After fertilization of the mammalian oocyte, the developmental programme leading to the conceptus begins with the early cleavage divisions. Lineage differentation begins after the early and mammal-specific event of compaction and continues throughout development of the blastocyst. This structure, unique to mammals, after implantation will give rise to the various embryonic and extraembryonic tissues of the developing conceptus. Soon after implantation, the orientation of the main body axis becomes apparent with the primitive streak, and the node at its anterior end. Gastrulation through the primitive streak results in the formation and segregation of the three germ layers and the establishment of the basic body plan of the developing fetus. Finally, we compare mammalian early development to other vertebrates.
Key concepts:
- Being higher animals, the early development of mammals prepares for and culminates in the generation of three tissue layers by gastrulation.
- Many aspects of early mammalian development are shared with fellow amniote animals (reptiles and birds) and even with anamniote vertebrates (e.g. amphibians).
- Mammals have however evolved specific adaptations to development in the mother's womb, in particular with regard to early stages of development and the role of the extraembyronic tissues.
Keywords: blastocyst; epiblast; primitive streak; node; notochord






