endocrine system - integrates corrdinates and controls body processes by chemical signals (hormones) that are released into the extracellular tissue spaces and carried by the vasculature to distant target cells (without the use of a duct system)

• as comp. to neural control mechanisms, typically long-lasting effects on target cells

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paracrine: endocrine secretion with localized dispersal in interstitial fluid or through short loops of blood vessels, (e.g. gastrin made by pyloric G cells reach target cells in the fundic glands)

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juxtacrine: signaling molecule remains on the secreting cell's surface or adjacent extracellular matrix  and effects target cells when the cells make contact (e.g. in developmental tissue interactions)

autocrine: cells may produce molecules that act on themsleves or on cells of the same type (e.g. IGF)

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protein-producing cells: look very much like other protein producing cells (secretory granules, lots of rER), but granules are basal and near a capillary

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steroid producing cells: cells have large quatities of smooth ER and mitochondria often have tubular-shaped cristae (instead of the usual flat plate-like cristae) => cristae are round

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pars distalis - one of the three parts of the anterior lobe; the bulk of the anterior lobe;

acidophils: somatotropic and mammotropic cells

basophils: gonadotrpic, corticotropic, and thyrotropic cells

pars intermedia - a part of the anterior pituitary; between the anterior and posterior pituitary

Express POMC -> MSH, γ-LPH, and β-endorphin

pars tuberalis - a part of the anterior pituitary that is a collar-like extension of the adenohypophysis that wraps around the infundibulum;

mostly secrete FSH and LH

pituitary stalk - same as the infundibulum; connects the hypothalamus with the posterior pituitary, where hypothalamic inhibitory & releasing factors are discharged into the primary capillary plexus to perfuse the anterior pituitary

embryonic origin: adenohypophysis is derived from the ectoderm of oropharynx (Rathke's pouch); neurohypophysis is derived from the neuroectoderm of the floor fo the 3rd ventricle--> neural tissue

histologic features: adenohypophysis (adeno == glandular); neurohypophysis has neural tissue

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distinct cell populations:

adenohypophysis: since glandular, stains acidophilic (somatotropes, lactotropes), basophilic (corticotropes gonadotropes, thyrotropes), and chromophobic;

neurohypophysis: contains non-myelinated axons and nerve endings arising from cells in the hypothalamus

hypophyseal portal system: Most of the anterior lobe is supplied indirectly via hypophysial portal veins

• primary capillary plexus: pars tuberalis, median eminence, & infundibulum supplied by the superior hypophysial arteries

• capillaries drain into hypophysial portal veins

hypophysial portal veins -> pars distalis of anterior lobe (also perfused by secondary capillary plexus)

• venous blood then drains to dural sinuses

pars nervosa: inferior hypophysial arteries supply a separate capillary plexus in neuohypophysis

pars nervosa vasculature:

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(1) somatotropes (GH cells - acidophils) - produce growth hormone

(+) hypothalamic growth hromone-releasing hormone (GHRH), and by throid hormone and cortisol

(-) hypothalamic somatostatin

(2) Lactotropes (acidophils) - secrete prolactin

(+) hypothalamic TRH, and by other mediators relating to pregnancy & nursing

(-) hypothalamic dopamine

(3) corticotropes (ACTH cells - basophils) - produce proopiomelanocortin (POMC) -> ACTH, ϐ-lipotrophic hormone, melanocyte stimulating hormone, and ϐ- endorphin

(+) corticotropin-releasing hormone (CRH)

(4) gonadotropes (FSH & LH cells - basophils) - produce both, or one or the other

(+) gonadotropin-releasing hormone (GnRH)

(5) thyrotropes (TSH cells - basophils) - produce thyroid stimulating hormone (TSH)

(+) stimulated by hypothalamic thyroid-releasing hormone (TRH)

Note: difference between tropic and non-tropic hormones

Tropic hormones (target cells are OTHER endocrine glands)

- Adrenocorticotrphic hormone (ACTH)
- Thyroid stimulate (thyrotropic) hormone
(TSH, thyrotropin)
- Follicle-stimulating hormone (FSH)
- Luteinizing hormone (LH)

Non-tropic hormones (targets are not endocrine cells)

- Growth hormone (GH) and Prolactin (PRL)

Recall: hormones ADH and oxytocin are NOT produced in the posterior lobe, just released there; produced in hypothalamus, then released in posterior pituitary

Herring bodies: Secretory nerve endings have swellings that contain secretory vesicles containing either oxytocin or antidiuretic hormone (ADH) that are stored there until they are released

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pars nervosa: arrows point to herring bodies

How to the hormones get to the herring bodies?

