Development and cytodifferentiation of the rabbit pars intermedia. I. Fetal and perinatal stages

The developing pars intermedia (PI) of rabbits of between 14 days post coitum (PC) and one day post partum (PP) have been studied by light and electron microscopy. Cell division is marked until 4th week PC, after which it is reduced. The (PI) loses its homogeneity by the 5th week PC due to localisation both intrinsic and invading cells and blood vessels. Four groups of parenchyma cells are recognisable: PI-glandular cells, interstitial cells, ACT-type cells and a heterogeneous group of dark cells. The ultrastructure of the ACT-type cells in the rabbit PI are here described for the first time. The differentiation of these cell types has been traced from 14 days PC to 32 days PC, in particular ACT-type cells are shown to develop in situ and the significance of this is discussed. In the PI-glandular cells dense cored granules appear first on the 15th day PC which is one day before any nerve connection is made with the primitive neural lobe and more than a week before vascularisation. Thereafter granularity increases until parturition when there is a phase of degranulation. Relatively electron lucent, larger vesicles are not found until 4th week PC.