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| |    The Developing Fetus, Weeks 9-24 by Robert Needlman, M.D., F.A.A.P. reviewed by Marjorie Greenfield, M.D. and Laura Jana, M.D., F.A.A.P. As the developing fetus' cells divide and divide again, growth occurs more and more rapidly. Clusters of cells are "remodeled" -- that is, they take on new shapes and functions. Each organ system has its own fascinating history. The intestines The intestines, which start out in the umbilical cord, go through a series of complicated twists and turns, as they fit themselves back into the abdominal cavity. The lungs The breathing tubes in the lungs grow out like limbs on a tree, branching into ever smaller divisions. These branches finally end in the tiniest alveolar sacks where the real work of absorbing oxygen can take place. It's not until after about 24 weeks that there are enough alveoli around for the infant to have a chance of breathing air. So about 23-24 weeks is the practical lowest age at which a premature infant has a chance of survival (although with medical advances this limit is being pushed back every year!). The heart The heart begins as a simple tube, then folds, pinches itself off into four chambers, makes valves, and develops its own set of nerve-like cells to coordinate its rhythmic beating. While it's doing this, it's twisting in the chest cavity, while certain blood vessels give off branches to the lungs and the body, and other vessels close off and disappear. The brain Some of the most fascinating developments during this period are occurring in the brain. By week 9, the major structure of the brain is in place. But more and more cells continue to develop at an astonishing rate. As the cells develop, they send out long extensions called axons that make connections with other cells. Some of these cells are near at hand, others may be in a completely different region of the brain. Groups of axons bunch together, for example, to send streams of information from areas of the brain stem up to the frontal lobe. Each axon can branch into many finer twigs, and each of these can connect with a different cell. The connection between cells take place at synapses. The cells don't actually touch, but nearly so. In the tiny gaps between cells, a flow of chemical messengers, called "neurotransmitters," carries signals from one cell to the next. There are literally billions of cells in the brain, and trillions of synapses. We know that a fairly precise "architecture" of nerve cells is necessary for normal brain development, but are only beginning to understand how that structure gets built. Because it is so complex, and is built over such a long period of pregnancy, the brain is especially vulnerable to potentially harmful influences from outside.
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