Book - Text-Book of Embryology 11
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Bailey FR. and Miller AM. Text-Book of Embryology (1921) New York: William Wood and Co.
- Contents: Germ cells | Maturation | Fertilization | Amphioxus | Frog | Chick | Mammalian | External body form | Connective tissues and skeletal | Vascular | Muscular | Alimentary tube and organs | Respiratory | Coelom, Diaphragm and Mesenteries | Urogenital | Integumentary | Nervous System | Special Sense | Foetal Membranes | Teratogenesis | Figures
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The Development of the Muscular System
Anatomy and Histology show that there are, in a sense, two muscular systems in the body, and Embryology teaches that the two systems have different origins.
- The skeletal musculature. This, as the name indicates, is closely associated with the skeletal system. It is made up of striated muscle fibers arranged to form definite bundles or muscles. The skeletal musculature is under the voluntary control of the central nervous system.
- The visceral musculature. This is. found in connection with and forms integral parts of certain organs. It is made up of two different kinds of fibers smooth muscle fibers or cells and striated fibers or cells (heart-muscle cells). The latter are found only in the wall of the heart. The visceral musculature is involuntary, being under the control of the sympathetic nervous system.
Both systems are derived from mesoderm but from distinct parts of the mesoderm. Furthermore, their developmental histories are quite different, as will be seen in the following paragraphs.
The Skeletal Musculature
In the chapters on the development of the germ layers it was said that throughout the length of the body region of the embryo the mesoderm on each side of the neural tube and notochord becomes divided into a definite number of segments the primitive segments or mesodermic somites (Figs. 24, 52, 51). These indicate the segmentation of the body, and the history of the greater part of the skeletal musculature dates from their differentiation from the axial mesoderm. Thus the skeletal musculature is, for the most part, primarily segmental in character.
At first the primitive segments are composed of closely packed, epitheliallike cells, and each segment contains a small cavity which represents a portion of the coelom (Fig. 103). The ventro-medial parts of the segments become differentiated to form the sclerotomes which are composed of more loosely arranged cells (Fig. 223), and which are destined to give rise to the vertebrae and to the various kinds of connective tissue in their neighborhood. The lateral parts of the segments become differentiated to form the cutis plates which are destined to give rise to a part of the corium of the skin. The remaining portions of the segments form the muscle plates or myotomes (Fig. 223), from which develop by far the greater part, at least, of the voluntary striated muscles.
The differentiation of the parts of the primitive segments begins in the cervical region by the end of the second week, and then gradually proceeds toward the tail. Three myotomes are also probably formed in the occipital region. The cells of the myotomes are at first of an epithelial character (Fig. 105). Contractile fibrils appear in the cells and the latter are transformed directly into muscle fibers. (For histogenesis see p. 276). The fibers later alter their direction in accordance with the particular muscle to which they belong. The muscle tissue first formed is thus segmented, being derived from the segmentally arranged myotomes, but as development proceeds the myotomes undergo extensive changes by which the segmental character is lost in the majority of cases. It is retained, however, in a few instances, such for example as the intercostal muscles. The course of the changes which obliterate the segmental character of the myotomes and give rise to the various muscles has not been observed in all cases. But since a nerve belonging to any particular segment and innervating the myotome of that segment always innervates the muscles derived from that myotome, it is possible to learn something of the history of the myotomes by studying the innervation of the muscles.
Fig. 223. Transverse section of human embryo of the 3rd week. Scl. 1 , Break in myotome at point where sclerotome is closely attached. Kottmann.
From a consideration of what is known concerning the individual histories of the muscles and concerning the innervation of the muscles, certain factors can be recognized, to one or more of which the changes in the myotomes may be referred. These factors are as follows:
1. Migration.- The myotomes may migrate in whole or in part, and the muscles derived from them may be situated far beyond their limits. For example, the latissimus dorsi is derived from cervical myotomes but ultimately becomes attached to the lumbar vertebrae and to the crest of the ilium. To this factor, possibly more than to any other, is due the loss of the segmental character in the musculature.
2. Fusion. Portions of two or more myotomes may fuse to form one muscle. For example, each oblique abdominal muscle is derived from several thoracic myotomes.
3. Longitudinal Splitting. Very frequently a myotome or a developing muscle splits longitudinally into two or more portions. The sternohyoid and the omohyoid, for example, are formed in this manner.
4. Tangential Splitting. A developing muscle may split tangentially into two or more plates or layers. The two oblique and the transverse abdominal muscles, for example, are formed in this way.
