Embryonic morphogenesis of the human pituitary.

Light microscopy methods were used to study the main stages in the organogenesis of the pituitary in human embryos at Carnegie stages (CS) 12-23. Rathke's pouch (RP) was shown to form as a traction fold over whole width of the roof of the stomodeum in embryos at CS 12 due to a flexure of the neural tube with which the epithelium had a tight anatomical relationship (the attached part of the anterior wall of the RP) in the median plane of the embryo. The rudiment of the hypothalamic infundibulum and neurohypophysis formed at CS 15, as a thickening of the posterior wall of the diencephalon. Transorientation of the positions of brain components, including the rudiment of the hypophysis, occurred at CS 20-23. The attached part of the anterior wall of the RP then formed the pars intermedia and pars tuberalis of the anterior lobe, while the epithelium of the orifice of the RP and its posterior wall formed the pars distalis. From CS 20 to 23, the RP epithelium formed the structural-functional units of the adenohypophysis, i.e., the epithelial cords, by invagination.