Structure of the Pituitary Gland.

·         The pituitary gland, or hypophysis, is an endocrine gland about the size of a pea and weighing 0.5 g.

·         It is a protrusion off the bottom of the hypothalamus at the base of the brain, and rests in a small, bony cavity (sella turcica) covered by a dural fold (diaphragma sellae).

·         The pituitary fossa, in which the pituitary gland sits, is situated in the sphenoid bone in the middle cranial fossa at the base of the brain.

·         It is considered a master gland. The pituitary gland secretes hormones regulating homeostasis, including tropic hormones that stimulate other endocrine glands. It is functionally connected to the hypothalamus by the median eminence.

Figure 1:  Mesal Aspect of a Brain Sectioned in the Median Sagittal Plane

Refer to Handout 15.1:  Medial View of Brain showing the position of Hypothalamus and Handout 15.2:  Pituitary Stalk.

Figure 3:  Hormones Produced by the Anterior and Posterior Lobes of the Pituitary

·         Located at the base of the brain, the pituitary is composed of two lobes: the anterior pituitary (adenohypophysis) and the posterior pituitary (neurohypophysis). The pituitary is functionally linked to the hypothalamus by the pituitary stalk, whereby hypothalamic releasing factors are released and, in turn, stimulate the release of pituitary hormones. Although the pituitary gland is known as the master endocrine gland, both of its lobes are under the control of the hypothalamus.

 

 Anterior Pituitary (Adenohypophysis)

·         The anterior pituitary synthesizes and secretes important endocrine hormones, such as ACTH, TSH, PRL, GH, endorphins, FSH, and LH. These hormones are released from the anterior pituitary under the influence of the hypothalamus.

·         Hypothalamic hormones are secreted to the anterior lobe by way of a special capillary system, called the hypothalamic-hypophyseal portal system. The anterior pituitary is divided into anatomical regions known as the pars tuberalis, pars intermedia, and pars distalis.

Pituitary Master Gland

Hormone	Target Organ or Tissue
Adrenocorticotropic hormone (ACTH)	Adrenal glands
Antidiuretic hormone	Kidney
Beta-melanocyte–stimulating hormone	Skin
Endorphins	Brain and immune system
Enkephalins	Brain
Follicle-stimulating hormone	Ovaries or testes
Growth hormone	Muscles and bones
Luteinizing hormone	Ovaries or testes
Oxytocin	Uterus and mammary glands
Prolactin	Mammary glands
Thyroid-stimulating hormone	Thyroid gland

Figure 4:   The Pituitary Gland

Figure 5:  Pituitary Gland and Hypothalamus

The Anterior Lobe of the Pituitary Produces and Releases (secretes) Six Main Hormones:

·         Growth hormone (GH),  which regulates growth and physical development and has important effects on body shape by stimulating muscle formation and reducing fat tissue

·         Thyroid-stimulating hormone  (TSH), which stimulates the thyroid gland to produce thyroid hormones

·         Adrenocorticotropic hormone (ACTH, also called corticotropin, which stimulates the adrenal glands to produce cortisol and other hormones

·         Follicle-stimulating hormone  (FSH)and luteinizing hormone (LH) (the gonadotropins), which stimulate the testes to produce sperm, the ovaries to produce eggs, and the sex organs to produce sex hormones (testosterone and estrogen)

·         Prolactin  (PRL), which stimulates the mammary glands of the breasts to produce milk

·         The anterior lobe also produces several other hormones, including one that causes the skin to darken (beta-melanocyte–stimulating hormone) and ones that inhibit pain sensations and help control the immune system (endorphins).

Posterior Pituitary (Neurohypophysis):  The posterior pituitary stores and releases,

·         Oxytocin, most of which is released from the paraventricular nucleus in the hypothalamus

·         Antidiuretic hormone (ADH, also known as vasopressin and AVP, arginine vasopressin), the majority of which is released from the supraoptic nucleus in the hypothalamus.

o   Oxytocin is one of the few hormones to create a positive feedback loop. For example, uterine contractions stimulate the release of oxytocin from the posterior pituitary, which, in turn, increases uterine contractions. This positive feedback loop continues throughout labor.

o   Oxytocin also stimulates contractions of the milk ducts in the breast, which move milk to the nipple (the let-down) in lactating women.

