C.N. Saxena

B.sc. IVth Sem. Assistant Professor,

Unit - iii Department of Zoology

Sri J.N.M. P.G. collage (KKC),

Luclnow

Pituitary gland

For many years the pituitary gland was called the master endocrine

gland because it secrete several hormones that control other endocrine gland.

We now know that the pituitary gland itself has a master hypothalamus. This is

small region of the brain below the thalamus, is the major integrating link

between the nervous and endocrine system.

The hypothalamus is an important regulatory centre in the nervous

system is an important endocrine gland. The pituitary gland or hypophysis lies

in the hypophyseal fossa of the Sella turcica of the sphenoid bone. It is

attached to the hypothalamus by a stalk the infundibulum.

The pituitary gland is a pea shaped, reddish grey small oval shaped

structure. It measures about 0.5 to 0.6 gram in weight. The gland develop

embryologically as a fusion between an up growth of ectodermal cells from the

roof of the primitive pharynx (Rathke’s Pouch) which forms adenohypophysis

or anterior pituitary and the same time Rathke’s pouch is devoloping, another

finger of ectodermal tissue evaginates ventrally from the diencephalon of the

developing brain. The extension of the brain become the posterior pituitary or

neurohypophysis.

In the 16th century vesalius named this gland pituitary. It has two anatomical and functionally separate lobe, anterior pituitary accounts for about 75% of the total weight of the gland while posterior pituitary constitutes about 25%.

Division of the pituitary gland –

The pituitary gland comprises two parts

1. Adenohypophysis

2. Neurohypophysis

These divisions are distinctly different in their embryonic origin and

histological structure. Adenohypophysis develops from the oral ectoderm and

the neurohypophysis from neural ectoderm. Adenohypophysis is comprises of

three parts.

1. Pars distalis

2. Pars tuberalis

3. Pars intermedia

Neurohypophysis is divisible into two parts

1. Paras nervosa

2. infundibulum

Diagram Showing Pituitary gland

Division of Pituitary gland

ADENOHYPOPHYSIS

1) Pars Distalis

This part of adenohypophysis is made up of parenchymatous cells which

are again divisible into two types of cells.

A) Chromophobe cells - This constitutes about 50% cells. These are so

named because these cells are not stained with either basic dyes are

acidic dyes they do not secrete any kind of hormone.

B) Chromophil cells – Chromophil cells are so named because they show

affinity towards dies. On the basis of their staining properties the cells

are again of two types Acidophil cells and Basophil cells.

Acidophil cells – The acidophil cells are so named because they have got

affinity towards the acidic dyes. Acidophils constitute 35% of the cell.

These cells consist of two types of cells somatotroph cells and lactotroph

cells.

a) Somatotrophs cells – These are secretory cells which are round and

ovoid in shape. They are present in very high concentration. The

cytoplasm is packed up with dense granules. They are concerned with

the secretion of the growth hormone. The cell secretes growth hormone

which is a protein in nature containing 191 amino acids in a single chain.

b) Lactotrophs - These are ovoid secretory cells having great affinity for

carmine stain. The granules of the leactotrophic cells are of larger size in

comparison to that of somatotrophs.

Basophil cells - These are so named because they have great affinity

towards basic dyes. These cells are concerned with the secretion of

different kinds of tropic hormones such as AC-TH TSH FSH and LH.

1. Corticotroph cells- These are of different shape and sizes. They are a

small and mostly oval cells with vesicles or tubules. The nucleus are

rounded or pear shaped. These cell secrete adrenocorticotropic

hormone (ACTH) to stimulate the adrenal cortex.

.

2. Thyrotroph cells- These are polygonal cells with a small nucleus. They

have large secretory granules and large endoplasmic reticulum. These

cells are primarily concerned with the secretion of thyroid stimulating

hormone (TSH).

3. Gonadotropic cells - The cells are very small all and rounded or

polygonal in shape. Gonadotropic cells comprise the interstitial cell

stimulating hormone or luteinizing hormone (LH). Gonadotropic also

comprises of follicle stimulating hormone (FSH).

Growth hormone (GH) - Growth hormone is also called as somatotropic

hormone. It is originated from Pars distalis of adenohypophysis. It is

secreted from acidophil cells. Growth hormone is a protein in nature

containing 191 amino acids in a single chain. Its molecular weight is

about 22124 dalton. Regulation of growth hormone secretion is

performed by hypothalamus and some other factor such as

hypoglycemic condition etc. The target cell or organ for this hormone

are somatic cells. The main function of this hormones are –

a) It increases the rate of protein synthesis in all cell of body by

increasing the amino acid transport through the cell membrane.

b) This hormone is basically concerned with the growth of body tissue

and bones

c) It influences glycogenesis and gluconeogenesis in liver.