5. Degeneration. Myotomes may degenerate as a whole or in part and be converted into some form of connective tissue, such as fascia, ligament or aponeurosis. The aponeuroses of the transverse and oblique abdominal muscles are probably due to a degeneration of portions of the myotomes from which the muscles are derived.
6. Change of Direction. The muscle fibers may change their direction. As a matter of fact, the fibers of very few muscles retain their original direction.
Muscles of the Trunk
The myotomes are at first arranged serially along each side of the notochord and spinal cord (compare Fig. 2 24 with Figs. 105 and 223). By the end of the second week fourteen myotomes are differentiated in the human embryo. Differentiation continues until, by the end of the fourth week, the total number thirtyeight is present. Of the thirty-eight, three are occipital, eight cervical, twelve thoracic, five lumbar, five sacral, and five (or six) coccygeal. The occipital myotomes are transient structures that appear in relation with the hypoglossal (XII) nerve. The cervical, thoracic, lumbar, sacral and coccygeal myotomes correspond individually to the spinal nerves (Fig. 224). As stated on page 148, the myotomes alternate with the anlagen of the vertebrae. Consequently in the cervical region there are eight myotomes, corresponding to the eight cervical spinal nerves, and only seven vertebrae. The myotomes in the neck and body regions are destined to give rise to the dorsal musculature, to the thoracoabdominal musculature, to a part of the muscles of the neck, and to the muscles of the tail region. There is a possibility that they give rise also to the muscles of the tongue.
As the myotomes continue to develop, they become elongated in a ventral direction. Those of the thoracic region extend into the connective tissue of the somatopleure, or in other words, into the lateral body walls (compare Figs. 224 and 225). During the fifth week the myotomes give rise to a dorsoventral mass of developing muscle tissue, in which the segmental character largely disappears. The muscle mass then becomes divided longitudinally into two parts, (i) a dorsal and (2) a ventro-lateral (Figs. 224, 225 and 226).
Fig. 224. Lateral view of human embryo of 9 mm. (4! weeks). Bardeen and Lewis.
- The area from which the skin has been removed is drawn from reconstructions. The myotomes have fused to a certain extent, so that segmentation is becoming less distinct. Note that the myotomes correspond to the spinal nerves. The developing muscle mass (the myotomes collectively) extends ventrally in the body wall in the thoracic region, and is divided by a longitudinal groove into two parts a dorsal and a ventro-lateral (see text).
- In the region of the upper extremity, dense masses of " premuscle " tissue are represented which later form the muscles. In the region of the forearm and hand the " premuscle " tissue has been removed to disclose the anlagen of the skeletal elements (radius, ulna, and hand plate). In the region of the lower extremity the superficial tissue has been removed to disclose the border vien, the anlagen of the os coxae, and the lumbo-sacral nerve plexus.
Fig. 225. Diagrammatic cross section through the sth-6th thoracic segments of a human embryo of 9 mm. (4! weeks). Bardeen and Lewis.
Fig. 226. Drawing from a reconstruction of the region of the lower extremity of a human embryo of 9 mm. (4 weeks). Bardeen and Lewis.
- The visceral organs and the greater part of the left body wall have been removed. The 8th thoracic to the 5th sacral segments are shown. On the right side of the body the costal processes, the spinal nerves (including the lumbo-sacral plexus), and the lower extremity are shown. On the left side the costal processes, the spinal nerves, and the nth and i2th thoracic myotomes are represented. Note the dorsal, lateral, and sympathetic branches of the spinal nerves.
1. The dorsal part is destined to give rise to those dorsal muscles of the trunk that are not associated with the extremities, and is innervated by the dorsal rami of the spinal nerves (Fig. 225).
2. The ventro-lateral part again divides longitudinally into (a) a lateral and (b) between (a) a ventral part, although the line of division is not so distinct as the original (i) dorsal and (2) ventro-lateral parts (Fig. 227). The lateral part subdivides tangentially and gives rise in the cervical region to the longus capitis, longus colli, rectus capitis anterior, to the scaleni, and to parts of the trapezius and sternomastoideus (Figs. 228 and 229). In the thoracic region it gives rise to the intercostaks and to the transversus thoracis (Figs. 227 and 230); in the abdominal region to the psoas, quadratus lumborum, and to the obliqui and transversus abdominis (Figs. 229 and 230).
Fig. 227. Diagrammatic cross section through the 6th~7th thoracic segments of a human embryo of 17 rnm. (5^ weeks). Bardeen and Lewis.
(b) The ventral part gives rise in the cervical region to the sternohyoideus, omohyoidem, sternothyreoideus and geniohyoideus. In the abdominal region the ventral part gives rise to the r edits abdominis and to the pyramidalis (Figs. 227 and 229). In the thoracic region there are no muscles derived from the ventral part, corresponding to those in the abdominal region. This is probably due to the development of the sternum.
