PRENATAL DEVELOPMENT OF THE SHEEP PINEAL-GLAND - AN ULTRASTRUCTURAL-STUDY

The ultrastructure of the pineal gland of 32 sheep embryos was studied from day 54 of development through birth. Embryos were arranged in fo ur age-groups,defined in terms of the most relevant histological featu res: group 1 (54 to 67 days of prenatal development), group 2 (71 to 9 2 days), soup 3 (98 to 113 days), and-group 4 (118 to 150 days). A pri mary cell type, designated the pinealoblast, was observed from 54 days until birth; ultrastructurally, this cell was found to contain all th e organelles required for hormone synthesis. A second cell population, classified as interstitial cells by virtue of their location among pi nealoblasts, appeared at 78 days gestation and persisted until birth. Interstitial cells were scarce and exhibited tropism for the perivascu lar space; From 118 days gestation until birth, a third cell type, ter med the pigmented cell, was visible. Pigmented cells, whose ultrastruc tural characteristics differed from those of pinealoblasts, contained a large number of pigment granules of varying size and shape. The pine al gland of developing sheep embryos showed considerable innervation a nd abundant vascularization; this, together with certain ultrastructur al characteristics, suggests that the gland has a secretory function i n uterine life.