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Abstract
This study shows the distribution patterns of apoptotic cells and biomarkers of cellular senescence during the ontogeny of the retina in the zebra finch (T. guttata). Neurogenesis in this altricial bird species is intense in the retina at perinatal and post-hatching stages, as opposed to precocial bird species in which retinogenesis occurs entirely during the embryonic period. Various phases of programmed cell death (PCD) were distinguishable in the T. guttata visual system. These included areas of PCD in the central region of the neuroretina at the stages of optic cup morphogenesis, and in the sub-optic necrotic centers (St15–St20). A small focus of early neural PCD was detected in the neuroblastic layer, dorsal to the optic nerve head, coinciding with the appearance of the first differentiated neuroblasts (St24–St25). There were sparse pyknotic bodies in the non-laminated retina between St26 and St37. An intense wave of neurotrophic PCD was detected in the laminated retina between St42 and P8, the last post-hatching stage included in the present study. PCD was absent from the photoreceptor layer. Phagocytic activity was also detected in Müller cells during the wave of neurotrophic PCD. With regard to the chronotopographical staining patterns of senescence biomarkers, there was strong parallelism between the SA-β-GAL signal and p21 immunoreactivity in both the undifferentiated and the laminated retina, coinciding in the cell body of differentiated neurons. In contrast, no correlation was found between SA-β-GAL activity and the distribution of TUNEL-positive cells in the developing tissue.
Highlights
Programmed cell death (PCD) and cellular senescence during vertebrate embryogenesis are transient phenomena that contribute mainly to tissue remodeling [1,2,3] through the degeneration of temporary structures in the embryo
PCD during visual system morphogenesis and retinogenesis is completely restricted to the embryonic period
In order to identify dying cells in the developing T. guttata visual system, we used some of the methods for detecting PCD in embryonic tissues [3]
Summary
Programmed cell death (PCD) and cellular senescence during vertebrate embryogenesis are transient phenomena that contribute mainly to tissue remodeling [1,2,3] through the degeneration of temporary structures in the embryo. It has been described that PCD processes are accompanied by cell senescence in interdigital regression [4,5,6], heart morphogenesis [7], pronephros and mesonephros degeneration [8,9,10,11], and degeneration of structures in the developing otic vesicle [12,13,14]. The vertebrate visual system constitutes an excellent model for investigating the mechanisms involved in cell degeneration and the phases of PCD that affect different structures (for a review, see [3]). Similar studies have been conducted in the chicken [32,33,34,35,36,37,38] and in the quail [39], two precocial bird species In these species, PCD during visual system morphogenesis and retinogenesis is completely restricted to the embryonic period. The last wave of cell death follows different gradients that resemble the spatiotemporal patterns of cell differentiation [34,39]
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