Transcript PITUITARY GLAND

PITUITARY GLAND
The pituitary is a pea-sized gland that is housed within a
bony structure (sella turcica) at the base of the brain. The
sella turcica protects the pituitary but allows very little room
for expansion.
The pituitary controls the function of most other endocrine
glands and is therefore sometimes called the master gland.
In turn, the pituitary is controlled in large part by the
hypothalamus, a region of the brain that lies just above the
pituitary. By detecting the levels of hormones produced by
glands under the pituitary's control (target glands), the
hypothalamus or the pituitary can determine how much
stimulation the target glands need.
Functions of pituitary gland:
Hormones secreted from the pituitary gland help control the
following body processes:
• Growth Excess of HGH can lead to gigantism.
• Blood pressure.
• Some aspects of pregnancy and childbirth including
stimulation of uterine contractions during childbirth.
• Breast milk production .
• Sex organ functions in both men and women.
• Thyroid gland function .
• The conversion of food into energy (metabolism).
• Water and osmolarity regulation in the body.
• Secretes ADH (antidiuretic hormone) to control the absorption
of water into the kidneys .
• Temperature regulation .
HORMONES OF PITUITARY GLAND:
The pituitary is divided into two distinct lobes. The anterior
pituitary , also know as the adenohypophysis, consists of
glandular epithelial tissue which accounts for 80% of the
pituitary gland's weight. The back The posterior pituitary
comprises of nervous tissue and is also know as the
neurohypophysis.
The lobes are connected to the hypothalamus by a stalk that contains
blood vessels and nerve cell projections (nerve fibers, or axons). The
hypothalamus controls the anterior lobe by releasing hormones
through the connecting blood vessels. It controls the posterior lobe
through nerve impulses.
The anterior lobe of the pituitary produces and releases
(secretes) six main hormones:
Growth hormone
Human growth hormone (HGH; also called somatotropin) is a protein of 191
amino acids. The GH-secreting cells are stimulated to synthesize and release
GH by the intermittent arrival of growth hormone releasing hormone (GHRH)
from the hypothalamus. GH promotes body growth by:
• binding to receptors on the surface of liver cells .
• this stimulates them to release insulin-like growth factor-1 (IGF-1)
• IGF-1 acts directly on the ends of the long bones promoting their growth
• In childhood,
hyposecretion of GH produces the stunted — but
normally well-proportioned — growth of a
midget.
hypersecretion leads to gigantism.
• In adults,
hypersecretion of GH or GHRH leads to
acromegaly.
Thyroid Stimulating Hormone (TSH)
TSH (also known as thyrotropin) is a glycoprotein consisting of:
• A beta chain of 118 amino acids.
• An alpha chain of 92 amino acids. The alpha chain is identical to that found
in two other pituitary hormones, FSH and LH as well as in the hormone chorionic
gonadotropin. Thus it is its beta chain that gives TSH its unique properties.
The secretion of TSH is
• stimulated by the arrival of thyrotropin releasing hormone (TRH)
from the hypothalamus.
• inhibited by the arrival of somatostatin from the hypothalamus.
ACTH — the adrenocorticotropic hormone
ACTH is a peptide of 39 amino acids. ACTH acts on the cells of the
adrenal cortex, stimulating them to produce
• glucocorticoids, like cortisol
• mineralocorticoids, like aldosterone
• androgens (male sex hormones, like testosterone)
• in the fetus, ACTH stimulates the adrenal cortex to synthesize a
precursor of estrogen called dehydroepiandrosterone sulfate (DHEA-S)
which helps prepare the mother for giving birth.
Production of ACTH depends on the intermittent arrival of
corticotropin-releasing hormone (CRH) from the
hypothalamus.
Hypersecretion of ACTH is a frequent cause of Cushing's
disease.
Prolactin (PRL)
Prolactin is a protein of 198 amino acids. During pregnancy it
helps in the preparation of the breasts for future milk production.
After birth, prolactin promotes the synthesis of milk.
Prolactin secretion is
• stimulated by TRH.
• repressed by estrogens and dopamine.
Gonadotropins
Follicle-Stimulating Hormone (FSH)
Complex of two different proteins.
• the same alpha chain found in TSH (and LH)
• a beta chain of 118 amino acids, which gives it its unique properties.
FSH in females
In sexually-mature females, FSH (assisted by LH) acts on the follicle to stimulate it to
release estrogens .
FSH in males
In sexually-mature males, FSH acts on spermatogonia stimulating (with the aid of
testosterone) the production of sperm.
