The pituitary gland, also known as the hypophysis, is a roundish organ that lies immediately beneath the hypothalamus, resting in a depression of the base of the skull called the sella turcica ("Turkish saddle"). In an adult human or sheep, the pituitary is roughly the size and shape of a garbonzo bean.
Careful examination of the pituitary gland reveals that it composed of two distinctive parts:
The anterior pituitary (adenohypophysis) is a classical gland composed predominantly of cells that secrete protein hormones.
The posterior pituitary (neurohypophysis) is not really an organ, but an extension of the hypothalamus. It is composed largely of the axons of hypothalamic neurons which extend downward as a large bundle behind the anterior pituitary. It also forms the so-called pituitary stalk, which appears to suspend the anterior gland from the hypothalamus.
The anterior and posterior pituitary have separate embryological origins. In many mammals, there is also an intermediate lobe (pars intermedia) between the anterior and posterior pituitary.
A key to understanding the endocrine relationship between hypothalamus and anterior pituitary is to appreciate the vascular connections between these organs. As will be emphasized in later sections, secretion of hormones from the anterior pituitary is under strict control by hypothalamic hormones. These hypothalamic hormones reach the anterior pituitary through the following route:
A branch of the hypophyseal artery ramifies into a capillary bed in the lower hypothalamus, and hypothalmic hormones destined for the anterior pituitary are secreted into that capillary blood.
Blood from those capillaries drains into hypothalamic-hypophyseal portal veins. Portal veins are defined as veins between two capillary beds; the hypothalamic-hypophyseal portal veins branch again into another series of capillaries within the anterior pituitary.
Capillaries within the anterior pituitary, which carry hormones secreted by that gland, coalesce into veins that drain into the systemic venous blood. Those veins also collect capillary blood from the posterior pituitary gland.
Pituitary Gland - Secretes hormones to stimulate the adrenals, thyroid, pigment-producing skin cells and gonads (ovaries and testes). Also secretes a growth hormone, an antidiuretic hormone, prolactin (a hormone which affects milk production after childbirth) and oxytocin (a hormone which plays a role in childbirth).
Pancreas - Secretes insulin and glucagon, which affect the body's absorption of glucose, the body's main source of energy.
Adrenal Cortex - Secretes hydrocortisone, which affects metabolism. Also secretes androgen hormone and aldosterone, which affect blood pressure and saline balance.
Thyroid gland - Secretes thyroxin, triiodothyronine and calcitonin, which affect metabolism, body heat, and bone growth.
Parathyroid glands - Secretes a parathyroid hormone, which affects calcium levels in the blood.
Gonads - The male and female reproductive glands (testes and ovaries). The testes secrete testosterone, which stimulate sperm production and other male characteristics. The ovaries secrete estrogen and progesterone, which affect many aspects of the female body, including menstrual cycles and pregnancy.
The exocrine glands actually secrete their substances through ducts to particular areas. Examples of exocrine glands include the salivary glands and the sweat glands. The endocrine glands, on the other hand, secrete the hormones they produce directly into the bloodstream. Most endocrine glands are controlled by trophic (stimulating) hormones secreted by the pituitary gland. The pituitary gland, in turn, is controlled by hormones secreted by the hypothalamus in the brain.
 |
 |
|
Growth Booster Plus Home |