Answer
I would explain to Marcus that the shaft (long middle part) of the humerus is called the diaphysis; that the the ends of shaft are called epiphyses; that the part of the bone ( wider part close to the ends is called the metaphysis, and that long bones continue to grow in length until about the age of 20 for men(16-18 for women). The elongation of bones is possible because in the metaphysis --between the epiphysis and diaphysis-- there is a thin layer of cartilaginous cells that retain their mitotic ability; this is the epiphyseal plate. As the diaphysis ossifies progressively, the epiphyseal plate continues to supply new cells near the ends of the bone causing it to continue to grow in length. At about the time of sexual maturity, this growth plate, too, begins to ossify and the cells lose the power to proliferate and cause the bone to elongate. Then we say the epiphyseal plates have closed; as a consequence, growth in length ceases.
Work Step by Step
Marcus' right arm is longer than his left because the epiphyseal plate in his left humerus was damaged and could not maintain the normal mitotic rate of the comparative growth plate in his right humerus. Marcus was only 14 at the time of his injury. I would explain to him that, absent the injury, the humeri in both arms should have kept growing at the same rate until about age 18-20 years. However , since the growth plate in the left humerus was damaged at age 14 it was not able to maintain the rate of mitosis of the right epiphyseal plate. Therefore, for four to six years the right epiphyseal plate outperformed the left epiphyseal plate and made his right arm longer than his left.
