Abstract 480 Endocrinology I Poster Symposium, Saturday, 5/1

The growth plate is composed of three layers: resting zone, proliferative zone, and hypertrophic zone. We previously showed that the spatial polarity of these layers does not depend on signals from adjacent bone but instead is maintained by mechanisms intrinsic to the cartilage.

Here we explore another characteristic of growth plate cartilage, its spatial orientation. Proliferative chondrocytes are arranged in columns parallel to the long axis of the bone. This orientation is critical for longitudinal bone growth. The mechanisms maintaining this spatial orientation are not known. We hypothesized that resting chondrocytes produce a factor which diffuses into the proliferative zone and that the resulting chemical gradient determines the orientation of the proliferative columns.

To test this hypothesis, resting zone cartilage from the rabbit distal ulna was surgically transplanted from its normal position above the proliferative columns to an ectopic position alongside the columns. Ten days after surgery, histological examination revealed that the proliferative cells adjacent to the ectopic resting zone were arranged in columns perpendicular to their original orientation. As a control, hypertrophic zone was placed alongside proliferative columns; no shift in spatial orientation occurred. The shift in spatial orientation was also demonstrated in organ culture. Rabbit costal growth plates were excised and the anatomical relationships were similarly manipulated. After a one-week incubation in serum-free medium, resting zone induced a 90 degree shift in spatial orientation of adjacent proliferative columns whereas hypertrophic zone did not.

We conclude that resting zone cartilage determines the spatial orientation of adjacent proliferative columns. Our findings suggest that the resting chondrocytes secrete a "growth plate-orienting factor."