The human brain is characterized by an extremely developed cortex, which contains 170 billion cells. The number of CNS neurons and glial cells decreases with physiological aging due to a progressive decrease in the regenerative capacity of progenitor cells. Since the process of neuroregeneration is biologically very similar to the developmental processes, studies of cortical development in animal models represent a promising approach in regenerative medicine. A laboratory model of the opossum, Monodelphis domestica possesses the ability to regenerate the CNS during the first 2 weeks of life and is therefore considered extremely suitable for neuroregenerative studies. Cellular characterization and localization of the postnatal opossum are insufficiently investigated, and the only available data on the total number of cells and neurons is that quantified at the age older than a postnatal day (P) 18. Using the isotropic fractionator (IFR) method and fluorescence microscopy, we quantified the number of neuronal and “non-neuronal” cortical cells of different postnatal opossums aged 6, 18, and 34 days. We confirmed the time period in which the neurogenesis process (P18) occurs, and the completion of which is succeeded by the gliogenesis process (P18 – P34). With the IHC method and fluorescence microscopy, we localized heterogeneous cell populations of the opossum cortex of postnatal days 6, 17, and 30. We used immunohistochemical markers SOX2, SOX9, GFAP, and NeuN which indicated different localization and cell migration during corticogenesis. The SOX2 stem cell marker and the SOX9 immature astrocyte marker showed a localization at the ventricular zone. We observed migration of SOX9 + cells toward the outer cortex at P30. The GFAP marker indicated the localization of mature astrocytes at P30 throughout the cortex.