Non-myelinated axons and nerve endings arising from cells in hypothalamus are unusual for NOT terminating on neurons or other target cells; rather, they release hormones near a capillary network

ADH release is controlled primarily through hypothalamic osmoreceptors and feedback from sensory
bodies (carotid body, juxtaglomerular apparatus); ADH (arginine vasopressin) acts on distal convoluted tubule & principal cells of collecting ducts to reabsorb water

Oxytocin release - Sensory feedback from cervix, vagina and nipple; Oxytocin promotes smooth muscle contraction, especially uterine smooth muscle & myoepithelial cells of mammary gland (“milk letdown”)

Describe the histologic features and function of the pineal gland.

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pineal gland

Histologic features: (1) pinealocytes, arranged in clumps, ocgered by the fibrous tissue of the pia mater; (2) interstitial (glial) cells; (3) calcified deposits (brain sand), which inc. with age

Function of pineal gland: helps regulate daily body rhythm, such as sleep; responds to light & dark cycles via retinohypothalamic tract; produces melanin during dark cycles

thyroid - has 2 lateral lobes, a connecting isthmus (+/- a pyramidal lobe)

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thyroid follicles- spheres of cuboidal cells with a central mass of extracellular material ("colloid"), where the accumulation of the secretory product is stored

follicular cells- produce thyroid hormone (T3 and T4)--which regulate metabolism; and regulated by pituitary TSH

colloid -contains thyroglobulin, the precursor for active thyroid hormones

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Thyroid hormone production:

(1) Thyroglobulin (precursor) is produced by follicular cells and excytosed into the follicle lumen

(2) Uptake of circulating iodine from blood by follicular cells, which is then oxidized to reactive iodine, then released to the follicle lumen

(3) trosine residues on thyroglobulin are iodinated

(4) Folding of thyroglobulin allows close proximity of iodinated tyrosines which are then coupled to form T4 & T3

(5) Stimulated by TSH, follicular cells endocytose thyroglobulin with coupled iodinated tyrosines by receptor-mediated endocytosis

(6) Endocytotic vesicles fuse with lysosomes & thyroglobulin is degraded by proteases, freeing T4 & T3

(7) Freed T4 & T3 cross the basal plasma membrane & enter the blood, where they are bound by thyroxin-binding protein, to other plasma proteins and a small amount is left free

Describe the parafollicular cells/C-cells of the thyroid. Describe their location in the thyroid gland, their secretory product and its action and controls.

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Parafollicular cells (C-cells) - interior to basal lamina but exterior to follicular cells  not exposed to follicle lumen; thus part of the follicle

Make calcitonin, wich lowers blood calcium levels by suppressing the action of osteoclasts (bone resorption) and promitng osteoid calcification; triggered by elevated Ca²⁺ levels

What gets sweeter as you go inside? The adrenal cortex: outer - salt regulation, middle - sugar regulation, inner - sex

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zona glomerulosa - Outermost, round balls of cells
Secretes mineralocorticoids: aldosterone (salt regulation)
Regulated by renin-angiotensin-aldosterone system

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zona fasciculate - Mid portion of cortex; cells arranged in long cords (‘fascicles’)
Secretes glucocorticoids: cortisol
Regulated by CRH-ACTH hypothalamic-pituitary system

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zona reticularis - Innermost; arranged in a network of interconnected cords of cells (a ‘reticulum’)
Secrete gonadal hormones, mostly the weak
androgen dihydroepiandrosterone (DHEA) and some glucocorticoids
Regulated by CRH-ACTH

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blood supply of medulla: Medullary veins have more muscle in their media than most other veins, arranged longitudinally, contraction of which promotes increased movement of medullary blood into the circulation

Adrenal medulla - Arises from neural crest cells; can be thought of as a modified sympathetic ganglion

Structure:

Medullary (chromaffin) cells are modified neurons but secretory products enter bloodstream rather than directly contacting target cells
Secretory products:
-Catecholamines  - (EPI production is >> than NE)
-Chromogranins:  a family of proteins characteristic of neuroendocrine cells throughout the body

Cholinergic (ACh) stimulation by preganglionic fibers innervating chromaffin cells releases medullary hormones

**Ganglion cell neuron cell bodies are often present in the medulla