Periosteal structure and development in a rat caudal vertebra.

Female Sprague-Dawley rats from birth to 300 days were used to study the bone/soft tissue interrelationships of the 14th caudal vertebra with particular emphasis on the periosteum throughout growth, development and maturation. The growth of the rats follows a sigmoid curve with three phases, a developmental, a rapid growth and a maturation phase. The width/length ratio of the bone and the thickness of the periosteum are closely concurrent, with a rapid decrease during the developmental phase and a levelling off during the rapid growth phase. SEM studies established that the caudal vertebra has symmetrical lateral sides and a pronounced concavity on the ventral surface where the main vascular plexus is located. Morphological changes in the periosteum cna be described as occurring in three layers and reflect the stages seen in general somatic growth. The inner cambial layer initially contains elongated but functional osteoblasts; these become cuboidal during the rapid growth phase and ultimately are flattened and quiescent. The mid-zone with its vessels, undifferentiated and mononuclear phagocytic cells also attains its maximum development in the rapid growth period and then gradually involutes. The fibrous periosteum consists of a syncytial arrangement of fibroblasts in a collagenous matrix which becomes increasingly dense although reduced in width. Sharpey fibre bundles connect the bone with the fibrous periosteum and these become thicker with age. The mid-zone of the periosteum has not been described previously. Besides having a nutritive role and providing progenitor cells it is thought to act as a buffer modulating the interaction between bone and the covering soft tissues. With age and the deletion of the mid-zone a less sensitive periosteal response to stress can be expected.