RESUMO
In early theropod dinosaurs-the ancestors of birds-the hallux (digit 1) had an elevated position within the foot and had lost the proximal portion of its metatarsal. It no longer articulated with the ankle, but was attached at about mid-length of metatarsal 2 (mt2). In adult birds, the hallux is articulated closer to the distal end of mt2 at ground level with the other digits. However, on chick embryonic day 7, its position is as in early theropods at half-length of mt2. The adult distal location is acquired during embryonic days 8-10. To assess how the adult phenotype is acquired, we produced fate maps of the metatarsals of day 6 chicken embryos injecting the lipophilic tracer DiI. The fates of these marks indicate a larger expansion of the metatarsals at their proximal end, which creates the illusory effect that d1 moves distally. This larger proximal expansion occurs concomitantly with growth and early differentiation of cartilage. Histological analysis of metatarsals shows that the domains of flattened and prehypertrophic chondrocytes are larger toward the proximal end. The results suggest that the distal position of the hallux in the avian foot evolved as a consequence of an embryological period of expansion of the metatarsus toward the proximal end. It also brings attention to the developmental mechanisms leading to differential growth between epiphyses and their evolutionary consequences.
Assuntos
Evolução Biológica , Dinossauros/anatomia & histologia , Pé/embriologia , Ossos do Metatarso/embriologia , Adaptação Fisiológica/fisiologia , Animais , Embrião de Galinha , Ossos do Pé/embriologiaRESUMO
Forty seven skeletons belonging to human fetuses, whose age ranged from eight to thirty eight weeks, were studied in order to establish the beginning of ossification in the bones. Red alizarin was employed in order to stain bone calcifications. Our observations agree with other authors considered as references nonetheless, we also have found differences among the beginning and the sequence of development of some of the primary centers of ossification in the fetal foot. The appearance of the ossification centers was as follows: 1) the five metatarsals: 9th. week; 2) distal phalange first digit; 11th. week; 3) a) proximal phalange 1, 2, 3, 4, 5 digits: 12th. week, b) distal phalange 2, 3, 4, 5 digits: 12th. week; 4) middle phalange second digit: 17th. week; 5) a) middle phalange fourth digit: 18th. week, b) middle phalange third digit: 18th. week, y 6) middle phalange fifth digit: 21th. week.