Book - Aids to Embryology (1948) 13
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Baxter JS. Aids to Embryology. (1948) 4th Edition, Bailliere, Tindall And Cox, London.
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Chapter XIII The Muscular and Skeletal Systems
There are three types of muscle cells present in the adult - smooth, cardiac, and skeletal. The mesoderm is the source of all? these, except for the muscles of the iris and those associated with the sweat glands.
Smooth muscle fibres develop from the visceral mesoderm through the intermediary of a myoblast. This cell contains an oval nucleus embedded in a granular cytoplasm. In the further stages of development of this cell, the granules are seen to become arranged in rows, and then these granules fuse to become myofibrillae, the cell nucleus retaining a central position. Connective tissue and reticular fibrils develop to bind the smooth muscle fibres into bundles.
Cardiac muscle fibres develop in a very similar manner in the myo-epicardial mantle, but at an early stage the individual cells lose all trace of cell autonomy and unite to form a syncytium. The myofibrils of cardiac muscle develop a cross-striation like those of voluntary muscle.
The skeletal muscle fibres are derived from mesodermal cells of the somites or from the branchial arch mesoderm. The nuclei of the myoblasts occupy a central position as in the early developmental stages of other types of muscle, and fibrillae arise in the cytoplasm. These fibrillae become cross-striated and increase in number very rapidly so that the nucleus is crowded to one side of the cell. While this process goes on the nucleus divides repeatedly so that the mature striated muscle cell is multi-nucleated.
The somites are laid down on either side of the neural tube as a series of hollow masses of mesoderm (p. 19). In further development the greater part of the medial and ventral walls of each somite are carried medially to form a sclerotome from which the axial skeleton is formed. Cells proliferating from the deep aspect of the remainder of the somite become the myotome, while the rest of the original mass is termed the dermatome. The latter becomes the corium and superficial fascia of the corresponding body segment.
Fig. 40. - The Development of the Muscle Sheets.
1, Dorsal muscle mass ; 2, ventro-lateral sheet ; 3, rectus sheet.
Each myotome is a rectangular block of tissue, and very early the segmental nerve growing outwards from the neural tube establishes connection with it. The myotome now divides into a dorsal and a ventrolateral part, the segmental nerve dividing correspondingly into posterior and anterior primary rami. The ventro-lateral part of each myotome sends out an extension into the ventral and lateral parts of the body wall. These ventral processes split into three layers which represent the three sheets of muscle in the abdominal wall, and the external intercostal, internal intercostal, and innermost intercostal muscles of the thoracic wall. The ventral ends of the three laminae become fused to form a rectus sheet on each side of the middle line. These rectus columns are represented in the adult by the rectus abdominis, the infra-hyoid, and some of the supra-hyoid, musculature. The thoracic part of it is occasionally present in the form of the sternalis muscle.
The Limb Muscles
On comparative grounds the limb muscles are considered to be of segmental origin arising from the ventro-lateral extensions of the somites, but in human development they differentiate in situ from the mesenchymal core of the limb. Each limb has a pre-axial or cephalic border, and a postaxial or caudal border. The muscles arising in relation with the pre-axial border are innervated by the nerves of the upper segments contributing to the limb, while the post-axial muscles are supplied by the lower segmental nerves. The musculature of the entire limb is split into a dorsal extensor and ventral flexor group of muscles, and this is reflected in a splitting of the segmental anterior primary rami in the root plexus. Subsequent migration and fusion of the primitive muscle masses of the limbs gives rise to the more complex adult pattern of the brachial and lumbo-sacral plexuses.
Histogenesis of Bone
There are two modes of development of bone, the intramembranous and the endochondral. The first indication of membrane bone formation is a condensation of mesenchyme cells to form a sheet. Fibres of collagen are laid down between the cells which become reoriented in rows along bundles of these fibres. The individual cells now begin to function as osteoblasts, and to deposit an osseous matrix among the fibres. The mesenchyme around this centre of ossification becomes highly vascular and penetrates between the spicules of newly formed bone. At the same time the more peripheral mesenchyme becomes transformed into a layer of cells which may be termed the* periosteum. This contains osteoblasts in its deeper parts which lay down a compact layer of subperiosteal bone. Eventually a bony sponge-like structure is formed, limited by compact bone externally and having within it the primary marrow cavity. Numerous osteoclasts (multi-nucleated giant cells) appear in the primary marrow, and by their absorptive powers remodel the bone as successive layers of compact bone are laid down externally by the deep layer of the periosteum. Some of the osteoblasts become entangled in the bone as this process goes on. They lie in little spaces called lacunae, and may themselves now be termed osteocytes. As the bone increases in thickness the outer periosteal layer is constantly being reinforced by cells derived from the surrounding undifferentiated mesenchyme. It should be emphasized that, in the growth of a simple membrane bone like the parietal, deposition of new bone on the outer surface is always accompanied by a co-ordinated resorption of bone on the inner surface, and this, resorptive activity is always associated with the presence of osteoclasts.