Abnormalities of growth hormone secretion

Hypo and hyper secretion of growth hormone causes following abnormalities.

Hyposecretion – It causes following disorders.

Dwarfism - It results from deficiency of growth hormone secretion

during childhood. Such people are normal with their mental status but if

deficiency occur in the first month of the foetal life which causes total

mentally retarded child because at this time the lymphatic or immune

system of the body is not yet developed and they are generally sterile.

Simmonds disease - This disease occurs during old age due to hypo

secretion of growth hormone. In this disease there is almost complete

destruction of the anterior hypophysis because of severe necrosis i.e.

the death of some or all the cells in an organ or tissue. Due to this the

body become very weak, muscular weakness, wrinkling of skin takes

place and the individual seems to be very old than its real age.

Acromicria - It occurs in adult it is marked by emaciation and retarded

growth of bones Limbs. But this is very rare.

Hyperactivity of growth hormone

Gigantism -

1) It is caused by the hyperactivity of the acidophil cells in young. This

disease is marked by overgrowth of the bones. All body tissue grow

rapidly including bones , the person become giant with the height of

about 7 to 8 feet or even more, the person become hyperglycemic.

Acromegaly -

It is caused by the hypersecretion of the growth hormone from

acidophil cells in adults. This disease can be marked by protruding jaw.

Face often look like Gorilla body hair increases, about 4% of the female

patient develop lactation without pregnancy, skin become thick and

loses its elasticity.

Cushing Syndrome -

It is due to the hyperactivity of basophil cells of adenohypophysis

it is very common in females and this disease can be marked by face

turns moons like due to excessive deposition of fat, skin become thinner

and emotionally sterile.

Flow chart of abnormal secretion of growth (GH) hormone

Thyroid Stimulating Hormone (TSH) -

The origin of thyroid stimulating hormone is the Pars distalis of

adenohypophysis and the cellular source is basophil cells. Thyrotropin is

a glycoprotein in nature, which is synthesized and stored in thyrotrophic

cells. The target organ of TSH is the thyroid gland, molecular weight is

about 28000 dalton. Hormones promotes and maintains growth and

development of the thyroid gland.

Control of release -

Thyrotropin release is controlled by the hypothalamic releasing

factor thyrotropin release factor(TRF). Its secretion is also under the

control of thyroxine level in the blood. High thyroxine content in the

blood inhibits while low thyroxine content in the blood stimulates its

secretion.

Primary action –

Its primary action is to stimulate the thyroid gland to secrete its

two hormone triiodothyronine (T3) and thyroxine (T4) in the

bloodstream.

Adreno Corticotropic Hormone (ACTH) -

The origin of this hormone is Pars distalis of adenohypophysis and

the cellular source is the basophil cell of adenohypophysis. This

hormone is polypeptide in nature consisting of 39 amino acids which is

synthesized and stored in corticotroph cells, molecular weight is about

4500 in certain mammals including man.

Control of release –

The release of corticotropic hormone is controlled mainly by

hypothalamus i.e. by corticotropin-releasing factor(CRF) . Various

neurotransmitter including acetylcholine and noradrenaline are involved

in the regulation of thyrotropin release factor. Beside these blood level

of steroid hormones play an essential role in modifying the activity of

adrenal pituitary axis. The existence of steroid adrenocortical tropic

hormone feedback mechanism has been evidenced by the following

observation, atrophy of the adrenal gland by increased dose of cortical

hormone corticosterone and cortisol etc.

Primary action –

Its primary function is to stimulate the two inner most zones of

adrenal cortex i.e. zona fasciculata and zona reticularis which secretes

glucocorticoids and small quantities of sex hormones. This hormone

promotes and maintain the growth and development of the adrenal

cortex. This hormone has important lipolytic action and lipid

mobilisation in the liver is increased.

Gonadotropic hormones (LH/FSH) –

These hormones are secreted from gonadotropic cells. It is

originated from basophil of Pars distalis of adenohypophysis. Two

gonadotropins produced in the adenohypophysis are luteinizing

hormone (LH) and follicle stimulating hormone (FSH).

Luteinizing hormone (LH) –

This gonadotropic hormone is a glycoprotein. In female it acts

primarily to precipitate ovulation by acting synergistically with follicle

stimulating hormone and then maintained the secretory function of the

Corpus luteum.

In male this hormone is often called interstitial cell stimulating

hormone(ICSH) because it acts primarily by stimulating the interstitial

leydig cells in testis which secrete testosterone.

Follicle stimulating hormone (FSH) –

This hormone is also glycoprotein in nature. Follicle stimulating

hormone is basically concerned with the folicular development of ovary.

In the males it has a possible action that it induces the development of

seminiferous tubules and also stimulate the process of spermatogenesis

it has however no effect on leydig cells and secretion of androgen.

