The Role of Autophagy and Apoptosis During Embryo Development

Programmed cell death (PCD) and cell survival are two sides of the same coin. Autophagy and apoptosis are crucial processes during embryo development of Invertebrates and Vertebrates organisms, as they are necessary for the formation of a new organism, starting from a fertilized egg. Fertilization triggers cell remodeling from each gamete to a totipotent zygote. During embryogenesis, the cells undergo various processes, thus allowing the transformation of the embryo into an adult organism. In particular, cells require the appropriate tools to suddenly modify their morphology and protein content in order to respond to intrinsic and external stimuli. Autophagy and apoptosis are involved in cell proliferation, differentiation and morphogenesis. Programmed cell death is a key physiological mechanism that ensures the correct development and the maintenance of tissues and organs homeostasis in multicellular organisms. PCD has been classified into three types, according to the morphology that the dying cells acquire and the molecular machinery involved: PCD type I or apoptosis; PCD type II or autophagy and PCD type III or necrosis (not involved in physiological development). These different types of cell death have specific features that can be used to be identified and characterized. Apoptosis is a highly conserved, genetically-controlled process through which certain cells destroy themselves. Autophagy is an evolutionarily conserved pathway used by eukaryotes for degrading and recycling various cellular constituents, such as long-lived proteins and entire organelles, that was mainly detected in those tissues where abundant cell death is required. Both autophagy and apoptosis are induced under stress conditions as an adaptive response against stress. Usually, environmental stress cause severe effects on embryonic development. Embryos of different species, exposed to different types of physical or chemical stress, temporarily suspend their development and activate several protective strategies, including PCD II and PCD III. Research has yet to elucidate the interplay between these key processes. Not all types of PCD are always detected in association with a developmental process. Unlike the degeneration of tissues of some invertebrates, the tissues of vertebrates undergo PCD preferentially through an apoptotic mechanisms. In this chapter, we will briefly describe some specific features of apoptotic and autophagic processes. We will focus our attention in some useful model systems of invertebrates and vertebrates organisms, where autophagy and apoptosis occur both in physiological and stress conditions; specifically, we will analyze embryos of: the nematode Caenorhabditis elegans, the insect Drosophila melanogaster, the sea urchin Paracentrotus lividus, the fish Danio Rerio, the mouse mammalian model, and finally we will consider the differentiation of the male and female embryonic germlines in humans.