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Hamilton WJ. Boyd JD. and Mossman HW. Human Embryology. (1945) Cambridge: Heffers.

   Human Embryology (1945): 1 Introductory Concepts | 2 Formation Maturation and Structure of Germ Cells | 3 Cyclic Changes in Female Genital Tract | 4 Fertilization Cleavage and Formation of Germ Layers | 5 Implantation of Blastocyst and Development of Foetal Membranes Placenta and Decidua | 6 Fate of Germ Lavers and Formation of Essential (Primary) Tissues including Blood | 7 Growth of Embryo Development of External Form Estimation of Embryonic and Foetal Age | 8 Determination Differentiation Organizer Mechanism Abnormal Development and Twinning | 9 Cardio Vascular System | 10 Alimentary and Respiratorv Systems Pleural and Peritoneal Cavities | 11 Urogenital System | 12 Nervous System | 13 Skeletal System | 14 Muscle and Fascia | 15 Integumentary System | 16 Comparative Vertebrate Development | Figures
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Chapter XIV Muscle and Fascia

The muscular tissue or the Ml ss with the exception of the muscles of the iro and the Mhlwt nils of the sneat glands (pi^e 3GC) 11 demetl from the mesoderm iV there arc important differences m the once and m the hwioqcncsn of toluniai^ (stnpeti or sep mental) molunlar) (smooth sisctral or non «.c^ental), and carrftac muscle tt is neccssir^ to describe Uicir development separately

Voluntary Musculature

This musculature is mainly derived from the pam^ial mesoderm by way of the sntmcntally arranged somites but as will be described later certain important parts of jt eg that of the limbs tongue and orbit and the segmental musculature avsoriated with the branchial arches* appear to arise mdependentlv of the somites differentiating in iilu m the mescnchvtnc

Mesodermal Somites

The segmentation of the paraxial mesoderm » a fundamental feature of the chordates and it appears early in development fbe embryonic somites arc fonned (page 52), from the paraxial mesotlerm and each of them diffetenuates into \ motomr (which gives ongm to muscle) a dermatome (giving ongin to integumenury tissues) and a sclrrotomt (concerned m the develop, ment of the axial skeleton) It is important therefore to have a thorough understanding of the manner of origin and developmental Imtorv of i)ic somites m order to undcntarid the segmental anitomy of the body Mesodermal segmentation is mirrored in the brain and spinal cord and « closelv related to the segmentation of the catlv excretory svstem

In most human embryos there are ^ occipital and 8 cervical and usually 12 thoracic, 5 lumbar 5 sacral and 0 to 10 coccygeal pair5 of somites (Kunitomo, 1918) The commoner variations from these numbers arc the presence of 11 or 13 thoracic 4 or 6 lumbar and 4 or 6 sacral somites As a rule a variation from the normal somite number in one region is iccom panicd by a compensatory variation sn an adjoining region an additional lumbar somite js compensated by the presence of one less sacra! and an additional thoracic by one less lumbar, etc Thus the total number of somatic segments represented in the average ii mm human embryo (the time of maximum number) is 42 to 44 (fit, 391) The first occipital (Arcy, 1938) and the last seven or eight coccygeal somites undergo nrh retrogression ind seem to disappear enlttely giving n e to no permincni skeletal structures Hie first somite to appear is the first occipital and the succeeding ones conimue to be formed in in uninterrupted cramo-caudal senes (page 52} and difTeremiation also follows this gradient the structures derived from the somites being first produced antenorly However the difTcrential gradient is not so much m favour of the cranial somites as might be expected as it has been shown that the later somites arc pro restively more advanced at the umr of their formation than are the cranial ones (Butcher, 1929) Finally u mav be said that the eirly dificrcntiation of each somite into a dermatome myotome and sclerotome is csventiatly the same throughout the persisting segments (that is second occipital to third or fourth coccygeal) but that the later history of the mjotomic derivatives vanes greatly in dinercnl legjom Somite dilfereniiauon is most typical in the mid thoracic region and therefore, this region wdl be considered first The oncm of the segmental musculature is summarized m Table II

• Thij branchial muscutaiare u probably visceral visceral origin explains why the branchio-mntor nerves ai

i^ngin although its fibres become sinaied Tbjj referred to as ipccial visceral effmtit (p ijS)

Differentiation of a Typical Thoracic Somite

In a human embryo of about 5 mm the cells of the ventro-medial portion of, for example, the fifth thoracic somite lose their epithehal-hke character and become mesenchymal (Fig. 369). They migrate medially towards the notochord and soon extend between it and the neural tube dorsally and the aorta ventrally to fuse with similar tissue from the opposite somite (Fig 223). This secondarily mesenchymal portion of each somite is the sclerotome which, through fusion with that of the opposite side, forms part of the pnmordium of an intervertebral disc and the major part of the vertebra immediately caudal to the disc (Chapter XIII). The remaining dorso-lateral epithelially-arranged portion of the somite is the dermo-myotome, from the dorsal and ventral edges of which cells proliferate to form a mass lying against its medial or deep surface and separating it from the sclerotomic tissue (Fig 370). The cells of this mass are embr^'onic muscle cells or myoblasts and the mass as a whole is called a myotome (Fig 27-^, Uj The less proliferative lateral portion of the original dermo-myotome is now called the dermatome. As soon as the myotome becomes well differentiated the dermatome rapidly loses its epithchoi character and probably becomes, like the sclerotome, a secondary mass of mesenchyme 11s spreads beneath the overlying ectoderm, clinging to it, so that there is little doulit that i^^is destined to become the dermis and subcutaneous tissue (Chapter XV). In p^t, however, the function of the dermatome is to act as a germinative zone for the myotome (Glucksmann, ' 934 )

