Printed in (‘.8.1. DISTRIBUTION OF MUCOSUBSTANCES IN THE DEVELOPING RAT HEART’

Mucosubstances (MS) were examined in 10 2-14 ’ rday embryonic rat hearts utilizing nonaqueous fixatives or formaldehyde vapor-fixed frozen sections hydrated in concen-trated 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 102 embryos principally contained a hyaluronic acid-like carboxylated mucosub-stance whose alcianophilia at pH 2.5 was abolished by hyaluronidase but was resistant to NaOH extraction and neuraminidase and trypsin digestion. A critical electrolyte concen-tration of 0.2 M MgC12 abolished alcianophilia. On days l3’---l4- 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 sur-faces and such staining was prevented by strong (60#{176}C)methylation. Hyaluronidase sen-sitivity of sulfated MS decreased with gestation. The critical electrolyte concentration varied from 0.5-0.7 M MgC12 on days 112-12 ’ 2 to 0.8-0.9 M MgCl2 after day 12 ’ 2. The sul