An immunohistochemical study on the cytogenesis of adenohypophysial cells in fetal rats

An immunohistochemical study was undertaken in fetal rats to determine the time of differentiation of various types of adenohypophysial cells and the site where they first appear and proliferate during development. A cell count was carried out on the first and second days of cytodifferentiation in each type of cell. Both the time and the site of cytodifferentiation were peculiar to respective cell types. ACTH cells appeared on Day 15 of gestation in the ventral region of the pars distalis where it faces mesenchymal tissue. While TSH cells first appeared exclusively in the posterior half on Day 16, further proliferation of this cell type also occurred predominantly in the posterocentral portion of the gland. LH and FSH cells appeared on Days 17 and 19, respectively; both cell types showed a similar localization, i.e., they were almost concentrated in the ventral region in the anterior half and posteriorly they were distributed sparsely and homogeneously. GH cells appeared on Day 18 in the central region of the pars distalis. Prolactin cells failed to be seen in the fetal adenohypophysis, and even in newborns 0–1 days of age, this type of cell was not consistently seen, although sometimes a few cells were encountered in the central region of the gland. Of these cell types, TSH, LH, and FSH cells persisted to be concentrated even after birth in the area where they first appeared. These observations are discussed in relation to data previously reported by other investigators.