Avian embryos differ from those of other vertebrates in several respects among them, they have a large mass of yolk, the cells initially divide from the centre of a disk with cleavage planes that open into the yolk, and the embryo has great ability to regulate until very late stages of development. After approximately 20 000 cells have been generated, gastrulation begins. This process generates the primitive streak which defines bilateral symmetry, and through which cells from the superficial layer (epiblast) ingress to generate two new layers of embryonic cells (mesoderm and endoderm). This period of development starts to define the three axes of the future embryo (headtail, dorsoventral and leftright), and it is at this time that many cell fates start to be fixed.
- Avian embryos cleave meroblastically: the cleavage planes are initially open to the yolk and generate a disc with smaller cells in the middle and larger, yolky cells outside.
- Gastrulation is the process by which the embryo generates three germ layers: ectoderm, mesoderm and endoderm. It involves massive cell movements as well as specification of cell fates through differential gene expression.
- As cells move around the embryo, they change their patterns of gene expression according to their current position. Therefore during early development, gene expression marks cell states rather than cell fates.
- The mechanisms of early development are largely conserved between different vertebrate classes but there are also some differences.
- Amniote (reptiles, birds and mammals) embryos have a unique capacity to regulate, meaning that when an embryo is cut into half or smaller pieces each piece can generate a whole embryo. Embryos retain this ability right up to the start of gastrulation and this is probably one of the processes responsible for generating identical twins.
- Neural induction subdivides the ectoderm into neural (future nervous system) and non-neural (future skin and sensory organs in the head) territories. Neural-inducing signals in avian embryos arise from the organizer, Hensen's node.
- The final headtail axis of the embryo does not correspond to the axis of the primitive streak (which correlates better with axial and lateral fates). Mesodermal cells that will occupy more cranial positions emerge earlier from the streak than those that will end up in the trunk and tail. After gastrulation, the embryo has mechanisms that convert time information into positional identity.
Keywords: gastrulation; primitive streak; epiblast; mesoderm; endoderm; hypoblast; Hensen's node; neural induction; neurulation