Contributions of early versus later generated cortical layers to the development of laminar patterns of ferret somatosensory cortex

The inside-out sequence of generation and formation of the cortical layers is well documented and has been observed in many species. While it is recognized that the early-generated components of cerebral cortex are essential for the formation of important architectural structures, the contributions of later generated cortical layers have not been completely addressed. This study examines the role of individual cortical layers in the development of the cortical laminae by disrupting the formation of specific cortical layers with the antimitotic methylazoxymethanol (MAM). Chapter 1 introduces the ferret as a model for studying key aspects of cortical development, and presents findings from birthdating studies that determined the timing of final mitosis and migration for cortical neurons in each layer of the ferret somatosensory cortex. The findings from these experiments were used to target the formation of specific cortical layers through timed in utero injections of MAM. Chapter 2 presents findings that layer 4 formation is disrupted after in utero injections of MAM on embryonic day 33 (E33). Treatment with MAM on E33 resulted in a greatly reduced thickness of layer 4, and reduced density of neurons within the remaining layer 4. Neuronal genesis resumed after E33 MAM treatment; neurons generated after the treatment migrated to appropriate locations and the overall laminar patterns of somatosensory cortex were preserved. Chapter 3 reports findings from experiments that further explore the ability of cortical cells to migrate in E33 MAM-treated cortex. Cortical precursor cells obtained from the ventricular zone of normal ferrets were transplanted into organotypic slices prepared from both layer 4-disrupted and normal ferret neonates. The transplanted VZ cells tended to migrate close to the cortical sites they would normally populate according to their date of birth. Chapter 4 presents results from studies that disrupted the formation of earlier generated layers of cortex with MAM treatment on embryonic day 24 (E24) or 28 (E28). In contrast to the E33 MAM treatment, E24 treatment severely disrupted formation of the cortical laminae. The cortical plate was much thinner compared to normals, and contained ectopic groupings of cells within the cortex. The disrupted cortical laminae were accompanied by distorted radial glial cells, and misplaced Cajal-Retzius cells. Further analysis suggests that interference with early development of neocortex leads to premature differentiation of radial glial cells into astrocytes, as demonstrated by the presence of glial fibrillary acidic protein (GFAP). A discussion comparing the relative contributions of early versus later-generated layers to the development of cortical lamination follows in Chapter 5.