Book - Developmental Anatomy 1924-11

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Arey LB. Developmental Anatomy. (1924) W.B. Saunders Company, Philadelphia.

Developmental Anatomy: Chapter I. - The Germ Cells and Fertilization | Chapter II. - Cleavage and the Origin of the Germ Layers | Chapter III. - Implantation and Fetal Membranes | Chapter IV. - Age, Body Form and Growth Changes | Chapter V. - The Digestive System | Chapter VI. - The Respiratory System | Chapter VII. - The Mesenteries and Coelom | Chapter VIII. - The Urogenital System | Chapter IX. - The Vascular System | Chapter X. - The Skeletal System | Chapter XI. - The Muscular System | Chapter XII. - The Integumentary System | Chapter XIII. - The Central Nervous System | Chapter XIV. - The Peripheral Nervous System | Chapter XV. - The Sense Organs | Chapter XVI. - The Study of Chick Embryos | Chapter XVII. - The Study of Pig Embryos | Figures
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Chapter XI The Muscular System

I. The Histogenesis Of Muscle

The muscular system is composed of specialized cells, called muscle fibers; these form a tissue in which contractility has become the predominant function. The fibers are of three types: (i) smooth, found principally in the walls of the viscera and blood vessels; (2) cardiac, forming the myocardium of the heart; (3) skeletal, chiefly attached to the elements of the skeleton. Of these, cardiac and skeletal muscle are banded with cross stripes ; only skeletal fibers are under voluntary control. All three differentiate from myoblasts of the mesoderm; the only exceptions are the smooth muscles of the iris and sweat glands, which are ectodermal.

Smooth Muscle

Certain stellate cells of the mesenchyme enlarge and elongate. The resulting, spindle-shaped cells remain attached to each other by cytoplasmic bridges. In the superficial layer of their cytoplasm coalesced granules form coarse, non-contractile myoglia fibrils, similar to the primitive fibrillge of connective tissue (Fig. 224 A). Fine myofibrils then differentiate uniformly throughout the cytoplasm of the myoblasts (Fig. 224 B). They increase in number as development proceeds, while the coarse type diminishes. The cytoplasmic bridges later give origin to white connective-tissue fibers which envelop the muscle cells and bind them together. In older fetuses new muscle elements also arise by mitotic division of existing fibers and by the transformation of apparent interstitial cells.


Fig. 224. Stages in the histogenesis of smooth muscle (adapted from (McGill). 4 , 13 mm. pig embryo ( X 550) : coalescing granules give rise to coarse myoglia fibrils. B. 27 mm. pig embryo (X 850): both myoglia fibrils and fine myofibrils are present.

Cardiac Muscle

The cardiac type of involuntary muscle develops from the splanchnic mesoderm that invests the primitive heart tubes (Fig. 155). The cells of this myocardial anlage at first form a syncytium in which myojibrils differentiate from the linear union of cytoplasmic granules (Fig. 225, A, B). The myofibrillae arise at the periphery of the syncytial strands of cytoplasm and soon extend long distances through the syncytium (D). They multiply rapidly (C) and form alternate light and dark bands, as in skeletal muscle. The syncytial character of cardiac muscle persists in the adult and the nuclei remain central in position. The intercalated discs, typical of adult cardiac muscle, probably appear in the early months of fetal life.

Skeletal Muscle

All striated voluntary muscle is derived from the mesoderm - either from portion of the mesodermal segments (muscles of the trunk, and, possibly, limbs), or from the mesenchyme (muscles of the head). According to Bardeen, the remainder of the primitive segment not involved in forming skeletal tissue constitutes the myotome, and its cells become myoblasts (Fig. 211). On the contrary, Williams finds that in the chick only the cells of the dorsal and mesial walls of a mesodermal segment comprise the myotome (Fig. 212).


As to the composition of the individual muscle fibers, there is also a difference of opinion. It is generally believed that the myoblasts elongate, and, by the repeated mitotic division of their nuclei, become multi nucleate. Godlewski, however, holds that several myoblasts unite to form a single muscle fiber. At the beginning of differentiation the nuclei lie centrally, surrounded by granular sarcoplasm (Fig. 226 A). These granules become consolidated in rows as the myofibrillce, which increase in number by longitudinal splitting (Fig. 226 B, C). The myofibrillae soon acquire the characteristic transverse bands, and the individual fibrils become so grouped that, in the third month, their dark and light stripes coincide (Fig. 226 C). During development, the muscle fibers increase enormously in size, the nuclei migrate to the surface, and the myofil)rilla} are arranged in bundles, or muscle columns. The fibrils of each column are said to result from the longitudinal splitting of single, primitive myofibrils. For a time new muscle fibers arise also by the division of those already formed.


II. Morphogenesis of the Muscles

The muscles of the body are distributed in two systems ; the visceral musculature, and the skeletal musculature.

