At about two weeks, the embryo enters a new phase that signals the start of a body blueprint¹. Cells from the ICM (inner cell mass) change and organize into three kinds of cells called germ layers², illustrated above. Cells from each germ layer begin to further develop into tissues and organs of the body (only major cell types and structures are listed):

o Cells of the ectodermal³ layer become nerve and skin cells and form the inner ear, eye, mammary glands (Milchdrüsen), nails, teeth, and the nervous system, including the spinal cord (Rückenmark) and brain.
o Cells of the mesodermal³ layer become blood, muscle, and bone cells, and make the heart, skeleton, gonads Keimdrüsen), urinary system, fat, and spleen (Milz).
o Cells of the endodermal³ layer become cells inside the gut (Darm), liver, pancreas (Bauchspeicheldrüse), bladder, lungs, tonsils (Mandeln), pharynx (Rachen, Schlund), and parathyroid glands (Nebenschilddrüsen).

At one month of age, the human embryo is three millimeters long, roughly the size of a pea. Now cells rarely function as individual actors; they start to work in concert with their neighbors. During this time, a primordial⁴ heart begins to beat. In a few days, it will pump blood through a graceful triad⁵ of circulatory arcs. Even the embryonic heartbeat, without a functioning system of blood vessels, is essential to development. Recent research in zebrafish completed at the University of California, San Francisco, has found that the very action of a beating heart is a signal for the formation of proper functioning valves. Apparently, blood flow sets certain cells on the trajectory to become part of a heart valve⁶. If true, this information could lead to the prevention of in utero defects⁷ caused by the interruption of the embryonic heartbeat in humans. The early days are crucial. Signals between cells increase, causing more cells to differentiate. The basic form of the body is laid down when, along with a primordial heart, the embryo develops structures that will become the future spinal cord and brain and small protuberances⁸ (called limb buds) where the limbs will be. The optic vesicle⁹, a slight depression where the eye and optic nerves will grow, rests in a cusp along the mushroom-like swelling that will become the cerebrum¹⁰. The embryonic stem cells have done much work in just a few weeks.

During the next four weeks, all the major organ systems appear. The embryo changes from a beanlike crescent¹¹ to a form with undeniably human characteristics. At the end of two months, the embryo has recognizable limbs and a heart, along with a brain, eyes, ears, and a nose. Perhaps the most astonishing accomplishment during this phase is the formation of the nervous system. Neural anatomy is not a single organ - it is an intricate system of organs and conduits¹² routed throughout the body. Following the fate of embryonic stem cells along one pathway in the laboratory shows how convoluted¹³ their journey can be.

The outermost germ layer, the ectoderm, eventually becomes part of the body's entire sensory and motor apparatus. An opening cascade of cell signals causes the topmost portion of the ectoderm to further diversify into specialized cells of the nervous system. Brain-related structures must be planned and executed simultaneously by other embryonic cells. For example, the brain needs a container to grow in, so genes in a different layer, the mesoderm, must coordinate the development of the skull. Other genes must coordinate the pace of growth so that the skull will develop at the same rate as the brain. As they gain mass through cell division, the brain and skull will require the support of muscles and tendons¹⁴ formed by different kinds of mesodermal cells.
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Source: Stem Cell Now by Ch. Th. Scott, A Plume Book, USA, 2006, pp. 29-31