Epithelial tissues are formed by closely apposed polygonal cells with little or no intercellular material. They occur as membranes and as glands. Membranes are formed by sheets of cells and cover an external surface or line internal surface. Glands develop from epithelial surfaces by downgrowths into underlying connective tissue, and usually the connection to the surface remains as the duct of the gland. Such are exocrine glands, the secretion passing externally to the surface. In some cases, the surface connection is lost and the gland secretes internally into the vascular system; these are endocrine glands.
All epithelia lie upon or are surrounded by a basal lamina that separates the epithelium from subjacent connective tissue, blood vessels, and nerves lying in that connective tissue. Functionally, epithelia form the coverings or linings of surfaces, provide secretions from both membranes and glands, and are involved in the process of absorption. A few specialized epithelial cells are contractile (myoepithelial cells), and a few are sensory (neuroepithelia).
MEMBRANES (SURFACE EPITHELIUM)
Being packed closely together, most epithelial cells are polygonal in outline although they may be highly irregular. Their shape and arrangement in layers are the two factors that provide the basis for classification of membranes.
With respect to shape, epithelial cells basically are squamous, cuboidal, or columnar, with intermediate forms. Squamous cells are very flat, with height much less than width. In profile, they show a central thickening at the site of the nucleus. Cuboidal cells are box-like, with height and width approximately equal, and columnar cells have a much greater height than width. Nuclei of all are parallel to the main axis of the cell with a shape corresponding to that of the cell. Thus, nuclei are spheroidal
Cells are arranged in membranes in one or more layers. Simple epithelia are those with cells in a single layer; all cells contact the basal lamina and reach the surface. Stratified epithelia are those with cells in two or more layers, only the cells of the deepest or basal layer contacting the basal lamina. Pseudostratified epithelia are those in which all cells contact the basal lamina but not all reach the surface. Basically, several cell types are present in a single layer, usually with nuclei at different levels giving the false appearance of several layers.
SIMPLE EPITHELIA
Simple squamous epithelium. Simple squamous epithelium is composed of very thin, flat cells of irregular outline fitted closely together to form a continuous sheet.
Simple cuboidal (cubical) epithelium. Simple cuboidal (cubical) epithelium is so termed because of its appearance in sections at right angles to the surface of the membrane, each cell appearing box-like or cube-like. From the surface, the cells appear as polygons.
Simple columnar epithelium. Simple nonciliated columnar epithelium has a similar appearance in surface view to the simple cuboidal type, but in perpendicular sections it is seen to be composed of tall cells, the nuclei of which usually are all approximately at the same level and situated nearer to the basal than to the apical (luminal) surface.
PSEUDOSTRATIFIED EPITHELIA
Pseudostratified columnar epithelium is composed of more than one type of cell with the cell nuclei lying at different levels in a perpendicular section, thus giving the impression that the membrane is composed of more than one layer of cells. Some of the cells may not reach the lumen, although all are adjacent to the basal lamina.
STRATIFIED EPITHELIA
Stratified squamous epithelium. This type of epithelium is made up of several layers of cells. The cells of the deepest (or basal) layer rest on the basement membrane: they are usually columnar in shape. Lying over the columnar cells there are polyhedral or cuboidal cells. As we pass towards the surface of the epithelium these cells become progressively more flat, so that the most superficial cells consist of flattened squamous cells.
Stratified squamous epithelium can be divided into two types: non-keratinized and keratinized. In situations where the surface of the epithelium remains moist, the most superficial cells are living and nuclei can be seen in them. This kind of epithelium is described as non-keratinized. In contrast, at places where the epithelial surface is dry (as in skin) the most superficial cells die and lose their nuclei. These cells contain a substance called keratin, which forms a non-living covering over the epithelium. This kind of epithelium constitutes keratinized stratified squamous epithelium.
Stratified squamous epithelium (both keratinized and non-keratinized) is found over those surfaces of the body that are subject to friction. As a result of friction the most superficial layers are constantly being removed and are replaced by proliferation of cells from the basal (or germinal) layer. This layer, therefore, shows frequent mitoses.
Transitional epithelium. This is multi-layered epithelium and is four to six cells thick. It differs from stratified squamous epithelium in that the cells at the surface are not squamous. The deepest cells are columnar or cuboidal. The middle layers are made up of polyhedral or pear-shaped cells. The cells of the surface layer are large and often shaped like an umbrella. Transitional epithelium can be stretched considerably without being damaged. When stretched it appears to be thinner and the cells become flattened or rounded.
