The chapter on myogenesis starts with the development of the skeletal or voluntary muscle tissue. To show the development of the trunk and vertebral column skeletal musculature, an approximately 32-day-old embryo is cut transversally, and the sectioned surface is observed under moderate magnification. This diagram is the last to address the development of the Mesoderm and shows the myotomes, cartilaginous body of a vertebra, urogenital ridges, gut, splanchno- and somatopleura, and the newly developed skin, among other details.

The next sequence, ending in an approximately 5-week-old embryo, shows the division of the myotomes into epimeres and hypomeres by intermuscular septa. The spinal nerves innervating the muscles split into dorsal and ventral primary rami. In addition, numerous myoblasts migrate ventrally. Over the course of two next weeks, the epimeres become the extensor muscles of the vertebral columns; from the hypomeres, the outer, intermediate, and inner muscular layers of the thorax and abdomen develop. The tunica muscularis of the gut differentiate from the splanchnopleura. The repartition of the innervation of the muscles and the skin by the ventral and dorsal primary rami of the spinal nerves is also animated.

The next sequences are dedicated to the development of the limb musculature. An approximately 28-day-old embryo is cut transversally at the level of the upper limb bud and observed under moderate magnification. Apical ectodermal ridges are visible on the tip of the upper limb buds along with other details such as dermomyotomes. During the next two days, the dermis and hypodermis form from the dermatomes and the somatopleura. Then, in a 7-week-old embryo, the myoblasts migrate, and the extensor and flexor muscles of the vertebral column as well as the extensors and flexor muscles of the upper limbs develop around a cartilaginous model.

The next sequence shows the spreading of the skeletal musculature as viewed over the whole embryo. The first diagram of this sequence shows the profile of an approximately 30-day-old embryo, in which the epimere and the hypomere are visible by transparence. Then, the gradual development of the epimere and the hypomere lead to the formation of the limbs and body musculature, which is illustrated in the last diagram of the sequence corresponding to an approximately 7.5-week-old embryo.

To animate the histogenesis of a skeletal muscle fibre viewed with a light microscope, a transversal section of an embryo is shown at the beginning of the 6th week. Then, several presumptive myoblast are framed an observed under high light microscopic magnification. The first diagram shows a row of presumptive myoblasts.Several of these cells are true myoblasts, while others are satellite cells. One cell entering the screen is the future Schwann cell. The next sequence shows the mitoses and the migration of the myoblasts and the satellite cells to form a primary myotube. In an approximately 7-week-old embryo, the primary myotube forms after the fusion of the myoblasts. Satellite cells adhere to the primary myotube. An axon, which becomes myelinated, grows toward the primary myotube. Within the primary myotube, the first myofilaments appear. In an approximately 8-week-old embryo, the myotube thickens; the satellite cells come to lie in the depressions of the myotube; and the number of myofibrils increases. The myelinization of the motor nerve fibre, its contact with the myotube to form the motor end plate, and the appearance of the basal lamina around the myotube are shown. Once in contact with the nerve fibre, the primary myotube becomes the secondary myotube. Then, during the next two weeks, the satellite cells divide, and the postmitotic nuclei of the satellite cells penetrate the secondary myotube. In an approximately 12-week-old fetus, the secondary myotube thickens, and the transversal striation (A-bands, I-bands, Z-lines) appears along the myofibrils. At both ends of the secondary myotube, invaginations gradually form and then penetrate the collagen fibres. In an approximately 18-week-old fetus, the nuclei of the secondary myotube migrate to the periphery, while the myofibrils gather in the centre of the secondary myotube, which transforms into the skeletal muscle fibre.

