extraembryonic tissues
organisms life cycle is as subject to the effects of evolutionary pressures as any other although it is easy to concentrate our attentions on adult forms and behaviors . The study of these processes, known as embryology, is beyond our scope here, but we can outline a few common themes. If fertilized eggs develop outside of the body of the mother and without parental protection, these new organisms are highly vulnerable to predation. In such organisms, early embryonic development generally proceeds rapidly. The eggs are large and contain all of the nutrients required for development to proceed up to the point where the new organism can feed on its own. To facilitate such rapid development, the egg is essentially pre-organized, that is, it is highly asymmetric, with specific factors that can influence gene expression, either directly or indirectly, positioned in various regions of the egg . Entry of the sperm the male gamete , which itself is an inherently asymmetric process, can also lead to reorganization of the cytoplasm SEP marks sperm entry point in the figure early frog development . Maternal and fertilization-driven asymmetries are stabilized by the rapid cycles of DNA replication and cell division, with growth being dependent upon the utilization of maternally supplied nutrients. As distinct cells are formed, they begin to become different from one another as i they inherit different determinants, ii the presence of these determinants leads to changes in gene expression, and iii cells secrete and respond to different factors that drive their differentiation further into different cell types, with different behaviors based on differences in gene expression. On the other hand, in a number of organisms, and specifically mammals, embryonic development occurs within the mother, so there is no compelling need to stockpile nutrients within the egg and the rate of development is generally dramatically slower. In such developmental systems, it is not the asymmetries associated with the oocyte and fertilized egg that are critical, but rather the asymmetries that arise during embryonic development. As the zygote divides, a major factor that drives the differentiation is whether a cell comes to lie on the surface of the embryo or within the interior . In mammals, the cells on the exterior form the trophectoderm, which goes on to form extraembryonic tissues, in particular the membranous tissues that surround the embryo and become part of the placenta, the interface between the embryo and the mother. Cells within the interior form the inner cell mass that produces to the embryo proper. Changes in gene expression will lead to changes in the ability to produce and respond to inductive signals, which will in turn influence cell behavior and gene expression. Through this process, the cells of the inner cell mass come to form the various tissues and organs of the organism; that is, skin, muscle, nerve, hair, bone, blood, etc. It is easy to tell a muscle cell from a neuron from a bone cell from a skin cell by the set of genes they express, the proteins they contain, their shapes morphology , their internal organization, and their behaviors. biofundamentals coreBIO.