Prenatal development of the sheep pineal gland: An ultrastructural study

Abstract

The ultrastructure of the pineal gland of 32 sheep embryos was studied from day 54 of development through birth. Embryos were arranged in four age-groups, defined in terms of the most relevant histological features: 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). A primary cell type, designated the pinealoblast, was observed from 54 days until birth; ultrastructurally, this cell was found to contain all the organelles required for hormone synthesis. A second cell population, classified as interstitial cells by virtue of their location among pinealoblasts, appeared at 78 days gestation and persisted until birth. Interstitial cells were scarce and exhibited tropism for the perivascular space. From 118 days gestation until birth, a third cell type, termed the pigmented cell, was visible. Pigmented cells, whose ultrastructural characteristics differed from those of pinealoblasts, contained a large number of pigment granules of varying size and shape. The pineal gland of developing sheep embryos showed considerable innervation and abundant vascularization; this, together with certain ultrastructural characteristics, suggests that the gland has a secretory function in uterine life.

Structure of the ovine pineal gland during prenatal development

“…It has not been possible to determine the kind of capillaries by light microscopy. Redondo et al [1996] in an electron microscopic study of sheep pineal gland at 98 days of gestation report nonfenestrated capillaries similar to those encountered in Carnivores [Calvo et al, 1990;Boya et al, 19951. VIP nerve fibers were observed at around 98 days of prenatal development. The localization of these fibers was similar to the those reported by Redondo et al [1996] in the prenatal development of the ovine pineal by using electron microscopy.…”

“…If we admit the prenatal differentiation on the pigment cells as a new cellular type, different from the pinealoblasts and interstitial cells as proposed by Calvo et al [ 1988a,b] in the postnatal development in dogs' pineal, and Redondo et al [1996] and Regod6n et al [1998] in the embryonic development of the ovine pineal, we would suggest that these cells have a priority role in melanosynthesis with regard to the cells containing pigment.…”

“…Arguments in favor of a hypothetical functionality involve all the cellular types that make up the ovine pineal parenchyma during ontogenesis. Redondo et al [1996] describe in pinealoblasts a considerable development of the organoids involved in proteosyntesis. In the interstitial cells, the proximity of the vascular structures [Calvo and Boya, 1983;Boya and Calvo, 1993;Redondo et al, 19961 could be the basis of the morphologic support of a hypothetical functionality, relating to the interchange of substances between the pineal parenchyma and the blood [Xu Zang, 19851 .…”

“…The pineal gland, which in mammals develops from the diencephalic ependyma and during embryonic development is located between the anterior and the posterior commissure, has recently been the object of considerable research [Redondo et al, 1996;Franco et al, 1997;Regodh et al, 19981. It is precisely in seasonally polyestrous species such as sheep that the pineal gland assumes great importance since it is involved in photoperiod regulation through secretion of melatonin [Bittman et al, 1983;Reiter, 1993;Cozzi et al, 19941. Accordingly, the aim of this study was to acquire a better understanding of the ontogenesis of the sheep pineal gland and therefore of its potential functional significance, by obtaining morphological data at varying stages of development, defined in terms of the most relevant structural features.…”

“…Two reasons led us to carry out a study about the pineal gland during prenatal life. The first is that few studies of these characteristics exist [Jordan, 191 1;Ariens-Kappers, 1960;Anderson, 1965;Redondo et al, 1996;Franco et al, 1997;Regod6n et al, 19981. The second reason is that any investigation on the functional aspects of the pineal gland necessarily implies wide knowledge of its morphological configuration, even of its differentiation from the diencephalic ependyma of the third ventricle [Pevet, 1977;Ueck, 19861. The structural and immunohistochemical analy-sis of the ovine pineal gland during prenatal life is the aim of this paper.…”