An experimental study of tendon growth in the rabbit.

The post-natal growth of muscles and tendons is of interest both from the anatomical and from the orthopaedic point of view. But whereas much information is available on the growth alld especially the differentiation of muscle, tendinous tissue has been relatively neglected. Ill fact, current opinion about tendon growth is largely based on the analogy of muscle. Present knowledge of the development of muscle and tendon has resulted mainly from histological examination of material from young animals. Many authors have described the appearance of the end of a growing muscle belly where it joins the tendon, and the literature has been reviewed by Prenant, Bouin and Maillard (1911) and by Haggquist (1931). In the undifferentiated end of a muscle fibre there is a high concentration of nuclei and this is regarded as the region of most rapid muscle growth. Felix (1889) nevertheless believed that ill the foetus some growth occurs throughout the length of the muscle fibre. The high concentration of nuclei extends into the tendon near its junction with the muscle, and it has been assumed that this indicates a multiplication of fibroblast nuclei which must be accompanied by a rapid deposition of collagen fibres. It seems unlikely, however, that the end of tile tendon is the only site of fibroblast proliferation ; and it would be reasonable to expect that new collagenous tissue might be laid down throughout the whole length of the tendon, an(l a means of interstitial longitudinal growth thus provided. Nevertheless this is not a generally accepted opinion (Haines 1932). Mason and Shearon (1932), investigating the capacit ’ of tendinous tissue to form new collagenous fibres, divided the tendons of adult rabbits and found that although the greater part of the repair was accomplished by fibroblasts invading the lesion from without, some new collagen was laid down and organised into normal tendinous tissue by intrinsic fibrobiasts. They thus showed that even in the adult new tendon fibres can originate from cells lying along the length of the tendon. In dealing with a relatively slow dynamic process such as tendon growth, the examination of histological sections may be misleading. Additional sources of information should be sought, and a second possible approach to the problem is to examine living muscles and tendons in situ in the growing animal. Speidel (1938) observed the myomeres in the tails of living tadpoles over a long period and confirmed the hypothesis that the main site of muscle growth was at the end of the muscle fibre. He was not primarily concerned with tendon growth, but he recorded his impression that the growth of the tendinous part of the myomere also occurred mainly at its junction with the muscle. A third method of investigation, which forms the basis of this paper, is to mark tendons along their length in young animals and to observe whether the marks become separated during growth. This is an adaptation of Hunter’s fundamental experiments on bone growth. A similar method was used by Warwick and Wiles (1934) to study the growth of periosteum. They made marks with indian ink along the periosteum of young rabbits and measured the distance between the marks at the time of the operation and again after an appropriate period of growth. They were able to demonstrate that growth of periosteum occurs along the whole length of the bone and not merely at the region of the epiphysial cartilage. In this investigation the same method has been applied to the study of tendon growth. Small marks were made with indian ink along the length of various tendons in young rabbits. The distance between them was measured at the time of operation and after death two to five months later; and the amount of growth and its situation was determined.