Programmed cell death is a well established key process required for proper development of the nervous system. The regulatory and executor mechanisms controlling survival/ death of projection neurons, as well as of other types of differentiated neurons and glial cells, have been studied intensely during neural development. Much less attention has been paid to earlier cell death events affecting neuroepithelial cells and recently born neurons and glial cells. We review here the reports on cell death during vertebrate retina development, our model system for many years, which has provided clear evidence of the importance of early neural cell death. We tentatively categorize the available observations in three death phases, namely morphogenetic cell death, early neural cell death and neurotrophic cell death. The magnitude and the precise regulation of the early phases of cell death are fully comparable to the much better characterized neurotrophic cell death. Therefore, early neural cell death deserves a profound dedicated study; this will help to obtain an integrated understanding of the development of the retina and other parts of the vertebrate nervous system.
The circadian system is responsible for the temporal organisation of physiological functions which, in part, involves daily cycles of hormonal activity. In this review, we analyse the interplay between the circadian and endocrine systems in fishes. We first describe the current model of fish circadian system organisation and the basis of the molecular clockwork that enables different tissues to act as internal pacemakers. This system consists of a net of central and peripherally located oscillators and can be synchronised by the light-darkness and feeding-fasting cycles. We then focus on two central neuroendocrine transducers (melatonin and orexin) and three peripheral hormones (leptin, ghrelin and cortisol), which are involved in the synchronisation of the circadian system in mammals and/or energy status signalling. We review the role of each of these as overt rhythms (i.e. outputs of the circadian system) and, for the first time, as key internal temporal messengers that act as inputs for other endogenous oscillators. Based on acute changes in clock gene expression, we describe the currently accepted model of endogenous oscillator entrainment by the light-darkness cycle and propose a new model for non-photic (endocrine) entrainment, highlighting the importance of the bidirectional cross-talking between the endocrine and circadian systems in fishes. The flexibility of the fish circadian system combined with the absence of a master clock makes these vertebrates a very attractive model for studying communication among oscillators to drive functionally coordinated outputs.
Programmed cell death occurs during both early and late neural development. The mechanisms for the regulation and execution of the early cell death as well as its developmental role are still not fully understood. In this work we have studied the early programmed cell death in the retinal neuroepithelium. Apoptotic cells were selectively located around the optic nerve head in the retinal neuroepithelium of 2- to 6-day-old chick embryos. TUNEL-positive cells and cells which were immunostained for activated caspase-3 showed overlapping distributions suggesting that caspase-3 is involved in the early retinal cell death. Caspase-3 involvement in early retinal cell death was also demonstrated by in vivo treatment with caspase inhibitors z-DEVD-fmk and Boc-D-fmk. After 6 h of treatment, the number of TUNEL-positive cells was reduced by 50%. Sustained treatments (20 h) resulted in a slight widening in the central part of the neural retina but the retinal ganglion cell axons maintained their organization and navigation towards the optic fissure. The most prominent result after inhibition of cell death was an increase in the number of retinal ganglion cells which also produced an enlargement of the ganglion cell layer and an increased number of ganglion cell axons. In conclusion, our results show that caspase-dependent programmed cell death occurs in the embryonic chick retina and that it plays a role to modulate the generation of retinal ganglion cells.
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