Quantitative Analysis of the Heart Development of Podocnemis unifilis (Podocnemididae, Testudines): A Stereology Study.

The present study investigated the cardiac development of the Amazonian turtle Podocnemis unifilis during its embryonic phase, from the 5th to the 59th day of incubation, with emphasis on the volumetric dynamics estimated by stereology. Development was organized into six morphological stages (E1-E6), from the folding of the heart tube to the formation of the prehatching heart. In the early stages, the heart showed a predominance of the myocardium (44%), mesenchyme (27%), and cardiac jelly (27%), which are components that undergo progressive regression during development. In contrast, in the advanced stages, expansion of the myocardium (up to 70%) and of the lumen (up to 27%) was observed, in addition to the late appearance of the atrioventricular valves (3%) and cartilage (1%). Analysis of the absolute volumes corroborated this finding, with an increase in the total heart volume from 0.015 mm3 (E1) to 1.33 mm3 (E6). Multivariate analysis (PCA) (i) clearly distinguished the heart components of the early stages from those close to hatching, (ii) statistically validated that the developmental stages represent distinct phases, and (iii) showed that the main change between them is the transition from a heart rich in jelly to a heart rich in myocardium and lumen. Together, the integration of embryological and stereological data provides a detailed quantitative reference of cardiogenesis in P. unifilis.