Table II. Origin of the Segmental Musculature

Muscles Myotomes Nerves
Orbital (extrinsic eye muscles) 3 Hypothetical anterior head myotomes (Direct evidence in the shark ) Cranial III, IV and VI
Tongue 3 Occipital (Direct evidence disputed ) Cranial XII
Intrinsic of back (extensors of vertebral column) Dorsal divisions of Cervical i to Sacral 3 or 4 Cervical 1 to Sacral 3 or 4
Cervical Prevertebrals


Geniohyoid and infrahyoid

Ventro-lateral divisions of Cervical i to 8 Prevertebral portion
Mam lateral sheet Ventral edge or rectus column Cervical 1 to 8
Diaphragm Ventro-lateral divisions of Cervical 3 to 5 Cervical 3 to 5
Anterior and lateral thoracic and abdominal wall Intercostals, obliqui and transversus

Rectus abdominis

Ventro-lateral divisions of Thoracic i to Lumbar i
Mam lateral sheet. Ventral edge (rectus column) Thoracic i to Lumbar i
Posterior abdominal wall (quadratus lumborum) Ventro-lateral divisions of Lumbar i to 5 Lumbar i to 5
Pelvic diaphragm (coccygeus and levator am) Ventro-lateral divisions of Sacral 2 to Coccygeal I (Direct evidence questionable )

Sacral 2 to Coccygeal i

Anal sphincter and external genital

Probably same as pelvic diaphragm, but has been traced back only to cloacal sphincter said to be a “skin” muscle

Sacral 2 to Coccygeal i

Upper limb and shoulder girdle

Mesenchyme of limb-bud in man, but phylogenetically from Cervical 3 to Thoracic i

Cervical 3 to Thoracic i

Lower limb and pelvic girdle

Mesenchyme of limb-bud in man, but phylogenetically from Lumbar 2 to Sacral 3

Lumbar 2 to Sacral 3

The crlls or the ra)otome u.uillt elongate in a p.ttnUel to the Ion; a-tn of the emhoo rhet Itceomc relatnch thtcl. tpmdle, tpherteal nuota and cttopbtnt Net, carlt (Strait and Wctidell toio) these mtohlaiu detelop the poner to Sntract upon st.muhuon althou;!, tt .. unltVeK tint under normal thm are st.muhled unttl serertl t-eehs liter tWindle tnin) Tlte further d.lTerentiat.on of tmobla-ti

mto mu'clc fibres is described on 3*^7 , n i .1 the diiappeirancc of the dermatome the miotome enllr;ra rapidh bo h dorsall) nanlan; the neural tube and lentnlh nhrre It extends into the lomalopleilre At the fibres of the oth thoracic nrrxe prmsini; out fiom the neural tulie rnafe contact oittt tne

m^oiomic cells Tins connexion once established is a peirmncnt one (Chapter \II) Between the fifth and sixth wceh (7-8 mm) the mjotome liecomcs disidcd h) a slight lonttitudmal constriction into a dorsal portion {ffimert) and a \cmro-lalcral portion [hti^mere) (Iiij 392) The nerve likewise becomes split into a dorsal or postenor pnmary ranttu and a ventral or nn/mor primary ramiis connected to the corrcspondint» portions of the mvotome Processes of sclerotomic tissue extend laterall> between the fifth thoracic mjotomc and the fourth (alwve) and sixth (below) m the plane of the rtoovc divadini; the epimcres and hvpomcrcs Tliesc processes wall become the transverse processes of the fifth and sixth thoracic vertebrae As the epimercs and h>pomeres become more complctcl) separated the mcscnch>-me left between them forms a sheet or intermuscular septum which is attached craniall> and caudall> to the two transverse processes and separates permanently the dorsal division of the muscle from the ventro-lateral division (Fig. 392). This is the rudiment of the lumbar fascia of this segment. Thus the division of the myotomes into dorsal and ventro-lateral portions apparently determines three things . (i) the primary branching of the spinal nerves; (2) the plane of formation of the transverse processes of the vertebrae; and (3) the first intermuscular septum of the fascia of the body, the lumbar fascia Soon after the appearance of the transverse processes the rib elements are laid down in sclerotomic tissue which migrates into the ventral portions of the original intersomitic clefts. As the ribs reach their maximum development in the thoracic region the original segmentation of the hypomeres is retained more obviously in this region. The dorsal divisions of the myotomes (epimeres) divide further into a medial and lateral group which will together give rise eventually to the extensors of the vertebral column The medial group, by the fusion of the relatively fe%v consecutive segments and subsequent longitudinal and tangentia sp itting, gives rise mainly to short oblique muscles {semispinalis, multifidus and rotatores) and to the all of which are supplied by the medial branches of the posterior primar>' rami o spinal nerves. The lateral group, by the fusion of a larger number of segments an subsequent splitting, gives nse to the longer muscles (the iho-cosialis, longisstmus a splemus) ivhich are supplied by the lateral branches of the posterior primary rami ( igs. 3J and 394.).

