The anterior pituitary (also called the adenohypophysis, from Greek adeno, "gland"; hypo, "under"; physis, "growth") is the anterior lobe of the pituitary gland. The term "pars distalis" is sometimes used as a synonym for the anterior pituitary, but this is not quite correct. The anterior pituitary is usually divided into the pars distalis ("distal part") comprising most of the anterior pituitary, and the pars tuberalis ("tubular part") - which extends up from the pars distalis and wraps around the pituitary stalk.
Under the influence of the hypothalamus, the anterior pituitary produces and secretes several peptide hormones that regulate many physiological processes including stress, growth, and reproduction.
Cells of the anterior pituitary gland produce six major peptide hormones.
- Growth hormone is produced by somatotroph cells
- Thyroid stimulating hormone (TSH), is produced by thyrotrophs
- Prolactin is produced by lactotrophs
- Adrenocorticotrophic hormone (ACTH), is produced by corticotrophs
- Luteinizing hormone (LH) and
- Follicle stimulating hormone (FSH), is produced by gonadotrophs
These hormones are all packeged in large dense-core secretory vesicles which are secreted by calcium-dependent exocytosis, following calcium-entry into the cell via voltage-dependent calcium channels and/or mobilisation of intracellular calcium stores.
Hormone secretion from the anterior pituitary gland is mainly regulated by factors secreted from neuroendocrine neurons of the hypothalamus into the hypothalamo-hypophysial portal vessels from the nerve endings of neurosecretory neurons. The secretion of LH and FSH is regulated by Gonadotrophin releasing hormone (GnRH, also known as LH releasing hormone, LHRH).
Growth hormone is regulated by an inhibitory factor, Somatostatin (from neurons of the periventricular nucleus) and by a stimulatory factor, Growth hormone releasing hormone(from neurons of the arcuate nucleus). Growth hormone secretion can also be stimulated in response to ghrelin secretion into the systemic circulation from the gastro-intestinal tract.
ACTH secretion is regulated by two synergistically interacting releasing -factors, Corticotrophin releasing hormone (CRH)(from neurons of the paraventricular nucleus), and vasopressin also released from parvocellular neurons of the paraventricular nucleus),(often from the same neurons that produce CRH). In some species (including the horse) systemic concentrations of vasopressin resulting from secretion from the posterior pituitary gland can be significant influences on ACTH secretion.
Prolactin secretion is mainly regulated by an inhibitory factor, dopamine, released by neurons of the arcuate nucleus (the so-called tuberoinfundibular dopamine neurons), but there are also stimulatory factors (including TRH). Prolactin secretion is also governed by a paracrine mechanism involving secretions from the pars tuberalis acting on the lactotrophs of the pars distalis.