes of Epithelia

Epithelia can be divided into two main groups: covering/lining epithelia and secretroy/glandular epithelia. This is an arbitrary division, for there are lining epithelia in which all the cells also secrete or in which glandular cells are distributed among the lining cells (mucous cells in the small intestine or trachea).

Epithelial cells that function mainly to produce and secrete various macromolecules may occur in epithelia with other major functions or comprise specialized organs called glands. Products to be secreted are generally stored in the cells within small membrane-bound vesicles called secretory granules. Structures of glandular epithelia are shown in Table 4-4. Epithelial cells in multicellular glands have three basic mechanisms for releasing their product, and cells involved in each type of secretion are easily recognized histologically:

  • Merocrine secretion: This is the most common method of protein secretion and involves typical exocytosis of proteins or glycoproteins from membrane-bound vesicles.
  • Holocrine secretion: In this process cells accumulate product as they mature and undergo terminal cell differentiation, culminating in complete cell disrutpion with release of the product and cell debris into the gland's lumen. This is best seen in the sebaceous glands of skin.
  • Apocrine secretion: Here product accumulates at the cells' apical ends, portions of which are then extruded to release the product together with a bit of cytoplasm and plasma membrane.  This is the mechanism by which droplets of lipid are secreted in the mammary gland.

Screen Shot 2016-05-18 at 1.38.32 PM

Renewal of Epithelial Cells

Epithelial tissues are relatively labile structures whose cells are renewed continuously by mitotic activity and stem cell populations.

Functions of the Skin

May 9, 2016 Histology, Physiology and Pathophysiology No comments , , , , , , , , , , , , ,

Screen Shot 2016-05-04 at 8.58.32 PMHistology of Skin


The epidermis consists mainly of a stratified squamous keratinized epithelium composed of cells called keratinocytes. There are also other three much less abundant epidermal cell types: pigment-producing melanocytes, antigen-presenting Langerhans cells, and epithelial cells called Merkel cells.

From the dermis, the epidermis consists of four layers of keratinocytes (or five layers in thick skin). The basal layer (stratum basale) is a single layer of basophilic cuboidal or columnar cells on the basement membrane at the dermal-epidermal junction. The stratum basale is characterized by intense mitotic activity and contains, along with the deepest part of the next layer, progenitor cells for all the epidermal layers. In addition to the basal stem cells for keratinocytes found here, a niche for such cells also occurs in the hair follicle sheaths that are continuous with the epidermis. The human epidermis is renewed about every 15 to 30 days, depending on age, the region of the body, and other factors. An important feature of all keratinocytes in the stratum basale is the cytoskeletal keratins, intermediate filaments about 10 nm in diameter. During differentiation, the cells move upward and the amount and types of keratin filaments increase until they represent half the total protein in the superficial keratinocytes.

The spinous layer (stratum spinosum) is normally the thickest layer, especially in the epidermal ridges, and consists of generally polyhedral cells having central nucleoli and cytoplasm actively synthesizing keratins. Just above the basal layer, some cells may still divide and this combined zone is sometimes called the stratum germinativum. The epidermis of thick skin subject to continuous friction and pressure has a thicker stratum spinosum with more abundant tonofibrils (keratin filaments at stratum germinativum assemble into microsocopically visible bundles called tonofibrils) and desmosomes.

The granular layer (stratum granulosum) consists of three to five layers of flattened cells, now undergoing the terminal differentiation process of keratinization. Their cytoplasm is filled with intensely basophilic masses called keratohyaline granules. Among the last activities of the keratinocytes, the lamellar granules (available in these keratinocytes) undergo exocytosis, producing a lipid-rich, impermeable layer around the cells. This material forms a major part of the skin's barrier against water loss. Together, keratinization and production of the lipid-rich layer also have a crucial sealing effect in skin, forming the barrier to penetration by most foreign materials.

The stratum lucidum, found only in thick sin, consists of a thin, translucent layer of flattened eosinophilic keratinocytes held together by desmosomes. Nuclei and organelles have been lost, and the cytoplasm consists almost exclusively of packed keratin filaments embedded in an electron-dense matrix.

The stratum corneum consists of 15 to 20 layers of squamous, keratinized cells filled with birefringent filamentous keratins. These fully keratinized or cornified cells (squames) are continuously shed at the epidermal surface as the desmosomes and lipid-rich cell envelopes breakdwon.

Thick skin is found on the palms of the hands, the soles of the feet, and corresponding surfaces of the fingers and toes. All five epidermal strata occur in thick skin. The epidermis of thick skin ranges  between 0.4 mm and 0.6 mm thick. Thick skin contains sweat glands, but no hair follicles or sebaceous glands. Thin skin covers most of the body. The epidermis lacks the stratum lucidum, so it has only four layers. Thin skin contains the following accessories: hair follicles, sebaceous glands, and sweat glands. The epidermis of thin skin ranges from 0.075 mm to 0.150 mm thick.

