LocationLocation A small gland which lies in the A small gland which lies in the

hypophyseal fossa hanging hypophyseal fossa hanging from the hypothalamus, to from the hypothalamus, to which it is connected.which it is connected.

Its dimensions are 1 x 1x 1.5 Its dimensions are 1 x 1x 1.5 cm and its weight is o.5 gm.cm and its weight is o.5 gm.

ComponentsComponents The pituitary consists of two The pituitary consists of two

embryologically, histologically and embryologically, histologically and functionally different parts:functionally different parts:

1.1. AdenohypophysisAdenohypophysis: from Rathke’s : from Rathke’s pouch (from oral ectoderm).pouch (from oral ectoderm).

2.2. NeurohypophysisNeurohypophysis: from neural : from neural ectoderm.ectoderm.

AdenohypophysisAdenohypophysis

1.1. Pars distalisPars distalis: 75 % of the gland.: 75 % of the gland.2.2. Pars intermediaPars intermedia.. The two parts are separated by The two parts are separated by

remnant of Rathke’s pouch remnant of Rathke’s pouch (epithelial cells which surrounds (epithelial cells which surrounds an amorphous colloid.an amorphous colloid.

3.3. Pars tuberalisPars tuberalis which surrounds which surrounds the infundibulum.the infundibulum.

NeurohypophysisNeurohypophysis

1.1. Pars nervosaPars nervosa: posterior to : posterior to pars intermedia.pars intermedia.

2.2. InfundibulumInfundibulum, connecting pars , connecting pars nervosa to,nervosa to,

3.3. Median eminenceMedian eminence, which , which connects the neurohypophysis connects the neurohypophysis to the hypothalamus.to the hypothalamus.

Anatomical Anatomical lobulationlobulation

Anterior lobeAnterior lobe::Pars distalis and pars Pars distalis and pars tuberalis.tuberalis.

Posterior lobePosterior lobe::Pars intermedia and pars Pars intermedia and pars nervosa.nervosa.

Blood circulationBlood circulation Superior hypophyseal arterySuperior hypophyseal artery: supplies : supplies

median eminence and infundibulum median eminence and infundibulum ((primary capillary plexusprimary capillary plexus) ) hypophyseal hypophyseal portal circulation portal circulation pars distalis ( pars distalis (secondary secondary capillary plexuscapillary plexus). It carries releasing and ). It carries releasing and inhibitory hormones from hypothalamus to inhibitory hormones from hypothalamus to adenohypophysis.adenohypophysis.

Inferior hypophyseal arteryInferior hypophyseal artery: supplies the : supplies the neurohypophysis.neurohypophysis.

Hypophyseal veinsHypophyseal veins drain to intracranial drain to intracranial venous sinus venous sinus general circulation. general circulation.

Function of hypophyseal portal Function of hypophyseal portal circulationcirculation

HypothalamusHypothalamus produces the produces the neurosecretory hormonesneurosecretory hormones which are then which are then stored in the median eminence. Through stored in the median eminence. Through the the hypophyseal portal circulationhypophyseal portal circulation, , these hormones are transferred to the these hormones are transferred to the anterior pituitaryanterior pituitary to affect its cells to affect its cells (either stimulate or inhibit hormone (either stimulate or inhibit hormone synthesis and secretion).synthesis and secretion).

So, portal circulation is the vascular So, portal circulation is the vascular system used for hormonal regulation of system used for hormonal regulation of the pars distalis by the hypothalamus.the pars distalis by the hypothalamus.

Hypothalamic neurosecretory Hypothalamic neurosecretory hormones (or factors) include:hormones (or factors) include:

1.1. Thyroid-stimulating hormone Thyroid-stimulating hormone releasing hormone (TRH).releasing hormone (TRH).

2.2. Corticotropin-releasing hormone Corticotropin-releasing hormone (CRH).(CRH).

3.3. Somatotropin-releasing hormone Somatotropin-releasing hormone (SRH).(SRH).

4.4. Gonadotropin-releasing hormone Gonadotropin-releasing hormone (GnRH).(GnRH).

5.5. Prolactin-releasing Hormone (PRH).Prolactin-releasing Hormone (PRH).6.6. Prolactin-inhibitory factor (PIF).Prolactin-inhibitory factor (PIF).

Pars DistalisPars Distalis Covered by fibrous capsuleCovered by fibrous capsule Composed of cords of parenchymal Composed of cords of parenchymal

cells surrounded by reticular fibrous cells surrounded by reticular fibrous and sinusoidal capillaries.and sinusoidal capillaries.

Its parenchymal cells are divided into:Its parenchymal cells are divided into:1.1. Chromophils:Chromophils:

a)a) Acidophils.Acidophils.b)b) BasophilsBasophils..

2.2. Chromophobes.Chromophobes.

AcidophilsAcidophils More in number, small rounded cells, More in number, small rounded cells,

having large acidophilic granules. There having large acidophilic granules. There are two types of acidophils:are two types of acidophils:

Somatotropes:Somatotropes: which produce which produce somatotropin (growth hormone)somatotropin (growth hormone) which which increases cellular metabolic rate, protein increases cellular metabolic rate, protein synthesis and elongation of long bones.synthesis and elongation of long bones.

