Endocrine System
The endocrine system controls body activities by releasing mediator molecules called Hormones. Hormones released into the bloodstream travel throughout the body and their results may take hours, but last longer.
Exocrine Glands– secrete products into ducts which empty into body cavities
or body surface– sweat, oil, mucous, & digestive glands
Endocrine Glands– secrete products (hormones) into bloodstream
• pituitary, thyroid, parathyroid, adrenal, pineal
– other organs secrete hormones as a second function• hypothalamus, thymus, pancreas, ovaries, testes, kidneys, stomach,
liver, small intestine, skin, heart & placenta
Introduction
The Pituitary Gland is also known as Hypophysis. It is a small gland
about 1 cm in diameter and 0.5-1 g in weight.
LocationIt lies in sella turcica (bony cavity at the base of the brain) of sphenoid
bone
Division Of HypophysisPhysiologically, it is divided into two lobes:
1. Adenohypophysis (anterior pituitary)
2. Neurohypophysis (posterior pituitary)
There is a zone between these divisions which is known as Pars
Intermedia.
Origin
Embryologically, two portions of pituitary gland originate from different
sources:
Posterior pituitary is derived from the neural tissues: During the embryological development , from the hypothalamic
portion of diencephalon there is an evagination which goes
downward and posteriorly.
Anterior pituitary is derived from the epithelial
tissues: It is a derivative of “Rathke’s Pouch”. From the nasopharynx
an epithelial evagination develops in upward direction,
eventually this evagination comes very close to posterior
pituitary.
Relationship b/w Hypothalamus & Pituitary Gland
The connection b/w hypothalamus and posterior pituitary is neural
connection.
The connection b/w hypothalamus and anterior pituitary is vascular
connection.
Hypothalamic Hypophysial Portal System
This system represents the connection between hypothalamus and
anterior pituitary. Hypothalamic arteries enter into hypothalamus and
break up into capillaries and fenestrated capillary network and this
capillary network recollects as long and short hypothalamic hypophysial
portal vessels. These veins come down to anterior pituitary through the
pituitary stalk and again break up into another capillary network which
again recollects into general blood circulation.
Why it is called portal system ??? Because one group of arteries
breakup into capillaries which recollects into veins but these veins don’t
drain into systemic circulation, before going to systemic circulation these
draining veins again breakup into capillary network.
Importance of Hypothalamic Hypophysial Portal System
The cells of anterior pituitary are controlled by certain factors coming
from the hypothalamus.
These hypothalamic factors are released by neurons which are a part
of hypothalamic nuclei. These certain factors are taken up by the
primary capillary network and carried through the vascular system into
anterior pituitary, from here these factors go out and influence the
cells of anterior pituitary cells.
There is a small amount of factors which are released by hypothalamic
system into primary capillary network of hypothalamus and these
substances don’t get diluted into systemic circulation rather through
the portal they reached directly into the anterior pituitary. If the veins drain directly into general circulation before going into the
anterior pituitary then these factors become very much diluted and they cannot influence the anterior pituitary cells properly. By this system, hypothalamus controls the anterior pituitary not through the neural connection but through the vascular portal system.
Hypothalamus Controls Pituitary Secretions
Almost all secretions by the pituitary is controlled by either hormonal or
nervous signals from the hypothalamus.
Secretion from the posterior pituitary is controlled by nerve signals
that originate in the hypothalamus and terminate in the posterior
pituitary.
In contrast, secretion by the anterior pituitary is controlled by
hormones called hypothalamic releasing and hypothalamic inhibitory
hormones (factors) secreted within the hypothalamus and reached to
the anterior pituitary through the hypothalamic hypophysial portal
vessels.
The hypothalamus receives signals from many sources in the nervous
system. The hypothalamus is a collecting center for information
concerning the internal well-being of the body, and much of this
information is used to control secretions of the pituitary hormones.
Posterior Pituitary Hormones
Posterior pituitary hormones are synthesized in some nuclei of
hypothalamus while the posterior pituitary gland only stores and
release the hormones into the systemic circulation.
There are two types of nuclei in the hypothalamus responsible for the
synthesis of posterior pituitary hormone:
A) Supraoptic Nucleus ADH or Vasopressin
B) Paraventricle Nucleus Oxytocin
90% of the supraoptic nucleus cell bodies produce ADH and 10% of the
paraventricle nucleus may produce oxytocin.
90% of the paraventricle nucleus cell bodies produce oxytocin and 10%
of the supraoptic nucleus may produce ADH.
Cont…
Anti-diuretic hormone (ADH) or Vasopressin It acts on smooth muscle cells around blood vessels to cause muscle
contraction.
It acts within the kidneys to decrease water excretion in the urine
that’s why it is also a anti-diuretic hormone because it retains the
water from kidney in the body.
Oxytocin It stimulates contraction of smooth muscle cells in breasts which
results into milk ejaculation.
It stimulates contraction of uterine smooth muscles during labor.
Remember milk synthesis is under control of prolactin but
milk ejection is under control of oxytocin.
Anterior Pituitary Hormones
Anterior hormones are synthesized by the anterior pituitary cells but the
release of these hormones is under the influence of certain factors which
are released by some nuclei that are present in the hypothalamus.
Types Of Nucleus
A. Pre Optic Nucleus Gonadotropin releasing
hormone (GnRH)
B. Ventro medial nucleus Growth hormone (GH) &
Somatostatin
C. Para ventricular nucleus Thyrotropin releasing hormone
(TRH) & corticotropic releasing hormone (CTRH)
D.Arcute nucleus Prolactin (PRL)
Cont…
Types Of CellsThere are five types of cells in the anterior pituitary:
1) Somatotropes release Human Growth Hormone (GH)
2) Corticotropes release Adrenocorticotropic Hormone (ACTH)
3) Thyrotropes release Thyroid Stimulating Hormone (TSH)
4) Gonadotropes release Gonadotropic Hormones which include both
luteinizing hormone (LH) and follicle stimulating hormone (FSH)
5) Lactotropes release Prolactin (PRL)
Anterior Pituitary can be divided into chromophobic cells and
chromophilic cells
Chromophobic cells are usually inactive and chromophilic cells are active
and are divided into acidophilic cells and basophilic cells
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