Intermediate Lobe

·         There is also an intermediate lobe in many animals. For instance, in fish, it is believed to control physiological color change. In adult humans, it is just a thin layer of cells between the anterior and posterior pituitary.

·         The intermediate lobe produces melanocyte-stimulating hormone (MSH), although this function is often (imprecisely) attributed to the anterior pituitary.

Figure 7:  The Main System Connecting the Endocrine Secretory Activities of the Pituitary Gland

Thyroid Stimulating Hormone

·         It stimulates growth and activity of the thyroid gland, which secretes the hormones thyroxine (T4) and triiiodothyronine (T3). Release is lowest in the early evening and highest during the night.

·         Secretion is regulated by a negative feedback mechanism .When the blood level of thyroid hormones is high, secretion of TSH is reduced, and vice versa.

Adrenocorticotrophic Hormone

·         Corticotrophin releasing hormone (CRH) from the hypothalamus promotes the synthesis and release of ACTH by the anterior pituitary.

·         This increases the concentration of cholesterol and steroids within the adrenal cortex and the output of steroid hormones, especially cortisol.

·         ACTH levels are highest at about 8 a.m. and fall to their lowest about midnight, although high levels sometimes occur at midday and 6 p.m. This circadian rhythm is maintained throughout life.

·         It is associated with the sleep pattern and adjustment to changes takes several days, following, e.g., changing work shifts, travelling to a different time zone (Jet lag).

Prolactin 

·         This hormone stimulates lactation (milk production) and has a direct effect on the breasts immediately after Parturition (childbirth).

·         The blood level of prolactin is stimulated by prolactin releasing hormone (PRH) released from the hypothalamus and it is lowered by prolactin inhibiting hormone (PIH, dopamine) and, by an increase in blood level of prolactin.

·         After birth, suckling stimulates prolactin secretion and lactation. The resultant high blood level is a factor in reducing the incidence conception during lactation.

·         Prolactin together with oestrogens, corticosteroids, insulin and thyroxine is involved in initiating and maintaining lactation. Prolactin secretion is related to sleep i.e. it is raised during any period of sleep, night or day, Emotional stress decreases production.

Gonadotrophin

·         There are two gonadotrophins which are released by anterior Pituitary gland. These are Follicle stimulating hormone and Lutenizing hormone.

·         In both sexes follicle stimulating hormone stimulates production of gametes, Ova or Spermatozoa.

·         In females FSH and LH are involved in secretion of Oestrogen and Progesterone. In males, Lutenizing hormone stimulates the secretion of testosterone.

Posterior Pituitary

·         This is formed from nervous tissue and consists of nerve cells surrounded by suppporting cells called pituicytes.

·         Posterior pituitary hormones are synthesised in the nerve cell bodies, transported along the axons and then stored in vesicles within the axon terminals within the posterior pituitary their release by exocytosis is trigggered by nerve impulses from the hypothalamus.

·         The gland secretes Antidiuretic hormone and Oxytocin. These are synthesized hormones are synthesized in the hypothalamus and the stored in the axonal terminals within the posterior Pituitary gland.

Oxytocin:  This stimulates two target tissues during and after child birth. These tissues are uterine smooth muscles and muscle cells of lactating breast. Inhibition occurs after delivery when uterine contractions no longer dilate (stretch) the cervix.

Antidiuretic Hormone (ADH)

·         The main effect of ADH is to regulate fluid balance in the body by reducing the urine output, for instance during thirsty, hypotension and when there is high plasma osmolarity and during stress.

·         At high concentrations, for example after severe blood, ADH causes smooth muscle contraction, especially vasoconstriction in the blood vessels of the skin and abdominal organs. This has a pressor effect, raising systemic blood pressure; the alternative name of this hormone, vasopressin, reflects this effect.

E-mail: paulmasawe@gmail.com