Fetus to adult: The development and evolutionary significance of pedal muscle variation.

摘要

Primate anatomists have traditionally focused on the identification of morphologies in various taxa, and the incorporation of that data (often in coded form) into a phylogenetic analysis. This approach is problematic, as sample sizes are quite small for most species, and morphologies (especially soft tissue traits) are known to vary greatly. An alternative approach is to use the available literature to document the variety of forms exhibited in primates. This information can then be used, on a case-by-case basis, to infer possible developmental mechanisms that could give rise to the morphologies observed in these organisms. This revised approach was adopted in the current study. The main goals were: (1) to document the range of variation in the muscles of the fetal human foot, (2) to compare this range of variation with that documented for adult humans and nonhuman primates, and (3) to interpret the development and evolutionary significance of muscle variation in the human foot, using nonhuman primates and other mammals as outgroups for comparison. The sample consisted of 50 human fetuses (25 males, 25 females) ranging in age from fourteen to nineteen weeks. The extrinsic and intrinsic muscles of the foot were dissected in the right and left limbs of each cadaver. The presence/absence of various anomalous muscles, variations in origins and insertions, and the number of bellies and tendons were recorded. These data were then compared with published reports of adult humans and nonhuman primates. Detailed descriptions of the fetal muscle anomalies are presented, and the clinical, functional, and evolutionary significance of these structures are discussed. On the whole, differences between adult and fetal frequencies (e.g., of absence/presence) were not statistically significant. In addition, the majority of anomalies occurred unilaterally in the fetal sample, indicating that local (non-genetic) factors play a key role in the development of lower limb musculature. In light of the comparative data, numerous hypotheses are offered regarding potential developmental mechanisms responsible for the evolution of the muscles in the human foot.