A quantitative analysis of bone tissue formation in different regions of the spongiosa in the dog skeleton.

An investigation has been carried out on the relationship between the extent of bone tissue formation and the architecture of the "tubular" and "lamellar" components of the spongiosa within shaft bone ends, the anterior and posterior epiphyses and the intermediate portions of vertebral bodies in 3 dogs (aged 6 months, 2 and 3 years) treated for 3 consecutive days with tetracycline. The ratio of the extent of labelled surfaces to the sum of labelled plus unlabelled ones indicates the bone formation rate, referred to as osteogenic activity (OA). In the growing animal, a higher osteogenic activity is displayed by the "tubular" spongiosa, which lies peripherally in the metaphyses of shaft bones and consists of strong trabeculae nearly parallel to one another, and apparently arranged according to the distribution of mechanical stresses. In the adult dogs, the relatively higher osteogenic activity recorded in "lamellar" spongiosa appears to depend on a marked reduction in the osteogenic activity in the "tubular" spongiosa. In the vertebrae, osteogenic activity is higher in the central portion than in extremities solely during the period of growth in length: in the adult, the value of osteogenic activity was found to be practically uniform throughout the vertebral body. The "tubular" spongiosa, therefore, would be more actively involved in the remodelling of the metaphyses while the bone increases in length; the same would occur in the intermediate portion of the vertebral body. The reconstruction of the "lamellar" spongiosa, presumably less involved mechanically than the "tubular" spongiosa, would rather be an expression of bone mineral metabolism. A close relationship between spongiosa structure and osteogenic activity would occur at the ends of long bones and within the vertebral body.