Page:Tumors of the pituitary gland.djvu/20

The Normal Pituitary

The nerve supply of the pituitary is unique and crucial in the regulation of pituitary function (1). Despite this fact, the human adenohypophy- sis has no direct nerve supply, apart from small sympathetic nerve fibers that are associated with and presumably innervate capillaries. Thus, neural connections may affect blood flow to the adenohypophysis but apparently have no direct role in the regulation of adenohypophysial hormone secretion.

The posterior lobe, in contrast, is composed almost exclusively of axons and nerve fibers that arise from the hypothalamus. It is these neural connections that are required for the normal secretion of the two hormonal productions of the posterior pituitary, oxytocin and vasopressin, as well as for the transport of the other hypotha- lamic peptides that regulate adenohypophysial function (1,14).

The hypothalamo-hypophysial tract consists primarily of nerve fibers from the supraoptic and paraventricular nuclei and carries vasopressin and oxytocin to the posterior lobe of the pituitary, where the hormones are released into capillar- ies. The tubero-infundibular tract, originating from neurosecretory neurons which produce hypophysiotropic hormones, projects from sev- eral nuclei to the median eminence where the

Figure 1.12 RATHKE'S CLEFT IN A FETUS AT 5 WEEK S OF GESTATION

Columnar cells line Rathke's cleft and the connection with the stomodeum (S) has been obliterated. Blood vessels hormones are released into the hypophysial por- tal vascular system. (arrows) adjacent to the primitive diencephalon (D) and the pituitary anlage are the precursors of the hypophysial portal system. (Fig. la from Asa SL, Kovacs K. Functional morphol- ogy of the human fetal pituitary, Pathology Annual 9 (part 1) 1984:275-315.)

The adenohypophysis derives from Rathke's pouch, an endodermal invagination of the primi- tive oral cavity. At the third week of gestation, endoderm from the roof of the stomodeum thick- ens and begins to invaginate; by 5 weeks, Rathke's pouch is a long tube with a narrow lumen and a thick wall composed of stratified cuboidal epithelium (fig. 1-12). By 6 weeks, the connection with the oropharynx is totally obliter- ated and Rathke's pouch establishes direct con- tact with the downward extension of the hypo- thalamus that gives rise to the infundibulum. The two tissues are enclosed by the cartilage anlage of the sphenoid bone, which separates them from the stomodeum, and the sella turcica is formed by 7 weeks (18).

It was suggested that Rathke's pouch arises from the ventral neural ridge in the pharyngeal region, thus sharing with the hypothalamus and posterior pituitary a common neuroectodermal origin (26, 38). The use of avian allografts, biologic markers, and serial sections of early chick embryos has provided indirect evidence for this theory; how- ever, further proof is required for its validation.

As the cells of Rathke's pouch proliferate, the anterior portion forms the pars distalis and pars tuberalis whereas the posterior wall lies in direct contact with the posterior lobe anlage and be- comes the pars intermedia (18). The growth of the anterior limb extends laterally and follows a triradiate pattern: the lateral borders become