Talk:Book - A Laboratory Manual and Text-book of Embryology 11

From Embryology


The Morphogenesis of the Skeleton and Muscles

The Skeletal System

The sketeton comprises (1) the axial skeleton (skull, vertebra, ribs, and sternum), and (2) the appendicular skeleton (pectoral and pelvic girdles and the limb bones). Except for the Sat bones of the face and skull, which develop directly in membrane, the bones of the skeleton exhibit first a blastemal or membranous stage, next a cartilaginous stage, and finally a permanent osseous stage.

For a detailed account of the development of the various bones of the skeleton the student is referred to Bardeen, Keibel and Mall, vol. i.

Axial Skeleton

The primitive axial skeleton of all vertebrates is the nolochord or chorda dorsalis, the origin of which has been traced on pp. 33 and 35 The notochord constitutes the only skeleton of Amphioxus, whereas in fishes and amphibians it is replaced in part, and in higher animals almost entirely, by the permanent axial skeleton. In the development of mammals, this transient elastic rod disappears early except in the intervertebral discs where it persists as the nuclei pulposi.

Fig. 313.— Frontal sections throu^ the mesodermal segmenls of the left side of human embryos. A, at about 4 nun. showing the difleTentiation of the sclerotomes into less dense and denser regions; B, at about 5 nun. iUustnting the union of the halves of successive sdeiotoines to form the aolages of Um

Vertebrae and Ribs

The mesenchyme derived from the sclerotomes grows mesad (Figs. 290 and 323) and comes to lie in paired segmental masses on either side of the notochord, separated from similar masses before and behind by the iiUersegmetUal arteries. In embryos of about 4 mm. each sclerotome soon differentiates into a caudal compact porUon and a cranial less dense half (Fig. 313 A).

From the caudal portions, horizontal tissue masses now grow toward the median J line and enclose the notochord, thus establishing the body of each vertebra. , Similarly, tiorsal extensions form the vertebral arch, and ventro-lateral outgrowths, J the costal professes. The looser tissue of the cranial halves also grows mesad and I fills in the intervals between successive denser regions.

The denser caudal half of each sclerotomic mass presently unites with the less 1 dense cranial half of the sclerotome next caudad to form the aniages of the j definitive verlebrm (Fig. 313 B). Tissue bordering the cranial and caudal portion j of the original sclerotome gives rise 1 tebrol discs. Since a vertebra is 4

formed from parts of two adjacent = erotomv it is evident that the interseg- J mental artery must now pass over the Dody of a vertebra and the myotomes and j vertebrae alternate in position.

Following this blastemal stage centers of chondrification appear, two centers J in the vertebral botly, one in each half of the vertebral arch, and one in each costal \ process. These centers enlarge and fuse to form a cartilaginous vertebra; the union of the costal processes, which will give rise to ribs, with the body is, however, temporary, an articulation forming later. Transverse and articular processes grow out from the vertebral arch, and the rib cartilages, having in the meantime formed tubercles, articulate with the transverse processes somewhat later, Tha various ligaments of the vertebral column arise from mesenchyme surrounding the vertebra:.

Finally, at the end of the eighth week, the stage of ossification sets in. A single center appears in the body, one in each half of the arch, and one near the angle of each rib (Fig. 296 A). The replacement of cartilage to form a solid mass is not completed until several years after birth. At about the seventeenth year secondary centers appear in the cartilage still covering the cranial and caudal ends of the vertebral body and form the disc-like, bony epiphysis. These unite with , the vertebra proper to constitute a single mass at about the twentieth year.

While the foregoing account holtls for vertebra; in general, a few deviations occur. When the atlas is formed a body differentiates as well, but it is appropriated by the body of the epistropheus (axis), thereby forming the tooth-like dens of the latter. The sacral and coccygeal vertebra; about the twenty-fifth year the sacral vertebrae and a similar fusion occurs between the rudiment

The ribs, originating as ventro-lateral outgrov reach their highest development in the thoracic they are short; their tips fuse with the transverse the vertebral bodies, thus leaving intervals — the transverse foramina — through which the vertebral vessels course. In the lumbar region the ribs are again diminutive and are fused to the transverse processes. The rudimentary ribs of the sacral vertebra are represented by flat plates which unite on each side to form a pars lateralis of the sacrum. With the exception of the first coccygeal vertebra, ribs are absent in the most caudal vertebrae.


