Abstract
With rat fetuses of the 8th to 20th day of gestation and with neonates and postnates of one to seven days of age, the cytogenesis of anterior pituitary cells was investigated light and electron microscopically. The activity of pituitary trophic hormones was evaluated in reference to the histological findings on the developing target organs.
The anterior lobe of rat fetuses, outgrowing from Rathke's pouch on the 15th day of gestation, consists of a single kind of agranular primordial cell. From the 17th day of gestation, they begin to granulate and turn into ambiguous cells containing minute granules, whose developmental orientation is still unknown. From the 17th to 20th day of gestation, the growing granules suggest their transformation into undifferentiated basophils. During late pregnancy the cellular transformation proceeds exclusively along the basophil axis, and undifferentiated thyrotrophs gain β-granules, and the follicles of the thyroid show a gradual development throughout perinatal life. In the late embryonal life, gonadotrophs with their characteristic large granules develop considerably in males. As this is timed by a transitory increase of Leydig cells in the testis, a discharge of gonadotrophin at this period is suggested. The involution of some gonadotrophs after birth is concurrent with that of Leydig cells. The development of gonadotrophs is delayed in female fetuses. No gonadotrophs reappear throughout postnatal life in either sex.
The development of acidophils is retarded to that of the cells along the basophil axis during embryonic life. The ambiguous cells developing along the acidophil axis are generally ellopsoid or elongated in shape, and begin to contain dense granules. After birth, the cellular transformation along the acidophil axis is activated.
Within 5 days of age, numerous young acidophils full of α-granules are produced. On the other hand, the fetal adrenal cortex has been substituted by the permanent cortex by the postnatal third day. The activated acidophil axis thus seems to be related to ACTH secretion.
Pituitary primordial cells in embryonal life are characterized by abundant rosettes and the absence of dense granules. From the late fetal period on, they decrease in number, having been transformed into granulated cells. Some primordial cells are retained without completing their defferentiation into the so-called chromophobes, while the target organs are still developing. Mitosis takes place frequently in primordial cells and ambiguous cells, but not in mature chromophils. The present findings support the theory of two developmental transformation axes of the anterior pituitary cells.
