Endocytosis of Sugars in Embryonic Skeletal Tissues in Organ Culture

Abstract

Cultivation of limb-bone rudiments in medium containing various non-metabolizable or poorly metabolizable sugars caused vacuolation of the perichondrial and articular cells and was accompanied by an increased synthesis and profuse secretion of lysosomal enzymes. Experiments with [14C] dextran indicated that the cytoplasmic vacuolation was due not to a higher rate of endocytosis, but rather to an abnormally long persistence of the pinocytotic vacuoles. Metachromatic material was lost from the cartilage only in the immediate vicinity of the vacuolated articular chondrocytes and the amount of hexosamine and hydroxyproline released into the medium was not much increased. However, the hexosamine and hydroxyproline of the sucrose treated rudiments was much more susceptible to extraction with neutral salt than those of paired controls. Sucrose taken up by the cells was liberated when the rudiments were returned to normal medium; this release, unlike the secretion of lysosomal enzymes, was unaffected by the presence of hydrocortisone. It is probable that the primary lysosornes, formed in the enlarged Golgi region, are the vehicles for the secretion of lysosomal enzymes. No appreciable amount of enzyrnic activity was released into the medium from rudiments grown in the presence of 0.08 M sucrose, before 36 h; at, or just before this time, however, a net increase in synthesis of enzyme was observed. The role of endocytosis in the resorption of skeletal matrix is discussed.