EN02 - Histology of Endocrine

What are main endocrine organs?

Pituitary gland, thyroid gland, parathyroid gland, pineal gland, adrenal gland, endocrine pancreas, testis, ovary, and diffuse endocrine cells in other organs.

What is the pituitary gland (hypophysis)?

A small ovoid organ attached to the hypothalamus by a thin stalk called the infundibulum.

What are the two main components of the pituitary gland?

Anterior pituitary and posterior pituitary.

What are the subdivisions of the pituitary called?

Pars.

What controls pituitary gland function?

The hypothalamus.

What is another name for the anterior pituitary?

Adenohypophysis.

What is the embryological origin of the anterior pituitary?

Oral epithelium from Rathke’s pouch (ectodermal origin).

What happens to Rathke’s pouch during development?

The connecting stalk degenerates, leaving epithelial cells that differentiate into the adenohypophysis.

What type of tissue makes up the anterior pituitary?

Epithelial tissue.

What are the subdivisions of the anterior pituitary?

Pars distalis (pars anterior), pars intermedia, and pars tuberalis.

What percentage of the pituitary is the pars distalis?

75% of the mass.

What is the arrangement of cells in the pars distalis?

Glandular epithelial cells arranged in irregular clumps or cords between fenestrated capillaries with large irregular lumens.

How much connective tissue is in the pars distalis?

Very little connective tissue between cells.

What are the three cell types in the pars distalis?

Acidophils (40%), basophils (10%), and chromophobes (50%).

How do acidophils appear with H&E staining?

Red-pink.

How do basophils appear with H&E staining?

Blue-purple.

How do chromophobes appear with H&E staining?

Not stained. (“color-fearing”)

What does electron microscopy show about acidophils and basophils?

They have different sized granules with different electron densities.

What are chromophobes thought to represent?

Acidophils or basophils that are dormant or recently degranulated.

What do acidophils synthesize and secrete?

Polypeptide hormones.

What do basophils synthesize and secrete?

Glycoprotein hormones.

What do somatotropes secrete?

Somatotropin (growth hormone).

What are somatotrope granule characteristics?

Dense 350 nm granules.

What do mammotropes (lactotropes) secrete?

Prolactin.

What are mammotrope granule characteristics?

Dense 600 nm granules and variable cell size.

What do corticotropes secrete?

ACTH.

What are corticotrope characteristics?

Round or oval cells with 100–300 nm granules and lipid droplets.

What do gonadotropes secrete?

FSH and LH.

What are gonadotrope granule characteristics?

200–250 nm granules.

What do thyrotropes secrete?

TSH.

What are thyrotrope characteristics?

Large polygonal cells with 150 nm granules.

What is the pars intermedia?

The boundary between the pars distalis and pars nervosa.

How developed is the pars intermedia in humans?

Not well developed.

What cell type is found in the pars intermedia?

Basophils.

What histologic feature is seen in the pars intermedia?

Colloid-filled structures representing remnants of Rathke’s pouch.

What hormone is predominantly secreted by the pars intermedia?

Melanocyte-stimulating hormone.

What is the pars tuberalis?

A region that wraps around the pituitary stalk.

What cell types are in the pars tuberalis?

Basophils (gonadotropes and thyrotropes).

What hormones may be produced by the pars tuberalis?

LH and FSH.

What is the functional significance of melatonin receptors in the pars tuberalis?

They allow response to light/dark (photoperiodicity).

What is another name for the posterior pituitary?

Neurohypophysis.

What is the embryological origin of the posterior pituitary?

Neuroectoderm.

What is the posterior pituitary analogous to?

Neural tissue and an extension of the hypothalamus.

What are the components of the posterior pituitary?

Pars nervosa, median eminence, and infundibular stalk.

What composes the pars nervosa?

Axons of the hypothalamo-hypophyseal tract.

Where are the cell bodies of hypothalamic neurosecretory neurons located?

Paraventricular and supraoptic nuclei (hypothalamus)

What type of axons are in the pars nervosa?

