Pituitary Gland pdf - D. Lamees

The Pituitary Gland

It is the master endocrine gland , consists of 3 lobes ( anterior,

intermediate  and  posterior)  which  are  more  or  less  separated  endocrine
glands.
The intermediate lobe is rudimentary in humans.
The anterior and posterior lobes are entirely distinct physiologically.

The Anterior pituitary

( Adenohypophysis)

Classification:

anterior pituitary gland secretes three hormone groups:

1- Protein hormone group:

a.  Growth hormone.
b.  Prolactin.
c.  Placental lactogen(chorionic somatomammotropin).

2- Glycoprotein hormone group:

a.  TSH.
b.  LH.
c.  FSH.
d.  Chorionic gonadotropin.

3- Pro-opio-melano-cortin(POMC) peptide group:

a.  ACTH.
b.  Melanocyte stimulating hormone (MSH).
c.  β-lipotropin.
d.  Endorphins.
e.  Enkephalin.

Growth hormone “ GH”

GH secretion is controlled by 2 hypothalamic hormones:
1. GHRH (growth hormone releasing hormone)
2.GHRIH

(

growth

hormone

release

inhibitory

hormone

,”Somatostatin” ).

GH is a single poly peptide synthesized by the somatotropes ( anterior
pituitary acidophilic cells).

Secretion of GHRH, and therefore of GH is

pulsatile occurring about seven or eight times a day usually associated
with the following:

a) exercise,

b) onset of deep sleep,

c) in response to the falling plasma glucose concentration an hour

after meals.

 Effects of GH

1. Protein metabolism:-

GH is protein anabolic H. It increases the transport of amino acids
into muscle cells.

2. Carbohydrate metabolism :-

GH  antagonizes  the  effects  of  insulin,  it  decreases  the  peripheral
glucose  utilization,  and  increases  hepatic  glucose  production  via
gluconeogenesis. GH also increases liver glycogen.

3. Lipid metabolism :-

GH  promotes  the  release  of  fatty  acids  and  glycerol  from  adipose
tissues  and  increases  circulating  fatty  acids.  It  also  increases  fatty
acid oxidation in the liver.

4. Mineral metabolism :-

It promotes positive Ca, Mg, and P balance, and causes retention of
Na,  K  and  Cl.  It  promotes  the  growth  of  long  bones  at  the
epiphyseal plate in growing in children and acral growth in adults.
It also enhances cartilage formation in children.

5. PRL – like effects:-

It bind to lactogenic receptors, and has many of PRL effects such
as stimulation of mammary gland and lactogenesis

Pathophysiology:

GH deficiency

1. GH deficiency in childhood result in short stature, a condition called

Dwarfism

2. GH deficiency in adulthood result in lethargy, muscle weakness and
increased fat mass

GH excess

1. If occurs before the closure of epiphyseal plate leads to an increase

in the growth of long bones, a condition called Gigantism.

2. If it occurs after epiphyseal plate closure, then it results in

Acromegaly in which

3. An increase in bulk of bone and soft tissues causing the

characteristic appearance (protruding jaw, enlarged nose, and also
enlargement of hands, feet and skull).

   Excessive hair growth and sebaceous gland secretion.
  Menstrual disturbances.
  Impaired glucose tolerance.

Prolactin “ PRL”

Is  a  protein  hormone  secreted  by  the  lactotropes  (  Pituitary  acidophilic
cells).
PRL  secretion  is  under  predominantly  inhibitory  control  by  Dopamine
which  is  secreted  by  hypothalamus.  However,  TRH  (Thyrotropin
releasing  hormone),  secreted  by  hypothalamus  may  ,  in  some  instances,
stimulate PRL secretion.

It  is  important  during  pregnancy&  post  partum  period.  Prolactin
secretion  increases  progressively  after  the  eight  week  of  pregnancy
probably because of the high estrogen concentration. At term, it  may be
10 to 20 times that of non pregnant woman.

Effects

Women:- Initiation and maintenance of lactation .
Men:- not clearly known yet.

Pathophysiology

Women:-
Excessive RRL ( hyperprolactinaemia) causes amenorrhoea and
galactorrhoea.

Pathological causes of hyperprolactinaemia:-

1- A prolactin secreting micro adenoma of the pituitary gland.
2-  Failure  of  hypothalamic  inhibitory  factors  to  reach  the  anterior
pituitary gland.
3- Other pituitary tumors.
4-  Drugs  such  as:  -  estrogen.  dopaminergic  agonist  (phenothiazin,
mitoclopramide).
5- Chronic renal failure.
6- Severe primary hypothyroidism.

Men:-

Excessive PRL results in erectile dysfunction, impotence and
gynaecomastia. Also, it may result in oligospermia.

Thyroid – Stimulating hormone “ TSH”

Is  a  glycoprotein  hormone.  It’s  secretion  is  controlled  by  the
hypothalamic  TRH.  It  is  synthesized  by  the  thyrotropes  (basophilic
pituitary cells).
Effects:-
Stimulate  the  growth  of  thyroid  gland,  and  the  synthesis  and  release  of
thyroid hormones ( T

& T

The secretion of TSH from the anterior pituitary gland is controlled by:-

 Circulating concentration of thyroid hormone.
 thyrotrophin releasing hormone (TRH) which produced In the

hypothalamus and stimulate TSH secretion.

