Post-implantation human early embryogenesis is a crucial developmental period that begins around day 6 to 10 after fertilization. During this phase, the blastocyst successfully implants into the uterine wall, marking the start of rapid cellular differentiation and the establishment of the basic body plan that will guide all subsequent development.
Following successful implantation, the inner cell mass undergoes its first major differentiation event. It separates into two distinct layers: the epiblast, which forms the dorsal layer, and the hypoblast, which forms the ventral layer. Together, these create a flat structure called the bilaminar germ disc. The amniotic cavity develops within the epiblast, while the yolk sac forms below the hypoblast.
Gastrulation is the most critical event in early embryogenesis, occurring around week 3. During this process, cells from the epiblast migrate inward through a structure called the primitive streak. This migration creates three distinct germ layers: the ectoderm, which will form the nervous system and skin; the mesoderm, which gives rise to muscles, bones, and circulatory system; and the endoderm, which forms the digestive and respiratory tracts.
After gastrulation, neurulation begins as the ectoderm forms the neural plate, which folds to create the neural groove and eventually fuses into the neural tube - the precursor to the brain and spinal cord. Simultaneously, body folding transforms the flat trilaminar disc into a cylindrical three-dimensional embryo. The mesoderm segments into somites that will form vertebrae and muscles, while the endoderm forms the primitive gut tube.
By week 4, early organogenesis begins as major organ systems start developing from the three germ layers. The heart begins beating, limb buds appear, brain regions form, and the digestive system develops. Simultaneously, the placenta and umbilical cord establish crucial maternal-fetal circulation, enabling efficient nutrient and gas exchange that supports the rapidly growing embryo throughout the remainder of development.