Luteinizing Hormone (LH)
LH is synthesized within the same pituitary cells as FSH and under
the same stimulus (GnRH). It is also a heterodimeric glycoprotein
consisting of
• the same 92-amino acid alpha subunit found in FSH and TSH
(as well as in chorionic gonadotropin).
• a beta chain of 121 amino acids that is responsible for its
properties.
The effects of LH also depend on sex.
LH in females
In sexually-mature females, a surge of LH triggers the
completion of meiosis I of the egg and its release (ovulation) in
the middle of the cycle.
stimulates the now-empty follicle to develop into the corpus
luteum, which secretes progesterone during the latter half of
the menstrual cycle.
LH in males
LH acts on the interstitial cells (also known as Leydig cells) of the
testes stimulating them to synthesize and secrete the male sex
hormone, Testosterone.
LH in males is also known as interstitial cell stimulating
hormone (ICSH).
The posterior lobe of the pituitary releases two hormones, both
synthesized in the hypothalamus, into the circulation.
Vasopressin
Vasopressin is a peptide of 9 amino acids (Cys-Tyr-Phe-Gln-AsnCys-Pro-Arg-Gly). It is also known as arginine vasopressin (AVP)
and the antidiuretic hormone (ADH).
Vasopressin acts on the collecting ducts of the kidney to facilitate
the reabsorption of water into the blood. This it acts to reduce the
volume of urine formed (giving it its name of antidiuretic
hormone).
A deficiency of vasopressin or
inheritance of mutant genes for its receptor (called V2)
leads to excessive loss of urine, a condition known as
diabetes insipidus. The most severely-afflicted patients
may urinate as much as 30 liters (almost 8 gallons!) of urine
each day. The disease is accompanied by terrible thirst, and
patients must continually drink water to avoid dangerous
dehydration.
Oxytocin
Oxytocin is a peptide of 9 amino acids
(Cys-Tyr-Ile-Gln-Asn-Cys-Pro-Leu-Gly).
It acts on certain smooth muscles:
• stimulating contractions of the uterus at the
time of birth;
• stimulating release of milk when the baby
begins to suckle.
Oxytocin is often given to prospective
mothers to hasten birth.
The hypothalamic-pituitary-adrenal axis (HPA axis) is a major part
of the neuroendocrine system that controls reactions to stress and
regulates many body processes, including digestion, the immune
system, mood and emotions, sexuality, and energy storage and
expenditure. Species from humans to the most ancient organisms
share components of the HPA axis. It is the mechanism for a set of
interactions among glands, hormones and parts of the mid-brain
that mediate a general adaptation syndrome.
The hypothalamic-pituitary-thyroid axis (HPT axis )is part of
the endocrine system responsible for the regulation of
metabolism.
The hypothalamus senses low circulating levels of thyroid hormone
(T3 and T4) and responds by releasing thyrotropin releasing
hormone (TRH). The TRH stimulates the pituitary to produce thyroid
stimulating hormone (TSH). The TSH, in turn, stimulates the thyroid to
produce thyroid hormone until levels in the blood return to normal.
Thyroid hormone exerts negative feed back control over the
hypothalamus as well as anterior pituitary thus controlling the
release of both TRH from hypothalamus and TSH from anterior
pituitary gland
The hypothalamic-pituitary-gonadal axis is referring to the effects of the
hypothalamus, pituitary gland , and gonads as if these individual endocrine
glands were a single entity as a whole. Because these glands often behave
in cooperation, physiologists and endocrinologists find it convenient and
descriptive to speak of them as a single system.
The hypothalamic-pituitary- gonadal axis is a critical part in the
development and regulation of a number of the body's systems, such as the
reproductive, and immune systems. Fluctuations in the hormones cause
changes in the hormones produced by each gland and have various
widespread and local effects on the body. The hypothalamus produces
gonadotropin-releasing hormone (GnRH). The anterior portion of the
pituitary gland produces luteinizing hormone (LH) and follicle-stimulating
hormone (FSH), and the gonads produce estrogen and testosterone.
Hormone
Target Organ or Tissue
Adrenocorticotropic hormone
(ACTH)
Adrenal glands
Antidiuretic hormone (ADH)
Kidney
Follicle-stimulating hormone(
FSH)
Ovaries or testes
Growth hormone
(GH)
Muscles and bones
Luteinizing hormone
(LH)
Ovaries or testes
Oxytocin
mammary glands
Prolactin
Mammary glands
Thyroid-stimulating hormone
(TSH)
Thyroid gland