Endocrine Glands: Histology and Function

Thyroid

Origin: Endodermal derivative, initially from the floor of the pharyngeal gut. It is a solid organ.

Capsule: Connective tissue rich in collagen fibers (fibrosis), surrounding the thyroid tissue.

Stroma: Septa extend inward (including into the parathyroid), containing reticular fibers surrounding the thyroid follicles. It is highly vascularized (by the superior and inferior thyroid arteries) with fenestrated capillaries.

Parenchyma: The gland consists of thousands of thyroid follicles, lined by simple cuboidal epithelium (flat or cylindrical) on a thin basal lamina. The cavity contains thyroid colloid.

Cells:

• Follicular cells: Cuboidal [low height indicates underactivity, high height indicates overactivity (less colloid)].
• Parafollicular cells: Do not form a continuous layer; they secrete calcitonin (which lowers serum calcium).

Parathyroid

There are four parathyroid glands.

Origin: 3rd and 4th pharyngeal pouches, posterior to the thyroid.

Capsule: Separated by a thin connective tissue trabeculae (containing blood vessels, nerves, and lymphatics).

Stroma: Adipocytes increase with age (with decreased parenchyma). Cells are arranged in radial dense clusters, solid nests, and sometimes follicles with colloid. Reticular fibers are found between glandular cells.

Cells:

• Chief cells (Principal cells): Most abundant, present until puberty. They appear clear (inactive) or dark (active). PAS staining reveals PAS-positive active cells. They secrete parathyroid hormone (which increases serum calcium).
• Oxyphilic cells: Appear after puberty, are larger, and are eosinophilic (many mitochondria). They have no secretory granules.

Epiphysis (Pineal Gland)

Located in the posterior third ventricle of the diencephalon, connected by a short peduncle. It is covered by pia mater (containing vessels and unmyelinated fibers). It has incomplete lobules, with groups of cell cords surrounded by a fine network of fenestrated capillaries.

Cells:

• Pinealocytes: (95%) Basophilic, lobed nucleus, containing granules of melatonin and serotonin.
• Astrocytes: (5%) Many extensions and intermediate filaments, basophilic cytoplasm, abundant in the pineal stalk, connecting pinealocyte nests.

Brain sand (corpora arenacea or acervuli) forms in adults. These are extracellular clusters of phosphate, calcium carbonate, and magnesium.

Adrenal Gland

A paired organ that produces glucocorticoids (zona fasciculata and reticularis), mineralocorticoids (zona glomerulosa), and sex steroids (zona reticularis). The medulla produces adrenaline and noradrenaline.

Capsule: Dense connective tissue that penetrates the organ, resulting in thin connective partitions (carrying vessels and nerves).

Vascularization:

• Superior adrenal artery (from the inferior phrenic artery)
• Middle adrenal artery (from the aorta)
• Inferior adrenal artery (from the renal artery)

Nodes are found in the capsule, septa, and around veins (not in the parenchyma).

Cortex: [Mesodermal origin] There are three zones:

1. Zona Glomerulosa: (10-15%) Cells form arched cylindrical cords surrounded by capillaries.
2. Zona Fasciculata: (75%) Strands of 1-2 cells thick, between blood vessels (sinusoids with continuous fenestrated endothelium with a diaphragm). Lipid droplets are apparent (spongiocytes).
3. Zona Reticularis: (5-15%) Ramifications of the cords, with capillaries in between. They show lipofuscin granules and fewer but larger lipid droplets.

Medulla: Well-defined limits (no separation structure). Cell cords:

1. Chromaffin cells: Large epithelioid cells.
2. Ganglion cells: Modified postganglionic neurons of the sympathetic nervous system.

Pituitary Gland (Hypophysis) Irrigation

Blood supply comes from two pairs of vessels arising from the internal carotid artery:

1. Superior hypophyseal arteries: Irrigate the pars tuberalis and infundibulum. They form an extensive capillary network, the primary capillary plexus of the median eminence.
2. Inferior hypophyseal arteries: Mainly irrigate the posterior lobe, though they also send some branches to the anterior lobe.

The hypophyseal portal veins drain into the primary capillary plexus of the median eminence, carrying blood to the secondary capillary plexus, located in the pars distalis. [Both plexuses are composed of fenestrated capillaries].

CRE (Corticotropin-Releasing Element)?:

• I: Between capsule and trabeculae.
• II: In the middle cortex.
• III: In the deep cortex and cortico-medullary junction [II and III: present autoantigens and MHC I and II molecules on immature T cells].
• IV: In the superficial medulla and cortico-medullary junction.
• V: Polymorphic nuclei and perinuclear chromatin define this region.
• VI: Forms Hassall's corpuscles.