ENDOCYTOSIS OF SUGARS IN EMBRYONIC SKELETAL TISSUES IN ORGAN CULTURE

SUMMARY Earlier experiments have shown that when limb-bone rudiments from 8£-day embryonic chicks are cultivated in medium containing an indigestible sugar, the perichondrial cells become intensely vacuolated, but this vacuolation does not appear in the chondrocytes, except in the articular region. Appleton's radioautographic method for demonstrating water-soluble materials was used to study the uptake, distribution and loss of 14 C-labelled sucrose in 8£-day limb-bone rudiments cultivated in a chemically denned medium (BGJ5). After 48 h exposure to ( 14 C)sucrose, both perichondrium and cartilage were heavily labelled. When such explants were either washed for 1 h in BGJ5 or cultivated for a further 24 h in non- radioactive medium, labelling was almost unaffected in the perichondrial cells but was greatly reduced in the cartilage, the cells of which were almost devoid of radioactivity. The presence in the medium of 0-08 M 'cold' sucrose did not alter the perichondrial grain counts in the washed explants. This finding supports the view that in the perichondrial cells sucrose is taken up non-specifically by endocytosis, and that the intense vacuolation of the cytoplasm is due to abnormal persistence of the pinocytotic vesicles in the presence of the sugar. It is concluded that failure of the chondrocytes to take up sucrose is due, not to lack of penetration of the sugar into the cartilage, but to a weak endocytotic activity of the cells.