Fig 392 — A schematic representation of the developing musculature in a 10 mni human embryo; somatic musculature, red, branchial musculature in other colours The presumed migration of the somite musculature is indicated by interrupted lines. X c 21.

The ventro lateral divisions (hypomerts) of all the thoracic and the first lumbar mj otomes extend into the somatopleure and gi\e nse to the ventral and lateral trunk muscles i e the flexors of the \ ertebral column These muscles are supplied bj the anterior pnmary rami of the spinal nerves In the thoracic region where the nbs are full^ developed the ^ musculature largely retains its segmental arrangement as the intercostal muscles but IS spht tangentially into three layers (Fig 393) ® superficial external intercostal an inter mediate internal intercostal and a deep intra costal or transversus thoracis muscle In the abdominal region owing to the absence of nbs the vcntro lateral musculature fuses to form a contmuous muscle sheet This nou 393 -A schemsuc .ceuon through the ^orucic

, rcgioa of a 15 mm human embryo to show the becomes suodivideQ into a narrow ventral arran^tnmt of the muscle sheets and the diitnbu

portion {rectus abdominis) which retains some nonoftbespinalnerves to them (after Br>cc 1923)

of its segmental character m the tendinous

mtersecuons and a broad lateral portion which subsequently becomes split tangentially into three layers the external oblique the internal oblique and the transtersus abdominis In the upper thoracic region the rectus sheet normally disappears but is occasionally represented bv the stemahs muscle

Attention must be drawn here to the experimental investigations of Rawics and Strauss (1948) which indicate that in the chick it is only the dorsal part of the trunk musculature that IS denved from the somites According to their observations the ventral trunk musculature arises directly from the lateral plate mesoderm

The Lumbar Sacral and Coccygeal Myotomes

Only the first lumbar myotome develops in the same manner as the thoracic myotomes.

Its ventro lateral portion giving nse to the most caudal portion of the fraiurmf and oblique muscles of the abdominal vvall The remainder of the lumbar my otomes have very small ventrolateral portions which give nse mainly to the quadratus iumborvm (Fig 394) The dorsal divisions give nse to all the lumbar extensor musculature the mam portions of which in this region are the sacro spinalis the multifidus and the roiatores

In the more caudal sacral and m all the coccygeal segments the extensor musculature shows earh degeneratn e or retrogressiv e changes and IS represented m the adult by the dorsal sacral ligaments Apparently the ventrolateral dmsions of the third sacral to the first coccygeal segments persist to form the muscles of the pelvic diaphragm {levator am and coccygeus). Probably the voluntary anal sphincters and the voluntary muscles of the external genitalia are formed from these same segments, although these muscles have not been traced back further than the cloacal sphincter which is said by Popowsky (1899) to be a “skin” muscle (Figs. 397 and 398). At least the medial part of the levator am is considered homologous with the rectus column. On the basis of their comparative anatomy and adult innervation,* it seems justifiable to assume that the muscles of the pelvic diaphragm and the striated musculature of the anus and genital organs are of myotomic origin

The dorsal divisions of the cervical myotomes give origin to the extensor musculature of the back of the neck. This is more elaborately developed than the corresponding musculature of the thorax. The ventro-lateral portion of the myotome in the neck is highly atypical due to the development of the pectoral girdle and limbs, to the recession of the coelomic cavity from the neck and also to the presence of visceral arch structures and muscles. The important muscles derived from these ventro-lateral portions of the myotomes are the scalenes and prevertebrals {longus capitis and cervicis), which correspond to the intercostal and ventrolateral abdominal muscles, and _ _ the geniohyoid and infrahyoid

1 DORSAL LIMB muscles which correspond to

V-Hl r<NF>nDLE MUSCLES abdomiuis (Fig.

395 ) \ 1 the head myotomes

The only head myotomes

EXTENSOR which can be recognized in , muscles of limb mammalian embryos

_ ^ • are four in the occipital region

I'm 395 — A schematic section through the cervical region of a ('■p'ltr onr'l Of these the first

15 mm human embryo to show the arrangement of the muscle ,

rudiments in the neck and upper limb (after Bryce, 1923) completely disappears, prOD ably by dedifferentiation, ^ at

about the 20-somite stage (Arey, 1938). The other three differentiate into fairly typical sclerotomes, dermatomes and myotomes.

