Establishment of early embryonic lineages and the basic body plan

Embryonic development transforms an apparently featureless fertilized egg into the complex form of the fetus. Several important processes need to occur in a coordinated manner to achieve this. First among them is the proliferation of cells, with every cell of the body being derived through the repeated division of the zygote and its descendants. These cells undergo a process of gradual specialization, so that their structure and physiology can be tuned to a variety of different functions. This is coordinated with patterning, which ensures that this emerging cellular heterogeneity within tissues is temporally and spatially organized. Patterning is primarily achieved through modulating the transcriptional state of the cells within tissues. Morphogenesis shapes these tissues to generate the basic body plan and the various organ primordia. This is brought about through coordinated movement, generation of forces, and the modulation of biomechanical properties of the component cells of embryonic tissues. In this chapter, we give a broad outline of the patterning and morphogenetic events that occur from fertilization up to the end of gastrulation. We describe the origin of the tissues that go on to lay the foundations of the various organ systems and how the basic body plan of the fetus is established. As a companion to the Kaufman atlas, this chapter focuses on the developmental origin of anatomical features of the mouse embryo. Wherever possible, we draw comparisons to what is known about human development.