Fig. 228. Lateral view of a human embryo of n mm. (about 5 weeks). Bardeen and Lewis. The area from which the skin has been removed is drawn from reconstructions. The dorsal musculature has been removed from the region of the upper extremity, exposing the 4th to the 8th cervical and the ist to the 3d thoracic vertebrae. The dorsal musculature has likewise been removed from the 5th lumbar and first three sacral segments. Segmentation is practically lost in the dorsal musculature in the thoracic region, but is still evident in the lumbar, sacral and coccygeal regions. The ventro-lateral musculature is distinctly separated from the dorsal, and is beginning to differentiate into the muscles of the thorax and abdomen.
The ventro-lateral portions of the lumbar myotomes and of the first two sacral myotomes, corresponding to the ventro-lateral portions of the thoracic myotomes, apparently do not take part in the production of muscles which belong to the body wall proper. It is even questionable whether they give rise to any muscles of the lower extremities. The ventro-lateral portions of the third and fourth sacral myotomes give rise to the levator ani, the coccygeus, the sphincter ani externus and the perineal muscles. The dorsal parts of the myotomes as far as the fifth sacral probably give rise to the sacrospinalis (Fig. 228). THE DIAPHRAGM. In addition to certain structures which are considered in connection with the pericardium (parietal mesoderm, mesocardium and common mesentery Chapter XIV), two myotomes on each side enter into the formation of the diaphragm. These are the third and fourth cervical myotomes, parts of which grow into the developing diaphragm in the earlier stages when it is situated far forward in the cervical region (p. 346 and Fig. 298), and give rise to its muscular elements.
Fig. 229. Drawing from a reconstruction of a human embryo of 20 mm. (about 7 weeks). Bardeen and Lewis. The superficial tissues have been removed from the extremities, the body wall, and the back.
Muscles of the Head
Primitive segments (mesodermic somites) are not clearly demonstrable in the heads of human embryos, nor, in fact, in the heads of any of the higher Vertebrates. In some of the lower forms, however, they are very distinct. It seems possible, even probable, that their indistinctness in the higher animals is due to an abbreviation or condensation in the development of the head region. Such condensations are known to occur in the development of other structures. In a human embryo 3.5 mm. long, three structures, resembling segments have been seen somewhat caudal to the region of the ootic vesicle on
Fig. 230. Drawing from a reconstruction of the right side of a human embryo of 20 mm. (about 7 weeks). Bardeen and Lewis.
the left body wall and viscera have been removed. Note especially the following muscles: The deltoid and biceps, just to the left of the brachial plexus and below the clavicle; the internal intercostals; the diaphragm, attached to the body wall; the transverse abdominal and the rectus abdominis; the quadratus lumborum, just to the right of the transverse abdominal; the psoas, cut just above the lumbo-sacral plexus; the levator ani, running obliquely upward from the coccygeal region.
one side. On the other side there were seven similar but smaller structures. All were composed of epithelial-like cells surrounding small cavities. Whether these segment-like structures bear any relation to the mesenchymal condensations which appear regularly in the occipital region (p. 157). seems not to have been determined.
Although the transformation of head segments into muscles has not been followed in detail in mammalian embryos, it may be inferred from the study of lower forms that three segments are involved in the formation of the eye muscles. The most cephalic (anterior) segment gives rise to the recti superior, inferior and medialis (internus) and to the obliquus inferior, all of which are innervated by the occulomotor (III) nerve. The next segment gives rise to the obliquus superior which is innervated by the pathetic (IV) nerve. The most caudal segment gives rise to the rectus lateralis (externus) which is innervated by the abducens (VI) nerve.
The development and innervation of the other muscles of the head and" of the hyoid musculature present certain peculiarities which have caused these muscles to be considered as more closely related to the visceral musculature than to the myotomic musculature. In the first place they are derived from the branchial arches (hence are often called branchiomeric muscles), and not directly from the myotomes of the neck region. This places them in closer relation to the visceral muscles, although they are structurally and functionally different from the latter. In the second place the nerves which supply them are fundamentally different from those which supply the myotomic muscles (Chap. XVII).
Fig. 231. Transverse section through the eighth cervical segment of a human embryo of 2.1 mm. Lewis
The first branchial arch on each side gives rise to the temporalis, masseter and pterygoidei, to the mylohyoideus and digastricus (venter anterior) and to the tensor tympani and tensor veli palatini. All these muscles are innervated by the trigeminal (V) nerve.
