VISUALIZING MUSCULAR AND SKELETAL ANOMALIES OF A TRISOMY 18 CYCLOPIC HUMAN FETUS IN AN EVO-DEVO CONTEXT WITH TRISOMY COMPARISON
Smith, Christopher M.
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The study of abnormal development, such as trisomy syndromes, allows us to explore the mysteries and mechanisms of normal evolutionary/developmental (evo/devo) processes, and provides insight into how morphology changes throughout evolution. One theory suggests that abnormal morphology can be predicted due to internal constraints, i.e. that there is a “logic” in normal and abnormal evolution and development, because there is a limited range of outcomes for a given set of developmental perturbations (Alberch 1989). An alternate theory, “homeostasis”, states that abnormal morphology is often random and that disorders result in overall chaotic effects throughout the body (Shapiro 1983). I tested these theories through the dissection of a rare 28-week human fetus with trisomy 18 (extra copy of chromosome 18) with cyclopia (abnormal craniofacial structure resulting in fused eyes). I examined the muscular origins and insertions in the head, neck, and limbs and discuss my results in the context of other evo/devo theories to broadly test if the patterns observed support the “logic” or “homeostasis” hypotheses. Prior publications on trisomy anatomy lack clear visualization. Photographs of dissections and simple line drawings are occasionally included, but neither adequately convey the three-dimensional structures. My 46 color and seven black and white illustrations fill this gap in visualization by presenting my results clearly, with enhanced form, allowing the viewer to better understand the subtleties of the abnormal musculature when compared to normal anatomy. This thesis expands research on skeletal muscle identity and attachments, by extending documentation into head and neck muscles in human trisomies. I compared these muscular arrangements with those found in other individuals with trisomy 18, 13, and 21. My prediction was that muscle attachments in the head of the human fetus with cyclopia were primarily constrained by developmental anlage (commonly accepted today), in contrast to determination by topological position as previously shown in the limbs. However, this prediction was contradicted for the muscles of facial expression. My results show a remarkable finding that sheds light on the development of muscles of facial expression and addresses how the study of both head and limbs could be integrated in developmental and evolutionary biology.