Immunohistochemical and ultrastructural characterization of pinealocytes in sheep prenatal development*

The ultrastructural characteristics and immunoreactivity to synaptophysin, neuron-specific enolase and non-neuron enolase of pinealocytes in 32 sheep embryos were studied from day 54 of development through birth. Embryos were divided into four age-groups : group 1 (54 to 67 days of prenatal development), group 2 (71 to 92 days), group 3 (98 to 113 days) and group 4 (118 to 150 days). At 98 days of prenatal development and until birth, ultrastructural morphological characteristics reflected the hypothetical functional activity of pinealocytes during intrauterine life. These characteristics consisted in a gradual increase (in line with ontogenesis) of the organelles involved in synthesis, perivascular arrangement of the terminal bulbs of the pinealocytic processes and the presence of gap junctions between the terminal bulbs of these processes. The immunohistochemical characteristics of these cells during prenatal life confirm the neuroendocrine nature of pinealocytes ; their potential neurosecretory capacity and their greater affinity for synaptophysin (SYN) and non-neuron enolase (NNE) than for neuron-specific enolase (NSE). Differences in numeric density and distribution over the pineal surface, together with greater abundance of cells staining positively to the three markers used in the medullar area, suggest that the pineal cortex and medulla are functionally dissimilar.