Endocrine System I. Introduction: A. Characteristics of endocrine glands: Ductless. Cells are arranged in cord, follicle, aggregation or net-like structure, among them are spaces containing rich blood and lymph Caps.

B. Secrete hormone at a specific time and in small amounts, which act on target organ, tissue or cells. C. The secretion activity is often regulated by other hormones or nerve impulses. D. Make up an endocrine organ, or exist in other organs as tissue or dispersed cells.

E. Two types of endocrine cells: 1. Peptide-secreting cells: Well developed G, RER and secreting granules. Release hormone that chemically contains amino acids.

2. Steroid-secreting cells: Foamy cytoplasmic appearance under LM 2. Steroid-secreting cells: Foamy cytoplasmic appearance under LM. Rich in SER, lipid droplets andMT containing tubular-like cristae. Secrete hormone that chemically contains steroids.

II. Thyroid gland: Largest endocrine gland in the body. A. General structure: Fibrous capsule protrudes and divides the parenchyma into lobules containing follicles and rich capillaries. B. Follicle: Colloid is surrounded by singer layer epithelium. 1. Epithelium: Simple squamous (hypoactive), cuboidal or columnar (hyperactive), depending upon the functional status.

Epithelial cell has rich RER, G, secreting granules, microvilli, phagosomes, LYS, iodide pump, and receptors for thyrotropin (TSH).

2. Colloid: Homogeneous secretion of epithelium 2. Colloid: Homogeneous secretion of epithelium. Acidophilic and PAS (+). Contains iodated thyroglobulin. 3. Formation of triiodothyronine (T3) & tetraiodothyronine (T4): a. AA→ (RER)→ thyroglobulin → (G, secreting granules) → colloid. b. Iodide → (iodide pump) → iodide oxidation in cytoplasm → colloid → bound to tyrosine residues of thyroglobulin → iodated thyroglobulin.

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c. iodated thyroglobulin → (TSH, microvilli) → phagosomes → LYS → T3 & T4. 3. Modulation of T3, T4 secretion: hypothalamus-pituitary-thyroid axis. 4. Function of T3, T4: a. elevates the basal metabolic rate. b. influence the body growth and nervous system development during fetal life. Cretins: Arrested mental and physical development in adult because of hypothyroid from birth.

C. LCT around follicles: Rich in capillaries. Parafollicular cells: C cells. Situated often in groups among follicles, or among follicular epithelium cells but are not adjacent to colloid. Argyrophilic. Lightly stained.

Contain rich RER and secretory granules Contain rich RER and secretory granules. Secrete calcitonin, which can decrease the blood Ca2+ level by inhibiting the activity of osteoclasts.

III. Adrenal gland: Centrally located medulla is derived from ectoderm and surrounded by mesoderm-derived cortex and fibrous capsule. A. Cortex: 3 layers are not sharply defined. Cells produce and release steroid-like hormone only upon demand.

1. Zona glomerulosa: Cells are arranged in clusters beneath the capsule. The secretion, the mineralocorticoids (e.g. aldosterone), participates in electrolyte balance regulation. Its secretion activity is regulated by renin-angiotensin system.

2. Zona fasciculata: Constitutes about 65% of cortex 2. Zona fasciculata: Constitutes about 65% of cortex. The typical steroid-secreting cells in the body are arranged in straight cords, and run perpendicularly to the capsule, with rich Caps among them.

Their glucocorticoids release (e. g Their glucocorticoids release (e.g. Cortisol & Corticosterone), which is regulated by ACTH, influences the protein, fat and carbohydrate metabolism, and impairs the immune responses.

3. Zona reticularis: Adjacent to medulla with obscure outline 3. Zona reticularis: Adjacent to medulla with obscure outline. Cells are arranged in net-like structure and contain more lipofuscin pigment granules.

Their androgen (with small amount of estrogen & glucocorticoids) release, which is regulated by ACTH, influences the sexual development.

B. Medulla: Contains central vein and sympathetic ganglion B. Medulla: Contains central vein and sympathetic ganglion. Two types of cells, which are also called chromaffin cells because the chromaffic granules can be found in their cytoplasm when fixed by chromaffic salt, are arranged in cords or clusters, among them exist capillaries.

Epinephrine-secreting cells and norepinephrine-secreting cells secrete adrenaline/epinephrine and noradrenaline/norepinephrine, respectively. The cytoplasm granules in former cells are smaller and slightly stained.

The hormones, the adrenaline and noradrenaline, which are stored in granules and whose secretion is stimulated by innervating sympathetic nerve, can immediately elevate the heart beating rate and blood pressure, respectively.

IV. Pituitary gland: Weight 0.5g. Most important endocrine gland. A. General structure: 1. Adenohypophysis: pars distalis (anterior lobe) + pars tuberalis + pars intermedia. 2. Neurohypophysis: pars nervosa (constitute the posterior lobe, together with pars intermedia) + infundibulum.

B. Adenohypophysis: 1. Pars distalis: Aggregated cells are surrounded by sinusoidal Caps, the secondary Cap network of hypophyseal portal circulation. a. acidophilic cell: 40%. Contains acidophilic secreting granules. The secretions are peptides. 2 types: * somatotrophic cell: secretes somototropin (GH), which can stimulate the growth of bone, muscles and CT, etc. Lack of GH in childhood results in midget. * mammotrophic cell: secretes prolactin, which can stimulate the development of mammary glands.

b. basophilic cell: 10%. Larger, contains basophilic and PAS (+) secreting granules. The secretions are glycoproteins. 3 types: * thyrotropic cell: secretes thyrotropin (TSH). * corticotropic cell: secretes corticotropin (ACTH) * gonadotropic cell: secretes gonadotropin, which includes follicle-stimulating hormone (FSH) that can stimulate the formation of oocyte and sperm, and luteinizing hormone (LH) that can stimulate the release of sex hormone. c. chromophobe: 50%. Smaller and inactive.

2. Pars intermedia: Contains follicles. Functions unknown. 3. Pars tuberalis: Secretes gonadotropin. C. Neurohypophysis: 1. Pars nervosa: a. pituicytes: specific glial cells.

b. unmyelinated nerve fibers: are the axons of neurons, whose cell bodies locate in paraventricular and supraoptic nuclei in hypothalamus. The axon dilations, the Herring Bodies, are light acidophilic and contain many secretory granules occupied by peptide-containing hormone. The oxytocin, which is produced by paraventricular nuclei, can shrink smooth muscles in uterus when parturition. The antidiuretic hormone (ADH, vasopressin), which is produced by supraoptic nuclei, can facilitate water re-absorption in kidney.

2. Infundibulum: Composed of median eminence and neural stalk. Contains primary Cap network of hypophyseal portal circulation.

D. Regulation of hormone release in pars distalis: a. the neuroendocrine cells in hypothalamus and endocrine cells in pituitary are functionally connected by hypophyseal portal circulation.

The neurons in tuberal nuclei in hypothalamus can produce releasing and inhibiting hormone, which are released into the infudibulum, where the primary Cap network of hypophyseal portal circulation exists. b. positive and negative feedback.