Book - A Text-book of Embryology 17

From Embryology

Chapter XVII. The Development of the Muscular System

Heisler JC. A text-book of embryology for students of medicine. 3rd Edn. (1907) W.B. Saunders Co. London.

Heisler 1907: 1 Male and Female Sexual Elements - Fertilization | 2 Ovum Segmentation - Blastodermic Vesicle | 3 Germ-layers - Primitive Streak | 4 Embryo Differentiation - Neural Canal - Somites | 5 Body-wall - Intestinal Canal - Fetal Membranes | 6 Decidual Ovum Embedding - Placenta - Umbilical Cord | 7 External Body Form | 8 Connective Tissues - Lymphatic System | 9 Face and Mouth | 10 Vascular System | 11 Digestive System | 12 Respiratory System | 13 Genito-urinary System | 14 Skin and Appendages | 15 Nervous System | 16 Sense Organs | 17 Muscular System | 18 Skeleton and Limbs

Early Draft Version of a 1907 Historic Textbook. Currently no figures included and please note this includes many typographical errors generated by the automated text conversion procedure. This notice removed when editing process completed.

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The Striated or Voluntary Muscles

The voluntary muscular system, genetically considered, is divisible into (1) the muscles of the trunk and (2) those of the extremities. The muscles of the trunk include two distinct sets : (a) the muscles of the trunk proper, or the skeletal muscles, and (b) the muscles of the visceral arches or the branchial muscles.

To arrive at a proper comprehension of the evolution of the muscular system it is necessary to revert to an important fundamental emhryological process, the segmentation of the body of the embryo, or, as it is sometimes expressed, the segmentation of the coelom, or body-cavity. As pointed out in Chapter IV, this process of segmentation occurs in all vertebrate animals and in some invertebrates.

The Muscles of the Trunk Proper

At a very early stage of development the tracts of mesodermic tissue situated one on each side of the median longitudinal axis of the future embryonic body, the paraxial mesodermic tracts, undergo division or segmentation, in lines transverse to the long axis, into 'a series of pairs of irregularly cubical masses of mesodermic cells. These masses are the mesoblastic somites or primitive segments, often inappropriately called the protovertebrse. The somite first formed corresponds with the future occipital region, the second one lies immediately in front of the first, while two others, situated still more anteriorly, that is, near the cephalic end of the embryonic area, and seven more, behind the first, are added almost simultaneously. The formation of the primitive segments

tlicn prcociU tiiilwiinl until a considerable number have l)e(.'ii luldi'd. Tliorie in front of the one first formed are di'iiomiimted tlip head-segments, while the others are known iis the trunk-segments. Kacli mniito ii> at first triangular in iToss-stt;fion, the haso of the triangle looking toward the chunla dor.sidi:*. Siibsc<iiiently they assume a more ciiboidal whiiiM'. In the lower vertebrates — amphibians and fishes — the somite is hollow, its cavity being in these cases a constricted-iift" portion of the IxKly-cavity (hence the term " 8^

el'iRciiuui tlMie iiiiting tnirt rb! roin wbo«e wall.

mentation of the eieloni" to exjin.-is this iirtHrsf). In the higher vert eh rates, liowevcr. the eavitv is obliterated by the eneroiielinieiit of the cells of the wall> of the somite.

The cells of the somites soon iin(h'r<;o ditfereiitiation nnd rearm iifreii lent. It is nsiiiilly stated that, preparatory to the segmentation of the paraxial mesodermic tract, this tract has become separated from the remaining lateral plate of the mesoderm. The separation is not complete, however, and therefore, after the appearance of the primitive segments, each segment is connected with the more laterally placed lateral plate — by the separation of which latter into two lamellae the coelom is formed — by a smaller mass of tissue, the iieplirotome, also called the middle plate, or intermediate cell-mass (Fig. 17-!, vb). As development progresses the distinction between the primitive segment proper and the nephrotome becomes more sharply expressed, and the former is designated the myotome. The primitive segment on its mesial surface, near the point of union with the nephrotome, sends forth cells which form a mass called the sclerotome (Fig. 174, sk). The sclerotomes spread out and blend with each other, forming a continuous mass of tissue which envelops the chorda and the neural canal, and which also extends laterally between the myotomes, separating them from each other and constituting the ligamenta intermuscnlaria {vide p. 375) ; this tissue, being concerned in the production of the permanent vertebrae, has no further interest in this connection.

What remains of the primitive segment after the formation of the nephrotome and of the sclerotome is the myotome proper or the muscle-plate. Although, as previously stated, the primitive segments of the higher vertebrates contain no cavity, the myotome and the nephrotome each enclose a space, that belonging to the former being known as the myocoel. The myotomes or muscle-plates are so called because they give rise to the voluntary musculature of the trunk. But not all of the cells of the muscle-plate undergo transformation into muscular tissue. While the cells on the mesial or chordal side of the myoccel are going through certain alterations preparatory to their metamorphosis, the cells nearer the body-wall become rearranged to form a characteristic layer which is known as the cutis-plate from the fact that it contributes to the formation of the corium of the skin (Fig. 174, cp). The cutis-plate and the remaining part of the muscle-plate are conliniiniis around the myoctpl, the trunsition from one to the other being more or less gradMal. To suminarize, the primitive segment is differentiated into the nephrotome, the sclerotome, the myotome or mnaclaplate, and the cutis-plate.