On the basis of comparative anatomy (Edgeworth, 1935) and adult innervation it seems fairly certain that the ventro-lateral divisions of these three occipital myotomes give origin to the tongue muscles, both intrinsic and extrinsic (Fig. 392). However, it has been denied by many who have studied the problem that any migration of these myotomes to the floor of the mouth can be observed during human development. Yet since the hypoglossal nerve, the motor nerv’e of the tongue, is formed by the grouping together of the segmental nerve bund es which grew out towards these myotomes, it seems reasonable to accept the myotomic origin of tongue muscles as a working hypothesis in human ontogeny.

A further speciahzation occurs m the development of the extrinsic muscles of tlie eye (orbital muscles) for here typical somites have not been found m any mammalian embryo

Tlic inncrL'ation of adult muscles is used as a clue to their embryonic origin This is fact that embryonic muscle masses receive their motor inners'ation very early in ontogeny, and that, wui / fcLs exceptions, the adult muscles derived from them retain this innervation regardless of how hir during development “The nerve supply to a muscle is the best index to its morphology. u infallible . ” (Bryce, 1923) See also Straus (1946).

Flexor Muscles of Limb

Fig. 395 — A schematic section through the cervical region of a 15 mm human embryo to show the arrangement of the muscle rudiments in the neck and upper limb (after Bryce, 1923)

mnervated b> the Illrd IVth and Vlth cranial nerves (Neal, 1918) Since these muscles an

Fig 396 — A schematic representation of the atrangetnent of the muscle groups deri\ ed from various sources The colour scheme is suDiIax to that rf Figs 391 and 39'*

nen. es are cIosel> analogous to the corresponding ones of higher \ ertebrates it ma> be assumed that the orbital musdes of man are derived ph^logeneticall^ from three pairs of anterior (pre otic) head myotomes The orbital muscles of man are first recognizable as a small aggregation of

condensed mesencb>mem the region of the embryonic e>eat about the hmb bud stage (Fig 391)

(For a detailed description of the development of the extrinsic ocular muscles, see Gilbert, 1947 ) The tongue muscles, according to most authorities, appear m a similar fashion in the mesenchyme of the developing tongue. In other words, in higher vertebrates the methods of formation of tongue and extrinsic eye muscles have been condensed and abbreviated so that these muscles now appear to differentiate in situ from mesenchyme.

The Visceral Arch Musculature

The pharynx of lower vertebrates is characterized by a secondary segmentation {branchiomensrri) caused by the presence in its wall of paired gill slits. In the higher amphibia these openings disappear at metamorphosis ; in reptiles and birds they are present only m the embryonic states and, in mammals, they are represented by internal pouches and external grooves (pharyngeal pouches and branchial grooves. Chapter X) which approach one another but are normally separated by the so-called pharyngeal membrane, so that actual opemngs from the pharynx to the outside do not occur. The pharyngeal wall tissue between these consecutive grooves forms a series of visceral arches, the ist arch separating the ist cleft from the stomatodaeum. Since, in the primitive vertebrates, most of these arches bear functional gills, each arch consists of the following structures —

  1. An arterial arch (aortic arch) primarily to supply blood to the gills
  2. A skeletal arch to support the pharyngeal wall and gills.
  3. Muscles to move the arch.
  4. A nerve to innervate the muscle and adjacent skin and mucous membrane.

In mammalian embryos these four fundamental elements are still present even though the pharyngeal structures differ greatly from those of embryos of fishes where the gills function in the adult. In human embryos of 7 to 10 mm. condensations of mesoderm are found in the dorsal end of each of the five (or six, see page 178) visceral arches The mesoderm of the condensations is of lateral plate origin. In lower vertebrate types, however, they are connected to some of the head somites. The condensations differentiate into myoblasts, and the voluntary motor portion of a special visceral cranial nerve grows into each of the muscle rudiments The complicated growth, subdivision and migration of each of these muscle masses are summarized diagrammatically in the series of Figs 391, 392 and 396; for convemence the derivatives of the visceral arches, both skeletal and muscular, are given in Table III It mu^t be pointed out that there is considerable uncertainty about some of the items set forth m Tables II and III As in the case of the muscles of the tongue and orbit, it has been difficult to trace the exact embryological origin of many of these branchial (special visceral) muse ^ from the primary rudiments appearing in the arches Controversy exists chiefly m regar to the derivatives of the more caudal arches. This is because these are relatively rudimentary in mammals and their parts are not as clearly differentiated as in the first two arches.

The Limb and Girdle Musculature

The muscles of the upper and lower limbs (including the limb girdles) are segmenta y innervated by the spinal nerves, but in mammals it has not been possible to trace their ongm back to the myotomes In some of the fishes, however, this has been accomplished T ere or^, the facts of comparative anatomy and comparative embryology and the adult innervation mus be cited as justification for the working hypothesis that the limb muscles are of segmenta It has already been explained (page 359) that the musculature of the ventral body be of lateral plate, and not of somite, origin , it is possible that the limb musculature is 1 c' primarily of lateral plate (i.e , somatopleure) origin.

