Sulfated extracellular matrix production in the embryonic heart and adjacent tissues

The incorporation of S 35 ‐sulfate into tissues and extracellular matrix of chick embryos ranging in age from stage 8 (4 somites) to stage 12 (16 somites) was examined. 0.05 mc S 35 were added to embryos grown in vitro and after appropriate incubation periods the embryos were fixed and embedded in plastic. Radio‐autographs of 1 μ plastic sections of embryos between stages 8 and 9 (4 to 9 somites) showed heavy S 35 incorporation by all tissues, especially the notochord, neural fold and precardia. Between stages 10 (10 somites) and 12 (16 somites) the incorporation of S 35 decreases in all tissues examined except the myocardium. These results show that the production of sulfated extracellular matrix is not restricted to chondrogenic tissues in very young embryos, but that this function becomes selectively repressed as the embryo matures. This suggests that S 35 incorporation during cartilage production by chondrogenic tissues (such as notochord) in older embryos involves the re‐expression of an earlier embryonic trait, rather than the acquisition of a wholly new capability. The myocardium, on the other hand, continues to secrete a sulfated matrix even after the cells contain well developed myofibrils and become functional myocytes. Embryonic cardiac muscle is therefore a secretory as well as contractile tissue and the two “specialized” functions are not mutually exclusive.