Other Cells in Epidermis


The color of the skin is the result of several factors, the most important of which are the keratinocytes' content of melanin and carotene and the number of blood vessels (hemoglobin) in the dermis. Melanocyte is a specialized cell of the epidermis found among the cells of the basal layer and in hair follicles. Melanocytes synthesize melanin and transfer them into nearby keratinocytes.

The first step in melanin synthesis is catalyzed by tyrosinase (the source of melanin is tyrosine), a transmembrane enzyme in Golgi-derived vesicles. Tyrosinase activity converts tyrosine into 3,4 – dihydroxyphenylalanine (DOPA), which is then further transformed and polymerized into the different forms of melanin. Melanin pigment is linked to a matrix of structural proteins and accumulates in the vesicles until they form mature elliptical granules abuot 1 um long called melanosomes. Melanosomes are then transported via kinesin to the tips of the cytoplasmic extensions (Figure 18-7, one melanocyte plus the keratinocytes into which it transfers melanosomes make up an epidermal-melanin unit and the density of such units in skin is similar in all individuals). The neighboring keratinocytes phagocytose the tips of these dendrites, take in the melanosomes, and transport them by dynein toward their nuclei. The melanosomes accumulate within keratinocytes as a supranuclear cap that prior to keratinization absorbs and scatters sunlight, protecting DNA of the living cells from the ionizing, mutagenic effects of UV radiation.

Langerhans Cells

Antigen-presenting cells (APCs) called Langerhans cells, which are usually most clearly seen in the spinous layer, represent 2% to 8% of the epidermal cells. Cytoplasmic processes extend from these dendritic cells between keratinocytes of all the layers, forming a fairly dense network in the epidermis. Like other APCs, they develop in the bone marrow, move into the blood circulation, and finally migrate into stratified squamous epithelia where they are difficult to identify in routinely stained sections.

Langerhans cells bind, process, and present antigens to T lymphocytes in the same manner as immune dendritic cells in other organs. Microorganisms cannot penetrate the epidermis without alterting these dendritic cells and triggering an immune response. Langerhans cells, along with more scattered epidermal lymphocytes and other APCs in the dermis, make up a major component of the skin's adaptive immunity.

Merkel Cells

Merkel cells, or epithelial tactile cells, are sensitive mechanoreceptors essential for light touch sensation. They are abundant in highly sensitive skin like that of fingertips and at the bases of some hair follicles.


Screen Shot 2016-05-08 at 7.17.20 PMThe dermis is the layer of connective tissue that supports the epidermis and binds it to the subcutaneous tissue. The thickness of the dermis varies with the region of the body and reaches its maximum of 4 mm on the back. The surface of the dermis is very irregular and has many projections (dermal papillae) that interdigitate with projections (epidermal pegs or ridges) of the epidermis, especially in skin subject to frequent pressure, where they reinforce the dermal-epidermal junction.

A basement membrane always occurs between the stratum basale and the dermis, and follows the contour of the interdigitations between these layers. Nutrients for keratinocytes diffuse into the avascular epidermis from the dermal vasculature through this basement membrane.

The dermis contains two sublayers with indistinct boundaries. The thin papillary layer, which includes the dermal papillae, consists of loose connective tissue, with types I and III collagen fibers, fibroblasts and scattered mast cells, macrophages, and other leukocytes. From this layer, anchoring fibrils of type VII collagen insert into the basal lamina, helping to bind the dermis to the epidermis. The underlying reticular layer is much thicker, consists of dense irregular connective tissue (mainly bundles of type I collagen), with more fibers and fewer cells than the papillary layer. A network of elastic fibers is also present, providing elasticity to the skin. Between the collagen and elastic fibers are abundant proteoglycans rich in dermatan sulfate.

Blood and Lymphatic vessels

Both dermal regions contain a rich network of blood and lymphatic vessels. Nutritive vessels form two major plexuses. 1.Between the papillary and reticular dermal layers lies the microvascular subpapillary plexus, from which capillary branches extend into the dermal papillae and form a rich, nutritive capillary network just below the epidermis. 2.A deep plexus with larger blood and lymphatic vessels lies near the interface of the dermis and the subcutaneous layer.

Lymphatic vessels begin in the dermal papillae and converge to form two plexuses located with the blood vessels.

Subcutaneous Tissue

The subcutaneous layer consists of loose connective tissue that binds the skin loosely to the subjacent organs, making it possible for the skin to slide over them. This layer contains adipocytes that vary in number in different body regions and vary in size according to nutritional state. The extensive vascular supply at the subcutaneous layer promotes rapid uptake of insulin or drugs injected into this tissue.

The integument is more than just a wrapping around the body. It serves many varied functions, including protection, prevention of water loss, temperature regulation, metabolic regulation, immune defense, sensory reception, and excretion.


The skin acts as a physical barrier that protects the entire body from physical injury, trauma, bumps, and scrapes. It also offers protection against harmful chemicals, toxins, microbes, and excessive heat or cold. Paradoxically, it can absorb certain chemicals and drugs. Thus, the skin is said to be selectively permeable because some materials are able to pass through it, while others are effectively blocked. The epidermis is designed to withstand stresses and regenerate itself continuously throughout a person's lifetime. The skin also protects deeper tissues from solar radiation, especially ultraviolet rays. When exposed to the sun, the melanocytes become more active and produce more melanin, thus giving the skin a darker, tanned look. Even when you get a sunburn, the deeper tissues (muscles and internal organs) remain unaffected.