The limb musculature can first be identified histologically in human embryos ol o-io as mesenchymal condensations near the base of each limb bud According to some aut these condensations are found round the terminal ends of the spinal nerves which are ex cn into the limb-bud at this time (Fig. 332). Since muscles will develop m limbs devoi innervation in certain types of monsters and in experimental animals (Hamburger, 1939) it IS unhkeK that the nerves are concerned with oi^nizint, pnmap^ muscle location

The early limb buds are somevshat flattened dorso ventrally, with cephalic (preaxial) and caudal (postaxial) borders They have a cramo caudal attachment to the body opposite a number of m>otomes (Figs 105 and 392) The superior limb buds lie opposite the lower SIX cervical and the first and second thoracic segments while the inferior buds are opposite the second to the fifth lumbar and the upper three sacral segments Branches of the nerves supplying these myotomes reach the base of their respective limb bud and as the bud elongates to form a limb they extend mto it m such a manner that the limb muscles of the preaxial border

Table III Origin of the Visceral Arch Musculture

I uceTal Arch



\rne of Museles


Quadrate cartilage— incus

Meckel s cartilage — malleus anterior ligamcrit of the mat leus spheno-mandibular liga ment (’) central core of body mandiole

Muscles of mastication (temporal tnasseler medial and lateral pterygoids) '

Mylohyoid and antenor belly of digastric

Tensor palati and tensor ty mpani

\ Trigeminal mandibular division — (Post tremawc)



Styloid process

Stylohyoid ligament

Lesser cornu and upper part of the body of the hyoid bone

Facial group (including buccinator extrinsic and intrinsic auricular muscles occipito frontalis and platysma)

Posterior belly of digastric and stylohyoid


VII Faual (Post trematic)




Greater cornu and lower pan ot ' the body of the hyoid bone


Probably part of upper pharyngeal muscles I

1 \ Glossopharyn geal (Post trematic)

4 3 and 6

Thyroid cartilage ’Other laryngeal cartilages

Pharyngeal and laryngeal muscles

\ Vagus (superior laryngeal and pharyngeal bran ches) [possibly XI)

Laryngeal muscles

\I Cranial fibres (possibly X) by way of the superior and recurrent laryngeal nerves

’ post 6ih

’Tracheal cartilages

^ Slemomastoid and trapezius

\I (Spinal fibres)

of the upper limb are innervated by the lower five cervical nerves while the muscles of the post axial side receive the last cervical and first thoracic fibres In the lower limb the preaxial group of muscles receives fibres from the second to the fifth lumbar and the first sacral nerves the postaxial group from the first second and third sacral nerves Both pre and post axial muscles lend 10 be split into a ventral or limb flexor group and a dorsal or limb extensor group The nerves of the limbs are likewise divided into anterior and posterior branches supplying flexors and extensors respectively Thus the radval wexxe deivNwi from the posterior divisions of the trunks of the brachial plexus supplies mucics which with one exception are extensors Likewise the ulnar and median nerves which are derived from the anterior divisions of the trunks ot the brachial plexus supply flexor musdes

Modification of the prim, me segmental arrangement of the nenes entenng the limb buds has resulted m the formation of compbcaled plexuses due to caudal migration of the attachment of the hmb-bud and intrinsic migration of its individual muscles during development. Nevertheless, it is possible to demonstrate that the muscles in the adult limb have a fairly consistent segmental innervation. This is clearer in the upper limb than in the lower where apparently more extensive migration and rearrangement of muscle has occurred.

Fig 397 Schemes of the development of the perineal muscles and their nerves of suppl) in the undifferentiated stage and in the male (after Popowsky, 1899) A undifferentiated stage, 2 months B 3 months C — ^ months D —

5 months.

The Diaphragm

In somite embryos a transverse septum is formed by the caudal portion of the pericardium and the cephalic wall of the yolk sac (Fig. 84) It consists chiefly of the loose mesenchyme surrounding the terminal parts of two pairs of extra-embryonic veins, vitelline and umbilical, entering the caudal end of the heart. At the time of its formation the dorsolateral parts of this septum he opposite the third, fourth and fifth cervical somites (Figs. 217 and 219). In the early hmb-bud stage the myotomes of this region have split into dorsal and ventro-lateral portions and groups of myoblasts from the latter migrate into the cranial surface of the septum carrying their nerve fibres with them. They spread out over this cranial area at the time when the liver bud is occupying the bulk of the septal tissue. The coelomic cavity rapidly extends into the septal tissue lateral, dorsal and anterior to the liver separating from the septum a bulky hepatic mass with its ligaments and mesenteries and leaving a thin cephalic sheet containing the muscle tissue derived from the cervical myotomes Thus a muscular diaphragm is formed. During this time the heart has receded from the neck to the thorax, apparently pushing the diaphragmatic musculature caudally. Although the diaphragm is finally situated in the lower thoracic and upper lumbar region it still retains its motor and, m part, its sensory innervation (phrenic nerve) from the third, fourth and fifth cervical nerves which is clear evidence of its cervical origin. The lower six thoracic nerves supply sensory fibres to the periphery of the diaphragm The final closure of the pleuroperitoneal canals by the diaphragm is discussed un er Coelom in Chaptei X.

