Epithelial cells line or cover all internal and external surfaces of the
body. They may function in protection, secretion, excretion and as components
of special senses. Since function is normally reflected in tissue structure,
you should be able to make an educated guess as to tissue function from
a microscopial study of any slide. Epithelia may then be classified as either
a) lining or covering epithelia or b) glandular epithelia.
The lining and covering epithelia are classified on the basis of, a) cell shape, b) number of cell layers, and c) cell surface specializations.
1.Cuboidal Epithelial Cells - Review your study of cuboidal cells p. 5 (Slide #16).
2.Columnal Epithelial Cells -
a) Simple Columnar Epithelium - These cells range in size from those whose heightis only slightly greater than their width, to those which are greatly elongated. They may be either ciliated or nonciliated.
Identify and study the columnar epithelium in the section of intestine (Slide #8).
Note the shape and positions of the nuclei. Are these cells ciliated?
b) Stratified Columnar Epithelium - is rare and difficult to find (see demonstration of the larynx). The basal cell layer is composed of squat, polyhedral cells, similar to the basal cells of stratified squamous epithelium. The superficial cells are columnar in shape and may be ciliated.
c) Pseudostratified Columnar Epithelium - (see Slide #130) In this type of epithelium, the nuclei lie at different levels and the cells may be variable in shape, thus giving the mistaken impression of being a stratified (multi-layered) epithelium. In actual fact, every single cell in this epithelium rests on the basal lamina; not every cell, however, reaches to the free surface.
In the larger passageways of the respiratory system (Olfactory epithelium - Slide #130; dog lung - Slide #70) there are numerous globlet cells (see pg. 8 - Glandular Epithelium - Unicellular glands).
Are any of these cells ciliated?
3. Squamous Epithelial Cells -
a) Simple Squamous - This eplithelium is composed of flattened, polygonal cells, which fit closely together. Examine the parietal layer of Bowman's capsule in the kidney (Slide #16 & Slide # 136) and the endothelial lining of the blood vessels located within the lamina propria (outer connective tissue layer) of the esophagus (Slide #4).
b) Stratified Squamous - Study sections of this type of epithelium and note that only the most superficial layers of cells are flattened and these may or may not be cornified (keratinized), depending on the location within the body. The deeper layers are polyhedral and the basal layer is cuboidal or low columnar.
Slide #26 - Human Skin
Slide #4 - Esophagus
Slide #101- Cornea (Convex surface)
Slide #108- Human Lip
What can you say about the function of each?
4. Transitional Epithelium -
is highly specialized and lines the walls of hollow organs which are subject to distension. Since the cells are capable of sliding over one another, in the contracted state the epithelium is multi-layered while in the distended condition only a few cell layers are observed. The most superficial cells are large and dome-shaped, while the intermediate and basal layers resemble stratified squamous epithelial cells.
Examine Slide #89
Was the bladder fixed in the contracted or the distended state?
5. Glandular Epithelium -
Secretion is one of the major functions of epithelia. Some secretions are delivered directly into the blood stream (Endocrine Glands) while others reach the surface of a membrane via ducts (Exocrine Glands).
a) Exocrine Glands may be classified on the basis of several criteria:
1. Number of Cells -
i) Unicellular Gland - Examine the goblet cells in the ileum (Slide #8).
ii) Multicellular Glands - (See Below)
2. Shape of the secretory units -
i) Tubular - examine the deep intestinal glands in the dog colon
(Slide #9) (simple tubular mucous) or sweat (Slide #26) of the skin (simple coiled tubular serous);
ii) Acinar (alveolar).
iii) Tubuloalveolar - examine the compound, serous glands in the human parotid (salivary) gland (Slide #66).
3. Type of secretion -
i) serous (Slide #66).
ii) mucous (Slide #8).
iii) mixed (mucous/serous) (Slide #95).
4. Multiplicity of secretory units and branching of ducts -
i) Simple or
ii) Compound = Branched
Examine the example of a branched tubular gland and compare it with the straight (simple) tubular gland.
b) Endocrine Glands - Examine the thyroid follicles (Slide #11) and the clump-cord type of construction in the cortex (outer region) of the adrenal gland (Slide #73). In both cases, note the rich vascularity of the organ.