Prevention of Water Loss

The epidermis is water resistant and helps prevent unnecessary water loss. Water cannot easily enter or exit the skin, unless it is specifically secreted by the sweat glands. The skin also prevents the water within the body cells and in the extracellular fluid from leaking out. When the skin is severely burned, a primary danger is dehydration, because the individual has lost the protective skin barrier, and water can escape from body tissues.

Although the integument is water resistant, it is not entirely waterproof. Some interstitial fluids slowly escape through the epidermis to the surface, where they evaporate into the surrounding air, a process called transepidermal water loss (TEWL). About 500 mL of water is lost daily by evaporation of moisture from the skin or from respiratory passageways during breathing. Insensible perspiration is the release of water vapor from sweat glands under "normal" circumstances when we are not sweating. In contrast, sensible perspiration is visible sweating. On most parts of the skin, water vapor released from sweat glands during insensible perspriation mixes with sebaceous secretions (sebum) to produce a thin, slightly acidic film (pH 4-6) over the surface of the epidermis. This film helps slow down TEWL by forming an oily barrier over the surface of the skin. The acidic nature of the barrier also prevents the invasion of certain bacteria.

Temperature Regulation

Body temperature is influenced by vast capillary networks and sweat glands in the dermis. When the body is too warm and needs to dissipate heat, the diameter of the blood vessels in the dermis enlarges to permit more blood flow through the dermis, and sweat glands release fluid onto the skin surface (evaporation). As relatively more blood flows through these dermals vessels, the warmth from the blood dissipates through the skin, and the body cools off by evaporation of the sweat. Conversely, when the body is cold and needs to conserve heat, the blood vessels in the dermis constrict to reduce blood flow. In an effort to conserve heat, more blood is shunted to deeper body tissues, and relatively less blood flows in the dermal blood vessels.

The arrector pili muscle, a small bundle of smooth muscle cells, extends from the midpoint of the fibrous sheath to the dermal papillary layer. Contraction of these muscles pulls the hair shafts to a more erect position, usually when it is cold in an effort to trap a layer of warm air near the skin. In regions where hair is fine, contraction of arrector pili muscles is seen to produce tiny bumps on the skin surface "goose bumps" where each contracting muscle distorts the attached dermis.

PS: The termoregulatory function of skin involves numerous arteriovenous anastomoses or shunts located between the two major plexuses. The shunts decrease blood flow in the papillary layer (see Figure ) to minimize heat loss in cold conditions and increase this flow to facilitate heat loss when it is hot, thus helping maintain a constant body temperature.Screen Shot 2016-05-09 at 12.49.15 PM

Metabolic Regulation

Vitamin D3 is a cholesterol derivative synthesized from cholecalciferol, which is produced by some epidermal cells when they are exposed to ultraviolet radiation. Calcitriol is synthesized from the cholecalciferol by some endocrine cells in the kidney. Calcitriol, the active form of vitamin D3, is a hormone that promotes calcium and phosphorus absorption from ingested materials across the wall of the small intestine. Thus, the synthesis of vitamin D3 is important in regulating the levels of calcium and phosphate in the blood. As little as 15 minutes of direct sunlight a day may provide your body with its daily vitamin D requirement.

Immune Defense

The epidermis contains a small population of immune cells. These immune cells (derived from a type of white blood cell), called epidermal dendritic cells, or langerhans cells, play an important role in initiating an immune response by phagocytizing pathogens that have penetrated the epidermis and also against epidermal cancer cells.

Sensory Reaction

The skin contains numerous sensory receptors. These receptors are associated with nerve endings that detect heat, cold, touch, pressure, texture, and vibration. Because your skin is responsible for perceiving many stimuli, it needs different sensory receptor types to detect, distinguish, and interpret these stimuli.

With its large surface and external location, the skin functions as an extensive receiver for various stimuli from the environment. Diverse sensory receptors are present in skin, including both simple nerve endings with no Schwann cell or collagenous coverings and more complex structures with sensory fibers enclosed by glia and delicate connective tissue capsules.

Unencapsulated receptors:

  • Merkel cells
  • Free nerve endings
  • Root hair plexuses

Encapsulated receptors:

  • Meissner corpuscles
  • Lamellated (pacinian) corpuscles
  • Krause end bulbs
  • Ruffini corpuscles

Excretion by Means of Secretion

Skin exhibits an excretory function when it secretes substances from the body during sweating. Sweating, or sensible perspiration, occurs when the body needs to cool itself off. Notice that sweat sometimes feels "gritty" because of the waste products being secreted onto the skin surface. These substances include water, salts, and urea, a nitrogen-containing waste product of body cells. In addition, the skin contains sebaceous glands that secrete an oily material called sebum, which lubricates the skin and hair.