The sternal aniages arise as paired mesenchymal bands, with which the first eight or nine thoracic ribs fuse secondarily (Whitehead and Waddell, Amer. Jour. Anat., vol. 12, 1911). After the heart descends into the thorax, these cartilaginous sternal bars, as they may now be termed, unite in a cranio-caudal direction to form the sternum, at the same time incorporating a smaller mesial sternal anlage (Fig. 314). Ultimately one or two pairs of the most caudal ribs lose their sternal connections, the corresponding portion of the sternum constituting the xiphoid process in part. At the cranial end of the sternum there are two imperfectly separated epistemal cartilages with which the clavicles articulate. These usually unite with the longitudinal bars and contribute to the formation of the manubrium. Variations in the ossification centers are not uncommon, although a primitive, bilateral, segmental arrangement is evident (Fig. 315). In the two cranial segments, however, unpaired centers occur.

Fig. 314. — Fonnation of the sternum in a Fig. 315. — Sternum of a child, showing centers of human fetus during the third month (modified ossification, after Ruge).

The Skull

The earliest anlage of the skull consists in a mass of dense mesenchyme which envelops the cranial end of the notochord and extends cephalad into the nasal region. Laterally it forms wings which enclose the neural tube. Except in the occipital region, where there are indications of the incorporation into the skull of three or four vertebrae, the skull is from the first devoid of segmentation.

Chondrificalion begins in the future occipital and sphenoidal regions, in the 1 median line, and extends cephalad and to a slight extent dorsad. At the same ' time, the internal ear becomes invested with a cartilaginous periolic tapsule which eventually unites with the occipital and sphenoidal cartilages (Fig. 316). The dwndrocranium, as it is termed, is thus coniint-d chiefly to the base of the skuU, the bones of the sides, roof, and the face being of membranous origin, Chondrification also occurs more or less extensively in the branchial arches, and, as will appear presently, the first two pairs contribute substantially to the forma- i tion of the skull.

In the period of ossijicalion, which now ensues, it becomes evident that some J bones which are separate in adult lower animals fuse to form compound bones in J

Fig. 316. — Reconstruction of the chondrocraniuni of a human cmbn'o of 14 mm. (Levi in McMurrich). us, Alisphenoid; bo, basi-ocdpilal; hs. basisphenoid; fo, exoccipiUil; «t, Meckel's cartilage; OS, orbitosphenoid; p, periotic; ps, presphenoid; so, sella turcica; s, supra-occipilal.

/ Eiaccipiia! Condyle ~Bati-orcipitai Fic. 317.^0ccipilnl botic of a humaji fetiM-l ilhs (after Sappey). The portions still f

++++a background.

the human skull. The sphenoid and temporal bone, for example, represent five I primitive pairs each. As such components may arise either in membrane or 1 cartilage the mixed origin of certain adult bones is explained.

Ossification of the Chondrocraniiun.— Ofci^iTu/ Sokc— Ossification begins ia J the occipital region during the third month. Four centers appear at right angles J about the foramen magnum (Fig. 317). From the veni. ' • .. . .

(basi-ocdpital) part of the future bone; from the li ::

cipital) parts which bear the condyles, and fro center the squamous (supra-occipital) part below squamous (inter parielal) part above that line is an origin. These several components do not fuse com]

Sphenoid Bone

Ten principal centers arise in the cartilage that corresponds to this bone (Fig. 318) : (1 and 2) in each ala magna {alisphenoid) ; (3 and 4) in each oAt parva {orbUosphenoid); (5 and 6) in the corpus between the aUe magns (basis phenoid) ; (7 and 8) in each lingula; {9 and 10) in the corpus between the ais parva; {presphenoid). Intramembranous bone also enters into its compo



Fic. 318. — Sphenoid bone of a human fetus Fic. 319. — Ethmoid bone of a human fetus of of nearly tout months (after Sappey). Parts still four months (modified after Kollmann).

cartilaginous are r^reaented in stipple.

sition, forming the orbital and temporal portion of each ala magna and the mesial laminx (Fawcett) of each pterygoid process (except the hamulus). Fusion of the various parts is completed during the first year.

Ethmoid Bone

The ethmoid cartilage consists of a mesial mass, which extends from the sphenoid to the tip of the nasal process, and of paired masses lateral to the olfactory fosss. The lower part of the mesial mass persists as the cartilaginous nas(U septum, but ossification of the upper portion produces the lamina perpendicuiaris and the crista gaUi (Fig. 319). The lateral masses ossify at first into the spongy bone of the ethmoidal labyrinths. From this the definitive honeycomb structure (ethmoidal cells) and the concha are formed through evaginations of the nasal mucous membrane and the coincident resorption of bone. (Similar invasions of the mucous membrane and dissolution of bone produce the frontal, sphenoidal, and maxillary sinuses.) Fibers of the olfactory nerve at first course between the unjoined mesial and lateral masses. Later cartilaginous, and finally bony trabecule surround these bundles of nerve fibers, and, as the cribriform plates, interconnect the three masses.

