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Endocrine glands

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ENDOCRINE GLANDS IN MAMMALS
-THE PITUITARY GLAND
In man, the pituitary gland or hypophysis cerebri is an ovoid structure about 0.5 gm in the adult and is
connected to the brain through a short stalk (the infundibulum). It has three lobes viz, anterior, median
and posterior. The anterior lobe is often referred to as the master gland, because in addition to
producing primary hormones it produces the tropic hormones which control the secretion of hormones
in many of the other endocrine glands.
Lobes of Pituitary Gland are in the following :
- Anterior Lobe
- Median Lobe
- Posterior Lobe
ANTERIOR LOBE :
Anterior lobe of pituitary secretes the following hormones:
1. Somatotropin hormone (STH) :
Somatotropin releasing factor (SRF) is secreted from the hypothalamus throughout life. When growth
has mostly ceased after adolescence, the hormone continues to promote protein synthesis throughout
the body. If produced in excess during early life, leads to gigantism or if later in life causes the abnormal
development of hands, feet, jaws, etc. (known as acromegaly). If there is under-secretion, dwarfism
results, as well as other symptoms associated with lack of thyroid and adrenal hormone.
2.Thyroid stimulating hormone(TSH):
Release of thyrotropin releasing factor from the hypothalamus is controlled by the levels of thyroxine in
the blood. In the presence of low levels of thyroxine, there is increasing production of TSH and vice
versa (Fig) It is secreted throughout life but particularly reaches high levels during the periods of rapid
growth and development. It acts directly on the cells of the thyroid gland, increasing both their numbers
and their secretory activity (Fig1)
3. Adrenocorticotropic hormone (ACTH)(Corticotropic hormone) :
Release of corticotropin releasing factor from the hypothalamus is controlled by steroid levels in the
blood and by direct nervous stimulation of the hypothalamus as a result of stress e.g. cold, heat, pain,
fright, infections. Excess and deficiency results in disturbance of normal adrenal functions.
4. Gonadotrophic hormones (GH) : These are follicle stimulating hormone (FSH), luteinising hormone (LH
also called interstitial cell stimulating hormone ICSH, in the male), prolactin (sometimes inappropriately
called luteotropic hormone, LTH). FSH and LH/ICSH share a common hypothalamic releasing factor.
Prolactin is continuously produced from the pituitary and is inhibited by prolactin inhibiting factor

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(PIH) from the hypothalamus. Prolactin stimulates milk production and acts with LH as described below.
FSH in females stimulates follicle development and secretion of oestrogens from the ovaries; in males it
stimulates development of the germinal epithelium of the testis and sperm production. LH works with
FSH to stimulate oestrogen secretion and rupture of mature follicles to release egg or ovum. It also
causes the luteinization (lit. "turning yellow) of the latter and acts synergistically with prolactin to
maintain the corpus luteum (and hence the progesterone it secretes). ICSH in the male stimulates the
interstitial cells of the testes to secrete testosterone.
MEDIAN LOBE :
Median lobe secretes the following hormone :
Melanophore stimulating hormone (MSH) :
Its inhibition of secretion is controlled by hypothalamus. External light governs its secretion. More
secretion in pregnancy stimulates melanocytes in skin to produce brown pigment, melanin, which
darkens the skin. Excess MSH is secreted in Addison's disease. One of the symptoms of which is
darkening of the skin.
POSTERIOR LOBE :
Posterior lobe of the pituitary gland secretes the following hormones:
Antidiuretic hormone (ADH) or Vasopressin:
Its secretion is caused by decrease in blood pressure, blood volume, and osmotic pressure of the blood
which is detected by osmoreceptors in hypothalamus. External sensory stimuli also influence
hypothalamic neurosecretory cells. Increased levels cause increased water reabsorption in distal parts of
nephron. A lack of this hormone produces diabetes insipidus, characterized by production of large
quantities of dilute urine and great thirst.
2. Oxytocin :
Its release is stimulated by distension of cervix, decrease in progesterone level in blood, and neural
stimuli during parturition and suckling. Primary action is on smooth muscle, particularly in the uterus
during childbirth, and also causes milk ejection from mammary glands.

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