Development of Caluromys philander (Didelphimorphia, Didelphidae) Foetuses with Estimated Ages of 20 and 25 days

Abstract

Background: Although Eutherian represents a higher-level taxon that includes all mammals except monotremes and marsupials, these two last groups especially the metatherians (marsupials) represent an important source of information on the evolution of the reproductive features in eutherian mammals. The aim of the present study was to describe the macroscopic and microscopic morphological features of Caluromys philander foetuses at two estimated developmental stages: 20 and 25 days of age. Methods: Twelve foetuses were obtained from two females and according to the Crown-rump lengths they were allocated in two Groups (Group I=about 20 days and Group II=about 25 days). The weight was also determined. After fixation in 70% ethanol or 10% formalin the specimens were examined by means of gross morphology and the external features were photo-documented. After that, the samples were dehydrated using increasingly concentrated ethanol solutions (70 to 100%), cleared in xylene, and embedded in paraplast. The paraffin blocks were sliced into 5 μm thick sections and the slides were stained using haematoxylin and eosin and Masson’s trichrome. Descriptive data were organised according to the body regions: cranial, thoracic, and abdominal. Results: Specimens from Group I showed more premature features, such as a more pronounced cervical curvature, than those in Group II. In addition, individuals from both groups exhibited more developed forelimbs than hind limbs. In relation to the cranial region, in both stages the retina was pigmented, but in Group II the ocular globe, the external ear and the eyelid projection were more prominent, as were the emergence of a row of hair on the eyelids and the presence of a darkened strip of hair between the eyes. Histological analysis showed that in Group II the skull bones were more calcified than in the Group I. In both groups, there was only a small opening in the mouth to allow for attachment to the nipples. The tongue was a well-developed organ and the onset of teeth formation was only visible in histological sections. Heart and lungs were observed in the thoracic region and they were composed by typical cells, structures, and layers. They were protected by the rib cage, which was undergoing ossification leading to rib formation. The organs inside the abdominal region mainly related to the digestory system (liver, stomach, and intestinal loops), as well as the kidney showed similar characteristics in relation to the stage of development and cellular composition between the two groups. The sexes in both groups were distinguished through the external genitalia. Conclusion: The results showed that at approximately 20 to 25 days of development, C. philander foetuses still present macroscopic features typical of preterm individuals. However, the vital organs required for survival in the external environment have formed and are able to perform their normal functions.