The Visceral Musculature

This group is associated chiefly with the hollow viscera and is under the involuntary control of the sympathetic nervous sytem. Except for the striated cardiac muscle in the wall of the heart, the visceral muscles are smooth. Their commonest arrangement is in orderly sheets or interlacing bundles.


The Skeletal Musculature

As the name indicates, these striated muscles come in intimate relation to the skeleton. With the exception of those muscles attached to the branchial arches, they originate from that portion of mesodermal segments designated a myotome, or muscle plate (p. 7; Figs. 21 1 and 212). Mesodermal segments first appear in the occipital region of embryos about 1.5 mm. long (Fig. 58), and the full number of nearly forty is acquired at 6 mm.( Fig. 63). At the latter stage of about five weeks, the myotomes first formed begin the differentiation of muscles. It will be convenient to consider their morphogenesis under three divisions: the muscles of the trunk, limbs, and head.


Fig. 225. The histogenesis of cardiac muscle in a 9 mm. rabbit embryo (adapted after Godlewski). A, Linear arrangement of granules; B, coalescence of granules into a fibril; C, fibril splitting; £>, long fibrils extending through syncytium.


Fig. 226. Stages in the histogenesis of skeletal muscle (after Godlewski). . 4 , Myoblast of a 13 mm. sheep embryo; B, myofibrils in a myoblast of a lo mm. guinea pig embryo; C, myoblast with longitudinally-splitting, striated myofibrils from an 8.5 mm. rabbit embryo.

Fundamental Processes

Although the primitive segmental arrangement of the myotomes is, for the most part, soon lost, their original innervation by the segmental spinal nerves is retained throughout life. For this reason, the history of adult muscles formed by fusion, splitting, or other modifications may be traced with considerable certainty.


The changes occurring in the myotonies during the formation of adult muscles are referable to the operation of the following factors;

  1. A change in direction of muscle fibers from the original craniocaudal orientation in the myotome. The fibers of but few muscles retain their initial orientation parallel to the long axis of the body.
  2. A migration of myotomes, wholly or in part, to more or less remote regions. Thus, the latissinius dorsi originates from cervical myotonies, but finally attaches to the lower thoracic and lumbar vertebrae and to the crest of the ilium. Other examples are the serratus anterior and the trajiezius.
  3. A fusion of portions of successive myotonies. The rectus abdominis and sacro-spinalis illustrate this process.
  4. A longitudinal splitting of myotonies into several portions. Examples are found in the sterno- and omo-hyoid and in the trapezius and sterno-niastoid.
  5. A tangential splitting into two or more layers. The oblique and the transverse muscles of the abdomen are formed in this common way.
  6. A degeneration of myotomes, wholly or in part. By this process fascias, ligaments, and a poneuroscs may be produced.


Muscles of the Trunk.- -Ventral extensions grow out from the cervical and thoracic myotonies (Fig. 212), and a fusion that is well advanced superficially occurs between all the myotonies in embryos of 10 mm. A dorsal, longitudinal column of fused myotonies, however, can still be distinguished from the sheet formed from the combined ventral prolongations (Fig. 227).


From the superficial portions of the dorsal column there arise by longitudinal and tangential splitting the various long muscles of the back and neck, innervated by the dorsal rami of the spinal nerves (Fig. 2 28). The deep portions of the myotomes do not fuse, but give rise to the several intervertebral muscles, which thus retain their primitive segmental arrangement.


Ddie muscles of the neck, other than those innervated by the dorsal rami and those arising from the branchial arches, differentiate from ventral extensions of the cervical myotomes. The muscles of the diaphragm, which in early stages lies at this level, appear to have a like origin. In the same manner, the thoraco-abdominal muscles arise from the more pronounced ventral prolongations of the thoracic myotomes that grow into the body wall along with the ribs (Fig. 228).

The ventral extensions of the lumbar myotomes (except the first) and of the first two sacral myotomes do not participate in the formation of the body wall. If they persist at all, it is possible that they contribute to the formation of the lower limb. The ventral portions of the third and fourth sacral myotomes give rise to the muscles of the perineal region.

Muscles of the Limbs

It is commonly stated that the muscles of the extremities develop from buds of the myotomes which grow into .


Fig. 227. Reconstruction of a 9 mm. human embryo, to show the partially fused myotomes and the premuscle masses of the limbs (Bardeen and Lewis). X 13. Distally, in the upper extremity, the radius, ulna and hand plate are disclosed; in the lower extremity the hip bone and the border vein show.

The limb anlages. In sharks this is clearly the case, but in birds and mammals distinct buds of this sort do not occur. The segmental nerve supply of the limb muscles of higher animals is merely suggestive, not proof, of a myotomic origin. Nevertheless, a diffuse migration of cells from the ventral portion of human myotomes has been recorded by various observers, recently by Ingalls. These cells soon lose their epithelial character and blend with the undifferentiated mesench}mie of the limb buds (Figs. 212 and 227). From this diffuse tissue, which at about 9 mm. forms condensed, premuscle masses, the limb muscles are differentiated, the proximal ones being the first to appear. The progressive differentiation into distinct muscles reaches the level of the hand and foot in embryos of seven weeks (Fig. 228). The upper limbs naturally maintain an advance over the lower throughout development.

