|MadSci Network: Anatomy|
Newborns do have kneecaps, Cathy. Kneecaps form about the fourth month of fetal life. However, they donít show up on x-ray very well because theyíre not ossified, or bony. At this point in life, the kneecaps are made of a cartilaginous material. The growth centers surrounding the kneecap form late in developmental life in utero and may not appear until just before or just after the infant is born.
Remember, infants are a work in progress. The potential for linear bone growth may continue until the late teens or early twenties. Although all the precursor tissues for the major bones are present at or immediately after birth, centers of ossification (where bone is laid down) continue to develop throughout childhood and beyond. For instance, the head of the femur appears at four months, the patella, or kneecap, starts showing signs of ossification at about 3 years in females and 4-5 years of age in males. Parts of the pelvic girdle (hips) donít appear ossified until adolescence with the tubercle of the pubis not appearing until 18-20 years of age.
Why does this progressive development happen? As usual, we donít know. There are clues that can lead to some speculation. When raised in tissue culture, the fetal tissues in question will form as cartilage, but will not ossify into bone. It is only with the presence of weight-bearing forces (along with the presence of chemical mediators) that these tissues ossify. In an article in Scientific American in 1995, researchers suggest that the reason that biological change happens is to promote the widest spread of DNA. In an evolutionary sense, it means the individual survives to reproductive age and can have children ("spread of DNA"). If creatures spent good energy ossifying bones before they were needed to bear weight, less energy would go to other developmental processes such as bodily growth or brain development, things that in the long run would be more likely to enhance the survival of the individual.
Imaging of the Newborn, Infant, and Young Child. Third edition, Leonard E. Swischuck, MD, 1988.
Lovell and Winterís Pediatric Orthopedics, 4th edition, Raymond T. Morrissy, MD and Stuart L. Weinstein, MD, 1996.
Diseases of the Fetus and Newborn, 2nd edition, edited by Reed et al.,1995.
Pediatric Orthopedics, 2nd ed., Mihran O. Tachdjian, M.S., M.D.,1990.
Scientific American, "Godís Utility Function" and "The Great Leveler," by Richard Dawkins, pp. 80-85. November, 1995
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