The structural and functional aspects of the adenohypophysis, which are essential for understanding the individual hormones, are discussed here. The pituitary gland is located below the hypothalamus and is connected to it via the pituitary stalk. It sits within the sella turcica, a depression in the sphenoid bone, and is covered by a dural reflection called the diaphragma sellae. The anterior pituitary, also known as the pars distalis, constitutes the majority of the pituitary gland. It is derived from epithelial tissue composed of acidophils (somatotropes and lactotropes) and basophils (thyrotropes, gonadotropes, and corticotropes). On the other hand, the posterior pituitary is derived from neural tissue and has a histological appearance resembling nonmyelinated nerves. The magnocellular neurons of the hypothalamus, specifically the paraventricular and supraoptic nuclei, project their axons down the infundibular process and terminate in the posterior lobe (pars nervosa). This is where they release their hormones, either ADH or oxytocin, into a capillary bed. It is important to note that posterior pituitary hormones are synthesized in the hypothalamus rather than the pituitary gland itself. Therefore, hypophysectomy (pituitary removal) does not necessarily permanently disrupt the synthesis and secretion of these hormones. Although secretion initially decreases following hypophysectomy, over time, histological modifications occur in the severed proximal end of the tract, and pituicytes form around the neuron terminals. Secretory vacuoles become visible, and hormone secretion resumes from this proximal end, potentially returning to normal levels. Conversely, a lesion higher up on the pituitary stalk can result in the loss of neuronal cell bodies in the paraventricular nucleus (PVN) and supraoptic nucleus (SON). In the context of the endocrine axes, the adenohypophysis exhibits both structural and functional organization. Each endocrine axis consists of three levels of endocrine cells: hypothalamic neurons, anterior pituitary cells, and peripheral endocrine glands. Hypothalamic neurons release specific hypothalamic releasing hormones (designated XRH) that stimulate the secretion of specific pituitary tropic hormones (XTH). In certain cases, production of a pituitary tropic hormone is secondarily regulated by a release-inhibiting hormone (XIH). The pituitary tropic hormones then act on specific peripheral target endocrine glands, stimulating them to release peripheral hormones (X). The peripheral hormone X serves two general functions: regulating various aspects of human physiology and providing negative feedback on the pituitary gland and hypothalamus, thereby inhibiting the production and secretion of tropic hormones and releasing hormones, respectively.
Before discussing the individual hormones of the adenohypophysis, it is important to understand the structural •Fig. 41.2 A,Grossstructureofthepituitarygland.Thepituitaryglandisbelowthehypothalamusandisconnectedtoitbythepituitarystalk.Theglandsitswithinthesellaturcica,afossawithinthesphenoidbone,andiscoveredbyaduralreflection,thediaphragmasellae.Theparsdistalismakesupmostoftheanteriorpituitary.B,Theparsdistalisisderivedfromepithelialtissuethatiscomposedofacidophils (A)(somatotropesandlactotropes)andbasophils(B)(thyrotropes,gonadotropes,andcorticotropes).C,Theposteriorpituitaryisderivedfromneuraltissueandhasahistologicalappearanceofnonmyelinatednerves.Cp,chromophobes;H,Herringbodies.(A,ModifiedfromStevensA.In:LoweJS[ed].Human Histology. 3rded.Philadelphia:Elsevier;2005.BandC,FromYoungBetal[eds].Wheater’s Functional Histology. 5thed.Philadelphia:ChurchillLivingstone;2006.) •Fig. 41.3 Magnocellularneuronsofthehypothalamus(paraven-tricularandsupraopticnuclei)projecttheiraxonsdowntheinfundibularprocessandterminateintheparsnervosa(posteriorlobe),wheretheyreleasetheirhormones(eitherADHoroxytocin)intoacapillarybed.(ModifiedfromLarsenPRetal[eds].Williams Textbook of Endocrinol-ogy. 10thed.Philadelphia:Saunders;2003.)Capilary bed Anterior lobe of pituitary Optic chiasm Hypothalamus Posterior lobe of pituitary Magnocellular neurosecretory cells
Becauseposteriorpituitaryhormonesaresynthesizedinthehypothalamusratherthanthepituitary,hypophysectomy (pituitaryremoval)doesnotnecessarilypermanentlydisruptsynthesisandsecretionofthesehormones.Immediatelyafterhypophysectomy,secretionofthehormonesdecreases.However,overaperiodofweekstheseveredproximalendofthetractwillshowhistologicalmodificationandpituicyteswillformaroundtheneuronterminals.Secretoryvacuolesareseen,andsecretionofhormoneresumesfromthisproximalend.Secretionofhormonecanevenpotentiallyreturntonormallevels.Incontrast,alesionhigheruponthepituitarystalkcanleadtolossofneuronalcellbodiesinthePVNandSON.
and functional organization of the adenohypophysis in the context of the endocrine axes (
Fig. 41.6 41.1 ). Each endocrine axis is composed of three levels of endocrine cells: (1) hypothalamic neurons, (2) anterior pituitary cells, and (3) peripheral endocrine glands. Hypothalamic neurons release specific hypothalamic releasing hormones (designated XRH in this generic scheme) that stimulate secretion of specific pituitary tropic hormones (XTH). In some cases, production of a pituitary tropic hormone is secondarily regulated by a release-inhibiting hormone (XIH). Pituitary tropic hormones then act on specific peripheral target endocrine glands and stimulate them to release peripheral hormones (X). The peripheral hormone X has two general functions: it regulates several aspects of human physiology, and it negatively feeds back on the pituitary gland and hypothalamus to inhibit production and secretion of tropic hormones and releasing hormones, respectively (see
Fig. 41.6