Follicle – Stimulating hormone “FSH”

and

Luteinizing hormone “LH”

Interestingly,  although  there  is  only  one  releasing  H.  for  both  FSH  and
LH called LH and FSH releasing H “ GnRH” ( LHRH, FSHRH) secreted
by  the  hypothalamus,  however,  LH  and  FSH  can  be  secreted
independently.
FSH  and  LH  are  glycoproteins secreted by  the  gonadotropes (basophilic
pituitary cells).

Effects of LH

Women:

Induces ovulation stimulate progesterone production by the

corpus luteum , and also, is required for the early maintenance of corpus
luteum.

Men:

Stimulate testosterone production by the Leydig cells of the testis.

Effects of FSH

Women:-

Stimulate the maturation of ovarian follicle causing an increase

in estradiol production by the granulosa cells of the follicle.

Men:-

FSH is essential for maintenance of spermatogenesis. FSH bind to

sertoli cells of Testis, promotes synthesis of ABP “ androgen binding
protein” which bind to testosterone and transport it to the lumen of
seminiferous tubules ( the site of spermatogenesis ).

Adrenocorticotropic hormone “ACTH”

Is  a  single  poly  peptide  secreted  by  the  cortictrops  basophilic  pituitary
cells  under  the  control  of  the  hypothalamic  CRH  (  corticotrophin-
releasing H.)

Effects:-

It regulate the growth and function of adrenal cortex, and enhances the
production of adrenal steroids ( glucocorticoids, mineralocorticoids &
dehydroepiandrosterone ). ACTH also has considerable MSH activity.

β - Lipotropin “β–LPH”

It is found only in the pituitary since it is rapidly converted to γ- LPH and
β - endorphin in other tissues.

Effects:-

causes lipolysis and fatty acids mobilization, but it physiological

role is minimal. It probably serves only as the precursor of β–Endorphin
which in turn behave as the precursor of α-endorphin and γ-endorphin.

Endorphins :

acts inside the CNS as neurotansmitters and are involved in

endogenous pain perception.

γ–Melanocyte – Stimulating hormone “γ-MSH”

They  increases  pigmentation  of  the  skin  and  mucous  membrane  by
stimulating  melanin  synthesis  in  the  melanocytes,  and  also,  causes
dispersion  of  the  intracellular  melanin  granules  resulting  in  skin
darkening.

The pituitary in mammals contain 3 MSHs:-
1. α –MSH
2. β -MSH
3. γ -MSH
γ-MSH  is  present  in  high  concentration  in  the  intermediate  lobe,  but  is
also present in the anterior pituitary. In adults , it appear that neither  α -
MSH nor β -MSH are secreted.

Hypopituitarism:-

The anterior pituitary gland has considerable functional reserve.
Clinical features of deficiency are usually absent until about 70% of the
gland has been destroyed.

Causes of hypopituitarism:-

1. Destruction of, or damage to, the anterior pituitary gland or the

hypothalamus by a primary or secondary tumor.

2.   Infarction most commonly postpartum (Sheehan ̓s syndrome).
3.   Pituitary surgery or irradiation.
4.   Less common causes include:-   a-head injury .

b-Infection or granulomas.

The Posterior Pituitary

( Neurohypophysis )

It  secretes  2  Hs  which  are  synthesized  by  the  hypothalamus  and  are
transported into  the nerve  endings in the posterior pituitary, where upon
appropriate stimulation, these Hs are released into the circulation:-

1. Oxytocin
2. Antidiuretic H. (ADH), “ Vasopressin ”
Each of them is a peptide of  9 amino acids.

Oxytocin

It's  secretion  is  stimulated  by  neural  impulses  resulting  from  nipple
stimulation, vaginal and uterine distension. And by oestrogen.
While progesterone inhibit oxytocin production.

Effects of

Oxytocin

Women : - Causes contraction of uterine smooth muscle. Thus ,it is used
therapeutically for the induction of labour also it stimulate the contraction
of myoepithelial cells surrounding the mammary alveoli promoting milk
ejection from the breast.
Men:- not clearly known yet

HOWEVER,  Current  evidence  suggests  that  oxytocin  is  involved  in
facilitating  sperm  transport  within  the  male  reproductive  system  and
perhaps  also  in  the  female,  due  to  its  presence  in  seminal  fluid.  It  may
also have effects on some aspects of male sexual behavior.

Antidiuretic H. (ADH), “ Vasopressin ”

ADH secretion is stimulated by :-
1. Increased plasma osmolality
2. Physical stress
3. Emotional stress
4. Pharmacological agents:-
  -Acetylcholine,- Nicotine, - Morphine.

Effects:

Acts on the distal convoluted tubules and collecting ducts of the kidneys
causing water reabsorption by the renal tubules , permitting osmotic
equilibrium of the cells of interstitium.

Pathophysiology

Diabetes Insipidus:

Deficiency of ADH or its action leads to Diabetes

lnsipidus (DI) which is characterized by excretion of large quantities of
extremely diluted urine ( of very low specific gravity).

DI could be:-

1. Cranial ( primary) DI:- due to insufficient ADH secretion.
2. Nephrogenic ( Secondary) DI:- ADH is normally secreted but,

either there is inherited defect or acquired damage in ADH
receptors.