PITUITARY GLAND: DEVELOPMENT, NORMAL APPEARANCES, AND MAGNETIC RESONANCE IMAGING

Origin of the Pituitary Gland

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raditional embryology states that the anterior lobe of the pituitary gland derives from an evagination (Rathke pouch) of the rostral stomodeum anterior to the buccopharyngeal membrane and the infundibulum/posterior lobe from a ventral caudal extension of the diencephalon. It is believed that contact between these 2 components is necessary and that it provides critical signaling for proliferation and determination of pituitary cell types later (Amar et al., 2003) this process seems to be regulated by the LIM homeobox genes. Although both components are derived from ectodermic germ cells, these cells express different phenotypes and histology. Those from Rathke pouch give rise to a ductless (exocrine) gland which integrates with posterior neurohypophysis at around the second month of life. The latter arises as a diverticulum of the third ventricle, which extends down (the infundibular stem) and terminates in a zone of expansion (infundibular process). Rathke pouch migrates rostrally via a canal in the body of the sphenoid bone (craniopharyngeal or basipharyngeal canal (Elsetr et al., 1993). this canal regresses (at approximately the sixth month of life) by virtue of expansion of the sphenoidal mesenchyme. Remnants of this canal may persist, and occasionally, glandular tissues may be found in those remnants (which explain the presence of sphenoidal adenomas or of functioning gland in the craniopharyngeal canal). It should be noted that some authors believe that Rathke pouch is not connected to the stomodeum, but that it represents an isolated vesicle. Others believe that the anterior lobe is actually of neuroectodermal origin, because some of its cells are capable of amine precursor uptake and carboxylation. At this time, the pars distalis enlarges and becomes the anterior lobe (Porter et al., 1977) The pars distalis extends superiorly along the anterior aspect of the infundibulum (eventually extending completely around it) as does the pars tuberalis. Thus, the pituitary stalk has components of both the anterior and posterior lobes (which explains the presence of adenomas arising in the stalk). Mesenchymal cells fill gaps found in the posterior aspect of the pars distalis (known as the fossa of Atwell), and they will eventually give raise to the gland’s portal vascular system (Amar et al., 2003). Adjacent to these cells, there is an area in the dorsal surface of the pars distalis that experiences lesser cell proliferation and becomes the intermediate lobe (pars). The pituitary fissure is a small potential space located between the anterior and the intermediate lobes, and this completes the formation of the adenohypophysis. Around the fourth month of life, the different cell populations arrange themselves around blood vessels in specific, bilateral, and nearly symmetrical groups. Acidophilic cells (prolactin releasing) are found in the lateral aspects of the anterior lobe; growth hormoneYreleasing cells are found in its anterior-lateral aspects, and adrenocorticotropic, follicle-stimulating, and thyroid-stimulating hormones are found in the lobe’s central aspect. It is possible, however, that a single cell type may be able to secrete more than one type of hormone (Aron et al., 1997) The adenohypophysis becomes functional between the first and second trimesters, whereas the activity of the neurohypophysis begins close to the time of birth. The infundibular process enlarges and contains neuroglial cells (pituicytes) and becomes the posterior lobe (pars nervosa). Nerve fibers from the hypothalamus also terminate in the neurohypophysis. Initially, the infundibular stem is hollow because of an inferior extension of the third ventricle. Eventually, its lumen is obliterated (but may persist even in adults), and all that is left behind is a depression in the anterior floor of the third ventricle, called the infundibular recess.