A human embryo forms after fertilization – a human sperm meeting a human egg. In this process, the 23 chromosomes in the one gamete combine with the 23 chromosomes of the other gamete to form the full 46 chromosomes required for normal human life and development. This initial joining consists of the diploid nucleus containing the chromosomes within the shell of the fertilized egg, the zygote. This usually occurs in the fallopian tube. The zygote then travels to the uterus and begins to implant by day 5 or 6. During these first days, the zygote divides into cells called blastomeres, eventually becoming a ball of cells called a blastocyst. The cells inside the blastocyst will become the embryo.
The embryo develops from fertilization to fetus over the course of 8 weeks as follows.
Blastocyst stage – Weeks 1 and 2
Considering fertilization as day 1, the blastocyst consists of 4 cells by day 3 and 8 cells by day 4. Twinning, splitting of the embryo into two, may occur in these first 4 days. The blastomeres undergo compaction and a first differentiation on day 5, which allows the formation of outer cells (trophoblast) and attachment to the endometrium by day 6. Differentiation creates stem cells that will become particular lineages of human cells (pluripotent). By days 7 and 8 the trophoblast is complete and the embryo begins to implant itself in the internal lining of the uterus. At this point the amniotic cavity is also formed and human chorionic gonadotropin (HCG) released - the mother will likely test positive for pregnancy.
By day 10, the endometrium where the embryo is implanting has been digested by enzymes secreted by the outer cells of the blastocyst. A yolk sac forms in the center of the embryo around this time and the embryo is nearly fully encapsulated by the cells that will become the protective membranes. By days 13 and 14, the yolk sac collapses, creating vesicles that will become the umbilical cord, and the chorionic cavity, which will become the interior of the placenta, is fully formed.
By the end of week 2 after fertilization the embryo is implanted in the uterus and ready to begin developing into a human fetus. It measures between one-tenth and two-tenths of a millimeter in diameter.
Embryonic development – Weeks 3 to 8
The first step in embryonic development is gastrulation - the ball of cells becomes three distinct germ layers (i.e. the three cell types: ectoderm, mesoderm, and endoderm) that separate to start differentiating into more specific cell types and tissues. The umbilicus forms and chorionic villi become distinct by day 17 after fertilization. By day 19 the embryo has doubled in size and hematopoiesis, the production of blood cells, begins. During the third week the internal organs begin to form, starting with the heart and gastrointestinal tract, as well as the central nervous system. By day 22 the embryo is nearly 2 mm in diameter and has a heart that beats, driven by the mother’s circulation and pumping the first blood cells through the embryo. The notochord, the beginnings of the spinal cord, is formed by week 4, as the embryo flattens out. The genitals begin to form around this time as part of the separation of cells on the embryonic axes (genital ridge).
Through week 5 the embryo doubles in size again - the limbs start to bud around day 28, cranial nerves form, the eyes and ears appear, and some bones begin to form. The cardiovascular system and brain continue to form. In week 6, feet and hands appear on the rudimentary limbs, as well as webbed digits, and the lungs begin to form. The mouth and nostrils begin to appear. The embryo is 10 to 12 mm in length at this point.
In week 7 nipples, hair follicles, elbows and toes appear. The genital ridge has become ambisexual gonads to be fully formed in the fetal period. In week 8, eyelids close over the eyes, external facial features are present, and all essential organs are undergoing formation - the intestines rotate into place, the first glomeruli of what will be the kidneys appear, and a number of other organs, including the brain, are ready to finish forming. The embryo is now a human fetus measuring 35 to 40 millimeters in length (about 1.5 inches).
The exact days when the stages occur vary by pregnancy. Two slide shows of interest for detailed pictures and explanations of the complex steps of embryonic development are VirtualHumanEmbryo and available from the University of Pennsylvania.