X-Ray Growth Zone Studies in the Rat Tail for the Appraisal of Chondrotropic Effects

Abstract

Indices of growth commonly are determined by the size and weight of the experimental animal without reference to the specific components contributed by the separate organs or tissues of the body. Growth hormones in the past have been appraised by their effect on body weight, whereas better standardization might have been obtained by the use of tables based on observations of the “growth mechanism” of bones. The effect of hypophysectomy, causing cessation of cartilage plate proliferation, had been recognized, but not until it was re-emphasized by such observers as Freud, Levie, and Kroon (9), was it made clear that the terms “growth hormone” and “chondrotropic hormone” should be used synonymously. The histologic study of chondrotropic effects in bone growth zones can be a long or a tedious procedure, while the x-ray examination of growth acceleration or arrest, without sacrifice of the growing parts to histologic study, can be accomplished rapidly. If both methods were to be combined, as Guerrant and Dutcher (10) actually have done in their “line test” examinations of rat tails, perhaps the optimum amount of information could be obtained. Levie (12) used the roentgen examination of the rat's tail for studying bone growth, but his standards and growth tables leave much to be desired. By a thorough reinvestigation of this method, it was our hope that a more reliable x-ray index of normal bone growth could be established as a reference for the assay of growth hormones or for the appraisal of abnormal bone growth due to any cause. Method Accurate measurements of tail length were made directly from the x-ray film; this was possible because the rats were placed in the supine position with the tail next to the film to prevent distortion. To get the best detail, the exposure time was reduced to 1/20 of a second, and finegrain screens, a small cone, and a filmtarget distance of 48 inches were used. Some exposures were made without screens, but these films were not found superior enough in detail to make up for the advantage of better contrast offered by the use of screens. Of prime importance in the demonstration of all the growth zone details are films which have the proper density of exposure. The reproductions in this paper, particularly Figure 3 show the characteristics of both properly and improperly exposed films. Errors of overexposure, if not too great, can be compensated by variations in the intensity of light in the viewing apparatus, whereas even slight degrees of underexposure result in films which fail to reveal the finer details in structure and outline of both bone and cartilage plates. In counting the number of tail segments, the first proximal segment was designated as that one just distal to a line drawn through the lowermost margins of the ischii. The sacro-caudal segments in this area often appear slightly distorted because of the hyperextension which occurs at this level of the spine when the rat is held supine against the film.