Pituitary Gland|Master Gland 

Overview:

The pituitary gland is a complex endocrine gland. It regulates the functions of other endocrine glands through its hormones. It controls the activity of different glands through its secretions. Therefore, it is also known as the 'Master gland' of the endocrine system. It is also known as Hypophysis.

 

Keywords:

Endocrine|Gland|Adenohypophysis|Neurohypophysis|CRH|Releasing factor| Hormones|Factors|Sella turcica|Hypothalamus|Master gland|

Location: The pituitary gland is situated at the base of the brain, behind the bridge of the nose. It is a continuity of the hypothalamus and seems to protrude from its bottom at the brain base, in the center of the Middle cranial fossa. This is present in the hypophysial fossa of the sphenoid bones, surrounded by a bony cavity known as 'sella turcica' and covered by 'diaphragm sella' –a dural fold. The hypophysial fossa is the deepest part of "sella turcica."

In Latin, Sella turcica (Sella=seat)and( turcica=Turkish) means Turkish saddle. In the sphenoid bone's body is a saddle-shaped depression known as Sella turcica.

Diaphragma sella is a dural fold that incompletely covers the sella turcica. It gives way to the infundibulum.

Size and shape

The size is like a pea or a kidney bean ( average 0.5gm). Its shape is variable, and it may be oval or flat.

The pituitary gland is anatomically and functionally related to the hypothalamus to the extent that the hypothalami-hypophysial is a single functional unit.

Parts of the Pituitary gland

The pituitary gland has three parts:

(i) Adenohypophysis: the anterior lobe or anterior pituitary contributes 75% of the pituitary gland's weight; the rest is from the posterior lobe.

(ii) Intermediate Lobe is present between the anterior and posterior lobes. It is relatively avascular and almost absent in humans.

(iii) Neurohypophysis: neural lobe or posterior lobe or posterior pituitary. A small tube known as pituitary stalk or infundibular stalk connects the median eminence' and hypothalamus.

Development of pituitary gland

Developmentally and functionally, anterior and posterior lobes are different.

The anterior pituitary is fleshy and glandular. It develops from an invagination of the oral ectoderm-Rathke's pouch.

The intermediate lobe arises from Rathke's pouch.

Neurohypophysis develops from the neuroectoderm.

Anterior and Intermediate Lobes arise from the ectoderm, whereas the posterior lobe originates from the neuroectoderm.

 

Adenohypophysis:

Adenohypophysis develops from ectoderm – an invagination of the oral ectoderm called Rathke's pouch.

The adenohypophysis has three parts:

(i)Pars distalis, or pars glandularis, forms the anterior lobe's bulk (80%) and is highly vascular.

(ii) Pars intermedia is between pars distalis and the neural tube. It is relatively avascular and all most absent in humans

(iii)Pars tuberalis (or pars infundibularis) is a highly vascular collection of secretory cells. It superficially surrounds the neural stalk.

Neurohypophysis

Neurohypophysis develops from neuroectoderm and forms the floor of the third ventricle.

Neurohypophysis consists of three components:

(i) Median eminence: it is a small, highly vascular protrusion of the base of the hypothalamus, present just below the third ventricle. It lies outside the blood-brain barrier. Portal vessels arise from the median eminence.

It is located beneath the third ventricle.

(ii) Infundibular stem, a funnel-shaped structure, arises from the median eminence. The floor of the third ventricle is known as the infundibulum ( a funnel).

(III)Pars nervosa is bulk of the posterior lobe.The infundibular process is a downward outgrowth that forms the bulk of the Posterior Lobe.

The neural stalk is composed of the median eminence and infundibular stem. It maintains the neural connection of the posterior lobe with the hypothalamus.

The hypophysial stalk = neural stalk + pars tuberalis. The hypophysial stalk attaches the pituitary gland to the hypothalamus.

Intermediate Lobe.

The intermediate lobe, or pars intermedia, is present between the anterior and posterior lobes and secretes melanocyte-stimulating hormones. This lobe is avascular and very small, almost absent in humans.

Hormones of the hypothalamus influence the adenohypophysis. These hormones come from the hypothalamus via 'hypothalamo-hypophyseal portal vessels.