The Development and Closure of the Body Wall

The body wall (somatopleure) of eaily vertebrate embryos is extremely thin and transparent, consisting of a layer of ectoderm and a layer 0 _ _ . , T’V.ic body wall parietal mesoderm (Fig. 50) This body va begins to thicken dorsally soon after the myotome form and the ventral edge of this thickening can be seen to advance over the sides of the viscer (Fig 207) until in the human embryo of 20 mm only a broad diamond-shaped transparen area remains round the attachment of the umbilical cord. The transition between the transparent and the thick body w'all is abrupt; the thick portion in the earlier stages contains advancmij ventro lateral portions of the in>otomcs and dermatomes In later embr> os these mvotomes difTerenliale to form the mass of body wall muscle the ventral edge of which forms the ventral ribbon musculature of the rectus column Only as the right and left edges of tfus true body wall meet and fuse are the edges of the rectus muscles brought close together This fusion of the body wall occurs first m the upper thoraac region and then suprapubically, and from these two ends approaches the umbilicus Complete replacement of the somatopleure by the true body wall occurs at about the end of the twelfth wceli. of mtra uterine life (70 mm ) In the thoracic region the ribs and right and left sternal bars of cartilage form in the growing body wall The sternal bars are first brought together in the region of the manubrium and gradually fuse tow ird the -viphoid end Failure of the final fusion of these sternal bars results m cleft or forked sternum which is fairly common More gross failure of union of the right and left thoracic wall results m ectocardia or cardiac hernia Failure of umon in the umbilical region results in greater or lesser degrees of umbilical hernia Incomplete closure near the pubis may produce herniation and incomplete closure of the bladder Incomplete development of the infra umbilical part of the antenor abdominal wall associated with incomplete development of the antenor wall ol the bladder results in ectopia vesicae and defects of the external gemtalia (Chapter \I) The line of normal fusion of nght and left true body wall in the abdominal region is the hnea alba

It should be realized that the somaiopleunc body wall contains neither muscles vessels nerves nor skeletal elements These all grow out with the myotomic muscles Even the ectoderm of the somatopleure is not true skin as it is merely a single layer of epithelium The dermis and stratified epidermis, skm glands and hair follicles only develop over the muscular wall although normally the true skin does advance a few milli metres beyond the muscles on to the umbilical cord after the wall is completed

Involuntary Musculature

All smooth and cardiac muscle arises mde 398— Schemw of the development of

pendentl, of seg^mal structures (somttes and pTiT.kf

Visceral arches) Most of it is derived from »^9l A — 4-5 months R — 6 months

visceral (splanchnoplcuric) mesoderni that is

the mesenchyme covering the yolk sac allantois (bodv stalk) gut and Us denwUves This includes the longitudinal and circular muscle and the muscularts mucosae of the intestine and the muscle of the trucAci and JronrAi As the embryonic heart aortae 'itellme and allantoic vessels also develop in this mesoderm their muscular coats are also of visceral mesoderm ongin Later hovvever vessels form in the somaiopleunc mesenchyme of the body wall limb buds and head and although their endothelium may be formed by sprouting from the original vessels (page It appears certain that their musculature is fornied from the surrounding mesenchyme Hcn?c it may be said that mesenchyme cv cry-w here is a potential source of the smooth musculature of blood v«sels Similarly smooth muscle of the embryonic and adult urogenital tract must m many places be derived from non splanchnopleunc mesoderm for example the smooth muscle of the vasa deferentia, uterus and ureters

SDCcfiV^ » ““'der separatd, Ac development of the smooth musculature of

nvrcfcl, olops t» Jita from the mesenchi-me sutroundmt; the parenchi-ma of the organ Attention is drawn elsewhere to variations in particular organs and regions. It must finally be noted that a number of investigators claim that the smooth muscle of the iris is of ectodermal origin (Retzius, 1893; Nussbaum, 1901; Lewis, 1903; Haggqvist, 1931). Such an origin is also generally accepted for the so-called “myo-epithelial” cells of the ducts of the sweat glands. As contractility is characteristic of living protoplasm it IS not surprising that cells other than mesoderm may develop specialized contractile elements. Further, rigid adherence to the germ-layer theory has been rendered unnecessary by modern embryological analysis (page 124). Vinnikov (1938), on the basis of tissue culture studies, has suggested that the iridial muscles are myoneural elements, not true smooth muscle cells. This explanation is not readily acceptable, however, in those vertebrates (birds and reptiles) where the sphincter and dilator pupillae muscles are striated. Whatever the real nature of the muscles of the iris may be there is little doubt that they differentiate from the ectoderm of the optic cup; it is quite otherwise with the ciliary muscle which arises from the mesoderm surrounding the cup.

