1. SIMPLE SQUAMOUS EPITHELIUM

Vascular endothelium
Aorta. The epithelium that lines the internal tunic or tunica intima is the endothelium. It’s a simple squamous epithelium and consists of a single layer of thin, flat cells. The cells appear thicker next to their nuclei.

2. SIMPLE CUBOIDAL EPITHELIUM

Glandular duct
Salivary gland. Between the seromucous acinar units there are numerous gland ducts lined by simple cuboidal epithelium. Note the cuboidal morphology of the cells and the central location of the nucleus.

Thyroid follicles
Thyroid gland. The thyroid gland consists of numerous follicles lined by follicular cells forming a simple cuboidal epithelium.

3. SIMPLE COLUMNAR EPITHELIUM

Intestine
Small intestine. The intestinal villi are lined by simple columnar epithelium. This epithelium consists of tall, narrow cells, and usually the nuclei are oval and are located near the base of each cell. The cells have prominent microvilli (only visible using electron microscopy); by light microscopy we call the numerous microvilli striated border. Because the glycocalyx is particularly well developed on microvilli of epithelial cells this striated border is PAS-positive. Goblet cells are dispersed among the columnar cells.

PAS stain

4. PSEUDOSTRATIFIED COLUMNAR EPITHELIUM

Trachea
Trachea is lined by a ciliated pseudostratified columnar epithelium (respiratory epithelium). Because the cells are irregular in shape and size, and their nuclei are located at various levels, this epithelium appears to have several layers. All cells rest on the basement membrane but not all reach the surface. Goblet cells are dispersed among the columnar cells reaching the surface.

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5. NONKERATINIZED STRATIFIED SQUAMOUS EPITHELIUM

Esophagus, dog
The esophagus in carnivores is lined by a nonkeratinized stratified squamous epithelium. This epithelium consists of several layers of cells, with only the superficial cells having a squamous shape. Because is nonkeratinized, the flattened superficial cells retain their nuclei. Remember that the degree of keratinization of the stratified squamous epithelium of the esophagus varies with the species.

6. KERATINIZED STRATIFIED SQUAMOUS EPITHELIUM

Esophagus. Rat; ruminant
The stratified squamous epithelium of the esophagus is keratinized to a high degree in rats and to a medium degree in ruminants.

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Epidermis
The epidermis, the outermost layer of the skin, is a keratinized stratified squamous epithelium. At least four layers can be identified (stratum basale, stratum spinosum, stratum granulosum, and stratum corneum). The cells on the surface layer have lost their nuclei and are filled with keratin. The thickness and degree of keratinization of the epidermis varies with the species and the anatomical region.

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7. TRANSITIONAL EPITHELIUM

Ureter and Urinary Bladder
Transitional epithelium is restricted to the urinary system. The urinary bladder is lined by several layers of transitional epithelial cells, depending on the species and degree of distension. When the urinary bladder is distended, the epithelial cells become flattened and elongated, and the total height of the epithelium decreases.

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8. GOBLET CELLS

Trachea. Intestine.
The goblet cell is an example of unicellular gland, and consists of a single secretory cell in a nonsecretory epithelium. It is a specialized epithelial cell that produces mucinogen, which is released onto the surface. We can observe numerous goblet cells dispersed among the columnar epithelial cells of trachea and intestine. Goblet cells are PAS-positive.

Trachea

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Intestine

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9. MUCOUS GLAND

Esophagus
Mucous glands are present in the submucosa layer of the esophagus in some species. The cells of the mucus-secreting units are filled with mucinogen, the precursor of mucus, which stains light with HE. The nuclei are displaced toward the basal part of the cell and are flattened against the cell membrane. Mucous glands are PAS-positive.

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PAS stain

10. SEROUS GLAND

Pancreas
The exocrine portion of the pancreas consists of serous glandular acini (secretory units). The cells of the secretory units have spherical nuclei near the center of the cells, and the apical cytoplasm is filled with small secretory granules called zymogen granules. The cytoplasm surrounding the nuclear region is basophil because contains a well-developed RER, necessary for the synthesis of pancreatic enzymes.

11. SEROMUCOUS GLAND

Salivary gland
Seromucous glands contain both mucous and serous cells. Some secretory units (acini) consist mainly of mucous cells, but few serous cells can be observed located at the periphery of the secretory unit being a half-moon-shaped cluster of cells called serous demilunes.

Other seromucous glands are composed of a mixture of all mucous acini and all serous acini, rather than each acinus having some serous and some mucous cells.