I. DEFINITION
A general classification of cells that are specialized to cover most of the body surfaces (internal and external), and form the parenchyma of its exocrine and endocrine glands. This fundamental tissue is characterized by its lack of vessels, little intercellular material, and by the presence of a basement membrane anchoring it to the underlying connective tissue stroma and separating it from its nutrient vessels.
II. SURFACE OR SHEET-LIKE EPITHELIA
1. Classification is based on:
a. Number of cell layers
1. Simple - one cell layer thick
2. Pseudostratified - all cells touch the basal lamina, but some do not reach the surface
3. Stratified - two or more cell layers thick
b. Shape of surface layer
1. Examples: simple columnar, stratified squamous, simple squamous(special forms of simple squamous epithelium are endothelium - that forms the inner surface of blood vessels; and mesothelium - that forms the surface of the serous cavities)
2. Transitional (urothelium) is a special type of stratified epithelium that lines the ureter and urinary bladder, and has large distinctive cells on its surface, that may have two nuclei.
2. Basement membrane support -
Description: A periodic acid Schiff (PAS) stainable, acellular sheet between the epithelium and the underlying connective tissue. In glands, it also encloses contractile cells called myoepithelium. The basement membrane is described by most histologists as being composed of two parts.
a. Basal lamina - A relatively amorphous layer only resolved with EM. Some of its components are type IV collagen, heparan sulfate, fibronectin and laminin. It is usually separated from the epithelium by an electron clear band (lamina rara) thought to be bridged by laminin and fibronectin. These molecules have sites that bind more or less tightly to collagen and heparan in the basement membrane and to integrins that span the cell membrane.
b. Reticular lamina - A rather thick layer of reticular fibers (type III collagen) in an amorphous matrix. It is produced by the connective tissue. Silver stains stain reticular fibers but they can also stain special forms of the basal lamina. Type 7 collagen has been observed spanning the space between the reticular lamina and basal lamina.
3. Association mechanisms - Epithelial cells are anchored to each other by a variety of mechanisms. There is also a means for cell-cell communication. Epithelial sheets separate compartments having different compositions. They therefore serve to protect and control the integrity and composition of those compartments by regulating what crosses that boundary.
a. Cement - glycoproteins (cell adhesion molecules) of the glycocalyx and calcium ions bind cells together
b. Interdigitation of cell processes of adjacent cells
c. Desmosomes, between cells, and hemidesmosomes or attachment plaques, at basal lamina, are localized spots where there is an accumulation of fibrillar and granular cementing material between cells or between the cell and the basal lamina. Cytoplasmic intermediate filaments of the tonofilament (cytokeratin) type, anchor into a region of increased density next to the plasma membrane in these regions.
d. Junctional complex holds cuboidal and columnar cells together at their apex
e. Gap Junctions or communicating junctions
Clusters of transmembrane proteins, conexins, form channels that directly connect the cytoplasm of adjacent cells so that small molecules can easily move from cell to cell.
f. Occluding junctions in simple squamous epithelium
These junctions vary from tissue to tissue in the tightness of the seal. In brain capillaries the seal is very tight but in muscle more diffusion between endothelial cells occurs.
1. Absorption, and solute transport (secretion is discussed in the next section)
a. Microvilli are variable in number (from tightly packed, intestine and kidney to sporadic) and regularity.
They are projections of cytoplasm containing longitudinally oriented but irregularly distributed microfilaments (actin) that are anchored in the filaments of the terminal web. Their surrounding plasma membrane contains proteins that serve to transport specific nutrients and ions into the cell (absorption)
b. Basal infoldings and interdigitations of plasma membrane (basal lamina does not follow folds)
These are extensive and associated with many mitochondria where transport of ions is a major function.
c. Cilia move fluids along surfaces
They are projections of cytoplasm surrounded by plasma membrane containing longitudinally oriented and precisely distributed microtubules. There are nine doublets around the perimeter and a single pair in the center. Movement, perpendicular to the plane of the central pair, occurs when the doublets interact via dynein arms. These are missing in "Kartagener's syndrome." Cilia on adjacent cells beat in a coordinated fashion because the cells are coupled through gap junctions. The microtubules arise from a centriole-like basal body found at the base of each cilium.
d. Secretion - examples are: goblet cells, stomach (gastric) lining cells and pancreatic acinus
III. GLANDULAR EPITHELIA - EXOCRINE
A. Morphology -
As an example study a mixed gland. Identify acini and ducts. Distinguish between serous and mucous cells. Identify serous demilunes.
