Tenascin-C lines the migratory pathways of avian primordial germ cells and hematopoietic progenitor cells

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Abstract
Tenascin-C is a large hexameric extracellular matrix glycoprotein associated with epithelial-mesenchymal interactions, connective tissue development, and the formation of the central nervous system. Tenascin-C also lines the pathways followed by migrating avian neural crest cells, although its role in neural crest morphogenesis remains unclear. In vitro, tenascin-C interferes with cell-fibronectin interactions, and promotes the motility of many cell types including the neural crest. To determine if tenascin-C is a consistent component of matrices through which invasive embryonic cells migrate, we have investigated if tenascin-C is associated with 2 additional populations of motile, embryonic cells: primordial germ cells and hematopoietic progenitor cells. We have found that HNK-1, a monoclonal antibody used as a marker of neural crest, also stains avian primordial germ cells. Double-label immunohistochemistry reveals that tenascin-C is found in the mesenchyme adjacent to the ventral half of the dorsal aorta where the primordial germ cells penetrate the vessel wall, and both tenascin-C and fibronectin are present in the extracellular matrix through which the primordial germ cells migrate to reach the genital ridges. Unlike fibronectin, which is found throughout the splanchnic mesoderm, tenascin-C is concentrated in the proximal part of the splanchnic region where the primordial germ cells are concentrated. In embryos where the gonadal anlagen are surgically removed before the primordial germ cells leave the bloodstream, ectopic primordial germ cells were found exclusively in head and trunk mesenchyme containing tenascin-C. Like primordial germ cells, a subset of hematopoietic progenitor cells migrate through the mesenchyme ventral to the dorsal aorta where they form hematopoietic clusters. Others bud directly into the lumen of the aorta. Anti-tenascin-C stains the mesenchyme surrounding the migrating cells as well as the basal surfaces of the cells that appear to be budding into the lumen. In situ hybridization with a tenascin-C-specific cDNA probe shows that the major sources of the tenascin-C mRNA in this region are the hematopoietic progenitor cells themselves as well as the cells in the wall of the ventral aorta. mRNAs encoding 3 major splice variants of tenascin-C were identified by reverse transcriptase polymerase chain reaction (PCR) in the embryonic aorta and adjacent mesenchyme dissected from both the region of primordial germ cell and hematopoietic precursor cell migration. These experiments indicate that tenascin-C is a component of the migratory environment for many motile cells in the early embryo, where it has the potential to mediate cell-fibronectin interactions.