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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.