Age-related morphometric changes in the pineal gland. A comparative study between C57BL/6J and CBA mice.

Relatively little is known about the effects of melatonin on the aging of the pineal, the organ which is the main place for synthesis of this hormone. Using simple morphometric methods, some parameters of the pineal gland, such as total volume, number of pinealocytes and pinealocyte volume were estimated in two mice strains: normal CBA and melatonin-deficient C57BL/6J. Two age groups, 6 weeks and 10 months, were studied in order to evaluate possible differential age-related changes between both strains. Pineals of both strains have similar morphometric and morphological features at 6 weeks of age. This suggests that pineal development, which has already concluded at 6 weeks of age, is not affected by the absence of melatonin synthesis in the pinealocytes. Later on, CBA pineal showed an increase in size caused by cellular hypertrophy. In contrast, the C57BL/6J pineal volume decreased by loss of pinealocytes in the same period of time. Semithin sections analysed by light microscopy did not show that this cell death was evident in the C57BL/6J strain at any of the ages studied. Thus, a gradual loss of pinealocytes could be hypothesised in these pineals. These results suggest that pineal melatonin could have a role in the maintenance of pinealocyte viability and the increase of pineal size which takes place after development. The abnormal pattern observed in the C57BL/6J pineal should be taken into account in future studies on this gland.

Ontogeny of a photic response in the retina and suprachiasmatic nucleus in the mouse.

The ontogeny of photic responsiveness in the retina and the suprachiasmatic nucleus (SCN) of C57BL/6J mouse was studied using the enhanced expression of the immediate early gene c-fos as a marker of neuronal activation. c-fos expression was assessed by immunocytochemical localisation of its protein product. Light induction of Fos-like protein in the retina and SCN occurred first at postnatal day four (PD 4). At this stage of development, some cells in the inner part of the neuroblastic layer and in the ganglion cell layer showed positive immunoreaction; the number of Fos-like positive cells increased with age until it reached adult levels by PD 15. Induction of Fos-like expression at PD 4 in the SCN mainly occurred in the ventrolateral region, the region that receives the greatest density of retinal innervation. These results indicate that retinal input can activate cells in the SCN even before eyelids open, and the SCN can be stimulated by photic inputs as early as day 4 after birth.