Endochondral ossification is seen typically in the long bones of a limb. Here the mesenchymal cells first become an axial condensation which is then transformed into the cartilaginous fore-runner of the bone. The (question now arises, if these axial mesenchymal cells are already pre-determined for chondrification, or simply indifferent cells which have taken up axial positions. Murray (1936) discussing this point, quotes an experiment by Fell, who cultivated fragments of teased mesenchyme from the limb buds of four-day chick embryos in vitro. Some of the fragments produced cartilage while others did not. The conclusion was that as all the fragments had been treated in the same way lying side by side in the same* medium, the differences in differentiation must have been intrinsic â€” i.e., that the mesenchyme which had chondrified had contained tissue predetermined for that process.
After the cartilaginous model of the bone has been formed changes take place in the surrounding perichondrium. The outer cells of this membrane become the fibrous layer of the future periosteum, while those of the inner layer differentiate to form osteoblasts which lay down compact bone on the outer surface of the shaft. The cartilage cells in the centre of the shaft become enlarged and calcium salts are deposited in the matrix around them. Vascular tissue derived from the deep layer of the periosteum penetrates into spaces in the matrix which have been formed by resorption, and this vascular tissue may be termed the primary bone marrow. Osteoblasts in the primary bone marrow deposit bone on the remains of the calcified cartilage. Giant cells (osteoclasts) later appear and collect along the surface of the spicules of primary bone. Remodelling of the central parts of the bone takes place so that a medullary cavity of increasing size is formed ; this process is accompanied by a co-ordinated deposition of subperiosteal bone on the outer surface of the shaft.
The Skeletal System
The skeleton may be considered as consisting of two morphological parts : (a) an axial skeleton made up of the vertebrae, ribs, sternum and skull ; ( b ) an appendicular skeleton, which is composed of the pectoral and pelvic girdles with the limb bones.
The Axial Skeleton
The vertebrae are derived from the sclerotomes. These are masses of cells which surround the notochord and are derived from the primitive mesodermal segments (p. 19). Each sclerotome is divisible into a dense caudal part and a less dense cranial part. The cranial part of one sclerotome fuses with the caudal part of the preceding one to give rise to the primordium of a vertebral centrum. The segmental spinal nerves lie between these vertebral primordia which are thus intersegmental in position. The denser part of each centrum gives rise to two outgrowths : one passes dorsally around the neural tube to meet its fellow of the opposite side to form the neural arch ; a second grows ventrolaterally as the costal process, but this only reaches its full development in the thoracic region where it forms a free rib. As the costal elements grow ventrally in the thoracic region they presently turn and grow medially. Their medial extremities become connected by a longitudinal bar, the sternal bar. There will be thus paired sternal bars which fuse in the midline from above downwards, forming in succession, the manubrium, the body of the sternum, and the xiphoid process. It must be understood that all the elements described are at first simply condensations of the mesoderm. These later chondrify and then ossify. The mesodermal tissue intervening between the primordia of the vertebral centra becomes transformed into the fibrocartilaginous intervertebral discs, in the centre of which notochordal tissue persists in modified form as the nucleus pulposus.
The skull consists essentially of two parts. There is first, a series of bones which form a protective investment for the brain and the organs of special sense (the neuro-cranium), and second, the bones of the jaws, or viscerocranium. Some of these bones develop in cartilage, others are formed in membrane, while yet a third group are compound, having some portions which ossify in membrane and others which pass through a cartilaginous stage.
The entire neurocranium of the human embryo is first indicated by a mesodermal condensation around the anterior end of the neural tube. In general, the basal part of this becomes transformed into a series of cartilages which later ossify, while the remainder forms a set of roofing bones for the skull ossifying directly from membrane.