The second arch, which is often called the hyoid arch, gives rise to a large sheet of myogenic tissue which produces many of the facial muscles, such as the platysma and epicranius, the muscles of expression quadratus labii superiority risorius, triangularis , mentalis, etc.; also two muscles connected with the hyoid bone digastricus (venter posterior) and stylohyoideus and the stapedius of the middle ear. The facial (VII) nerve corresponds to the second arch and supplies all these muscles.
The glossopharyngeal (IX) nerve corresponds to the third branchial arch, and this fact indicates the muscles derived from that arch. Some, at least, of the constrictor muscles of the pharynx are derived from the third arch. The stylo-pharyngeus is also a derivative of the same arch.
The vagus (X) nerve is associated with the fourth and fifth arches and consequently innervates the muscles derived from these arches, viz., the rest of the constrictors of the pharynx (see above), the laryngeal muscles and the muscles of the soft palate (except the tensor veli palatini which is derived from the first arch (p. 271). The glossopalatinus and chondroglossus are also derived from the fourth and fifth arches, while the rest of the extrinsic muscles of the tongue are of myotomic origin.
Two other muscles are probably derived in part from the branchial arches, for fibers of the spinal accessory (XI) nerve afford a part of their innervation. These are the trapezius and the sternomastoideus , the remaining parts of which are of myotomic origin (p. 267).
Muscles of the Extremities
The question as to whether the muscles of the extremities are derivatives of the myotomes or of the mesenchymal tissue in the limb buds has not been settled. In some of the lower Vertebrates, especially in some of the Fishes, it seems to have been pretty clearly demonstrated that bud-like processes from the myotomes grow into the anlagen of the extremities (fins), and there give rise to muscles. In other lower forms no such buds from the myotomes have been demonstrated, but the muscles are apparently derived directly from the mesenchymal tissue in the anlagen of the extremities. In the higher vertebrates, especially in Mammals, no distinct myotome buds have been traced into the extremities. Some investigators hold, however, that instead of myotome buds some cells from the myotomes myoblasts wander into the limb buds and give rise to muscles. Other investigators are inclined to the view that the musculature of the extremities is not of myotomic origin, but that it is derived from the mesenchymal tissue of the limb buds.
A most striking feature of the musculature of the extremities is its distinctly segmental nerve supply. This, of course, is in favor of, although it does not prove, its myotomic origin. If the muscles of the extremities are of myotomic origin, it is very probable that several myotomes take part in their formation.
In the first place among the lower Vertebrates the muscles of each extremity are derived from several myotomes and are innervated by segmental nerves corresponding to these myotomes. In the second place among the higher Vertebrates, although the myotomic origin of the muscles has not been clearly demonstrated, the nerve supply in each extremity comes through several segmental spinal nerves.
Knowledge concerning the development of the individual muscles of the extremities in the human embryo is incomplete. Especially is this true of the muscles of the lower extremities.
The upper limb bud first appears in embryos of 2-3 mm. (during the third week) as a slight swelling ventro-lateral to the myotomes in the lower cervical region (Fig. 231; see also Fig. 87). The swelling gradually enlarges and by the time the embryo has reached a length of 4-5 mm. lies opposite the last four cervical and the first thoracic myotomes. At this time it is filled with closely packed mesenchymal cells. No buds from the myotomes can be seen extending into the mesenchyme (Fig. 232).
Fig. 232. Transverse section through the eighth cervical segment of a human embryo of 4.5 mm. Lewis.
In succeeding stages the limb bud enlarges still more, and the mesenchymal tissue becomes denser (Figs. 233 and 234). During these stages no growths, either of buds or of individual cells, from the myotomes are apparent. Some of the cervical nerves, however, enter the limb buds (Fig. 234).
Apparently the tissue from which the muscles, as well as the skeletal elements, are to develop, is the condensed mesenchymal tissue. The first indication of differentiation occurs during the fourth week (embryo of about 8 mm.). The central portion or core of the mesenchymal mass becomes still denser to form the anlage of the skeletal elements of the extremity. The tissue of the core shades off into the surrounding tissue of a lesser density, which is destined to give rise to the muscles and which is known as the premuscle sheath.
During these processes of differentiation in the limb bud proper, masses of premuscle tissue have also become differentiated around the base of the limb bud. These are the forerunners of certain extrinsic muscles of the upper extremity, such as the pectoralisj levator scapula, trapezius, latissimus dorsi, serratus, etc. (Fig. 235; compare with Fig. 236).
Fig. 233. Transverse section through the 8th cervical segment of a human embryo of 5 mm. Lewis.
