Endocrine Histology and Embrology Flashcards
What are the divisions of the pituitary gland, their individual components, and the embryonic structures they derive from?
Anterior Pituitary (adenohypophysis): Pars Distalis Pars Intermedia Pars Tuberalis Derived from the endoderm (Rathke's pouch)
Posterior Pituitary (pars nervosa/infundibular process):
Infundibular stem/stalk
Median Eminence
Derived from the brain (hypothalamus)
What hormones does the pars distalis produce and what cells produce them?
The anterior pituitary is composed of cells that synthesize and release growth hormone (GH), prolactin (PRL), adrenocorticotropin (ACTH) and derivatives, thyroid stimulating hormone (TSH), and two gonadotropins, follicle stimulating hormone (FSH) and luteinizing hormone (LH). Five types of cells have been identified in various mammalian species. These are:
- Somatotrophs (GH)—make up about 50 % of the secretory cells.
- Lactotrophs (PRL)—make up about 20%
- Gonadotrophs (FSH, LH) about 5-10%
- Corticotrophs (ACTH), about 15-20%
- Thyrotrophs (TSH), about 5-10%
What is the structure of the pars distalis?
There is an extensive vasculature of small vessels/capillaries/sinusoids within the pars distalis. The hormone-secreting cells are arranged in rows around capillary endothelial cells that are fenestrated to allow rapid passage of hormones out from the various endocrine cells, but also enables diffusion towards the cells of releasing factors transported via the hypophyseal portal system (described below). This enables reasonably rapid hormonal responses by fast passage into/out of the capillary sinusoids of the anterior pituitary.
Where is the pars tuberalis located and what is its function?
The pars tuberalis is a collar of cells around the infundibular stalk containing blood vessels that lead from capillaries of the median hypothalamic eminence to small vessels/capillaries of the pars distalis. The blood entering the median eminence comes from the superior hypophyseal arteries (from the internal carotid). The capillaries of the median eminence thence lead to larger vessels in the tuberalis that deliver regulatory peptides (the releasing factors) secreted by hypothalamic neurons to the cells in the anterior pituitary. Thus, the pars tuberalis forms the hypophyseal portal system.
What hormones are found int he hypophyseal portal system?
TSH-releasing hormone (TSH-RH), gonadotropin releasing hormone (GNRH), corticotropin releasing hormone (CRH), growth hormone releasing hormone (GHRH) inhibitory factors: somatostatin and dopamine
What is the structure and function of the posterior pituitary/pars nervosa?
The posterior pituitary is essentially an extension of the hypothalamus. The hormones, antidiuretic hormone (ADH, vasopressin) and oxytocin, are released from the ends of axons that arise from cell bodies of neurons present in the hypothalamus. They are unmyelinated and comprise a bundle that extend alongside one another within the infundibular stalk. There are also nuclei that can be observed within the posterior pituitary that are the nuclei of pituicytes, which are supportive astrocyte-like glial cells (not producing hormones). The axons expand into bulbous structures that contain neurosecretory vesicles (Herring’s bodies).
Where and how are ADH/vasopressin and oxytocin created and secreted?
ADH and Oxytocin are produced in the hypothalamus (in the cell bodies) as large polypeptides that undergo cleavage during vesicular transport down the axons. The prohormones are called vasopressin-neurophysin and oxytocin-neurophysin (more in physiology). Vasculature in the posterior pituitary is evident, but is not as extensive as in the anterior pituitary.
What arteries and veins move blood to and from the thyroid?
Blood supply to the thyroid is via the inferior thyroid artery (from the thyrocervical trunk) and the superior thyroid artery (from the external carotid artery); drainage is from the inferior thyroid vein (to the subclavian vein) and the superior thyroid vein (to the jugular vein).
What is the structure of the thyroid gland?
The thyroid is a multi-lobed gland comprised of a series of follicles, each having a single layer of epithelial cells surrounding a central chamber referred to as the colloid. The epithelial cells are producers of the colloid and ultimately the thyroid hormone group. The gland is notable for its tremendous storage capability of potential hormone in the colloid. Scattered cells between follicles produce calcitonin.
What is thyroglobulin?
Thyroglobulin is a large protein rich in tyrosine residues, which are the sites of iodination and modification to generate the thyroid hormones.
How does TSH affect the level of T3 and T4 in the blood?
TSH stimulates synthesis of thyroglobulin and its uptake and breakdown from the colloid with consequent release of thyroid hormone (T3 and T4, containing, respectively, 3 or 4 iodine atoms per molecule) into the blood. Colloid droplets are taken up and processed in the interior of the epithelial cells by the lysosomal system resulting in production of thyroid hormones.
How does the extensive vascularization of the thyroid affect thyroid hormone synthesis and iodine uptake?
Extensive vascularization around the follicles enables iodide pumping from the blood and conversion to iodine by the epithelial cells and release of the thyroid hormones into the blood. The epithelium also synthesizes and secretes the protein thyroglobulin into the interior of the follicle and takes up and digests thyroglobulin to generate the thyroid hormones. The iodide pump is very effective. Within a few minutes a major portion of radioactive iodide is taken up by the thyroid making it possible to perform partial thyroidectomies.
What cells produce calcitonin in the thyroid and what does calcitonin do?
Calcitonin “C” cells have secretory granules containing calcitonin, a small protein. This hormone decreases release of calcium from bones (down regulates osteoclastic activity). It appears to act oppositely to parathyroid hormone which is centrally involved in increasing blood calcium levels.
What is the structure and cell types present in the parathyroid glands and what do the glands produce?
1 - Chief cells, which produce parathyroid hormone (PTH), a protein of 84 amino acids. It increases osteoclast release of calcium from bone, and increases calcium uptake in the GI tract and by the kidney, which elevates calcium levels. Opposite functions of calcitonin.
2 - Oxyphil cells, which contain a number of mitochondria, usually stain paler but whose functional significance remains unknown
3 - Adipose cells.
Blood vessels are seen, although not as extensive as the anterior pituitary or islets of Langerhans.
What is the general structure, function, and blood supply of the adrenal gland?
The adrenal gland, like the pituitary, is a dual origin gland, housing two organs, physically distinct as the CORTEX and the MEDULLA. The cortex produces and releases various STEROIDS whereas the medulla produces and releases amino acid derived hormones including EPINEPHRINE, NOREPINEPHRINE and ENKEPHALINS. Blood is delivered via the SUPERIOR, MIDDLE and INFERIOR SUPRARENAL ARTERIES, which branch and enter through the capsule via short cortical arteries into an outer subcapsular arterial plexus. Blood then passes via an anastomosing network of capillaries into the medullary region. Other arteries (long cortical arteries) take blood to medullary region more directly, with the blood ultimately entering a series of small capillaries/sinuosoids to the central medullary vein, which drains via the suprarenal vein.