The Metamorphosis of the Muscle-plate

By the terra inujKle-ftlate. ia meant here the thickened layer of cells on the chordal or mesial side of the myotome proper, which layer condtitiites what remains of the myotome after the differentiation of the eutis-plate. These cells having proliferated and increased in size, and having encroached thereby upon the cavity of the myotome, next undergo alteration in shape, becoming cylindrical, with their long axes parallel with that of the body of the embryo. The length of each cylindrical cell equals the thickness of the primitive segment, at least in the Amphibia and probably also | in the chick. The next step in the transformation is the ' acquisition of the transrerse Btriation characteristic of vertebrate voluntary muscle. Soon after this the protoplaf of tlic cell undergoes longitudinal division into minute] fibrillie — which latter do not necessarily correspond, however, with the primitive fibrillfe of mature muscle — and tlw I cell-nucleus likewise divides. The metamorphosis of the now tibrillated protoplasm into muscular tissue is first completed at the periphery of the fiber, so that a young muscle fiber contains a central core of undifferentiated material, including the daughter-nuclei resulting from the divis of the original nucleus. Soon after the appearance of striation and the fibrillation of the fil>er, the fibers begin to sepa^l rate from each other, and developing connective tissue witJtT young blood-vessels penetrates between them, the fibers now 1 showing aggregation into bundles. For some time longer the fibers are naked, since the earcolemma is not acquired until considerably later. The differentiation into muscular tissue gradually extends from the periphery of the fiber to its core, the process being complete in the human embryo at about the end of the fifth month for the muscles of the upper extremities and in the seventh month for tho.-ie of the lower.

The embryonic muscle-fibers are smaller than the mature elements and increase in size until the third month.

It is considered highly probable by most embryologists that muscle-fibers undergo multiplicatioii during embryonic life. There are several theories as to the method of this multiplication. The most generally accepted view is that put forth by Weismann, the essential feature of which is that the fibers multiply by longitudinal division or fission. Reference was made above to the repeated division of the nucleus of the cell as one of the initiatory steps in the formation of the muscle-fiber. According to the fission theory, there is one class of fibers in which the nuclei are arranged in a single row, and the fibers of this class do not undergo fission ; while there is another class, the fibers of which have their nuclei arranged in several rows. Fibers of the latter type divide longitudinally into as many daughter-fibers as there are rows of nuclei.

Although many of the details of the development of the muscular system are still involved in obscurity, it is a generally accepted fact that each fiber is derived from a single cell, the protoplasm of which develops the function of contractility to the subordination of the remaining vital properties of protoplasm. With this specialization of function there is nefcessarily a concomitant alteration of structure.

The muscular mass resulting from the transformation of each myotome grows in the ventral direction between the ectoderm and the parietal leaf of the mesoderm, or in other words into the somatopleure, to produce the muscular structures of the ventrolateral body-wall. The off-shoots of the myotomes which thus jKjnetrate the body-wall in the fourth week produce, in the fifth week, a muscle-mass which, for the most part, is non-segmental, and which gives rise to a dorsal and a ventrolateral division ; the dorsal division, derived from all the spinal myotomes, l)eing destined for the musculature of the back, while the ventrolateral division, springing from the thoracic myotomes alone, gives rise during the fifth, sixth, and seventh weeks to the muscles of the thoracic and abdominal walls (Banleen and Lewis *). The dorsal division extends in the dorsal direction, covering and acquiring points of attachment to the vert<}bral column, which has meanwhile Ix'cn forming. In addition to the ventnd and dorsal extension of the muscl(?-|)lates, each one grows both forward and backward — cephahid and caudad — in such manner that overhipping and intermingling result. During the differentiation of the various muscular masses from the myotomes, ventnd and dorsal l)ranches of the corresi)onding spinal nerves grow forth, their final distribution being to muscles developed from the particular myotcmies with which the respective nerves correspond. According to Bardeen and Lewis the structures of the body-wall are well differentiated by the end of the sixth week, although their extension to the mid-line is not completed until near the end of the third month.

What has been said above concerning the evolution of the trunk-musculature from the primitive s(»gments refers to those muscles that are develope<l from the segments of the trunk. As to the evolution of the head-segments comparatively little is definitely known. It is generally accepted that in ela^niobranchs — a group including sharks and rays — there and nine primitive segments in the region of the future head. The number present in mammalian embryos has not been clearly worked out. Three? oeeiptal and thirty-five spinal myotomes have been seen in human embryos of the fourth week, at which time the formation of myotomes is said to cease. In th(» l(>wer vertebrates each segment contains a eavitv lined with flattened e(»lls, the mesothelium, the metamorphosis of which into muscular tissue may be inferred to be essentially as alreadv outlined ai)ove. The first head-segment, which lies in contact with and partially envelops the optic vesicle, gives rise to the su|)erior rectus, the inferior rectus, antl the inferior t>bli(jue muscles of the eye-ball (innervated by the thinl cranial nerve) : the second segment produces the superior obli(jue (iiniervated by the fourth nerve); and the third, the external rectus (iiniervated by the sixth nerve). The fourth, fifth, and sixth segments al)ort and hence produce no adult structures ; while the seventh, the eighth, and the ninth segments become metamorphosed into the muscles that connect the skull with the shoulder-girdle.

