WKI Sales Training Feb 2014

62 Part 1 General Embryology

by cells in the node and streak, induces Nodal and LEFTY-2 expression on the left side and these genes upregulate PITX2 , a transcription factor and master gene for left-sidedness (Fig. 5.6).The neurotransmitter serotonin (5HT) also plays a role as a signal molecule upstream from FGF8. Disruption of 5HT levels or misexpression of PITX2 results in laterality defects, such as dextro- cardia, situs inversus, and cardiac abnormalities. Epiblast cells moving through the node and streak are predetermined by their position to become specific types of mesoderm and endo- derm.Thus, it is possible to construct a fate map of the epiblast showing this pattern (Fig. 5.7). By the end of the third week,three basic germ layers , consisting of ectoderm , mesoderm , and endoderm , are established in the head region, and the process continues to produce these germ layers for more caudal areas of the embryo until the end of the fourth week.Tissue and organ dif- ferentiation has begun, and it occurs in a cepha- locaudal direction as gastrulation continues. In the meantime, the trophoblast progresses rapidly. Primary villi obtain a mesenchymal core in which small capillaries arise (Fig. 5.12). When these villous capillaries make contact with capillaries in the chorionic plate and connect- ing stalk, the villous system is ready to supply the embryo with its nutrients and oxygen (Fig. 5.13).

Problems to Solve 1. A 22-year-old woman consumes large quantities of alcohol at a party and loses consciousness; 3 weeks later, she misses her second consecutive period.A pregnancy test is positive. Should she be concerned about the effects of her binge-drinking episode on her baby? 2. An ultrasound scan detects a large mass near the sacrum of a 28-week female fetus.What might the origin of such a mass be, and what type of tissue might it contain? 3. On ultrasound examination, it was deter- mined that a fetus had well-developed facial and thoracic regions, but caudal structures were abnormal. Kidneys were absent, lumbar and sacral vertebrae were missing, and the hindlimbs were fused.What process may have been disturbed to cause such defects? 4. A child has polysplenia and abnormal posi- tioning of the heart. How might these two abnormalities be linked developmentally, and when would they have originated? Should you be concerned that other defects might be present?What genes might have caused this event, and when during embryogenesis would it have been initiated?

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