Unmyelinated axons.

What other structures are found in the pars nervosa?

Pituicytes and fenestrated capillaries.

Where do axons in the pars nervosa terminate?

Near capillaries (not synaptic endings).

What hormone is produced by paraventricular nucleus neurons?

Oxytocin.

What hormone is produced by supraoptic nucleus neurons?

Antidiuretic hormone (ADH).

What are Herring bodies?

Dilated regions of axons that store hormones.

What are pituicytes?

Specialized glial cells of the posterior pituitary.

How many types of pituicytes exist?

Five types: major, dark, oncocytic, ependymal, and granular.

What do pituicytes do?

Surround unmyelinated axons and regulate release of oxytocin and ADH.

Where is the thyroid gland located?

In the anterior neck adjacent to the larynx and trachea.

What is the structure of the thyroid gland?

Two lateral lobes connected by an isthmus.

What structures form thyroid lobules?

Connective tissue capsule sends trabeculae to form lobules

How many follicles are in the thyroid?

About 20 million.

What is the size of thyroid follicles?

Up to 1 mm.

What surrounds the colloid in thyroid follicles?

Follicular cells.

What is located between follicles?

Parafollicular cells, fenestrated capillaries, and connective tissue.

What is the normal shape of follicular cells?

Simple cuboidal.

How does follicular cell shape change with activity?

Columnar when active and squamous when inactive.

What do follicular cells do?

Synthesize, store, process, and secrete thyroid hormones.

What is colloid in the thyroid?

A central core containing thyroglobulin.

What is thyroglobulin?

A storage form of T3 and T4.

What is unique about thyroglobulin?

It is an extracellularly stored precursor to active hormones.

What are parafollicular cells also called?

C-cells or clear cells.

Where are parafollicular cells located?

Between follicular cells surrounding the colloid.

How do parafollicular cells stain?

Pale with H&E.

What do parafollicular cells secrete?

Calcitonin.

What is the origin of parafollicular cells?

Neural crest.

What indicates the presence of parathyroid glands on the thyroid?

Small light-colored nodules on or protruding from the thyroid surface.

How many parathyroid glands are typical?

One superior and one inferior pair.

What percentage of people have extra parathyroid glands?

2–10%.

What structures are found in the parathyroid gland?

Thin connective tissue capsule, fenestrated capillaries, chief cells, and oxyphil cells.

What are chief cells?

The most numerous cells that secrete parathyroid hormone (PTH).

How do chief cells appear when loaded with hormone?

Dark.

How do chief cells appear after secretion?

Clear.

What are oxyphil cells?

Larger cells with many mitochondria that stain eosinophilic.

When do oxyphil cells appear?

Around age 6–7.

What is the function of oxyphil cells?

Unknown, may be transformed chief cells.

What is a histologic feature of parathyroid glands in adults?

Adipose infiltration, up to ~50% of gland weight.

Where is the pineal gland located?

Midline, attached to posterior roof of the third ventricle.

What surrounds the pineal gland?

Pia mater forming a capsule.

What does the capsule form inside the pineal gland?

Connective tissue septa dividing it into lobules.

What cell types are in the pineal gland?

Pinealocytes and glial cells.

What is another structure found in the pineal gland?

Brain sand (corpora arenacea).

What percentage of pineal gland cells are pinealocytes?

95%.

What do pinealocytes do?

Secrete melatonin.

What are pinealocyte characteristics?

Large, lightly staining nuclei.

What are pineal glial cells?

Astrocytes that support pinealocytes.

What are the staining characteristics of pineal astrocytes?

Small, dark nuclei.

What is brain sand?

Calcium and magnesium salt concretions.

What is unique about brain sand?

Increases with age and visible on imaging.

What is another name for the adrenal glands?

Suprarenal glands.

Where are adrenal glands located?

Embedded in perirenal fat at the superior pole of each kidney.

What are the two regions of the adrenal gland?

Cortex and medulla.

How do cortex and medulla differ?

Functionally different with different embryological origins.