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Endocrine glands

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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-THYROID GLAND

In mammals it consists of two lobes situated below the larynx (Fig. 2) It produces thyroxine (or

tetraiodo-thyonine: T4), tri-iodothyronine or T3 (which has a structure similar to thyroxine with 3 iodine

atoms rather than 4) and calcitonin hormone. The thyroid is active continuously but produces higher

levels of secretions during periods of rapid growth and sexual maturation and in stress situations such as

cold and hunger. Thyroxine and triiodothyronine, the two hormones act in essentially the same way.

They act on the basal metabolic rate by stimulating the breakdown of glucose and release of heat and

generation of ATP. They also act in conjunction with somatotropin in bringing about growth, and act

directly on brain cells causing them to differentiate. In amphibians, they bring about the process of

metamorphosis.If secretion of thyroid is deficient, tadpole larva of frog does not metamorphose to

develop into frog, but instead grow to a large sized tadpole.

Graves’ disease : Excess thyroxine produces a condition called GRAVES' DISEASE , with exophthalmic

goiter and increase in the basal metabolic rate. This can lead to cardiac failure if prolonged. The cause of

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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 r ...
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