By the end of the fifth week the premuscle sheath in the limb bud proper becomes more or less differentiated into muscles or groups of muscles. The differentiation is most complete at the proximal end. From this the transition is gradual to the distal end where the premuscle sheath is intact
By the end of the sixth week most of the muscles of the upper extremity are recognizable (Figs. 236 and 237).
By the end of the seventh week practically all the muscles can be recognized and are composed of muscle fibers.
During the differentiation of the muscles, the limb bud and certain extrinsic muscles migrate a considerable distance caudally. For example, the pectoralis and latissimus dorsi migrate from the base of the arm to the thoracic wall. Their nerves are naturally pulled with them. The trapezius muscle, which originates well forward in the cervical region, migrates so that it finally reaches as far as the last thoracic vertebra. The sternomastoideus also originates well forward in the cervical region, but finally extends to the clavicle and sternum. The migration of the upper extremity causes the brachial plexus to have a caudal inclination.
The lower limb buds arise very soon after the upper. As stated on page 115, the upper limbs always maintain a slight advance over the lower in development. As in the case of the upper, the lower limb buds appear as swellings on the ventro-lateral surface of the body, opposite the fifth lumbar and first sacral myotomes. The interior of each swelling is at first composed of closely packed mesenchymal tissue, but whether any part of the myotomes enters it is questionable. At all events several spinal nerves do enter the tissue and supply the nro.3cles. The differentiation of a central core as the anlage of the skeleton, and the differentiation of the surrounding tissue as the premuscle sheath, take place in the same manner as in the upper extremity (p. 274). From this premuscle sheath all the muscles of the lower extremity are developed.
Fig. 234. Transverse section through the 8th cervical segment of a human embryo of 7 mm. (about 4 weeks). Lewis.
Histogenesis of Striated Voluntary Muscle Tissue
The majority of the striated voluntary muscles of the body are derived from the myotomes. Some are derived from the mesenchymal tissue in the branchial arches, some possibly from the mesenchymal tissue in the limb buds. Thf primitive segments are at first composed of closely arranged, epithelial-like cells that radiate from a small centrally placed cavity (Fig. 103). The cavity represents part of the ccelom and from this point of view it can be said that the muscle is a derivative of the epithelial lining of the ccelom. A part of each primitive segment becomes the sclerotome and cutis plate. The remaining part becomes the myotome or muscle plate (Fig. $ 23).
Fig. 235. Drawing from a reconstruction of the upper limb region of a human
embryo of 9 mm. (4 weeks) ; ventral view. Lewis.
Inf. hy., infrahyoid; Lev. scap., levator scapulae; My., myotome mass; Rhom., rhomboid; Trap., trapezius.
The cells of the myotome are at first not essentially different from those of the rest of the primitive segment. Soon, however, changes take place in them and they become the so-called myoblasts or muscle-forming cells, which are destined to give rise to the muscle fibers. Opinions differ as to the manner in which the myoblasts produce the muscle fibers. It was once thought that each myoblast gave rise to a single muscle fiber in which there were several nuclei, all derived from the original myoblast nucleus by mitotic division. It was also thought that the muscle fibrillae represented highly modified and specialized parts of the cytoplasm, which arranged themselves longitudinally in the cell. Some of the later researches indicate that a muscle fiber represents a number of myoblasts fused together. This explanation is not, however, accepted by all investigators.
In contrast with the above, there is a quite general consensus of opinion in regard to the development of the internal structure of the muscle fiber. In the
Fig. 236. Lateral view of a reconstruction of the muscles of the upper extremity of a human embryo of 16 mm. (about 6 weeks). Lewis.
The trapezius is the large muscle arising from the transverse processes of the vertebrae (at the right of the figure) and converging to its insertion on the clavicle. Just below the insertion of the trapezius is the deltoid, which partly hides the subscapular (on the right) and the pectoralis major (on the left). Arising beneath the deltoid and running downward to the elbow is the triceps. To the right of the triceps is the teres major (composed of two parts). The large sheet of muscle extending down the forearm and sending divisions to the ad, 3d, 4th and 5th digits is the extensor communis digitorum.
cytoplasm of the myoblasts there appear granules which soon arrange themselves in parallel rows and unite to form slender thread-like fibrils (Fig. 238). These fibrils are at first confined to one myoblast area. If several myoblasts fuse, the fibrils probably extend in a short time from one myoblast area to another. If one myoblast produces a fiber, the fibrils naturally are confined to a single myoblast area throughout development. The fibrils are usually formed first at the periphery of the cell and later in the interior (Figs. 239 and 240.) At the same time they increase in number by longitudinal splitting. The cytoplasm among the fibrils becomes the sarcoplasm.