Temporal Bone

Several centers of osdfication in the periotic capsule unite to form a single center from which the whole cartilage is transfonned into the petrous and mastoid portions of the temporal bone ^Fig. 320). The mastoid process is formed after birth by a bulging of the petrous bone, and its internal aiWties, the mastoid ceiii, are formed and lined by the evaginated epithelial lining of the middle ear. The squamosal and tympanic portions of the temporal bone are of inlramembranous origin, while the styloid prottss origtoates from the proximal end of the second, or hyoid, branchial arch.


Fic. 320.— The left Umporal bone at birth. The portion of intncartilaginous origin is represented in

Membrane Bones of the SkoU. — From the preceding account it is evident that although the bones forming the ba:seuf iuUaii^chiellyiD cartilage, they

recei\'e substantial contributions from membrane bones. The remainder of the sides a nd roof of the skull is wholly of intramembroiKnis ori^ , each of the parietais


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the maocUla arise directly in membrane. Each palate bone develops from a single center of ossification. According to one view five centers contribute to the formation of each maxilla; Mall, however, maintains that there are but two centers, one giving rise to the portion bearing the incisor teeth, the other to the remainder of the maxilla.

The entire core of the mandibular process becomes a cartilaginous bar, Meckel's cartilage^ which extends proximally into the tympanic cavity of the ear (Fig. 321). Membrane bone developing distally in the future body encloses MeckeFs cartilage and the inferior alveolar nerve, whereas proximally in the ramus the membrane bone merely lies lateral to these structures — hence the position of the adult mandibular foramen. The portion of MeckeFs cartilage enclosed in bone disappears, while the cartilage proximal to the mandibular foramen becomes in order, the spheno-mandibular ligament, the malleus, and the incus (p. 389 and Fig. 387).

Each second branchial arch comes into relation proximally with the periotic capsule. This upper segment of the cartilage becomes the stapes and the styloid process of the temporal bone (Figs. 321 and 387). The succeeding distal portion is transformed into the stylo-hyoid ligament and connects the styloid process with the distal end of the arch, which also undergoes intracartilaginous ossification to form the lesser horn of the hyoid bone.

The cartilage of the third branchial arches ossifies and gives origin to the greater horns of the hyoid bone, while a plate connecting the two arches becomes its body.

The fourth and fifth branchial arches co-operate in the formation of the thyreoid cartilage of the larynx.

Appendicular Skeleton

Whereas the axial skeleton originates chiefly from the sclerotomes of the mesodermal segments, the appendicular skeleton is apparently derived from the unsegmented somatic mesenchyme. In embryos of 9 mm. mesenchymal condensations have formed definite blastemal cores in the primitive limb buds (Fig. 323). Following this blastemal stage the various bones next pass through a cartilaginous stage and finally an osseous one.

Upper Extremity

The clavicle is the first bone of the skeleton to ossify, centers appearing at each end. Prior to ossification it is composed of a peculiar tissue which makes it difficult to dedde whether the bone is intramembranous or intracartilaginous in origin.

The scapula arises as a single plate in which there are two chief centers of j ossification. One center early forms the body and spine. The other, after birth, gives rise to the rudimentary coracoid process, which in lower vertebrates extends from the scapula to the sternum. Union between the coracoid process and the body does not occur until about the fifteenth year.

The humerus, radius, aud ulna ossify from single primary- centers and two or more epiphyseal centers (Fig. 296 C-F).

In the cartilaginous carpus there is a proximal row of three, and a distal row of four elements. Other inconstant cartilage may appear and subsequently disappear or become incorporated in other car^xil bones. The pisiform is regarded as a sesamoid Ivone which develops in the tendon of the flexor carpi ulnaris: in the same category is the patella which forms in the tendon of the quadriceps extensor cruris.

Lower Extremity

The cartilaginous plate of the i>s coxa is at first so placed that its lon^ axi> is perpendicular to the vertebral column (Fig. 322). Later it rotates to a position parallel with the vertebral column and shifts slightly caadad to come into rcliiion with the first three sacral x-ertebrsE. .K retention of the membranou-; c^uKlition in the lower half of each prtniiti\Te cartilaginous plate accounts for the obturator membraiK which closes the foramen of the same n Three centers of o:>sification aj^xar. forming the iitMrn. ischium, and pMs. The three bones do not fuse completely until about puberty.

The general development of the femur, tibia, ^btila. tarsus, meiatarsm, and phalanges is quite similar to that of the OHresponding buoes of the upper cTtremity.

