Cleavage and Gastrulation in Avian Embryos
Claudio D Stern, University College London, London, UK
Published online: December 2009
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 (head–tail, dorsoventral and left–right), 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
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 head–tail 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
Keywords: gastrulation; primitive streak; epiblast; mesoderm; endoderm; hypoblast; Hensen's node; neural induction; neurulation
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