The cartilaginous part of the skull (chondrocranium) is first seen as a pair of bars lying on either side of the cranial end of the notochord. These are the parachordal cartilages and they terminate anteriorly just behind the hypophysis. The two cartilages enlarge and come together in the middle line surrounding the notochord and forming what is known as the basal plate. A second pair of cartilages appears flanking, and in front of the hypophysis. These are the trabeculae, and their caudal ends eventually fuse with the basal plate which also becomes supplemented at its posterior extremity by the union with it of three or four occipital sclerotomes. Laterally the basal plate fuses with the cartilaginous otic capsule. An opening between this and the first occipital sclerotome represents the jugular foramen. The hinder end of the basal plate gives rise to the basioccipital, while cartilaginous centres representing the ex-occipital and supra-occipital form in the mesodermal condensation lateral and dorsal to the developing hind brain. The anterior end of the basal plate forms the post-sphenoid. The pre-sphenoid element is formed from the trabecular bars. Centres of chondrification in the mesoderm lateral to these denote the orbito-sphenoid and the ali-sphenoid. The first of these extends medially around the optic nerve and forms the lesser wing of the sphenoid. The second is separated from the orbito-sphenoid by the third, fourth, sixth, and first two divisions of the fifth cranial nerve, that is, the structures passing through the primitive sphenoidal fissure. The second division of the fifth nerve is later encircled by bone so passing through a separate foramen rotundum in the adult skull. The ali-sphenoid gives rise to much of the greater wing of the sphenoid. Chondrification around the otocyst with subsequent ossification, forms the primordium of the petro-mastoid portion of the temporal bone. Chondrification also takes place in the mesoderm around the olfactory placodes so forming the nasal capsules which unite with each other, and posteriorly fuse with the trabeculae. The ethmoid and the cartilaginous portion of the nasal septum represent the nasal capsules in the adult. The optic capsule does not chondrify in the human subject.
The roofing bones of the neurocranium, that is, the frontal, the parietals, the squamous parts of the temporals and the interparietal portion of the occipital ossify in the mesodermal condensation without the intervention of a cartilaginous stage. Owing mainly to the relatively great growth of the brain before birth in man, the neurocranium, and especially the part formed by these membranous roofing bones, is large as compared with the viscerocranium of the infant.
Fig. 41. - The Primordial Chondrocranium.
1, Nasal capsule ; 2, optic capsule ; 3, hypophyseal foramen ; 4, trabecular cartilage ; 5, otic capsule ; 6, parachordal cartilage ; 7, occipital segments ; 8, cervical segments ; 9, notochord.
As was mentioned previously (p. 75), each branchial or visceral arch has a cartilaginous basis. The first two of these aid in the formation of the skull. In the case of the first arch cartilage, bones formed in membrane supplement and replace the cartilage. The dorsal extremity of this cartilage however, becomes subdivided into the primordia of the malleus and incus, which ossify during the fourth month, while the adjacent part of the second arch cartilage likewise ossifies to form the stapes and the styloid process.
The mandible is developed mainly from ossification in the dense mesodermal condensation on the outer side of the first arch cartilage (Meckel's cartilage). The following account of its formation is based on the description given by Fawcett (1924).
The inferior dental branch of the mandibular division of the trigeminal nerve lies parallel with, and on the outer side of the cartilage. Opposite the junction of the middle and anterior thirds of this, the nerve divides into mental and incisive branches. Bone makes its first appearance at the eighth week in the tissue at the angle between these two branches and grows rapidly backward underneath the mental nerve which thus lies in a groove. Inbending of the anterior border of this bony groove under the incisive nerve takes place. This forms the inner alveolar wall which insinuates itself between the developing teeth and Meckel's cartilage. A hook-like process over the mental nerve grows backwards from the anterior border of the mental groove and fuses with the outer alveolar plate so forming the mental foramen. A bridge of bone between the two alveolar walls closes in the incisive canal while two shelf-like projections of bone, one above and one below Meckelâ€™s cartilage, fuse to form a tunnel for the cartilage. This tunnel extends from the second milk molar tooth germ almost to the anterior end of the cartilage. Early in the third month osteoblasts invade Meckelâ€™s cartilage opposite the incisor tooth germs and ossification in the cartilage occurs here. The remainder of it in relation with the membrane bone degenerates and is absorbed. Secondary cartilages arise in relation with the condylar and coronoid processes and possibly at the angle. These are all ossified by extension from the neighbouring membrane bone.
