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 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 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 anterior lobe of the pituitary produces and releases (secretes) six main hormones :

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

Growth hormone

• 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.

Acromegaly

Acromegaly is a growth disorder caused by excessive production of growth hormone (GH) by

the pituitary gland.

In adults, hypersecretion of GH causes acromegaly, a disorder in which parts of the

body, especially the head, face, hands, and feet, gradually get bigger. GH affects growth by

stimulating the formation of other hormones. These hormones are mostly made by the liver in

response to GH stimulation and are called somatomedins (somatotropin-mediating hormones or insulinlike growth factors).

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

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 femalesIn sexually-mature females, FSH (assisted by LH) acts on the follicle to

stimulate it to release estrogens .

FSH in malesIn sexually-mature males, FSH acts on spermatogonia stimulating (with the aid of testosterone) the production of sperm.

Follicle-Stimulating Hormone (FSH)

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 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) .

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.

Vasopressin is a peptide of 9 amino acids (Cys-Tyr-Phe-Gln-Asn-Cys-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).

The posterior lobe of the pituitary releases two hormones, both synthesized in the hypothalamus, into the circulation.

Vasopressin

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)

HYPOTHALAMIC-PITUITARY-ADRENAL AXIS

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. It is the mechanism for a set of interactions among glands, hormones and

parts of the mid-brain that mediate a general adaptation syndrome.

HYPOTHALAMIC PITUITARY THYROID AXIS

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

HYPOTHALAMIC PITUITARY GONADAL AXIS.

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.

Target Organ or Tissue Hormone

Adrenal glands Adrenocorticotropic hormone (ACTH)

Kidney Antidiuretic hormone (ADH)

Ovaries or testes Follicle-stimulating hormone(

FSH)

Muscles and bones Growth hormone(GH)

Ovaries or testes Luteinizing hormone(LH)

mammary glands Oxytocin

Mammary glands Prolactin

Thyroid gland Thyroid-stimulating hormone (TSH)