Gastrulation

Gastrulation is an early embryological process forming three germ layers: the ectoderm, endoderm, and mesoderm. Each layer gives rise to specific organs and structures in the adult human. The ectoderm forms the integumentary system (skin, ears, nails, hair) and nervous system (brain, spinal cord, peripheral nerves, pituitary gland). The endoderm creates the epithelial lining of the lungs, digestive system, pancreas, bladder, liver, thyroid, parathyroid, and thymus. The mesoderm develops into the musculoskeletal system (bone, cartilage, three types of muscle), cardiovascular system (heart, blood vessels), excretory system, and reproductive system (gonads).

Gastrulation begins with the implantation of the blastocyst into the uterine endometrium. The blastocyst consists of the trophoblast (outer layer, forming chorion and placenta), inner cell mass (forming the germ layers), and blastocoel (fluid-filled cavity). The placenta provides nutrition and oxygen to the developing organism.

Trophoblast cells secrete digestive enzymes, allowing the blastocyst to embed entirely within the endometrium. The inner cell mass differentiates into the hypoblast (green cells, forming endoderm) and epiblast (blue cells, forming ectoderm and later mesoderm).

The upper portions of the hypoblast and epiblast migrate upward, forming the umbilical vesicle (previously yolk sac) and the amniotic cavity, respectively. The amniotic cavity will house the developing fetus. The trophoblast extends to form the chorion, which contributes to the placenta.

The mesoderm, the middle layer, forms between the epiblast and hypoblast. This occurs through invagination: epiblast cells move inward along a 'primitive streak', differentiating into mesoderm cells (red cells). This inward pushing spreads the mesoderm cells around the existing structures.

The inward movement of cells continues, Pushing the mesoderm around the umbilical vesicle (endoderm) and amniotic cavity (ectoderm). In the final stage, labeled diagram 6, the embryo folds inward, resulting in the three defined germ layers: the outer ectoderm (blue), middle mesoderm (red), and inner endoderm (green). The ectoderm gives rise to the skin and nervous system, the mesoderm to skeletal, muscular, cardiovascular, excretory, and reproductive systems, and the endoderm to the digestive and respiratory linings, liver, pancreas, and glands. Surrounding structures include the umbilical vesicle, chorionic cavity, chorion, embryonic stock (developing into the umbilical cord), and amniotic cavity (housing the fetus).