Hormones Secreted by the Anterior Pituitary Gland

Embryological and Anatomical Context

• The anterior pituitary gland, or adenohypophysis, originates embryologically from Rathke's pouch, which is an invagination of the oral ectoderm,.
• Rathke's pouch detaches from the oral epithelium and establishes a direct connection with the downward extension of the hypothalamus, which gives rise to the pituitary stalk.
• The anterior pituitary constitutes approximately 80% of the entire pituitary gland,.
• It consists of the pars distalis (the largest portion housing the majority of hormone-producing cells), the pars intermedia, and the pars tuberalis,.
• The anterior pituitary is connected to the hypothalamus via the hypophyseal portal venous system, a specialized vascular network through which hypothalamic regulatory hormones are delivered to the anterior pituitary cells,.
• A complex cascade of sequentially expressed transcriptional activation factors, including those resembling homeobox genes, directs the differentiation and proliferation of anterior pituitary cell types,.
• There are five distinct cell types within the anterior pituitary gland that produce six principal peptide hormones.

Specific Hormones and Their Cellular Origins

• Growth Hormone (GH): Produced by somatotrope cells, growth hormone is a single-chain polypeptide hormone that promotes linear growth, bone thickness, soft tissue growth, protein synthesis, and fatty acid release,,.
• Thyroid-Stimulating Hormone (TSH): Synthesized by thyrotrope cells, TSH is a glycoprotein consisting of two chains (alpha and beta),.
• The alpha-subunit of TSH is identical to that of other glycoproteins (FSH, LH, and human chorionic gonadotropin), whereas the beta-subunit is specific for TSH and confers its unique biological activity.
• TSH stimulates the release of thyroxine (T4) and triiodothyronine (T3) from the thyroid gland through cyclic adenosine monophosphate formation.
• Adrenocorticotropic Hormone (ACTH): Secreted by corticotrope cells, ACTH is a 39-amino acid single-chain peptide derived from the proteolytic cleavage of a 240-amino acid precursor molecule known as proopiomelanocortin (POMC),.
• ACTH acts on the adrenal cortex to stimulate the synthesis and secretion of cortisol and adrenal androgens,.
• Luteinizing Hormone (LH): Produced by gonadotrope cells, LH is a glycoprotein that shares a common alpha subunit with FSH and TSH but possesses a distinct beta subunit,.
• In females, LH promotes ovulation, luteinization of the ovary, and the development of the corpus luteum,.
• In males, LH binds to specific receptors on testicular Leydig cells to stimulate the production and secretion of testosterone,.
• Follicle-Stimulating Hormone (FSH): Also secreted by gonadotrope cells, FSH is a glycoprotein that mediates gonadal function,.
• In females, FSH stimulates the secretion of estrogen from the ovary and promotes follicular development,.
• In males, FSH acts on testicular Sertoli cells to stimulate gametogenesis (sperm production),.
• Prolactin (PRL): Produced by lactotrope cells, prolactin is a 199-amino acid peptide,.
• The primary physiologic role of prolactin is to promote milk synthesis, initiate and maintain lactation, and prepare the breasts for lactation,.

Transcriptional Regulation of Anterior Pituitary Hormones

• The specification, proliferation, and survival of anterior pituitary cell lineages are strictly regulated by transcription factors.
• The PROP1 gene encodes a transcription factor that is responsible for the early determination and differentiation of multiple anterior pituitary cell lineages, specifically affecting the secretion of GH, PRL, TSH, LH, and FSH.
• The POU1F1 (formerly PIT1) gene encodes a nuclear protein that binds to the GH and prolactin promoters, and is essential for the emergence and mature function of somatotropes, lactotropes, and thyrotropes,.
• The HESX1 gene is expressed in precursors of all five cell types early in embryologic development, and its alterations result in heterogeneous phenotypes including deficiencies in GH, TSH, LH, FSH, ACTH, and PRL,.
• The LHX3 and LHX4 genes are essential for pituitary development, with their respective defects causing combinations of GH, PRL, TSH, LH, and FSH deficiencies,.