Torrealba F, Parraguez V H, Reyes T, Valenzuela G, Serón-Ferré M
Facultad de Ciencias Biológicas, Universidad Católica de Chile, Santiago.
J Comp Neurol. 1993 Dec 8;338(2):304-16. doi: 10.1002/cne.903380212.
Circadian rhythms are present during fetal life in several mammalian species. To characterize the ontogeny of the neural mechanisms that account for circadian rhythmicity in a precocious species, we studied the prenatal development of the retinohypothalamic pathway in lambs (gestation period of 147 days), using horseradish peroxidase and wheat germ agglutinin as anterograde tracers. The suprachiasmatic nucleus was present as early as embryonic day 52 (E52). After E58, the suprachiasmatic nucleus reached its full number of neurons, estimated by the disector method in about 160,000 cells per nucleus at E62. The retinohypothalamic axons invaded the suprachiasmatic nucleus from E58, while neuroblasts were still migrating to the nucleus. At E62, there was a strong retinal projection that evolved until E121, when the retinal afferents established their definitive pattern of distribution in the ventral and central regions of the suprachiasmatic nucleus, and adjacent hypothalamic structures. The development of the retinohypothalamic pathway was delayed by about a week relative to the innervation of other subcortical visual centers. The present findings demonstrated an early prenatal development of the visual pathways in lambs, including the retinohypothalamic pathway, suggesting that the mechanisms for the visual entrainment of circadian rhythms in lambs may be functioning several weeks before birth.
在几种哺乳动物的胎儿期存在昼夜节律。为了表征早熟物种中负责昼夜节律的神经机制的个体发育，我们使用辣根过氧化物酶和小麦胚凝集素作为顺行示踪剂，研究了羔羊（妊娠期147天）视网膜下丘脑通路的产前发育。视交叉上核早在胚胎第52天（E52）就已存在。E58之后，视交叉上核的神经元数量达到最大值，通过分割法估计，在E62时每个核约有160,000个细胞。视网膜下丘脑轴突从E58开始侵入视交叉上核，而神经母细胞仍在向该核迁移。在E62时，有强烈的视网膜投射，这种投射一直发展到E121，此时视网膜传入纤维在视交叉上核的腹侧和中央区域以及相邻的下丘脑结构中建立了最终的分布模式。相对于其他皮层下视觉中心的神经支配，视网膜下丘脑通路的发育延迟了约一周。目前的研究结果表明，羔羊视觉通路包括视网膜下丘脑通路在产前发育较早，这表明羔羊中昼夜节律的视觉同步机制可能在出生前几周就已发挥作用。
