Pituitary gland - Wikipedia
The pituitary gland comprises two lobes; anterior lobe (adenohypophysis) and posterior lobe (neurohypophysis). The hypothalamus is. The hormones released from the anterior and posterior pituitary regulate vital Anatomical and functional relationship between the hypothalamus and the pituitary. Two types of neurons are important in mediating the endocrine functions of. You have your hypothalamus and pituitary gland to thank. glands, today we'll focus on just two: the hypothalamus and the pituitary gland. The pituitary consists of an anterior lobe and a posterior lobe, each of which have.
Its primary function is anabolic; it promotes protein synthesis and tissue building through direct and indirect mechanisms Figure Growth hormone GH directly accelerates the rate of protein synthesis in skeletal muscle and bones. Insulin-like growth factor 1 IGF-1 is activated by growth hormone and indirectly supports the formation of new proteins in muscle cells and bone.
A glucose-sparing effect occurs when GH stimulates lipolysis, or the breakdown of adipose tissue, releasing fatty acids into the blood. As a result, many tissues switch from glucose to fatty acids as their main energy source, which means that less glucose is taken up from the bloodstream. GH also initiates the diabetogenic effect in which GH stimulates the liver to break down glycogen to glucose, which is then released into the blood.
Blood glucose levels rise as the result of a combination of glucose-sparing and diabetogenic effects.
GH indirectly mediates growth and protein synthesis by triggering the liver and other tissues to produce a group of proteins called insulin-like growth factors IGFs. These proteins enhance cellular proliferation and inhibit apoptosis, or programmed cell death.
IGFs stimulate cells to increase their uptake of amino acids from the blood for protein synthesis. Skeletal muscle and cartilage cells are particularly sensitive to stimulation from IGFs. For example, gigantism is a disorder caused by the hypersecretion of GH before the growth plates have closed resulting in excessive growth of all bones. Abnormally low levels of GH in children can cause growth impairment—a disorder called pituitary dwarfism also known as growth hormone deficiency which affects all bones.
Achondroplastic dwarfism affects only the bones with growth plates long bones resulting in short arms and legs with normal sized trunk and head. Thyroid-Stimulating Hormone The activity of the thyroid gland is regulated by thyroid-stimulating hormone TSHalso called thyrotropin.
TSH is released from the anterior pituitary in response to thyrotropin-releasing hormone TRH from the hypothalamus. As will be discussed shortly, it triggers the secretion of thyroid hormones by the thyroid gland. In a classic negative feedback loop, elevated levels of thyroid hormones in the bloodstream then trigger a drop in production of TRH and TSH.
ACTH come from a precursor molecule known as pro-opiomelanotropin POMC which produces several biologically active molecules when cleaved, including ACTH, melanocyte-stimulating hormone, and the brain opioid peptides known as endorphins.
The release of ACTH is regulated by the corticotropin-releasing hormone CRH from the hypothalamus in response to normal physiologic rhythms. A variety of stressors can also influence its release, and the role of ACTH in the stress response is discussed later in this chapter.
Follicle-Stimulating Hormone and Luteinizing Hormone The endocrine glands secrete a variety of hormones that control the development and regulation of the reproductive system these glands include the anterior pituitary, the adrenal cortex, and the gonads—the testes in males and the ovaries in females.
Much of the development of the reproductive system occurs during puberty and is marked by the development of sex-specific characteristics in both male and female adolescents. Puberty is initiated by gonadotropin-releasing hormone GnRHa hormone produced and secreted by the hypothalamus. GnRH stimulates the anterior pituitary to secrete gonadotropins—hormones that regulate the function of the gonads.
The Hypothalamus and Pituitary Glands
The levels of GnRH are regulated through a negative feedback loop; high levels of reproductive hormones inhibit the release of GnRH. Throughout life, gonadotropins regulate reproductive function.
The gonadotropins include two glycoprotein hormones: FSH also promotes ovarian follicular growth in women; these follicles then release estrogens. Luteinizing hormone LH triggers ovulation in women, as well as the production of estrogens and progesterone by the ovaries.
LH stimulates production of testosterone by the male testes.
Prolactin As its name implies, prolactin PRL promotes lactation milk production in women. After birth, it stimulates the mammary glands to produce breast milk. However, the effects of prolactin depend heavily upon the permissive effects of estrogens, progesterone, and other hormones.
In elasmobranchs there is an additional, ventral lobe beneath the anterior pituitary proper. Here, the posterior pituitary is a simple flat sheet of tissue at the base of the brain, and there is no pituitary stalk.
Rathke's pouch remains open to the outside, close to the nasal openings. Closely associated with the pouch are three distinct clusters of glandular tissue, corresponding to the intermediate lobe, and the rostral and proximal portions of the anterior pituitary. These various parts are separated by meningial membranes, suggesting that the pituitary of other vertebrates may have formed from the fusion of a pair of separate, but associated, glands.
This may have a function in osmoregulation. 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.
Functional Anatomy of the Hypothalamus and Pituitary Gland
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.