Morphological effects of streptomyces hyaluronidase treatment on the outgrowth of the nasal processes in mouse embryos.

The role of hyaluronic acid (HA) in embryonic mouse nasal process outgrowth was assessed following administration of Streptomyces hyaluronidase, an enzyme that degrades HA. Enzyme-treated and control embryos were compared morphologically 4 and 24 hr after treatment on day 11 of gestation. After 4 hr the nasal processes of treated embryos were reduced in volume compared to controls. This size reduction was associated with a decrease in the amount of extracellular space in the nasal processes and a change in mesenchymal cell shape. Extracellular matrix material observed in controls included collagenlike fibers, 25-30-nm granules, and a delicate meshwork of 3-4-nm filaments. Basal laminae exhibited filamentous and granular material that extended to the surface of underlying cells. Similar matrix constituents were observed in treated embryos with the exception of the 3-4-nm filaments, which probably represent HA. By 24 hr after treatment, embryonic circulation had ceased and heart beat was slow. The nasal processes of these embryos were very small, but their configuration was such that fusion had often begun. Thus the presence of HA appears to be important in maintenance of the normal volume of the nasal processes and in maintenance of normal mesenchymal cell morphology, but other factors appear to contribute to the change in process shape requisite for fusion.