Teratogensis: Caudal Dysgenesis ( sirenomelia)
The third week of development is the period of greatest sensitivity for teratogensis (Sadler 2010). In caudal dysgenesis (sirenomelia) genetic abnormalities and toxins disrupt normal gastrulation. A more common name for sirenomelia is the mermaid syndrome. Sirenomelia is characterized by insufficient mesoderm formed in the caudalmost region of the embryo. The mesoderm contributes to the formation of the lower limbs, urogenital system and the lumbosacral vertebrate. Defect varies from simple cutaneous fusion of the limbs to absence of all long bones but one femur. The defect of the feet is proportional to the defect of the long bones, with cutaneous defect commonly presenting a double fused foot with ten toes, and more severe defect present with a more rudimentary foot and ectromelia. Thus, the fibulae when present are between the tibia and the sole of the foot is oriented “venturally” instead of “dorsally” (Sandra R. Silva, MD & Philippe Jeanty, MD, PhD 1999). Sirenomelia is thought to occur in about 1 in 65,000 livebirths (Oxford Medical Database; Dysmorphology). Sirenomelia may be caused by abnormalities in blastogenesis that affect the distribution of blood to the caudal region of the fetus. The “vascular steal theory” indicates that a single large artery assumes the function of the umbilical arteries, thus diverting blood flow from the caudal portion of the embryo to the placenta (Bibhuti B Das et al 2001). The presence of paired femurs indicates that the condition is probably caused by failed lateralization of the lower limbs, arising at approximately 4 weeks. When single large umbilical artery is observed in direct continuation with the abdominal aorta, it is classified as a vascular anomaly. This type of vascular anomaly is considered a remnant of the vitelline artery complex and is almost always associated with sirenomelia (Haider 20001). Sirenomelia is fatal due to severe pulmonary hypoplasia and...
Please join StudyMode to read the full document