There is another membranous ossification in the dorsal part of the mandibular arch which forms the tympanic part of the temporal bone.
The Maxillary Processes
It will be remembered that the first branchial arch bifurcates anteriorly, the upper portion being called the maxillary process. A number of membrane bones form in this. Superficially there are the premaxilla, the maxilla, the zygomatic, and the squamous portion of the temporal. More deeply, there develop the palatine and the vomer.
The bony centres for all these make their appearance relatively early in development during the eighth week.
The Appendicular Skeleton
The mesenchymal core of the limb bud from which the bones arise, becomes divided into segments as chondrification takes place. Reference has already been made to experimental work on the growth of bone (p. 159) and the results of similar investigations show that any embryonic bone normally grows to form its general adult pattern in response to intrinsic factors ; extrinsic factors may later modify somewhat details of the predetermined bony form.
The bones of the limbs, and of the limb girdles, pass through a cartilaginous stage before ossification occurs. The clavicle is an exception to this rule. It ossifies, for the most part, in membrane, the centre for it, detectable at the sixth week, being the first to appear in the embryonic body.
Anomalies of Development of the Skeleton
- Variations in the number of vertebrae are not uncommon. For example, thirteen thoracic vertebrae may be present instead of the normal twelve ; the lumbar series may be reduced to four or increased to six ; the sacrum may show six segments instead of five.
- Rachischisis. In this condition the halves of the neural arches fail to unite in part or all of the vertebral column, the contents of the vertebral canal then being exposed. When the cranial vault is also affected the condition is termed craniorachischisis. Associated abnormalities of the central nervous system (which are probably responsible for the bony defect) are mentioned on p. 159.
- Supernumerary ribs may be present, and are produced by excessive growth of the costal elements of the seventh cervical or first lumbar vertebra. The cervical rib is the common form, and is of clinical importance because of its relation to the lowest trunk of the brachial plexus and to the subclavian artery.
- Cleft sternum results from complete or partial failure of union of the sternal bars in the mid-line.
- Premature union of one or more cranial sutures produces various abnormal skull forms. Early ossification of the sagittal suture gives rise to a keeled or boat-shaped skull (scaphocephaly) ; if the coronal suture is affected a pointed skull (acrocephaly) results , plagiocephaly (a twisted skull) is due to early ossification of one-half of the coronal suture or lambdoid suture.
- Dysostosis cleido-cranialis. This is a very rare condition, of interest mainly because in it there are combined defective ossification of the membrane bones of the skull and the clavicle.
- Anomalies of the digits may occur. Syndactyly (fusion of digits) and polydactyly (supernumerary digits) are the most common. The importance of genetic factors in the production of digital abnormalities is mentioned in the next chapter.
- Congenital clubfoot may be due to some inherent defect in the limb blastema, or it may be the result of persistence of the foetal position of the limb. The former is the much more likely cause. The association of this condition with other abnormalities, particularly with lumbar rachischisis, should be noted.
- Gigantism and dwarfism may be general or local conditions. The general condition is most probably to be ascribed to an endocrine dysfunction associated with some genetic abnormality. In simple cases, there is wide variation from the normal in the period in which union takes place between the diaphyses and epiphyses of the long bones. If union be delayed, growth in length continues for a longer period than normal and gigantism results. If union occurs prematurely, the opposite condition of dwarfism will be found. There are, however, special forms of dwarfism such as achondroplasia and ateleiosis, which are rather more complicated. The achondroplasiac dwarf is characterised by the possession of short limbs as compared with the trunk ; there is disorganisation of the cartilaginous cells at the junction between epiphysis and diaphysis in the long bones of the limbs. The ateleiotic dwarf represents a persistent infantile condition where the epiphyseal cartilages have not united and are inactive. Local gigantism is an uncommon condition affecting perhaps one or more digits or a region of the body.
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Cite this page: Hill, M.A. (2021, April 20) Embryology Book - Aids to Embryology (1948) 13. Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/Book_-_Aids_to_Embryology_(1948)_13
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