Developmental plasticity of the placenta in the caviomorph rodent Microcavia australis

The diversity of placenta in hystricomorph rodents provides key insights into the evolution of reproductive strategies.

Here, we describe the placenta of the caviomorph rodent Microcavia australis (cuis chico) at three stages of gestation, with emphasis on morphological implications. Placentas from a mammalogy collection were used, processed for paraffin embedding, and histological, histochemical, and lectin histochemical stains were used for description. Morphometrics were also performed, but for descriptive purposes.

The chorioallantoic placenta was organized into trophospongium and labyrinth, the latter arranged in lobules that increased in number towards near-term gestation. This progressive expansion may be related to the increasing physiological demands associated with the development and growth of precocial offspring. A hemomonochorial maternal-embryo/fetal interface and a dual maternal-fetal vascular system was identified, becoming more closely associated as gestation progressed. Peripheral maternal blood chambers and a prominent Reichert's membrane were observed, suggesting a potential structural role during increasing hemodynamic demands. The subplacenta displayed a lobular organization and showed ontogenetic changes in vascularization. Multinucleated cells consistent with macrophage fusion were identified in remodeling areas. Notably, trophoblast was observed within a uterine gland, compatible with an endoglandular phenotype not previously reported in hystricomorph rodents.

These findings suggest that trophoblast invasion patterns in hystricomorph rodents are more evolutionarily labile than previously assumed. The decidua exhibited marked cellular heterogeneity across gestation. These findings reveal both conserved and species-specific features of placentation in M. australis, highlighting the role developmental plasticity in the evolution of reproductive strategies in hystricomorph rodents.