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