A COMBINED IMMUNOHISTOCHEMICAL AND ELECTRON-MICROSCOPIC STUDY OF THE 2ND CELL-TYPE IN THE DEVELOPING SHEEP PINEAL-GLAND

Ultrastructural and immunohistochemical techniques were used to study the second cell type in sheep embryo pineal glands. Thirty-two embryos were studied from day 54 of development through birth. Specimens were arranged in four age groups, defined in terms of the most relevant hi stological features: Group 1 (54-67 days of prenatal development), Gro up 2 (71-92 days), Group 3 (98-113 days), and Group 3 (118-150 days). At 98 days. a second cell type was observed which differed from pineal oblasts and showed uniform ultrastructural characteristics similar to those of astrocytes in the central nervous system. Ultrastructural hom ogeneity was not matched by the results of histochemical and immunohis tochemical analysis: while all Type II cells stained positive to phosp hotungstic acid hematoxylin, only 50% expressed glial fibrillary acidi c protein. In the course of ovine intrauterine development, the vascul ar affinity of this second cell population, composed of glial-like or astrocytic cells at varying stages of maturity, leads to the formation of a limiting pineal barrier. This barrier may constitute the morphol ogical expression of a hypothetical functional involvement in the exch ange of substances between blood and pineal parenchyma.