The influence of stress and strain in the early development of shaft bones. An experimental study on the chick embryo tibia.

In chick embryos from stage 23 to stage 27 the whole presumptive zeugopod - or its pre-axial (tibial) portion only - was proximodistally and dorsoventrally inverted by turning it 180 degrees round the anteroposterior axis of the limb bud. Development of the reoriented blastema of the tibia was consistently retarded and variously reduced: this skeletal piece appeared shorter and relatively thicker than the controlateral normal tibia. Chondrification, progress of differentiation of the cartilaginous model, onset and gradual spreading of the ossification processes were considerably delayed. Often the diminutive tibia underwent a degree of bending or angulation of up to 90 degrees - 100 degrees in the sagittal plane. In these bent tibiae - obviously developing under abnormal conditions of intrinsic and extrinsic mechanical stresses - cell hypertrophy appeared greatly retarded or hindered in sites of the diaphysial cartilaginous core which were presumably subjected to strong longitudinal compression. No rigorous temporal and topographical relationships were observed between chondrocyte hypertrophy and onset of perichondral osteogenesis. Apparently, a direct contact between hypertrophic cartilage and perichondrial cells was not strictly required to prompt osteogenesis; this process, in fact, often involved areas of the perichondrium enveloping parvicellular cartilage. Radial pressures exerted by the over-stretched outer layer of the perichondrium, or periosteum, on the subjacent prospective osteogenous layer reduced or prevented the deposition of bone. Conversely, radial stretching of the inner layer of the perichondrium, or periosteum, considerably enhanced cell proliferation, blood vessel formation, differentiation of osteoblasts and formation of bone matrix.