Preembryonic

Preembryonic refers to the earliest stage of prenatal development, from fertilization through the formation of the germ layers, before the embryo begins to generate distinct tissues and organs. In humans, this period roughly spans the first 14 days after fertilization and ends with the onset of gastrulation, when the three germ layers begin to form.

Key events during the preembryonic period include fertilization to form a zygote, followed by cleavage that yields progressively smaller cells (blastomeres). The embryo becomes a morula and then a blastocyst, which consists of an outer trophoblast layer and an inner cell mass. The blastocyst implants into the endometrium, with the inner cell mass differentiating to form the bilaminar germ disc (epiblast and hypoblast) and the initial development of extraembryonic membranes such as the amnion and chorion. The placenta begins to establish as part of supporting structures for the developing embryo, but true embryonic development proper has not yet begun.

Understanding the preembryonic period is important for fields such as reproductive biology and obstetrics, as well as for ethical and legal considerations surrounding embryo research. In many jurisdictions, research and culturing of human embryos are subject to the 14-day rule, which limits in vitro studies to embryos up to approximately two weeks after fertilization, before gastrulation.