Ontogenia de Iguana iguana (Linnaeus, 1758) : estágios embrionários e desenvolvimento do esqueleto

Investigations addressing the ontogeny of vertebrates aim to standardize the description of a developmental sequence, supporting discussions about the animals evolution. Aiming to clarify the pattern of the ontogeny of Iguana iguana iguana, embryos artificially incubated at a constant temperature of 30°C (± 0.1°C) were collected daily (days 1-10) and at intervals of two days (day 10 to hatching), fixed in formalin solution 10% and then kept in alcohol 70% totaling 142 specimens. The sample was evaluated to determinate morphological features and was cleared with KOH and stained with Alizarin red and Alcian blue. Some specimens were subjected to scanning electron microscopy and the others through usual histology protocol. A normal development table was made with 17 stages (28 to 44, the initial development stages 1 to 27 occurs prior to oviposition), based on external characteristics such pharyngeal archs, brain vesicles, craniofacial features, limbs, scales, pigmentation, among others. For the description of the adult skeleton, we used two specimens that died of natural causes. The skeleton was prepared following standard methods and subsequently described. Skull, scleral ossicles, hyoid, vertebrae and limbs have similar structure to that of other lizards, albeit with subtle variations. Additionally we ascertained the chondrogenesis and the ossification of forelimb and hindlimb. The condensation of the proximal elements of these followed the described pattern of Tetrapoda: formation of the primary axis femur/humerus, radius/ulna and fibula/tíbia, from the radius/fibula comes the radiale/fibulare, later the distal element IV, the digit/toe IV, continuing the pre-post axial direction and forming the digital arc. In the forelimb the formation of the distal elements was observed for all five digits. The dcI remained fused to the mcI. I. iguana iguana has one central and intermedium element of carpus. These were formed by an independent condensing apart from digital arch and its ossification occurs in post hatching. The chondrogenesis of hindlimb was similar to those reported for other lizards. The presence of two distal elements of the tarsus is shared with most reptiles. It was not possible to ascertain clearly the astragalus origin. We observed a clouded condensation area in the central region of the tarsus that originated this element, but the presence of individual structures was not confirmed. Astragalus and fibular merged to form the proximal tarsal, which started to ossificate in the embryonic period (stage 42) by two individual ossification centers. The distal elements III and IV were the only ones differentiated, the others remained fused with metatarsal cartilage. The digit V presented late development in all embryos. The ossification of the limbs and vertebrae occurred as described for other reptiles. The ossification gradient in the vertebrae was cranio-caudal with formation of independent ossification centers to vetebrae center, vertebrae arch and ribs. In the thoracic girdle, the scapula ossified first followed by coracoid. The ilium was the first to ossify in the pelvis, followed by pubis and isquium. The incubation period was approximately two months, in a controlled environment. Using similar methodology, it was possible to approximate these results with data of other lizards, verifying many similarities, what confirmed the conservative pattern of embryonic development for reptiles.