embryo

An embryo is the early stage of animal development that begins with fertilization and ends when organ formation is sufficiently advanced to be distinguished as a fetus. In humans, the embryonic period typically covers the first eight weeks after fertilization, after which the developing organism is generally referred to as a fetus.

Following fertilization, the zygote undergoes rapid cell divisions in a process called cleavage, producing progressively smaller cells. The resulting morula becomes a blastocyst, which implants into the uterine lining. The blastocyst has an outer layer of trophoblast cells that contribute to the placenta and an inner cell mass that will form the embryo proper. The placenta, amniotic sac, and umbilical cord support the developing embryo.

Weeks 3 through 8 encompass organ formation. Gastrulation creates three germ layers—ectoderm, mesoderm, and endoderm—from which all tissues derive. The ectoderm gives rise to the nervous system and skin; the mesoderm forms muscle, bone, and the circulatory system; the endoderm generates the gut and associated organs. The neural tube forms early in this period and develops into the brain and spinal cord; the heart begins to beat, and major organ systems take shape. By the end of the eighth week, gross body plans are recognizable, though the embryo remains small and organ systems are still immature.

In humans, after the embryonic period development continues as the fetus, with growth and maturation of organs. Embryology also applies to many other vertebrates, where the same general sequence of fertilization, cleavage, gastrulation, and organogenesis governs early development. In vitro fertilization and related research involve studying or culturing embryos outside the body, subject to ethical and regulatory considerations.