The time of appearance and the relative activity of isozymes of LDH were studied in portions of the skeleton of the embryonic and newly hatched chick by starch-gel electrophoresis. The following tissues were examined for presence or absence of isozymes: mesenchyme of the hind limb as it separated into chondrogenic and myogenic tissue; the development of the cartilaginous model of the tibia and its subsequent replacement by bone; the development of the quadratojugal (a membrane bone) with and without secondary cartilage.

LDHX and LDH2 were present in all tissues from the earliest time at which the tissues could be detected histologically. Thus these isozymes are ubiquitous. They were also the predominant isozymes during the first and second weeks of development of all the tissues and continued to predominate in the quadratojugal after hatching.

LDH3, LDH4, LDH5 appeared after the initial differentiation of the tissues, arose at earlier ages in cartilage than in bone, and became the predominant isozymes earlier in cartilage (14 days of incubation) than in bone (20 days of incubation).

These patterns of isozymes were related to metabolic activity of the developing tissues, especially to aerobic and anaerobic glycolysis.

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