A Practical Approach, Second Edition=Ronald D. Ho.pdf

POSTNATAL DEVELOPMENTAL MILESTONES 991until 3 months. After 3 months, there is a gradual decrease in growth rate up to 6 months whenthe growth ceases. In later studies, Rudicel et al., (76) report that the length of the rabbit femurrapidly increases between 2 and 4 weeks of age. Growth rate begins to plateau at 8 weeks of ageand by 10 to 14 weeks of age growth essentially ceases.As is true in all mammals, the growth rate of long bones in the rabbit decreases with increasingage (77). When compared to rats and humans, rabbits have the highest longitudinal growth rate/day(77). The rate of longitudinal growth in the rabbit ages 20-60 days is greater than that seen in therat ages 20-60 days, and than that seen in humans ages 2-8 years (77). At 20 days old, the growthrate in rabbits and rats were 554 and 375 µm/day, respectively, when measured in the proximaltibias (77). The growth rate in humans (femur) at 2 years of age was 55 µm/day. By 60 days oldthe growth rate in the rabbit and rat had decreased to 378 and 159 µm/day, respectively. The growthrate in humans at age 5-8 years decreased to 38 µm/day (77).4.4 RatThe structure of the epiphyseal growth plate in the rat is similar to that of the human. In the rat,at about the time of weaning, the epiphyseal growth plate has been formed (67, 68).A rapid increase in length of each long bone of the fore and hind limb has been seen up untilthe rats reach the age of 8 weeks (47). The growth of the long bones in male and female rats slowsalmost to the point of stopping between the age of 15 to 17 weeks (47).5 MetaphysisThe metaphysis is a transitional region of bone that develops between the epiphyseal growth plateand the diaphysis during postnatal growth of the long bones. The structure and function of themetaphysis is similar among mammals (60, 78, 79, 64, 80). The metaphyseal region is one of theactive sites of bone turnover in the long bone of growing mammals (79, 55).In long bones, the metaphyseal region consists of an area of spongy bone directly beneath theepiphyseal growth plate. Together, the epiphyseal growth plate and the metaphysis form the growthzone (3). During postnatal bone growth and development, the composition of the metaphysischanges significantly. Early in development, calcified cartilage is the predominant hard tissue inthe metaphysis; however, as development progresses the metaphyseal tissue is converted to bonewith minimal amounts of cartilage (79).During postnatal development, the cancellous bone of the metaphysis is divided into twodifferent regions, the primary spongiosa and the secondary spongiosa (81, 79, 6). As a long bonegrows in length, the primary spongiosa fills the areas that were previously occupied by the growthplate (6). The primary spongiosa contains many osteoblasts and osteoprogenitor cells and ischaracterized by thin trabeculae made up of bone covered calcified cartilage. Capillary loops at thefront of the primary spongiosa, play a role in introducing osteoprogenitor cells that later formosteoblasts and osteoclasts (60).The tissue outside the primary spongiosa is called the secondary spongiosa. The secondaryspongiosa is mainly composed of bone with small amounts of calcified cartilage (79). As boneformation proceeds at the growth plate, most of the primary spongiosa is converted into secondaryspongiosa (6).Debates have been raised regarding whether rat long bone metaphysis is an appropriate modelto simulate human cancellous bone remodeling. In humans the two processes, mini-modeling andremodeling, are responsible for developing and maintaining cancellous bone mass and architecture.Mini modeling changes trabeculae by removing old bone from one surface and adding new boneon the opposite surface. Remodeling removes pieces of old lamellar bone and refills the space(lacuna) with new lamellar bone (78). Mini-modeling is the mostly occurs before skeletal maturitywhile remodeling occurs mostly after skeletal maturity has occurred.© 2006 by Taylor & Francis Group, LLC