After the granules which first appear unite to form the fibrils, the latter are apparently quite homogeneous. Later they become differentiated into two distinct substances which alternate throughout their length and produce the characteristic cross striation. The nature of this differentiation is not known. One investigator holds that both substances are derived from the original granules that unite to form the fibrils, alternate granules being composed of like substance and united by delicate strands of the other substance.
Fig. 237. Medial view of a reconstruction of the muscles of the upper extremity of a human embryo of 16 mm. (about 6 weeks). Lewis.
The muscle arising on the scapula (at the left of the figure) and passing toward the right is the subscapular. The small muscle just below the subscapular is the teres major; below the latter and hanging downward is the latissimus dorsi. Note the cut end of the pectoralis minor just to the right of the narrow portion of the subscapular. Running from this cut end toward the right is the biceps. The muscle at the lower edge of the figure in the arm region is the triceps. In the forearm region, the muscle crossing the end of the biceps is the pronator teres. Below the pronator teres, extending from the elbow to the thumb region is the flexor carpi radialis. Below the latter and extending to a point opposite the thumb, is the palmaris longus. Beneath the palmaris longus and dividing into branches which pass to the 2d, ad, 4th, and 5th digits is the flexor sublimis digitorum. The muscle passing to the thumb is the flexor longus pollicis. The muscle at the lower border of the figure in the forearm region is the flexor carpi ulnaris.
Fig. 238. Myoblasts in different stages of development. Godleivski.
The upper cell represents a myoblast with granular cytoplasm (from sheep embryo of 13 mm) ; the middle, a myoblast with fibrils in process of formation (from guinea-pig embryo of 10 mm.); the lower, a myoblast with still further differentiated, segmented fibrils (from a rabbit embryo of 8.5 mm.). ++++++++++++++++++++++++++++++++++
While the fibrils are being formed, the nuclei of the myoblasts undergo rapid mitotic division. When the cells are about filled with fibrils, the nuclei migrate to the periphery where they are situated in the fully formed fiber (Fig. 278). Each fiber thus possesses a number of nuclei, whether it is derived from one myoblast or from several.
Fig. 239. From a cross section of developing voluntary striated muscle in the leg of a pig embryo of 45 mm., showing fibril bundles at the periphery of the cells. MacCallum.
Fig. 240. From a cross section of developing voluntary striated muscle in the leg of a pig embryo of 75 mm., showing fibril bundles more numerous than in Fig. 239. A, Central vesicular nucleus; B, peripheral more compact nucleus. MacCallum.
For some time at least, the number of fibers in a developing muscle increases by division of those already formed. This process would produce a certain degree of enlargement of the muscle as a whole. Later the increase in the number of fibers ceases, and the muscle grows by enlargement of the individual fibers. It is not certain at what period in development the increase in the number of fibers ceases.
In many muscles development is further complicated by a retrograde processa degeneration of some of the fibers. This occurs quite regularly in the extremities. A well fibrillated fiber first presents a homogeneous appearance, then becomes vacuolated, the nuclei disintegrate, and finally the whole structure disappears. Mesenchymal (or connective) tissue takes its place, and the remaining fibers are thus grouped into bundles and the bundles into muscles. This would account to a certain extent for the intermuscular connective tissue, the perimysium and endomysium, the epimysium being derived from the mesenchymal tissue which originally surrounded the muscle.
The Visceral Musculature
The visceral musculature is derived wholly from the mesoderm, but not from the myotomes. The striated involuntary muscle or heart muscle is derived from the mesothelial lining of the coelom, the smooth muscle from the mesenchymal tissue in various regions of the body. The heart muscle develops only in connection with the heart and consequently occurs in the adult only in that organ. Smooth muscle develops to form integral parts of certain structures such, for example, as the alimentary tube, glands, blood vessels, and skin.
Histogenesis of Heart Muscle
When the simple tubular heart is first formed, the splanchnopleure projects into the ccelom (primitive pericardial cavity) along each side (Fig. 165; also p. 196). The mesothelium covering these projections is destined to give rise to the myocardium. The mesothelial cells which are at first closely packed together with but little intercellular substance, assume irregular branching forms and the branches anastomose freely (Fig. 241). After the cells become loosely arranged, they again become closely packed to form a compact syncytium, individual cells apparently assuming the shape of heavy bands (Fig. 242). Irregular transverse bands next appear, dividing the syncytium into the so-called heart muscle cells. These may or may not represent the original cells or myoblasts. At all events the muscle fibrils are continuous across the lines. The nuclei proliferate in the syncytium but remain in the central part of the bands or cells, instead of migrating to the periphery as in striated voluntary muscle.