The next sequences are dedicated to the histogenesis of a skeletal muscle fibre as viewed with a transmission electron microscope. From a myoblast of an approximately 6.5-week-old embryo, a small peripheral segment is magnified. The diagram shows the sarcolemma, sarcoplasm, endoplasmic reticulum and mitochondria. An approximately 8-week-old fetus shoes the division of the mitochodria, formation of the basal lamina around the myoblast, subdivision of the endoplasmatic reticulum into the rough and sarcoplasmic reticulum, and appearance of actin myofilaments, ribosomes and initial segments of the T-tubules. In an approximately 12-week-old fetus, the constitution of the myosin myofilaments, myofibrils and further growth of the T-tubules are shown. The last diagram of this sequence details the ultrastructure of a skeletal muscle fibre as viewed in a longitudinal section but without the final development of the sarcoplasmic reticulum and the T-tubules. This is done in the next sequence, which is dedicated to the deepening of the T-tubules into the sarcoplasm and the organization of the sarcoplasmic reticulum. The triads are shown spread and in profile as is the sarcoplasmic reticulum with its cisternae. The complete ultrastructure of a skeletal muscle fibre is described. In the next sequence, the muscle fibre makes some contractions in order to illustrate the sliding filament theory of contraction.

The next animated sequence deals with the histogenesis of the cardiac muscle tissue as viewed in a light microscope. The cardiogenic area of an approximately 19-day-old embryo is shown with the pericardial cavity, cardiogenic plate, one of the endocardial tubes and numerous other details. Then, a segment of the cardiogenic plate is observed under high light microscopic magnification.

The first diagram of the new sequence shows a group of young cardiac myoblasts, among of which are future endothelial cells. The mitotic divisions of the cardiac myoblasts are animated with the development of the blood vessels in an approximately 20-day-old embryo. During the next two days, the cardiac myoblasts differentiate into the cardiac muscle cells (cardiomyocytes). The cardiac muscle fibres are arranged in a plexus, and the intercalated discs appear. The next sequence shows several contractions of the cardiac muscle fibres along with mitosis of a cardiac muscle cell in an approximately 23-day-old embryo.

To animate the formation of an intercalated disc as viewed with a transmission electron microscope, the neighboring portion of two cardiac myoblasts in an approximately 21-day-old embryo are observed. The first diagram shows these two neighboring portions of cardiac myoblasts with detailed descriptions of myofibrils and their striations. During the next sequence, the neighboring cardiac myoblasts come into contact, while their sarcoplasms fill with myofibrils and mitochondria. The beginning of the formation of the T-tubules is also animated.

In an approximately 22-day-old embryo, an intercalated disc forms, and the myoblasts differentiate into the cardiac muscle cells. The sarcoplasmic reticulum, T-tubules, dyads develop, and propagation of the bioelectric impulses occurs. The next sequence illustrates several contractions of the cardiac muscle cells in an approximately 23-day-old-embryo. The last diagram describes the mechanism of contraction.

The next sequences animate the histogenesis of the smooth muscle tissue as viewed with a transmission electron microscope. A small segment of the future tunica muscularis in an approximately 6-week-old embryo is shown. The first diagram details the ultrastructure of the precursor smooth muscle cells (sarcolemma, sarcoplasm, nucleus, nucleolus, mitochondria, Golgi apparatus, sarcoplasmic reticulum and rough endplasmic reticulum). The transformation of the precursor smooth muscle cells into the differentiated smooth muscle cells during the 7th week is animated, detailing the arrangement the actin myofilaments and desmin microfilaments and their fixation to the dense bodies. The formation of the endoplasm, basal laminae, nexuses and cavolae are also animated.

The next sequence shows smooth muscle cells at the beginning of contraction during the course of the 8th week and polymerization of the myosin myofilaments between the actin myofilaments. The actin myofilaments slide along the myosin myofilaments and approach the dense bodies. This displacement is transmitted to the desmin microfilaments, which act as a system of noncontractile levers causing the cells body and the nucleus to deform. Several contractions and relaxations of the smooth muscle cells illustrate the polymerization and depolymerization of the myosin myofilaments.

The following malformations of the muscle tissue are illustrated but not animated: aplasia and hypoplasia of the muscles, congenital torticollis, prune-belly syndrome and Duchenne muscular dystrophy.

(This animation is essential for students of medicine, veterinary medicine, and biology as well as for departments of anatomy, histology, embryology; it is also recommended for students of stomatology; departments or clinics of physiology, pathology, and kinesitherapy; and for research institutes in biomechanics.)