  • Amerirnn Jtntrnal nj AniitomUj vol. L, No. 1.

From recent studies^ it would appear that individual muscles undergo peculiar and significant migrations during their development, and that the origin of the nerve-supply of a muscle indicates the location of the particular myotome or myotomes from which it originated, since the segmental nerves are connected with their respective myotomes and supply the muscles derived from such myotomes. For example, the serratus magnus, being innervated by branches of the cervical nerves, develops from myotomes in the neck region, and subsequently moves down to become attached to the scapula and the ribs.

The Branchial Muscles

This term embraces the muscles of mastication and the various muscles connected with the hyoid bone, with the jaws, and with the ossicles of the middle ear. They result from the metamorphosis of the mesothelimn of the visceral arches and acquire connections with structures that have arisen from the so-called mesenchymal cells of these arches or, in other words, from the embryonal connective tissue which makes up the chief part of their bulk. For an account of the growth of the visceral arches the reader is referred to Chapter VII. From this account and from that found in Chapter IV., it will be seen that the formation of the visceral arches and clefts is in reality the segmentation of the ventral mesoderm of the headregion of the embryo, or to express it in another way, it is the segmentation of the ventral coelom of that region. It is interesting to note that whereas in the trunk the segmentation of the mesoderm is restricted to the dorsal part of the body, in the head-region the ventral mesoderm also participates in the process. Hence the visceral arches, as might be exj)ected, consist of so many masses of mesodermic tissue, each arch containing a small («ivity lined with mesothelium, which cavity is a constricted-off part of the body-cavity or

  • See '* Development of the Ventral Abdominal Walls in Man/* Franklin P. Mall, Johns Hopkins Papers, vol. iii., 1898.

Od'loiii. It is those mesotholial ct4l8 that produce, by their diift'rentiation, the niusoles under consideration. While so nuieh ooneerninjj: the origin of tliis group of muscles is practically assured bv ol)servations upon the embryos of the lower vertebrates, the details are still obscure. His assumes the origin of the palatoglossus, the styloglossus, and the levator palati from the second or hyoid arch ; of the stylopluayngeus, perhaps the palatopharsrngeus, the hyoglossus and the superior constrictor of the pharsrnx from the third arch ; and of the middle and inferior pharyngeal constrictors from the fourth arch. Further, it is held bv Rabl that the muscles of the i\u\\ including those of the scalp and the platysma — the muscles of expression — originate* from the mesothelium of the hyoid anrh in the form of a thin superficial sheet, which, gratlnally spreading out from the place of origin, breaks up into the intlividual muscles.

The Muscles of the Extremities

The relation of i\\o (It'vclopnirnt ot'tlic inn>cles of tiie limbs to the myotomes i> >till a (li<j)uttMl point. Sdihc authorities hold that the linil)-mu-cK> of inaiiinials orii^inate from the mvotomes, as \\{\< >li()\vn l)v l)«)lirn U) l)c the ca>e with the fin-musculaturc of Selachian-. A tact adduced as a strong argument in t'avnr «>f tht'ir niyotomic oriiiiii is that the ncrve-su|)[)ly of each liiiil) ('(U'rc-jjonds with the nerves of the number of myotoniie x'unients in relr.tion with which the limb-bud <lcvelops (rid, |). loi)). ( )n th(* other iiand, it is stated* that the myotome- do n(»t extend into th(» developini:- limb-buds, but that the inn.-ch'< ol" the liinl).- are diil'ci-entiated from the mesenchvinal core ol' the liinh-bud, thi- procos following the entrance of the motor nerve-fihi'rs into the member. The mn>chs of the npper lind) are so well advanced in their devel<»|)ment by th(*' >ixth week a< to be individually distingni-hablc. th(»-^e (if the lower limb reaciiing a corresponding stau'e in the >eventh week.

The Involuntary or Unstriated Muscular Tissue

This variety of muscular tissue, like that considered above, is of mesodermic origin. But while the voluntary muscles arise from the flattened or mesothelial cells of the primitive segments, involuntary muscle results from the transformation of the embryonal connective-tissue elements, the mesencliymal cells, of the mesoderm. It is for this reason that some authors speak of the voluntary muscles as the mesothelial muscles and designate the involuntary muscular tissue as mesenchymal muscle.

While it is a generally accepted fact that each of the fibercells which make up nnstriated muscle is a metamorphosed mesenchymal or connective-tissue cell, the details of the process have not been accurately worked out. One may assume that necessarily the young connective-tissue cell elongates and that its protoplasm must undergo such differentiation as will fit it for the exercise of its future function, contractility.

The Cardiac Muscle

The account of the development of the heart-muscle will be found in Chapter X.