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Endocrine System

  1. General morphology and development
    1. Linked by a stalk to the base of the brain, and lies surrounded by dural membrane (capsule) in the bony sella turcica.
    2. Stalk extends through the dural diaphragma sellae. Pituitary weighs 0.5-1.0 g.
  2. Adenohypophysis (histology and function)
    1. Pars tuberalis - wrapped around the neural stalk are cords of basophilic cells containing gonadotrophic hormones.
    2. Pars intermedia - rudimentary in man ; variable in width ; several colloid-filled cysts ; glandular cells - chromophobe or basophil ; basophilic cells may extend into the neural lobe ; function - unknown in man, but in fish and amphibia the melanocyte stimulating hormone (MSH) formed varies skin pigmentation.
    3. Pars distalis
      1. Thick, branching cords and plates of cells, supported on basal laminae and reticular fibres. Between the cords run wide sinusoidal capillaries of fenestrated endothelial cells on their own BLs.
      2. Classical division of the cells was into acidophils (40 per cent), basophils (10 per cent), and chromophobes (50 per cent).
      3. Chromophobes are sparsely granular, small, pale, and often clustered together. They are thought to be less active forms of the five secretory, granular, chromophil cell kinds.
      4. Chromophils can be distinguished by various stains, since some form peptide hormones, others glycoproteins ; by EM, from the size, density and shape of the granules ; and by immunostaining, for LM and EM.
        • ACIDOPHILS
          Somatotroph - makes growth hormone (GH)/somatotrophin (STH) ; stained by orange-G Lactotroph/Mammotroph - makes prolactin/mammotrophin (MTH) ; stained by erythrosin
        • BASOPHILS, staining also with PAS and aniline blue
          Thyrotroph gives thyrotrophic hormone (TSH/TH)
          Gonadotroph gives luteinizing hormone (LH) and follicle-stimulating hormone (FSH)/interstitial cell-stimulating hormone (ICSH)
          Corticotroph makes adrenocorticotrophin (ACTH) by cleaving pro-opiomelanocortin (POMC) appropriately
      5. Hypothalamic regulation of the adenohypophysis is via the hypothalamo-hypophyseal portal circulation, and for gonadotrophins, ACTH, and TSH, functions by negative feedback thus:
        Hypothalamic neurons are specialized to be sensitive to a blood deficiency of the target gland’s hormone, e.g. thyroxine.
        From the sensitive neuron’s terminal, a neurosecretory, chemical peptide releasing factor, e.g. TH-RH/TH-RF, passes into blood capillaries of the median eminence, whence it drains down via the portal circulation to the pars distalis.
        The releasing factor passes out of the blood to activate the appropriate chromophil cell, which produces more trophic hormone, e.g., TH. The trophic hormone passing in the blood to the target gland, thyroid, promotes an increased output of target gland hormone, thyroxine, whose raised blood level then reduces the activity of 1. the sensitive hypothalamic neurons, i.e., the system uses a negative feedback.
        This simplification ignores the inhibitory factors, such as hypothalamic somatostatin preventing the release of growth hormone.
      6. The hypothalamus thus acts as an endocrine organ.
  3. Neurohypophysis
    May be viewed as a downward extension of the hypothalamus, allowing for hormone storage and a complete breach of the blood-brain barrier for hormone release. Its structure follows:
    1. The neural stalk and posterior lobe consist of the unmyelinated axons (grouped as the hypothalamo-hypophyseal tract) of neurosecretory neurons of the hypothalamic supraoptic and paraventricular nuclei.
    2. The neurosecretion collects, and dilates some axons and their terminals into Herring bodies. Gomori staining or EM shows the presence of granules in these axons, but not in the Pituicytes - a neuroglial kind of cell.
    3. The secretion collects in terminals arranged as a palisade around blood vessels. Its release may involve electrical discharge in the axon and chemical factors in the ‘synaptic’ vesicles also present.
    4. Two polypeptide hormones in the secretion are:
      1. oxytocin/pitocin: makes mammary gland myoepithelial cells and uterine smooth muscle contract;
      2. vasopressin/pitressin/antidiuretic hormone (ADH): makes the kidney collecting tubule permeable to water, and influences vascular and gut smooth muscle.
    5. The neural lobe has a direct arterial supply from the inferior hypophyseal arteries to its fenestrated capillaries.

Thyroid Gland

  1. General morphology
    1. Develops from an endodermal downgrowth at the base of the tongue. The thyroglossal duct, connecting it with its point of origin, later disappears. Two lateral lobes, an isthmus (and sometimes a pyramidal lobe) are established.
    2. The inner, true, CT capsule sends in septa to partially enclose lobules.
    3. In the lobules are rounded or elongated bodies - follicles, in a loose stroma ofCT, with many blood vessels.
  2. Thyroid follicle
    1. In man, they vary between 0.02 and 0.9 mm in diameter. A gland has several million follicles.
    2. Filled with viscous fluid - thyroid colloid - variably acidophil or basophil, and often shrunken and showing knife chatters.
    3. Lined by basophilic cuboidal follicular cells, varying in height as a simple epithelium on
    4. a basal lamina, outside which is an extensive plexus of blood capillaries, and reticular fibres and fibroblasts.
    5. Follicular cells are polarized with respect to the follicle lumen; the nucleus is central, the Golgi complex supranuclear ; EM shows plenty of granular ER, some luminal microvilli, endocytotic vesicles, and lysosomes.
    6. Between the follicular cells and the BL, and sometimes outside the BLs, lie occasional C cells (clear/parafollicular cells), having no direct access to the lumen, and no colloid droplets, but with small argyrophil, secretory granules.
  3. Thyroid histophysiology
    1. C Cells
      1. APUD cells of neural crest origin,
      2. Produce the polypeptide calcitonin for the reduction of high plasma Ca2+ and phosphate levels.
      3. Although diffuse, in sum they form a gland antagonistic to the action of the parathyroids.
    2. Follicular cells
      1. Are stimulated by pituitary thyrotrophic hormone (TSH) to produce and release two iodinated amino-acid hormones - tetra iodo-thyronine (thyroxine/T4) and 3, 5, 3-triodo-L-thyronine (T3),
      2. Which are stored in the colloid, as component amino acids of the glycoprotein - thyroglobulin.
      3. The hormones accelerate general and specific metabolic processes of the body.
      4. Electron radioautography has shown the sites in the sequence of hormone production by the follicular cells:
        i. Iodide concentration - basal part of the follicular cell.
        ii. Iodide oxidation - throughout the cell.
        iii. Synthesis of thyroglobulin - basal cell, granular ER, Golgi body, by vesicle to the lumen.
        iv. In the luminal thyroglobulin, tyrosine residues are iodinated, and then pairs condense.
        v. Cellular retrieval of thyroglobulin from colloid storage - cell’s apical region by endocytosis.
        vi. Transport to lysosomes, where cathepsins degrade the large modified molecule.
        vii. Release of freed iodothyronines - out of the base of the cells into the blood.

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