1. General - exocrine glands consist of epithelium that forms tubes and sacs rather than sheets like surface epithelia. They consist of:
These have various shapes (spherical to tubular). Contractile myoepithelial cells are usually found between the secretory cells and the basal lamina. Rough endoplasmic reticulum is in the base of the cells, and secretion granules are stored in the apex of the secretory cells.
1. Acini (alveoli) - hollow ball-like or short cylindrical units
2. Tubules
3. Tubulo-alveolar - tubes ending in acini
These form a tree-like system of drainage tubes. The smaller ones, surrounded by secretory units, are intralobular, while the larger ones are found in connective tissue between lobules and are called interlobular and interlobar ducts. Small ducts are usually formed by simple cuboidal epithelium, but larger ones are sometimes stratified. Look for both kinds and learn to visualize and explain how the plane of section will influence the appearance of the epithelia forming the ducts. Simple glands have a single, unbranched duct while compound glands have a branched duct system.
2. Cytology
a. Organelles involved in synthesis and secretion.
1. RER - assembly of polypeptides
2. Golgi - addition of polysaccharides
3. Secretion granules: these are surrounded by a single membrane (membrane limited) and contain the secretory product. If the product is largely globular protein the secretion is watery and the cells and the secreting units are said to be serous; but if the product is largely polysaccharide the secretion is viscous and they are called mucous cells, mucous acini, or mucous glands. Many glands contain both serous and mucous secreting units.
B. Functions of exocrine glands and their morphological basis
1. Secretion mechanisms
a. Exocytosis or merocrine - membrane surrounding granules fuses with the plasma membrane, releasing granule contents into extracellular space.
b. Apocrine - a small blob of cytoplasm containing fat or other materials - is pinched off the cell apex. The released material is surrounded by plasma membrane - milk fat is secreted this way.
c. Holocrine - the entire cell filled with its product is pushed out of the gland or from a surface - sebaceous gland.
2. Control:
a. Regulated Secretion: Glandular epithelia that contain stored secretion granules that are released when stimulated by neural and/or hormonal signals are said to be regulated.
b. Constitutive (Anon-regulated) secretion: This occurs in cells that secrete their products continuously and do not contain noticeable secretion granules
3. Ducts modify ionic composition
IV. GLANDULAR EPITHELIA - ENDOCRINE
A. Definitions:
Hormones are substances that regulate the activity of cells bearing receptors for the hormone either on their surface or in their cytoplasm. Hormones are carried by the blood stream from their cells of origin to the target cells. They can be derivatives of amino acids ( for example, triiodothyronine, epinephrine), polypeptides (for example, calcitonin, oxytocin), proteins or glycoproteins (for example, thyroid stimulating hormone) or steroids (for example, cortisol, testosterone).
Paracrine (acting on neighboring cells) and autocrine (acting on the cell of origin) secretions also regulate cells via their receptors. However, since they do not depend upon the circulation to find their target cells these are not generally called hormones nor are their cells of origin included in the classical system of endocrine organs.
B. Morphology of endocrine organs-
Endocrine organs are composed of epithelial cells that produce hormones. Epithelial cells are organized in small clumps, irregular cords, irregular plates, or follicles surrounded by basal lamina and closely associated with numerous fenestrated capillaries. There are no ducts. Some hormones are produced by isolated cells within a surface epithelium - the diffuse neuroendocrine system.
C. Notes on function
1. Secrete into interstitium around capillaries that are usually fenestrated so that the secretion easily enters the blood.
2. Products may be small molecules, polypeptides, proteins, or steroids.
3. Control by:
a. Other hormones, paracrine factors, or neurotransmitters
b. Circulating level of substance controlled - for example: glucose and insulin. This is a feedback system.
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Last Revised: Tue, Nov 14, 2006