Mammotropes:Mammotropes: which produce which produce prolactineprolactine which promotes mammary gland growth which promotes mammary gland growth during pregnancy and lactation after during pregnancy and lactation after delivery.delivery.

BasophilsBasophils Less in number, located at periphery of pars Less in number, located at periphery of pars

distalis, There are three types of basophils:distalis, There are three types of basophils: Corticotropes:Corticotropes: large, rounded or oval cells large, rounded or oval cells

with eccentric nuclei, produce with eccentric nuclei, produce adrenocorticotropin (ACTH), lipotropic adrenocorticotropin (ACTH), lipotropic hormone (LPH).hormone (LPH).

Thyrotropes:Thyrotropes: angular in shape, have small angular in shape, have small granules, produce granules, produce thyrotropin (TSH).thyrotropin (TSH).

Gonadotropes:Gonadotropes: small and rounded in shape, small and rounded in shape, produces: produces: follicular stimulating hormone follicular stimulating hormone (FSH), Leuteinizing hormone (LH) and (FSH), Leuteinizing hormone (LH) and interstitial cell-stimulating hormone (ICSH).interstitial cell-stimulating hormone (ICSH).

ChromophobesChromophobes Small, weakly stained cells. Have Small, weakly stained cells. Have

less cytoplasm.less cytoplasm. The may represent non-specific The may represent non-specific

stem (precursor) cells, or stem (precursor) cells, or degranulated chromophil cells.degranulated chromophil cells.

Pars intermediaPars intermedia Lies between pars distalis and pars Lies between pars distalis and pars

nervosa.nervosa. Has colloid-containing cysts, which are Has colloid-containing cysts, which are

lined by cuboidal cells and represents lined by cuboidal cells and represents remnants of Rathke’s pouch.remnants of Rathke’s pouch.

Its has basophils arranged in cords Its has basophils arranged in cords around network of capillaries.around network of capillaries.

These basophils may secrete These basophils may secrete melanocyte stimulating hormone melanocyte stimulating hormone (MSH).(MSH).

Pars tuberalisPars tuberalis Surrounds the infundibulum, but Surrounds the infundibulum, but

it is thinner or may be absent it is thinner or may be absent posteriorly.posteriorly.

Formed of longitudinal cords of Formed of longitudinal cords of cuboidal parenchymal basophilic cuboidal parenchymal basophilic cells surrounded by sinusoids.cells surrounded by sinusoids.

These cells have small dense These cells have small dense granules and they may share in granules and they may share in secretion of secretion of FSH and LHFSH and LH..

NeurohypophysisNeurohypophysis It develops from downgrowth of It develops from downgrowth of

the hypothalamus.the hypothalamus. It consists of median eminence, It consists of median eminence,

infundibulum and pars nervosa.infundibulum and pars nervosa. It is connected to the hypothalamic It is connected to the hypothalamic

nuclei (supra-optic and nuclei (supra-optic and paraventricular nuclei) through the paraventricular nuclei) through the hypothalamo-hypophyseal tract.hypothalamo-hypophyseal tract.

It is composed of:It is composed of:1.1. Unmyelinated nerve fibersUnmyelinated nerve fibers (forming the (forming the

hypothalamo-hypophyseal tract).hypothalamo-hypophyseal tract).2.2. Herring bodiesHerring bodies: are distensions of the : are distensions of the

axons of the hypothalamo-hypophyseal axons of the hypothalamo-hypophyseal tracts containing membrane-bound tracts containing membrane-bound granules full of granules full of vasopressin and oxytocinvasopressin and oxytocin. . These granules are released to enter the These granules are released to enter the fenestrated endothelium of the capillaries fenestrated endothelium of the capillaries surrounding them, then to general surrounding them, then to general circulation.circulation.

3.3. PituicytesPituicytes: which are similar to neuroglial : which are similar to neuroglial cells, they support the axons of the pars cells, they support the axons of the pars nervosa.nervosa.

4.4. Blood capillariesBlood capillaries..

Function of pars Function of pars nervosanervosa Storing and release of Storing and release of oxytocinoxytocin which causes which causes

contraction of the myoepithelial cells contraction of the myoepithelial cells surrounding the alveoli of the mammary surrounding the alveoli of the mammary glands and contraction of the uterine glands and contraction of the uterine muscles after labour.muscles after labour.

Storage and release of Storage and release of vasopressin vasopressin (antidiuretic hormone, ADH)(antidiuretic hormone, ADH) which causes which causes reabsorption of water in distal convoluted reabsorption of water in distal convoluted tubules and collecting tubule of the kidney, tubules and collecting tubule of the kidney, and contraction of the smooth muscles of the and contraction of the smooth muscles of the blood vessels to elevate the blood pressure.blood vessels to elevate the blood pressure.

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