Proteoglycan-collagen relationships in developing chick and bovine tendons. Influence of the physiological environment.

Developing and mature flexor digitorum tendons of chick and bovine origin were analyzed for nucleic acid, hydroxyproline, hyaluronate, chondroitin sulfate and sulfated glycosaminoglycan hexuronate. Collagen fibril diameters were determined by electron microscopy. Results are compared with similar data from rat tail tendon. Tissue hydroxyproline (mg/g) increased rapidly in early embryogenesis, while collagen fibril diameters remained relatively constant and small. Hyaluronate and chondroitin sulfate were present in considerable quantities, (phase I). A rapid increase in fibril diameters in midterm 'pregnancy' coincided with a decrease in hyaluronate and chondroitin sulfate contents (phase II). The transition from phase I to phase II was associated with the first loading by active muscles of the tendon, independently of whether the animal was pre- or post partum. The probable existence of a developmental program intrinsic to tendon itself, is discussed. The data are compared with predictions from three different simple models. Hyaluronate is concluded to be entirely interfibrillar, chondroitin sulfate is probably similarly distributed, whereas dermatan sulfate is entirely fibril-surface associated. The 'small' dermatan sulfate proteoglycan is assigned to the 'd' band in the gap zone.