Table of Contents
The muscles, connective tissues, and skeleton arise from the mesoderm in the embryo at about two weeks of gestational age. The skeleton’s first form is as fibrous membranes (the bones of the cranium) or cartilage.
Through a process called ossification or osteogenesis, cartilage cells (chondrocytes) convert to bone cells (osteoblasts, osteocytes, and osteoclasts).
This early ossification uses the fibrous membrane (called intramembranous ossification) or the cartilage skeleton (called endochondral ossification) as a mold or template. Bone cells replace the connective tissue cells to form the bone matrix.
Musculoskeletal Changes With Advancing Age
Areas of specialized bone tissue called secondary ossification centers form in the long bones; these become the epiphyses or growth plates. After birth the epiphysis extends through the growth of cartilage, which ossification then replaces as bone. The process extends through nearly the first two decades of life. Disorders of ossification include achondroplasia, marfan syndrome, acromegaly, and osteogenesis imperfecta.
Muscle structures gain definition, mass, and strength as growth occurs. By four months of age a healthy infant can support his or her head and at about six months can sit unsupported and roll over from front to back or back to front. Between 8 and 12 months, an infant begins to crawl, throw things, and pull into a standing position.
By 14 months most infants are walking on their own, and by 18 months can run and jump. Motor skills—the ability to use the musculoskeletal system for mobility—continue to evolve throughout childhood. These skills, along with muscle mass and bone density, peak in the late 20s.
Musculoskeletal changes in women and men
By age 40, musculoskeletal structures and functions begin to decline. Joints begin to show the effects of wear. One health consequence of this is osteoarthritis, which can become severe enough to warrant joint replacement.
Softening of the ligaments and other connective tissues makes joints more vulnerable to injury. Muscle mass and bone density gradually decrease, as does strength and flexibility. In women these decreases become dramatic with menopause, with the sudden and significant decline in estrogen. (Estrogen is one of the hormones that influences the movement of calcium between the blood circulation and the bones.) The rate of decrease remains fairly constant in men, who have inherently larger amounts of muscle and bone.
However, by about age 75 or 80 gender differences balance out. Men and women alike have significantly less muscle tissue and bone structure, increasing susceptibility to injury from falls and other accidents. Though bone remodeling continues, it proceeds at a much slower pace.
Other changes in the body may result in bone resorption outpacing bone rebuilding. The risk for neuromuscular disorders such as Parkinson’s disease also rises. At age 80, a woman may have lost four inches or more of her height as a consequence of musculoskeletal changes. Men also lose height, though typically not as dramatically.
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