Muscles of the Head

Distinct mesodermal segments do not occur in the head region. It is possible, however, that a premuscle mass, from which the eye muscles of man are developed, is comparable to three myotomic ^segments having a similar fate in the shark. The muscles of the eyes are activated by the third, fourth, and sixth pairs of cerebral nerves.


Fig. 228. Reconstruction of the superficial muscles of a 20 mm human embryo (Bardeen and Lewis in Bailey and Miller). X 4.5.

The remaining muscles of the head differ from all other skeletal muscles in that they arise from the splanchnic mesoderm of the branchial arches and are innervated by nerves (visceral) of a different category than those (somatic) which supply myotomic muscles (p. 275). The muscles derived from the several arches retain their primitive branchial-arch innervation (Fig. 367). Hence it follows that the mesoderm of the first branchial arch gives rise to the muscles of mastication and to all other muscles associated with the (fifth) trigeminal nerve. Similarly, the muscles of expression, and other muscles supplied by the (seventh) facial nerve, originate from the second, or hyoid arch. The third arch appears to be the source of muscles, like the pharyngeal constrictors, which receive branches of the (ninth) glossopharyngeal nerve. The fourth and fifth arches shave the (tenth) vagus nerve; it innervates their derivatives, such as the laryngeal muscles and part of the pharyngeal and palate group.


The muscles of the tongue are supplied by the hypoglossal nerve, originally a member of the spinal series. For this reason, it is assumed that, at least historically, they are derived from myotonies of the occipital region. Yet, according to Lewis, “there is no direct evidence whatever for this statement, and we are inclined to believe from our studies that the tongue musculature is derived from the mesoderm of the floor of the mouth.


Segmentation of the Vertebrate Head. - The vertebrate head undoubtedly consists of fused segments. This was suggested to the earlier workers by the arrangement of the branchial arches {hranckiomerism), by the presence of supposedly significant - neuromeres' in the brain wall (p. 257), and by the discovery, in the embryos of lower vertebrates, of so-called head cavities, homologous with mesodermal segments.

Only the first three head cavities persist; they form the eye muscles. All the remaining muscles of the head are derived from branchiomeres. Even assuming that the branchiomeres represent portions of the primary head somites - and there are recent observations which tend to disprove this - their segmentation still is not comparable to that of the trunk, for the branchial arches are formed by the serial division of splanchnic mesoderm, tissue which in the trunk never segments. The branchial arches, therefore, represent a different sort of metamerism. From what has been said, it is evident that one cannot compare the relation of the cranial nerves to the branchiomeric muscles with the relation of a spinal nerve to its myotomic muscles. For this reason, the cerebral nerves furnish unreliable evidence as to the primitive number of cephalic segments. A'arious investigators have set this number between eight and nineteen.

Anomalies

Variations in the form, position, and attachments of the muscles are common. Most of these anomalies are referable to the variable action of the several developmental factors listed on p. 226.




Historic Disclaimer - information about historic embryology pages 
Mark Hill.jpg
Pages where the terms "Historic Textbook" and "Historic Embryology" appear on this site, and sections within pages where this disclaimer appears, indicate that the content and scientific understanding are specific to the time of publication. This means that while some scientific descriptions are still accurate, the terminology and interpretation of the developmental mechanisms reflect the understanding at the time of original publication and those of the preceding periods, these terms and interpretations may not reflect our current scientific understanding.     (More? Embryology History | Historic Embryology Papers)
Developmental Anatomy: Chapter I. - The Germ Cells and Fertilization | Chapter II. - Cleavage and the Origin of the Germ Layers | Chapter III. - Implantation and Fetal Membranes | Chapter IV. - Age, Body Form and Growth Changes | Chapter V. - The Digestive System | Chapter VI. - The Respiratory System | Chapter VII. - The Mesenteries and Coelom | Chapter VIII. - The Urogenital System | Chapter IX. - The Vascular System | Chapter X. - The Skeletal System | Chapter XI. - The Muscular System | Chapter XII. - The Integumentary System | Chapter XIII. - The Central Nervous System | Chapter XIV. - The Peripheral Nervous System | Chapter XV. - The Sense Organs | Chapter XVI. - The Study of Chick Embryos | Chapter XVII. - The Study of Pig Embryos | Figures

Reference

Arey LB. Developmental Anatomy. (1924) W.B. Saunders Company, Philadelphia.


Cite this page: Hill, M.A. (2019, March 21) Embryology Book - Developmental Anatomy 1924-11. Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/Book_-_Developmental_Anatomy_1924-11

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