Musculoskeletal Key

Most of the time, the sarcomere is in a state of relaxation. Because it is longer than a thick filament, there is a region at either end of the sarcomere…

Each muscle fiber is invested by a thin layer of connective tissue called the basal lamina, or basement membrane. It is now believed that the basement membrane contains molecules important…

AMPHIOXUS The extant adult amphioxus, or lancelet, is considered to resemble an ancient ancestor of the vertebrates (see Plate 1-1). It is a fishlike animal, about 2 inches long, that…

In the formation of membrane bone, individual shafts of bone, known as trabeculae, are laid down (see Plate 1-10). Trabeculae increase in length and thickness and join each other at…

The formation of muscles from the cervical hypaxial column of hypomeres, however, is quite different from what happens in the thorax; this is due to the development of the adjacent…

SKELETAL MUSCLE INNERVATION The establishment of neural contacts with developing skeletal muscle fibers is a critical developmental stage. The contacts enhance muscle development and are important for the complete differentiation…

At 6 weeks, the flexed limbs have not yet rotated out of their primary position (see Plate 1-8). Because the upper and lower limbs later undergo opposite rotations to reach…

2. Portions of successive myotomes commonly fuse to form a composite single muscle (the erector spinae and rectus abdominis muscles). The latter is formed by the fusion of the ventral…

After the transverse processes appear, the ribs form in the sclerotomal tissue that extends by differentiation into the ventral portions of the original clefts between the somites. The maximum development…

In humans, a striated cloacal sphincter muscle and levator ani muscle (pelvic diaphragm) arise from the third sacral to the first coccygeal myotomic hypomeres and are well developed by the…