A noma 1 its. — Vmi* lions in the ntunber of vrrtebnc (ncvpl cervical are not tnfrequeni. The L\st irni-kal tad first lumbu vtrtcbtv ocaisio<uB>- Iwur ribs, doc lo the con* tinueil de\-riopoiern ol the pntD>li%-e costal processes. Cleft ftcmum or deft npbokl process TVpresects an iocompktc fusioD of the slcinal bats. Ailditta^ finsos oc toes (polvdadyly) may w-vur: thr causv e obenue. Hue Kp aad deft p«bte have abrady bcva meatioaei ipp. 146, 149 .

n. The BfuscuiAR System

The skeletal muscles, with the eTception of tlMie attt ' ' »- •++++arvhes. ori^nate ban tbe myotomet of the mrst i

and Fig. 323). Ahfaoug^ the pninithv segmental arraagrBNl is. for the most part, sooo ket, their •.<rigiial innervatiiw b^- tm Qer\'es is retained thtoo^MMit fife. For tht^ mtsofi tbe IttftMJ formed by ftiaon. spfitting, cr other nxxfifioitioas may be tm

The development of the human musculature is fully described by W. H. Lewis in Keibel and Mall, vol. 1.

Fundamental Processes. — The changes occurring m tlie myotomes during the formation of adult muscles are referable to the operation of the foUowmg fundamental processes:

(1) A change in direction of the muscle fibers from their original cranio-caudal orientation in the myotome. The fibers of but few muscles retain this initial orientation.

(2) A migration of myotomes, wholly or m part, to more or less remote regions. Thus the latissimus dor si originates from cervical myotomes, 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 trapezius.

(3) A fusion of portions of successive myotomes. The rectus abdominis illustrates this process.

(4) A longitudinal splitting of myotomes into several portions. Examples are found in the sterna- and omo-hyoid and in the trapezius and sterno-mastoid,

(5) A tangential splitting into two or more layers. The oblique and the transverse muscles of the abdomen are formed by this common process.

(6) A degeneration of myotomes, wholly or in part. In this vrsiyfasciaSy ligaments, and aponeuroses may be produced.

Muscles of the Trunk

Ventral extensions grow out from the cervical and thoracic myotomes and a fusion that is well advanced superficially occurs between aU the myotomes in embryos of 10 mm. A dorsal, longitudinal column of fused myotomes, however, can still be distinguished from the sheet formed from the combined ventral prolongations (Fig. 322).

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

The muscles of the neck, other than those innervated by the dorsal rami and those arising from the branchial arches, dijBFerentiate from ventral extensions of the cervical myotomes. Reference has already been made to the probable contribution from cervical myotomes to the formation of the diaphragm (p. 188). In the same way the thoraco-abdaminal muscles arise from the more pronounced ventral prolongations of the thoradc myotomes which grow into the body wall along with the ribs (Fig. 322).

The ventral extensions of the lumbar myotomes (except the first) and of the j first two sacral myotomes Ho 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.

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MuKtM of th* Uwb*. It )u.v )c«:«r«itt\- brf« bctkxrd tint Hk mosclcs of (V v'Mivniilh-* AW itcwtv^vsi ft\»B\ bwts. *.•» tW mH-uKtOkK wfekli puw into tbe A»t.i^<.->k y4 t)K- bVttv t)i vtu^it tb» ks. t:tif«H>' tW c»^, and in man the srgmcntHlj uotw >U)<4>l.v t» i'Uj|aer«im\ Wt vk^ |wv^«J. «<i * wvvCubmc oc^^ According to Lewis, "there are no observations of distinct myotome buds extending into the limbs." A diffuse migration of cells from the ventral portion of the myotomes has been recorded by various observers, recently by Ingalls. These cells soon lose their epithelial character and blend with the undifferentiated mesenchyma of the limb buds (Fig. 323). From this diffuse tissue, which at about 10 mm. forms premuscle masses, the limb muscles are differentiated, the proximal muscles being the first to appear (Fig. 322).

Somatic mesoderm++++Fig. 323. — Transverse seciion ol a 10.3 mm. monkey embryo showing the myotome and ihe mcscntliyma a( Ihe arm bud (KoURmnn}. A, aorta: *, sclerotome.

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 (cf. p. 366).

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 of a different category than those which supply myotomic muscles. The mesoderm of the first branchial arch gives rise to the muscles of mastication and to all other muscles innervated by the trigeminal nerve. Similarly the- muscles of expression, and other muscles supplied by the fadal nerve, ori]pnatc from the second, or hyoid arch- The third arch probably gives origin to the pharyngeal musdcs, and the ikird and/ourtli arches to the intrinsic muscles of the larynx.

The muscles of the tongue are supplied by the n. hj-poglossus, and therefore it has been assumed that they are derived from myotomes of the occipital region. According to Lewis, "there is no evidence w.-hatc^'T for this statement, and we are inclined to believe from our studies that t] je musculature is derived from

the mesoderm of the floor of the mouth."

Anomalies. — Variations in the form, position, and attachments of the muscles are moil. Most of 1 hriK nnnmalies are referable to the variable action of the severe develop++++iiil fiictora lisu-J on p, iM.