DISTRIBUTION OF MUCOSUBSTANCES IN THE DEVELOPING RAT HEART

Mucosubstances (MS) were examined in 10½-14½-day embryonic rat hearts utilizing nonaqueous fixatives or formaldehyde vapor-fixed frozen sections hydrated in concentrated solutions of cetylpyridinium chloride. Ribonuclease-resistant, polyanionic sites were limited to the extracellular cardiac jelly, endocardium and fibroblastic cells (cushion tissue) associated with the endocardium. The cardiac jelly and endocardium of day 10½ embryos principally contained a hyaluronic acid-like carboxylated mucosubstance whose alcianophilia at pH 2.5 was abolished by hyaluronidase but was resistant to NaOH extraction and neuraminidase and trypsin digestion. A critical electrolyte concentration of 0.2 M MgCl 2 abolished alcianophilia. On days 13½-14½ carboxylated MS were restricted to cushion tissue and partially resisted mild methylation. Sulfated MS were limited to primitive endocardial cells which gave origin to cushion tissue. Dye deposits of aldehyde fuchsin, high iron diamine or Alcian Blue (pH 1.0) were localized on cell surfaces and such staining was prevented by strong (60°C) methylation. Hyaluronidase sensitivity of sulfated MS decreased with gestation. The critical electrolyte concentration varied from 0.5-0.7 M MgCl 2 on days 11½-12½ to 0.8-0.9 M MgCl 2 after day 12½. The sulfated MS of endocardial cells were preceded by a transitory accumulation of diastase-resistant, periodic acid-Schiff-positive material. Possible roles of MS in normal and abnormal cardiac septation processes are discussed.