Anterior pituitary
A major organ of the endocrine system, the anterior pituitary is the glandular, anterior lobe that together with the posterior lobe makes up the pituitary gland. The anterior pituitary regulates several physiological processes, including stress, growth, reproduction, and lactation. Proper functioning of the anterior pituitary and of the organs it regulates can often be ascertained via blood tests that measure hormone levels.
Structure
The pituitary gland sits in a protective bony enclosure called the sella turcica. It is composed of three lobes: the anterior, intermediate, and posterior lobes. In many animals, these lobes are distinct. However, in humans, the intermediate lobe is but a few cell layers thick and indistinct; as a result, it is often considered part of the anterior pituitary. In all animals, the fleshy, glandular anterior pituitary is distinct from the neural composition of the posterior pituitary.The anterior pituitary is composed of three regions:
Nota bene: The terms "basophil" and "acidophil" are used by some books, whereas others prefer not to use these terms. This is due to the possible confusion with white blood cells, where one may also find basophils and acidophils.
Development
The anterior pituitary is derived from the ectoderm, more specifically from that of Rathke’s pouch, part of the developing hard palate in the embryo.The pouch eventually loses its connection with the pharynx, giving rise to the anterior pituitary. The anterior wall of Rathke's pouch proliferates, filling most of the pouch to form the pars distalis and the pars tuberalis. The posterior wall of the anterior pituitary forms the pars intermedia. Its formation from the soft tissues of the upper palate contrasts with the posterior pituitary, which originates from neuroectoderm.
Function
The anterior pituitary contains five types of endocrine cell, and they are defined by the hormones they secrete: somatotropes ; Lactotropes ; gonadotropes ; corticotropes and thyrotropes. It also contains non-endocrine folliculostellate cells which are thought to stimulate and support the endocrine cell populations.Hormones secreted by the anterior pituitary are trophic hormones. Trophic hormones directly affect growth either as hyperplasia or hypertrophy on the tissue it is stimulating. Tropic hormones are named for their ability to act directly on target tissues or other endocrine glands to release hormones, causing numerous cascading physiological responses.
| Hormone | Other names | Symbol | Structure | Secretory cells | Staining | Target | Effect | - |
| Adrenocorticotropic hormone | Corticotropin | ACTH | Polypeptide | Corticotrophs | Basophil | Adrenal gland | Secretion of glucocorticoid, mineralocorticoid and androgens | - |
| Thyroid-stimulating hormone | Thyrotropin | TSH | Glycoprotein | Thyrotrophs | Basophil | Thyroid gland | Secretion of thyroid hormones | - |
| Follicle-stimulating hormone | - | FSH | Glycoprotein | Gonadotrophs | Basophil | Gonads | Growth of reproductive system | - |
| Luteinizing hormone | Lutropin | LH, ICSH | Glycoprotein | Gonadotrophs | Basophil | Gonads | Sex hormone production | - |
| Growth hormone | Somatotropin | GH, STH | Polypeptide | Somatotrophs | Acidophil | Liver, adipose tissue | Promotes growth; lipid and carbohydrate metabolism | - |
| Prolactin | Lactotropin | PRL | Polypeptide | Lactotrophs | Acidophil | Ovaries, mammary glands, testes, prostate | Secretion of estrogens/progesterone; lactation; spermatogenesis; prostatic hyperplasia | TSH and ACTH secretion |
Role in the endocrine system
Hormone secretion from the anterior pituitary gland is regulated by hormones secreted by the hypothalamus. Neuroendocrine cells in the hypothalamus project axons to the median eminence, at the base of the brain. At this site, these cells can release substances into small blood vessels that travel directly to the anterior pituitary gland.Aside from hypothalamic control of the anterior pituitary, other systems in the body have been shown to regulate the anterior pituitary’s function. GABA can either stimulate or inhibit the secretion of luteinizing hormone and growth hormone and can stimulate the secretion of thyroid-stimulating hormone. Prostaglandins are now known to inhibit adrenocorticotropic hormone and also to stimulate TSH, GH and LH release. GABA, through action with the hypothalamus, has been shown experimentally to influence the level of GH secretion. Clinical evidence supports the experimental findings of the excitatory and inhibitory effects GABA has on GH secretion, dependent on GABA’s site of action within the hypothalamic-pituitary unit.
Effects of the anterior pituitary
The homeostatic maintenance of the anterior pituitary is crucial to our physiological well being. Increased plasma levels of TSH induce hyperthermia through a mechanism involving increased metabolism and cutaneous vasodilation. Increased levels of LH also result in hypothermia but through a decreased metabolism action. ACTH increase metabolism and induce cutaneous vasoconstriction, increased plasma levels also result in hyperthermia and prolactin decreases with decreasing temperature values. follicle-stimulating hormone also may cause hypothermia if increased beyond homeostatic levels through an increased metabolic mechanism only.Gonadotropes, primarily luteinising hormone secreted from the anterior pituitary stimulates the ovulation cycle in female mammals, whilst in the males, LH stimulates the synthesis of androgen which drives the ongoing will to mate together with a constant production of sperm.
Main article Hypothalamic-pituitary-adrenal axis
The anterior pituitary plays a role in stress response. Corticotropin releasing hormone from the hypothalamus stimulates ACTH release in a cascading effect that ends with the production of glucocorticoids from the adrenal cortex.