Developmental biology part 1 : introduction and grey crescent formation

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Embryogenesis is the step in the life cycle after fertilisation -- the development of the embryo, starting from the zygote (fertilised egg). Organisms can differ drastically in how the embryo develops, especially when they belong to different phyla. For example, embryonal development in placental mammals starts with cleavage of the zygote into eight uncommited cells, which then form a ball (morula). The outer cells become the trophectoderm or trophoblast, which will form in combination with maternal uterine endometrial tissue the placenta, needed for fetal nurturing via maternal blood, while inner cells become the inner cell mass that will form all fetal organs (the bridge between these two parts eventually forms the umbilical cord). In contrast, the fruit fly zygote first forms a sausage-shaped syncytium, which is still one cell but with many cell nuclei.[18]

Patterning is important for determining which cells develop into which organs. This is mediated by signaling between adjacent cells by proteins on their surfaces, and by gradients of signaling secreted molecules.[19] An example is retinoic acid, which forms a gradient in the head to tail direction in animals. Retinoic acid enters cells and activates Hox genes in a concentration-dependent manner -- Hox genes differ in how much retinoic acid they require for activation and will thus show differential rostral expression boundaries, in a colinear fashion with their genomic order. As Hox genes code for transcription factors, this causes different activated combinations of both Hox and other genes in discrete anteroposterior transverse segments of the neural tube (neuromeres) and related patterns in surrounding tissues, such as branchial arches, lateral mesoderm, neural crest, skin and endoderm, in the head to tail direction.This is important for e.g. the segmentation of the spine in vertebrates.

Embryonic development does not always proceed correctly, and errors can result in birth defects or miscarriage. Often the reason is genetic (mutation or chromosome abnormality), but there can be environmental influence (like teratogens) or stochastic events. Abnormal development caused by mutation is also of evolutionary interest as it provides a mechanism for changes in body plan (see evolutionary developmental biology).

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