Histogenesis of Muscle

Smooth Muscle. Nearly all smooth muscle arises from mesenchyme, but there is a fundamental difference of opinion on the nature of mesenchyme which complicates any description of its histogenesis. Many investigators regard the mesenchyme as being essentially a syncytium m which the processes of adjacent mesenchymal cells are fused to form a protoplasmic continuum, while other workers (notably Lewis, 1922) maintain that while there is contiguity of the processes, there is no protoplasmic continuity If there is no primary syncytial arrangement the transition from a mesenchymal cell to an embryonic smooth muscle cell is easily followed as far as the microscopically observable changes are concerned. The myoblastic cells and their nuclei elongate, becoming fusiform, and arrange themselves in groups and layers which are orientated in the same direction In cells in the transitional stage rows of cytoplasmic granules are seen which coalesce to form myofibnllae. These may be artefacts as they have not been demonstrated in living smooth muscle cells which appear to possess a homogeneous cytoplasm. Later both coarse collagenous and fine reticular intercellular fibrils develop throughout the smooth muscle condensation. These connective tissue fibrils bind the smooth muscle cells into functional groups. Some embryologists regard these fibrils as products of myoblasts, but It is probable that they are produced by a fibroblastic differentiation of some of the mesenchymal cells in the original condensation If the former interpretation is correct it indicates a close relationship of smooth muscle cells to certain types of connective tissue cells.

The origin of smooth muscle cells from a mesenchymatous syncytium has been descnw by a number of workers including McGill (1907) and, more recently, Haggqvist (igsOthis view the assumed connexions between the processes of adjacent primitive mesenchyma cells in an area where smooth muscle is to be formed become progressively broader uim eventually the myoblastic mass presents the appearance of a multinucleate synctium. In t is syncytium there are no clear indications of cell walls and the myofibnllae which develop appear to pass through long stretches of cytoplasm and past many nuclei without interruption.

It is generally agreed that growth of smooth muscle can take place by the differentiation of myoblasts from mesenchyme until quite late in development, or by mitotic division of already differentiated smooth muscle cells or by the increase in size (hypertrophy) of individual cel s All of the changes have been described as occurring m the pregnant uterus, but the re importance of each in the growth of this organ in pregnancy has not yet been finally assesse In general in adult smooth muscle some power of mitotic activity is retained, but the capaci for regeneration is small and any extensive injury heals by fibrous tissue replacement (scarringj.

Cardiac Muscle. Although this muscle is involuntary it is cross-striated like muscle; furthermore, it appeals to form a syncytium* and the mesenchymal cells from w

• The syncytial nature of heart muscle is almost universally accepted, but there is some f particularly from tissue culture observations, that the syncytial appearance is more apparent . (.’ndent two cells, apparently united by an extensive cytoplasmic bridge have been observed to contract wiin 1 rhythms (Less is, 1926)

the muscle IS dcs eloped $ho%% a precocious fusion of their processes and extensive mtrac^ toplismic development of fibrib The fihnls of cardiac muscle develop like those of smooth muscle, thev arc first indicated rows of fme c>toplasnuc granules which coalesce to form the fibrils They form m the cytoplasm on all sides of the nuclei and extend uninterruptedly through the intercellular processes from one cell to another They multiply greatly in number, apparenUy by longitudinal splitting as well as by new formation Like the fibnls of voluntary muscle and unlike those of the smooth variety they become segmented bv the formation of allcmatmg light and dark portions The light bands of ncighbounng fibnls come to he opposite to one another the dark bands hkcvnse thus giving the cross smation effect of the muscle fibres Although the fibnls arc clearly seen m fixed cardiac muscle they are not icadilv demonstrated in the living muscle cells (Leviis I96)

A peculiar feature of adult cardiac muscle is the appearance of intercalated discs which at first sight seem to cut transversely across the intercellular processes separating the indmdual cells These are very scarce or absent m foetal hearts and only liecome numerous some years after birth They are not complete sepia across the processes and therefore do not change the syncytial nature of the muscle

The most unusual feature of the heart muscle and one of great practical importance is the conducting system {Putkinje tissue) concerned with the miliaiion and propagation of the cardiac impulse and assuring an efficient and rct3;uUf contraction sequence m the organ These cells are cardiac muscle cells specialized physiologically for conduction rather than contraction They become noticeable m late embryonic or early foetal hearts as bundles of muscle cells with relatively fewer fibnls and relatively larger diameters than the bulk of the cardiac muscle cells (Godlewski 1902 Sanabna 1036) They are mainly located just external to the endo* cardium and extend waihoul interruption from the atnum to the venincle as the aino rentncvlar bundle

Voluntary Muscle Myoblasts destined to form voluntary striated muscle fibres may develop from citlicr somite or non somite (eg branchial or possibly lateral plate) mcscnchyTOe The myoblasts derived from either of these sources are short spindle shaped cells with spheroidal nuclei and a relatively large amount of clear cvloplasm Rous o£ Fine granules soon appear m the cytoplasm and fuse to form fibrils which pass from one end of the cell ^on all sides of the nucleus) to the other (I ig 94) These granules are regarded by some as mitochondria but this seems improbable (Ueed J936) Neiertlieless the mitochondria arringe themselves in close association with the developing fibnls ^^hlch arc at fini homogeneous but later develop cross striations rcgularlv distributed along their length These are the precursors of Henscn s lines in the A discs of adult stn tied muscle Still later other