. Control of release –

The release of both the gonadotropins under the hypothalmic

control through gonadotropin release factor i.e. luteinizing release

factor and follicle stimulating hormone release factor. Dopamine for

example appears to have excitatory and inhibitory action with respect to

gonadotropin release factor. Oestrogen and progesterone both

influences luteinizing hormone release, through the feedback

mechanism at both hypothalamic and adenohypophyseal level.

Melanocyte stimulating hormone (MSH) -

Melanocyte stimulating hormone is secreted from Pars intermedia

of adenohypophysis. It is a polypeptide in nature. Two forms of this

hormone have been isolated, they are 𝛼 melanocyte stimulating

hormone and 𝛽 melanocyte stimulating hormone. 𝛼 melanocyte

stimulating hormone is a single chain polypeptide of 13 amino acid while

𝛽 melanocyte stimulating hormone is a single chain polypeptide of 22

amino acid in humans.

Control of release - The secretion of this hormone is primarily under the

control of hypothalamus which release specific factor called melanocyte

inhibiting factor which controls it.

Primary function - The only known function of melanocyte stimulating

hormone in humans is to increase pigmentation of the skin by increasing

melanin synthesis in the melanocytes. The melanin appears to move out

of the cells and is then dispersed in the surrounding dermal cells.

Prolactin - It is also called as lactogenic hormone, this hormone is

secreted from acidophil cells of Pars distalis of adenohypophysis.

Prolactin has 170 amino acid and Molecular weight is approx 22,000

dalton. In this way it is protein in nature. The target organ for this

hormone is mammary gland.

Control of release- Prolactin release is under the control of the

hypothalamus and the placental gonadotropin. Dopamine,

noradrenaline, histamine and serotonin all influence the release of

prolactin acting at either hypothalamic or pituitary level. Hypothalamus

releases a factor known as prolactin inhibiting factor which controls its

secretion.

Primary action – The only established function for prolactin is the

initiation and maintenance of lactation in females. Prolactin plays a

supportive role with LH in the maintenance of corpus luteum at different

periods. For this reason it is also called as luteotropic hormone. There

are specific biding site in the kidney for prolactin. This hormone can

influence the retention of fluid and electrolytes (Na & K) by the kidney.

Flow chart to show cellular source of hormones from pars distalis

Neurohypophysis

The neurohypophysis functionally consist of supra optic and para

ventricular nuclei in hypothalamus, the hypothalmo- hypophyseal nerve

tract and the pars nervosa of the hypophysis. However the term

neurohypophysis is often used to describe the posterior pituitary as a

separate unit to differentiate this lobe from adnohypophysis. It receives

its arterial blood supply from two main sources, the superior

hypophyseal artery in the median eminence region, and the inferior

hypophyseal artery in the lower part of pars nervosa.

The cell bodies of the supraoptic and paraventricular neurons

synthesizes two hormones.

1. Vasopressin

2. Oxitocin

1. Vasopressin –

This hormone is also called as anti-diuretic hormone. It is secreted

from supra optic nuclei and para ventricular nuclei of hypothalamus. It is

a polypeptide (Octapeptide) in nature. Molecular weight is about 1100.

The target organ for this hormone are kidney and blood vessels.

Control of release –

Central nervous system (CNS), has a controlling device over its

secretion. One mechanism involved in the release of vasopressin is a

variation of the plasma osmolality. Second mechanism involved in

control of vasopressin release, the concern changes in blood volume.

Primary action –

The principal physiological action of vasopressin is to stimulate the

reabsorption of water from the tubular fluid in the collecting ducts of

renal nephron in presence net reabsorption pressure. The blood level of

the hormone therefore directly determines the water balance of body.

In the presence of vasopressin the urine excreted by kidney is

small in volume and highly concentrated (anti-diuresis). Hence it is also

called as antidiuretic hormone. In this way hyposecretion of ADH causes

a disease known as Diabetes insipidus.

Oxytocin –

This hormone secreted from hypothalamic paraventricular nuclei

of neurohypophysis. It is also a polypeptide in nature (octapeptide).

Molecular weight of this hormone is 1000. The target organ for this

hormone is uterus and the mammary gland.

Control of release – Hypothalamus through supra optic hypophyseal

tracts controls the release of its secretion.

Oxytocin release is stimulated in the lactating mother by

suckling,the cell bodies in the paraventricular nuclei are then

stimulated, resulting in the release of oxytocin.

Primary action - Oxytocin which is present in the neurohypophysis of

both male and female , exerts its physiological effects only in females.

Oxitocin stimulates the contraction of the smooth muscles of the uterus

and the lactating mammary gland. Contraction of uterus in response to

oxytocin is only observed during the late stage of pregnancy. It is

believed that under the influence of progesterone it prepare uterus for

parturition (child birth). It is also concern with ejection of milk in

females.