Fig. 241. From a section of developing heart muscle from a rabbit embryo of 9 mm. Godlewski,
a, Cell body with granules arranged in series; b, cell body with centrosome and attraction sphere;
c, branching fibril; d, fibrils extending through several cells.
While the cells are still loosely arranged, rows of granules appear in the cytoplasm, and the granules in each row unite to form a fibril (Fig. 241). The fibrils are at first confined to individual cell areas, but as the cells come closer together to form the compact syncytium, they extend through several cell areas and run in different directions (Fig. 242). As development proceeds the fibrils become more nearly parallel (Fig. 243). They are first formed in the peripheries of the cells, but later also in the interior, except in a small area immediately surrounding the nucleus, where a small amount of undifferentiated cytoplasm remains. The latter is continuous with the cytoplasm or sarcoplasm among the fibrils. As in voluntary seriated muscle the fibrils become differentiated into two distinct substances which alternate with each other, thus producing the transverse striation.
Fig. 242. From a section of developing heart muscle in a rabbit embryo of 9 mm. Godlewski. The cells form a compact syncytium. ++++++++++++++++++++++++++++++++++
Histogenesis of Smooth Muscle
The mesenchymal cells which are destined to produce smooth muscle cells are not grouped into any particular primitive structures like the mesodermic somites. They are simply scattered through the general mass of mesenchymal tissue and, like other mesenchymal cells, possess irregular branching forms and distinct spherical nuclei. The internal changes by which these cells are converted into muscle cells are not well known. The contractile elements the fibrillae probably represent highly modified portions of the original cytoplasm but the manner in which the cytoplasm is transformed into fibrillae has not been determined. The external changes consist essentially in an elongation of the irregular mesenchymal cells. The result of this elongation is usually a spindle-shaped cell, but exceptionally cells forked at one or both ends are produced. The original spherical nucleus also shares in the elongation and becomes rod-shaped.
In some cases, for example in the muscular layers of the gastrointestinal tract, distinct bands or sheets of smooth muscle are formed in which the cells are closely packed and lie approximately parallel. In other cases, such as the mucosa of the intestine and the capsules of certain glands, the muscle cells develop in little groups or as isolated cells.
More or less of the muscular system is involved in some of the gross anomalies or malformations of the body. For example, congenital defects in the central nervous system (anencephaly, rachichisis, etc.) are necessarily accompanied by atrophy or faulty development of certain parts of the muscular system. In the case of ventral median fissure of the abdominal wall (gastroschisis) , the abdominal muscles are naturally involved. Such anomalies in the muscles are, however, secondary to the other malformations and are not due to primary defects in the muscles themselves.
Fig. 243. From a section of developing heart muscle in a rabbit embryo of 10 mm. Godlewski. The fibrils are segmented, indicating the beginning of the cross striation characteristic of heart muscle.
Many of the minor variations in the muscular system occur in the same form or in similar forms in different individuals, thus indicating their relation to a fundamental type. Many of these are more or less accurate repetitions of normal structures found in lower animals. Such variations are probably rightly attributed to hereditary influences. On the other hand, there are variations which cannot be referred to conditions found in any of the lower animals. These constitute a class of variations which must be accounted for upon some other basis than that of heredity. As pointed out in the chapter on Teratogenesis (Chap. XX), external influences undoubtedly play an important part in the production of anomalies and it is probable that similar influences act upon the development of the muscular system.
The scope of this book does not permit a description, or even mention, of the great number of variations in the muscles. A few are described here as examples; for others the student is referred to the more extensive text-books of anatomy.
Extrinsic Muscles of the Upper Extremity
The trapezius is sometimes attached to less than the normal number of thoracic vertebrae. Its occipital attachment may be wanting. Occasionally the cervical and thoracic portions are more or less separated as in some of the lower animals.
The latissimus dorsi sometimes arises from less than the usual number of thoracic vertebrae, and from less than the normal number of ribs. The iliac origin may be wanting.
The rhomboidei vary in their vertebral and scapular attachments.
The number of the vertebral attachments of the levator scapulae may vary. A small part of the muscle is sometimes attached to the occipital bone.
The pectoralis major not infrequently varies in the extent of its attachment to the ribs and sternum.
The serrati vary in their attachment to the ribs.
The above mentioned extrinsic muscles of the upper extremity vary principally in their attachments. Since they all appear well forward in the cervical region in the embryo, and, along with the extremity, gradually migrate caudally before acquiring their final attachments, it is not unlikely that the variations in their attachments are due to variations in the extent of migration.