thickenin'^s which become Krauses membranes in the 1 or 7 discs appear midviav bctvicen successive A discs The fibrils attain this segmental structure before they arc numerous enough in an individual cell to produce the appearance of cross stnation and while the cells are still tmi nucleate This is the state in the early foetal stage (third month) The fibnh multiply rapidly by new formation and by longuudmal spUutng and it the same time the mv oblasts become multinucleatcd This may be the result of secondary fusion of separate cells or the multiplication of the nuclei without cytoplasmic division (Weed 1936) or both These processes result in the production of multinucleatcd muscle fibres In the last three months of gestation the previously centrally located nuclei seem to be crowded to the surface of the fibre by the fibnls now densely packed asm the adult condition The muscle spindles (proprioceptors) ofvoluntan muscle can be distinguished at about the twelfth week of gestation vCuajunco 1940) Thev deselop practically in the same manner as the contractile elements but their nuclei remain central and their fibrils are coarser and not so denselv packed Sensory as well as motor nerve endings also become intimitely associated with the muscle spindles

Eanj-smaled musefa gro« b, d.lferoMlion ornE^. mj oblasts from the atijacem mesen chyme b) mitotic dmsions of early myoblasts before fibnls have formed and by the increase msiae of the indmdoal fibres Post natal gnnsth seems to be almost entirely due il etilargemcnt

of pre-existing fibres The regenerative capacity of adult striated muscle is limited and defects are mainly replaced by fibrous tissue (see Gehlen, 1937; Clark, 1946).


Fasciae are made up of fibrous tissues. (For a description of the histogenesis of connective tissues see Chapter VI ) Developmentally fasciae arise from the mesenchyme between, and surrounding, the various organ rudiments; e g , around the primordia of muscles, bones, vessels, viscera The mesenchymal cells may differentiate into one of the following —

(1) Dense fibro-elastic tissue which makes up most of the more definite named fasciae of the body, especially the muscle fasciae (epimysium).

(2) Areolar tissue which is the chief constituent of subcutaneous tissue (“superficial fascia”) and of fascial spaces where movement occurs between condensed fasciae

(3) Reticulum which is an even more delicate fascial material found as the supporting framework of fat masses and of the tissues of most organs,

(4) Adipose tissue

Since most of the anatomically and clinically important fasciae are associated with the primary muscles (1 e , group i) the development of this group only will be discussed Kuhn (1927) has shown that the prevertebral fascia can be traced back to that surrounding each prevertebral and scalene muscle mass; the deep fascia of the back of the neck or nuchal fascia to that mesenchyme separating the dorsal from the ventro-lateral divisions of the myotomes in this area and investing the muscle masses formed from the dorsal portions; the pretracheal (infrahyoid) cervical fascia to the mesenchyme enveloping the rectus column of the neck, and the investing layer of deep cervical fascia (external cervical) to that ongmally surrounding the sterno-mastoid-trapezius sheet The separation of this sheet into the more posterior trapezius and the more anterior sternomastoid muscle leaves the investing layer of deep cervical fascia covering the posterior triangle of the neck The lumbar fascia, as has been indicated earlier (page 358), arises from the mesenchyme separating the dorsal from the ventral division of the myotomes This intermuscular septum exhibits a characteristic of many other fasciae in that it comes to serve as an attachment for muscles, being greatly strengthened and thickened by the formation within it of sheets of collagenous or tendinous fibres. There is considerable reason to believe that the tendinous fibres develop in these fasciae in direct response to mechanical stresses. Such tendinous fasciae are called aponeuroses

Since mesenchymal tissue connects all adjacent fascial rudiments it is obvious that fasciae will become adherent to their neighbours during development. Thus in the neck the caroti sheath, prevertebral, pretracheal, and investing layers of the deep cervical fasciae are all more or less fused with one another. Many of these connexions seem to be due also to development of fibres in response to lines of stress Fasciae developed around a muscle mass in one region are usually continuous with fasciae developed in connection with the. homologous group m t e adj'oimng regions, e g., the nuchal fascia of the back of the neck is continuous with the lumbar fascia, as both are developed around the dorsal divisions of the myotomes.


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   Human Embryology (1945): 1 Introductory Concepts | 2 Formation Maturation and Structure of Germ Cells | 3 Cyclic Changes in Female Genital Tract | 4 Fertilization Cleavage and Formation of Germ Layers | 5 Implantation of Blastocyst and Development of Foetal Membranes Placenta and Decidua | 6 Fate of Germ Lavers and Formation of Essential (Primary) Tissues including Blood | 7 Growth of Embryo Development of External Form Estimation of Embryonic and Foetal Age | 8 Determination Differentiation Organizer Mechanism Abnormal Development and Twinning | 9 Cardio Vascular System | 10 Alimentary and Respiratorv Systems Pleural and Peritoneal Cavities | 11 Urogenital System | 12 Nervous System | 13 Skeletal System | 14 Muscle and Fascia | 15 Integumentary System | 16 Comparative Vertebrate Development | Figures
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Hamilton WJ. Boyd JD. and Mossman HW. Human Embryology. (1945) Cambridge: Heffers.

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