1. TRACHEA

The mucosa of the trachea is lined by pseudostratified ciliated columnar epithelium with goblet cells. The lamina propria-submucosa consists of dense irregular connective tissue. Tracheal seromucous/mucous glands may be present within the submucosal region. The most distinctive feature of the trachea are the cartilages, which in most species are C-shaped pieces of hyaline cartilage that are incomplete dorsally. The trachealis muscle is a band of smooth muscle whose fibers extend transversely between the dorsal free ends of the cartilage. A dense, irregular connective tissue surrounds the outside of the cartilage rings.

PAS stain

2. LUNG

The lung is encapsulated by a layer of connective tissue and mesothelial cells termed the visceral pleura.

The general appearance of the lung is a network-like organ due to the presence of numerous pulmonary alveoli.

Bronchi are lined by a pseudostratified ciliated columnar epithelium with goblet cells. The epithelial height and the thickness of the lamina propria progressively decrease proximally to distally. Submucosal glands are fewer and smaller in proximal bronchi as compared to the trachea. The amount of cartilage decrease in more distal bronchial generations while the relative abundance of smooth muscle increases.

Bronchioles are composed of epithelium (simple columnar or cuboidal), smooth muscle, and small amounts of connective tissue without glands or cartilage.

Respiratory bronquioles are similar to the terminal bronchioles, with the exception that the epithelium is interrupted by alveoli.

Alveoli are spheroidal airspaces separated by thin sheets of connective tissue called interalveolar septa. The interalveolar septa contained a capillary plexus and are covered on both sides by a layer of epithelium. The type I pneumocyte is a flat cell with a centrally placed nucleus. The type II pneumocyte is a cuboidal cell with a central nucleus.

3. AVIAN LUNG

Intrapulmonary primary bronchi (mesobronchi): typical respiratory epithelium with goblet cells. Mucous glands. Hemi-rings of cartilage.

Secondary bronchi: simple ciliated columnar epithelium. Cartilage is absent. Smooth muscle is well developed.

Tertiary bronchi (parabronchi): simple cuboidal epithelium. The parabronchial wall is perforated with openings to the atria.

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4. KIDNEY

In the cortex, numerous renal corpuscles (glomerulus + Bowman’s capsule) are present. The outer layer of the Bowman’s capsule is the parietal wall (simple squamous epithelium), which is separated from the visceral layer (podocytes) by the urinary space. The podocytes closely invest the capillary endothelium of the glomerulus. The proximal convoluted tubule is lined by simple cuboidal epithelium, showing brush border (PAS-positive). The distal convoluted tubule did not show brush border (PAS-negative).

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PAS stain

In the medulla, we can observe loops of Henle and collecting tubules.

Avian kidney

5. URETER AND URINARY BLADDER

The mucosa of the ureter forms longitudinal folds (stellate-shaped lumen in cross sections) and is lined with 5-6 layers of transitional epithelial cells. Under the lamina propria there is no muscularis mucosae. The tunica muscularis has three layers: inner and outer longitudinal layers and a middle circular layer. The outermost coat of the ureter may be either an adventitia or a serosa.

The urinary bladder is lined by a transitional epithelium. The lamina propria is composed of loose connective tissue. The presence of muscularis mucosae varies with the species (best developed in the horse bladder). The submucosa contains large blood vessels. The tunica muscularis is well developed and has three layers: inner and outer longitudinal layers and a middle circular layer. The outer layer of the urinary bladder may be adventitia or serosa.

Urinary bladder with muscularis mucosae (horse)

6. HYPOPHYSIS

The pars distalis of the adenohypophysis is the major constituent of the hypophysis. The dense connective tissue capsule is continuous with a fine network of reticular fibres supporting the cords and clusters of parenchymal cells. There are two groups of cells: chromophobes and chromophils.

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The pars intermedia and pars distalis are partially separated by the Rathke’s cleft. The parenchymal cells are basophilic. The tuberalis cells are small and basophilic.

The neurohypophysis is composed of numerous unmyelinated nerve fibres with cell bodies that are located in the hypothalamus. The axons are supported by pituicytes (neuroglial cells).

7. THYROID GLAND

The thyroid gland consists of numerous follicles, filled with acidophilic colloid and lined by follicular cells (simple cuboidal epithelium). The follicles are surrounded by a basement membrane, sparse connective tissue, and a network of capillaries.
Large pale cells with an eosinophilic cytoplasm, the parafollicular cells (C cells), lie between the follicular cells.

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8. PARATHYROID GLAND

Its parenchyma is organized in cords of densely packed basophilic epithelial cells along the rich capillary bed. There are two types of cells: chief cells and oxyphil cells. Chief cells are numerous and stain darkly basophilic. Oxyphil cells are large cells with an acidophilic cytoplasm, but usually are found only in the horse and large ruminants.

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9. ADRENAL GLANDS

They are covered by a connective tissue capsule. An outer cortex (zona glomerulosa, zona fasciculate, and zona reticularis) and an inner medulla can be identified.The zona glomerulosa (arcuata, multiformis) is the outer layer immediately beneath the capsule. It consists of curved cords of columnar cells or clusters of polyhedral acidophilic cells.

The zona fasciculata, the most extensive zone, is formed of cuboidal cells arranged in radial cords separated by sinusoids. The cytoplasm of these cells (also called spongiocytes) may appear foamy.

The zona reticularis is the innermost zone next to the medulla and the cells are small.

The adrenal medulla is composed mostly of columnar or polyhedral APUD cells that take up and stain strongly with chromium salts and have numerous brown granules in the cytoplasm (the chromaffin reaction demonstrates the presence of adrenalin and noradrenalin).