Abstract
Background:
The epiphysis cerebri (pineal gland) is a small-sized, photo neuroendocrine organ in the brain of most vertebrates. Their effect is through secretion of melatonin, a serotonin-derived hormone which is stimulated by darkness and inhibited by light and modulates the circadian rhythm; light and dark cycle like a biological clock, sleep patterns (sleep-wake cycle) and sexual development.
Aim:
This study aimed to identify and differentiate the different cell types filling the pineal gland parenchyma of mature male sheep.
Methods:
Pineal glands were collected and sliced parasagitally then processed histologically for light and electron microscopic examinations.
Results:
Two main cell types; pinealocytes and astrocytes were recognized within the gland parenchyma. Pinealocytes were the chief parenchymatus cells occupied the largest volume of the gland and were classified according to the nuclear pictures (activity status) into two subtypes; pinealocytes I (pale subtype, active) and II (dark subtype, in active). Astrocyte neuroglial cells had cytoplasmic processes which forms a huge supportive framework between the pinealocytes and clarified two types; type I were elongated cells with elongated snake shaped nucleus and type II were smaller in size, with oval nuclei. Another marginal cell type was identified as a neuron-like cell which appeared larger in size than others and distributed sporadically, has eccentric oval nucleus with prominent nucleoli and single, long cytoplasmic process that branched at its terminal forming T-shaped process looks like pseudo unipolar neuron. Moreover, aggregations of pigment granules were markedly observed in the intercellular spaces and also near the blood capillaries. With TEM a special characteristic feature of pinealocytes; synaptic ribbons were recognized that appeared as bands of electron-dense material with several synaptic spherules; vesicles adjacent to its surface helping in the multivesicular release.
Conclusion:
The gland parenchyma revealed two main cell types; pinealocytes and astrocytes. Each one was subdivided to two subtypes; I and II. The first one was classified according to their nuclear pictures (activity status) and the second one was according to their shape, size and cytoplasmic processes. Other cell types were also identified as neuronal and pigmented like cells in the pineal matrix.

Key words: Astrocyte, Epiphysis cerebri, Neuronal like cell, Pinealocyte, Synaptic ribbons.