This serves to illustrate a factor which is probably important in producing variations in the attachments of many other muscles. As stated in paragraph i, on page 264, the myo tomes frequently migrate very extensively during their transformation into muscles, before the muscles have acquired their permanent attachment. Variations in the extent of this migration would naturally produce variations in the final attachments of these muscles.
Other factors related to the changes in the myo tomes, such as fusion, longitudinal and tangential splitting (paragraphs 2, 3 and 4, p. 264) probably also play a part in the production of variations.
A greater than normal degree of fusion between two or more myotomes might result in the union of muscles which are usually separate; a less than normal degree of fusion might result in the separation of parts usually united. Variations in the splitting of myotomes might produce similar results.
At the same time, however, heredity may be the active factor in some cases where abnormal fusions or separations between muscles or parts of muscles produce results resembling conditions found in lower animals.
Reference for Further Study
V BARDEEN, C. R. : The Development of the Musculature of the Body Wall in the Pig, Including its Histogenesis and its Relation to the Myotomes and to the Skeleton and to the Nervous Apparatus. Johns Hopkins Hospital Reports, Vol. XI.
BARDEEN, C. R., and LEWIS, W. H.: Development of the Limbs, Body Wall and Back in Man. American Jour, of Anat., Vol. I, 1901.
BOLK, L.: Die Segmentaldifferenzierung des menschlichen Rumpfes und seiner Extremitaten. Morph. Jahrbuch, Bd. XXV, 1898.
FUTAMURA, R.: Ueber die Entwickelung der Facialismuskulatur des Menschen. Anat. Hefte, XXX, 1906.
GODLEWSKI, E.: Die Entwickelung des Skelet- und Herzmuskelgewebes der Saugetiere. Arch. f. mik. Anat., Bd. LX, 1902.
GRAFENBERG, E.: Die Entwickelung der menschlichen Beckenmuskulatur. Anat. Hefte, 1904.
HEIDENHAIN, M.: Structur der contractilen Materie. Ergebnisse der Anat. u. Entwick., Bd. VIII, 1898.
HEIDENHAIN, M.: Ueber die Structur des menschlichen Herzmuskels. Anal. Anz., Bd. XX, 1901.
KASTNER, S.: Ueber die Bildung von animalen Muskelfasern aus dem Urwirbel. Arch. f. Anat. u. Physiol., Anat. Abth., Suppl., 1890.
KEIBEL, F., and MALL, F. P.: Manual of Human Embryology, Vol. I, 1910.
KOLLMANN, J.: Die Rumpfsegmente menschlicher Embryonen von 13-35 Urwirbeln. Arch. f. Anat. u. Physiol., Anat. Abth., 1891.
LEWIS, W. H.: The Development of the Arm in Man. American Jour, of Anat. y Vol. I, 1902.
MAURER, F.: Die Entwickelung des Muskelsystems und der elektrischen Organe. Also Bibliography. In Hertwig's Handbuch der vergl. u. experiment. Entwickelungslehre der Wirbeltiere, Bd. Ill, Teil I, 1904.
MACCALLUM, J. B.: On the Histology and Histogenesis of the Heart-muscle Cell. Anat. Anz., Bd. XIII, 1897.
MACCALLUM, J. B.: On the Histogenesis of the Striated Muscle Fiber and the Growth of the Human Sartorius Muscle. Johns Hopkins Hospital Bulletin, Vol. IX, 1898.
MALL, F. P. : Development of the Ventral Abdominal Walls in Man. Jour, of Morphology, Vol. XIV, 1898.
McGiLL, CAROLINE: The Histogenesis of Smooth Muscle in the Alimentary Canal and Respiratory Tract of the Pig. Internal. Monatsch. Anat. u. Phys., Bd. XXIV, 1907.
McMuRRicn, J. P. : The Phylogeny of the Forearm Flexors. American Jour, of Anat., Vol. II, 1903.
McMuRRiCH, J. P.: The Phylogeny of the Palmar Musculature. American Jour, oj Anat., Vol. II, 1903.
MCMURRICH, J. P.: The Phylogeny of the Crural Flexors. American Jour, of Anat., Vol. IV, 1904.
MCMURRICH, J. P.: The Phylogeny of the Plantar Musculature. American Jour, oj Anat., Vol. VI, 1907.
POPOWSKY, I.: Zur Entwickelungsgeschichte der Dammmuskulatur beim Menschen. Anat. Hefte, 1899.
SUTTON, J. B.: Ligaments, Their Nature and Morphology. London, 1897. ZIMMERMANN: Ueber die Metamerie des Wirbeltierkopfes. Verhandl. d. Anat. Gesettsch. Jena, 1891.
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