GLANDS
Some epithelial cells may be specialized to perform a secretory function. Such cells, present singly or in groups, constitute glands.
From this is obvious that some glands are unicellular. Unicellular glands are interspersed amongst other (non-secretory) epithelial cells. They can be found, for example, in the epithelium lining the intestines.
Most glands are, however, multicellular. Such glands develop as diverticula from epithelial surfaces. The “distal” parts of the diverticulae develop into secretory elements, while the “proximal” parts form ducts through which secretions reach the epithelial surface.
Those glands that pour their secretions on to an epithelial surface, directly or through ducts are called exocrine glands (or externally secreting glands). Some glands lose all contact with the epithelial surface from which they develop: they pour their secretions into blood. Such glands are called endocrine glands, internally secreting glands, or duct-less glands.
When all the secretory cells of an exocrine gland discharge into one duct the gland is said to be a simple gland. Sometimes there are a number of groups of secretory cells, each group discharging into its own duct. These ducts unite to form larger ducts which ultimately drain on to an epithelial surface. Such a gland is said to be a compound gland.
Both in simple and in compound glands the secretory cells may be arranged in various ways.
(a) The secretory elements may be tubular. The tube may be straight, coiled, or branched.
(b) The cells may form rounded sacs or acini.
Exocrine glands may be:
1. Unicellular.
2. Simple tubular.
3. Simple alveolar (or acinar).
4. Compound tubular.
5. Compound alveolar.
6. Compound tubulo-alveolar (or racemose).
Exocrine glands may be classified on the basis of the nature of their secretions into mucous glands and serous glands. In mucous glands the secretion contains mucopolysaccharides. The secretion collects in the apical parts of the cells. As a result nuclei are pushed to the base of the cell, and may be flattened.
The secretions of serous glands are protein in nature. The cytoplasm of these cells is granular. Their nuclei are centrally placed. Some glands contain both serous and mucous elements.
Epithelia in secretory portions of glands show specializations of structure depending upon the nature of secretion as follows.
(1) Cells that are protein secreting (e.g. hormone producing cells) have a well developed rough endoplasmic reticulum, and a supranuclear Golgi complex. Secretory granules often fill the apical portions of the cells.
(2) Mucin secreting cells have a well developed rough endoplasmic reticulum (where the protein component of mucin is synthesized), and a very well developed Golgi complex (where proteins are glycosylated).
(3) Steroid producing cells are characterized by the presence of extensive smooth endoplasmic reticulum, and prominent mitochondria.
In addition to their classification on the basis of structure, and on the basis of the nature of their secretion, exocrine glands can also be classified on the basis of the manner in which their secretions are poured out of the cells. In most exocrine glands secretions are thrown out of the cells by a process of exocytosis the cells remaining intact: this manner of secretion is described as merocrine (sometimes also called eccrine or epicrine). In some glands the apical parts of the cells are shed off to discharge the secretion: this manner of secretion is described as apocrine. In some glands the entire cell disintegrates while discharging its secretion. This manner of discharging secretion is described as holocrine. Depending on the mode of secretion glands may, therefore, be described as merocrine, apocrine or holocrine.
The secretory elements of exocrine glands are held together by connective tissue. The glandular tissue is often divisible into lobules separated by connective tissue septa. Aggregations of lobules may form distinct lobes. The connective tissue covering the entire gland forms a capsule for it.
When a gland is divided into lobes the ducts draining it may be intralobular (lying within a lobule), interlobular (lying in the intervals between lobules), or interlobar (lying between adjacent lobes), in increasing order of size.
Blood vessels and nerves pass along connective tissue septa to reach the secretory elements. As a rule exocrine glands have a rich blood supply. Their activity is under nervous or hormonal control. The secretory cells of a gland constitute its parenchyma, while the connective tissue in which the former lie is called the stroma.
Questions:
1. Classification of the epithelial tissue.
2. Classification of membranes (surface epithelium).
3. Structural features and types of simple epithelium.
4. Structural features of pseudostratified epithelium.
5. Structural features and types of stratified epithelium.
6. Structural classification of glands.
7. Classification of glands on the basis of the nature of secretion.
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