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Human Body

19 18 Embryology one cell becomes human Uterine milk nourishes the embryo during the rapid cell division of the first few days. On day five the bundle of 32 cells bores a hole in its coat and implants into the wall of the womb, which feeds it, helped by the yolk sac, until the placenta, made jointly by mother and baby, is fully functional by the end of the first month. Three primordial germ layers below develop into all tissues ectoderm forms brain, nerves, and outer skin mesoderm makes solid organs, flesh, and bone endoderm forms most epithelial tissues. The influences that make embryonic cells specialize and grow in patterns are still secret, but guidance seems to come through prenatal master genes, local signal gradients near the developing blood vessel tree, and streams of pilot cells that die before birth. The busy infolding, migration, and reunion of diverse pockets, arches, and tubes of cells proceeds so quickly that by two months it has made nearly all its little organs, and is recognisably human. It just remains to make some gonads. In boys, a signal cascade in week seven is started by the Sry1 gene on the tiny Y chromosome, which acts on some cells that crawled from the yolk sac to the lower pole of the developing kidney, and results in male genitals, with all their hormonal consequences. In girls, Xdriven maturation continues, undiverted, to form womankind. Early cells are totipotent, meaning they can become any type of cell, but gradually lose this ability as they differentiate into germ layers, then tissues and organs. A midline stripe, the early nervous system, first gives the embryo a head and tail, and the fastest developing early organs are the brain and heart. Five arches migrate from the top of the heart to form head and neck structures, like ears, cranial nerves, and the thyroid, the earliest endocrine gland. Buds from the primitive gut form the lungs and liver, which begins making the first blood cells. The lower body develops from 44 paired segments which give rise to bones, muscles, nerves, and inner organs. Limb buds are forming by seven weeks, as is the young face. After eight weeks, growth rather than differentiation takes over.
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