Skeletal Muscles in Fetuses

There are few reports comparing the muscle fiber morphometry between human fetuses and adults. Data on fetal and adult muscle fibers would be valuable in understanding muscle development and various muscle diseases. This study investigated human muscle fiber growth to clarify the difference between the facial muscles and skeletal muscles. The specimens were obtained from three (6-month-old) male fetuses and 11 Japanese male cadavers between the ages of 43 and 86 years (average: 71.8 years). Human orbicularis oculi, orbicularis oris, and buccinators muscles (facial muscles) and masseter and biceps brachii muscles (skeletal muscles) were resected. We counted the muscle fibers and measured their transverse area. We also calculated the number of muscle fibers per mm2 (NMF) and the average transverse area of the muscle fibers (TAMF). The average NMF in the orbicularis oculi, orbicularis oris, buccinator, masseter, and biceps brachii muscles in fetuses was respectively, 35, 31, 19, 37 and 22 times greater than in adults. The average fetus/adult ratio of the TAMF in the orbicularis oculi, orbicularis oris, buccinator, masseter and biceps brachii muscles was 5.0%, 4.5%, 4.0%, 2.4% and 4.1% respectively. The average NMF in all of the muscles decreased after birth; however, the maximum NMF and decrease with the natural aging process it varied among muscles examined. The average TAMF for all of the muscles increased after birth. We concluded that the increase in the TAMF is related to the emergence of fetal movements and functional demands after birth. Chapter 1