ABSTRACT
In this chapter, the authors consider how the mammalian zygote gives rise to early lineages of more specialized cells, charting the initial steps of tissue differentiation and the formation of the three primary germ layers and germ cells against a background of mammalian embryonic development. They introduce various aspects of stem cells and cell differentiation. Most attention has focused on models that deal with post-compaction breaking of symmetry: either the symmetry of cells (cell polarity), or of their positions in the embryo (cell surface location versus interior location), or both. Tissue-specific stem cells are maintained in special supportive microenvironments, called stem cell niches, where chemical signals are conveyed from neighboring cells and extracellular matrix to receptors on the stem cell to support stem cell activity and renewal. Human blood cells are initially made in certain embryonic structures before the fetal liver takes over production.
