Hypophysis

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I. THE ADENOHYPOPHYSIS lias three subdivisions, called the pars distalis, pars tuber-alis, and pars intermedia.

A. The pars distalis contains important endocrine cells (Table 20-1).

B. The pars tuberalis surrounds the median eminence and infundibular stem of the neurohypophysis. It contains the portal venules of the hypophyseal portal system.

C. The pars intermedia contains numerous colloid-filled cysts (Rathke's cysts) as well as cells that secrete melanocyte-stimulating hormone (MSH). It is rudimentary in humans.

II. HORMONAL SECRETION from the adenohypophysis is controlled by hypothalamic neurons and rhe hypophyseal portal system.

A. Hypothalamic neurons

1. Cell bodies are located in the arcuate nucleus, medial preoptic nucleus, and paraventricular nucleus of the hypothalamus. They synthesize releasing hormones (RHs) and inhibiting hormones (IHs).

2. Axons project to the median eminence, where axon terminals secrete RHs and IHs into the primary capillaries of the hypophyseal portal system.

3. RHs and IHs control hormone secretion from the adenohypophysis.

B. The hypophyseal portal system has three components:

1. Primary capillaries (fenestrated)

a. Primary capillaries are formed by the superior hypophyseal arteries.

b. They are located in the median eminence.

C. They are the site where RHs and IFs are secreted into the bloodstream.

2. Portal venules a. Portal venules are located in the pars tuberalis.

b. They transport RHs and IHs to the pars distalis.

3. Secondary capillaries (fenestrated)

a. Secondary capillaries are located in the pars distalis.

b. They are the site where RHs and IHs leave the bloodstream to stimulate or inhibit endocrine cells of the adenohypophysis.

C. Types of RHs and IHs

1. Thyrotropin-releasing factor (TRF)

2. Gonadotropin-releasing factor (GRF)

Table 20-1

Characteristics of Endocrine Cells in Pars Distalis

Table 20-1

Characteristics of Endocrine Cells in Pars Distalis

Endocrine Cells

Hormone

Function

Somatotrophs

Growth hormone

Promotes skeletal growth and bone remodeling; hyposecretion causes dwarfism and hypersecretion causes gigantism or acromegaly

Mammotrophs

Prolactin

Promotes growth of the mammary gland during pregnancy and milk secretion in the lactating woman

Thyrotrophs

Thyroid-stimulating hormone

Stimulates triiodothyronine (T3) and thyroxine (T4) secretion

Corticotrophs

Adrenocorticotropin (is derived from a large precursor called proopiomelanocortin)

Stimulates Cortisol secretion

1

' Follicle-stimulating hormone

Women: Promotes secondary follicle to

Graafian follicle growth Men: Stimulates spermatogenesis and synthesis of androgen-binding protein in Sertoli cells

Gonadotrophs S

v Luteinizing hormone

Women: Promotes ovulation (luteinizing hormone surge), formation of corpus luteum, and progesterone secretion Men: Stimulates testosterone secretion from Leydig cells

3. Growth hormone releasing factor (GHRF)

4. Growth hormone inhibiting factor (also called somatostatin)

5. Corticotropin releasing factor (CRF)

6. Prolactin inhibiting factor (also called dopamine)

III. THE NEUROHYPOPHYSIS (Figure 20-1) receives axonal projections from neurons that have cell bodies that are located in the supraoptic nucleus and paraventricular nucleus of the hypothalamus.

A. The cell bodies synthesize oxytocin, which causes milk ejection (by stimulating myoepithelial cells in the mammary gland to contract) and uterine contraction during childbirth (by stimulating smooth muscle cells of the myometrium).

B. The cell bodies also synthesize antidiuretic hormone (ADH), which increases water re-absorption from tubular fluid to blood by the medullary collecting ducts of the kidneys.

C. Axons project to the neurohypophysis, where axon terminals secrete oxytocin and ADH into the bloodstream.

D. Axon terminals have large aggregations of neurosecretory vesicles (called Herring bodies) containing either oxytocin or ADH (plus a binding protein called neurophysin).

E. Pituicytes also are found within the neurohypophysis and may function as glial-type cells.

pV Hypothalamus

/ " s pV Hypothalamus

Figure 20-1. Diagram of the hypophysis (consisting of the adenohypophysis (pars disralis, pars tuheralis, pars intermedia) and neurohypophysis] and hypothalamus. Releasing hormones (RH) or inhibitory hormones (1H) from the arcuate nucleus (ARC), medial preoptic nucleus (MPO), and paraventricular nucleus (PV) of the hypothalamus enter the primary capillaries (pc) of the portal system. RH and IH travel to the secondary capillaries, (sc) where they either stimulate or inhibit endocrine cells of the pars distalis. Oxytocin (OXY) and antidiuretic hormone (ADH) from the supraoptic nucleus (SO) and PV nucleus of the hypothalamus travel down axons to the neurohypophysis, where they are secreted through axon terminals into the blood. ACTH = adrenocorticotropic ADH = antidiuretic hormone; FSH = follicle-stimulating hormone; GH = growth hormone; LH = luteinizing hormone; OXY = oxytocin; PRL = prolactin; TSH = thyroid-stimulating hormone.

OXY ADH

Figure 20-1. Diagram of the hypophysis (consisting of the adenohypophysis (pars disralis, pars tuheralis, pars intermedia) and neurohypophysis] and hypothalamus. Releasing hormones (RH) or inhibitory hormones (1H) from the arcuate nucleus (ARC), medial preoptic nucleus (MPO), and paraventricular nucleus (PV) of the hypothalamus enter the primary capillaries (pc) of the portal system. RH and IH travel to the secondary capillaries, (sc) where they either stimulate or inhibit endocrine cells of the pars distalis. Oxytocin (OXY) and antidiuretic hormone (ADH) from the supraoptic nucleus (SO) and PV nucleus of the hypothalamus travel down axons to the neurohypophysis, where they are secreted through axon terminals into the blood. ACTH = adrenocorticotropic ADH = antidiuretic hormone; FSH = follicle-stimulating hormone; GH = growth hormone; LH = luteinizing hormone; OXY = oxytocin; PRL = prolactin; TSH = thyroid-stimulating hormone.

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