Morphogenic and Immunocytochemical Studies on Secondary Neurulation in the Mouse Embryo: Localization of Fibronectin and Laminin
Murray, Susan M.
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The morphological changes involved in secondary neurulation, and the deposition and distribution of fibronectin and laminin during this process, were examined in mouse embryos (35-40 somites) using routine and immunocytochemical transmission electron microscopy. Secondary neurulation, which results in the formation of caudal regions of the neural tube (the precursor of the adult brain and spinal cord), occurred by a mechanism different from that observed in cranial levels. Following closure of the posterior neuropore, dorsal mesenchymal cells of the tail bud aggregated and subsequently cavitated to form a single lumen in continuity with the primary neural tube. This process occurred sequentially in a craniocaudal direction. Cells surrounding the lumen underwent further differentiation to form the pseudostratified neuroepithelium typical of the neural tube. Immunocytochemical techniques allowed limited localization of fibronectin and laminin, glycoproteins which may be involved in the mesenchymal-epithelial tissue type transition which occurs during secondary neurulation. Fibronectin appeared to be localized along cell processes and at junctions between cell processes and adjacent cells. Laminin also appeared to occur at such junctions, and in some areas of the extracellular matrix in the region of the basal lamina. Observed localization may suggest the involvement of fibronectin and laminin in cell aggregation, cavitation, or establishment of the basal lamina, which occur during secondary neurulation.