Neurohypophysis does not synthesize any hormone; it only stores and secretes two hormones. Neurohypophysis is connected to the hypothalamus by the hypothalamo-hypophyseal neural tract. The unmyelinated nerve fibers originating from the supraoptic and paraventricular nuclei of the hypothalamus form the hypothalamo-hypophyseal neural tract. The supraoptic and paraventricular nuclei synthesize hormones, ADH, and oxytocin. The neural tract carries hormones from the nuclei to the posterior pituitary for storage.

Hormones of the pituitary gland :

Adenohypophysis =Anterior pituitary

1. Growth hormone (GH) or Somatotrophin.

2. Thyroid stimulating hormone (TSH) Thyrotrophin.

3.Adrenocorticotrophic hormone(ACTH) Corticotrophin.

4.Follicle stimulating hormone(FSH).

5. Luteinizing hormone (LH), Interstitial cell-stimulating hormone (ICSH).

6. Prolactin, lactogenic hormone (LTH).

The intermediate lobe

1. alpha and beta melanocyte-stimulating hormone (MSH) melanotropin.

Neurohypophysis

1.Antidiuretic hormone (ADH),Vasopressin.

2. Oxytocin

Trophic or tropic hormones stimulate the secretion of other endocrine glands. Anterior pituitary hormones are Trophic or tropic hormones.

 

 

Cellular structure of Anterior pituitary:

Anterior pituitary cells are divided into two main groups: chromophils(75%), granular secretory cells, and chromophobes (25%).

Based on the staining property of cells, chromophils are of two types:

Acidophils (80%)-stain red or orange with acidic dye. They are

1. Somatotrophs secrete Somatotrophin or growth hormone.

2. Mammotrophs secrete prolactin.

Basophilic cells(20%) stain blue with basic dyes. They are

1.Corticotrophs secrete ACTH and beta lipotropin

2. Thyrotrophs secrete TSH

3. Gonadotrophs secrete LH and FSH.

Chromophobes do not show staining properties. They are small degranulated secretory cells.

Nerve supply: postganglionic sympathetic fibers supply the blood vessels of the anterior pituitary.

Blood supply

By internal carotid artery.

Cavernous sinuses and jugular veins drain the venous blood.

Regulation of endocrinal cells of anterior pituitary ;

Regulatory hormones released by Parvocellular neurosecretory cells in the hypothalamic capillaries regulate the endocrinal cells of the anterior pituitary.

The regulatory hormones diffuse from the second capillaries of the hypothalami-hypophysial portal vessels, bind to the endocrinal cells of the anterior pituitary, and stimulate or inhibit their secretions.

The paraventricular nucleus secretes corticotrophic releasing hormone (CRH). CRH travels in the hypothalami-hypophysial portal vessels and stimulates corticotrophs to secrete ACTH and beta-lipotropic hormones.

The paraventricular nucleus secretes thyrotrophic releasing hormone TRH). TRH ravels in the 'hypothalami-hypophysial portal vessels and stimulates thyrotrophin from TSH.

The arcuate nucleus secretes growth hormone-releasing hormone (GHRH). GHRH travels in the 'hypothalami-hypophysial portal vessels' and stimulates somatotrophs to synthesize Somatotrophin, i.e., growth hormone.

The arcuate nucleus also secretes prolactin inhibitory hormone (PIH). PIH travels in the hypothalami-hypophysial portal vessels and inhibits lactotrophs to reduce prolactin synthesis.

The preoptic nucleus secretes gonadotrophic releasing hormone (GnRH). GNRH travels in the 'hypothalami-hypophysial portal vessels' and stimulates Gonadotrophs to secrete LH and FSH. When the frequency of GnRH is high, it secretes LH; when it is low, it forms FSH.

The same nucleus secretes inhibitory hormones in response to secondary triggers. For example, the arcuate nucleus forms GHIH, also known as somatostatin.

In the following article, we will learn about oxytocin.

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https://blog.totalphysiology.com/2022/04/growth-hormone-gigantismacromegaly.html

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External link:

Pituitary gland – Health Blog. http://kress.tomsk.ru/pituitary-gland/

Pituitary gland - Wikipedia. https://en.wikipedia.org/wiki/Pituitary_gland