Book - Embryology of the Pig 12
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Patten BM. Embryology of the Pig. (1951) The Blakiston Company, Toronto.
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Patten BM. Developmental defects at the foramen ovale. (1938) Am J Pathol. 14(2):135-162. PMID 19970381
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Chapter 12. The Histogenesis of Bone and the Development of the Skeletal System
I. Histogenesis of Bone
Histologically bone belongs to the group of tissues known as the connective and supporting tissues. In spite of their widely varying adult conditions these.tissues are all similar in that the secreted parts, rather than the cells themselves, carry out the functional role characteristic of the tissues. It is the secreted, fibrous portion of the binding connective tissues which ties together various other tissues and organs ; it is the secreted matrix of cartilage and of bone which affords rigid support and protection to soft parts and furnishes a lever system on which the muscles may be brought into play.
The cellular elements of these tissues must not be overlooked, however, in emphasizing the functional importance of the cell products. The cells are, so to speak, the power behind, in that they extract the appropriate raw materials from the circulation, elaborate them within their cytoplasm, and deposit the characteristic secretion as an end-product. Moreover after the fiber is formed or the matrix is laid down, it is dependent on the cells for maintenance in a healthy active conditidn.
Embryologically the entire connective-tissue group arises from mesenchymal cells. It is not surprising, in view of their closely related functions and their derivation from a common type of ancestral cell, that one type of connective tissue may be converted into or replaced by another. This facility for changing the type of specialization is sometimes referred to as plasticity.
The plasticity of the connective-tissue series is well exemplified in the development of bone. Bone does not form in vacant spaces. It is always laid down in an area already occupied by some less highly specialized member of the connective-tissue family. The formation of some bones begins in areas already occupied by connective tissue — such bones are said to be intramembranous in origin, or are spoken of as membrane bones. Other bones are laid down in areas already occupied by cartilage. In this case they are said to be pndochondral in origin, or, are called cartilage bones. It should be clearly borne in mind that these terms apply solely to the method by which a bone develops and do not imply any differences in histological structure, once the bone is fully formed.
Likewise we should know at the outset what histologists mean when they speak of cancellous bone and compact bone. These terms refer not to the method of origin of the bone but to its density when fully formed. Developmen tally all bone goes through the spongy or cancellous stage. Some bones later become compact, others remain cancellous. Most bones are compact in some areas and cancellous in others.
The subject of bone development can be presented more simply if we take up first the formation of primary cancellous bone intramembranously ; then the method by which this same type of spongy bone is formed within cartilage, and finally the changes by which cancellous bone, formed in either of the above ways, may become secondarily compact.
Intramembranous Formation of Primary Cancellous Bone
In an area where intramembranous bone formation is about to begin we find an abundance of mesenchymal cells congregated and numerous small blood vessels present. The mesenchymal cells soon exhibit a tendency to cluster together in more or less elongated groups here and there throughout the area. If we study a group of this type which has been aggregated for a short time we can make out the beginning of a definite plan of organization. Near the axis of the cord delicate fibers appear, produced by the secretory activity of the cells. As this fibrous strand becomes more definite, the cells tend to become ranged against it (Fig. 151, A). In so doing they retract the cytoplasmic processes which are so characteristic of undifferentiated mesenchymal cells and become rounded. In this stage we have essentially a connective tissue in which the fibrous strands are for the most part rather widely separated from one another, and in which each strand has, lined up against it^ the cells responsible for its production.
The actual deposition of bone matrix begins very soon after the establishment of these primordial strands of mesenchymal cells and fibers. In fact one usually finds the formation of bone beginning on the older part of a strand while the strand itself is still being extended at one end by the aggregation of more mesenchymal cells (Fig. 151,
I'k;. 151, Formation of trabeculae of membrane bone. Projection drawings from the mandible of a pig embryo 130 mm. in length (cf. Fig. 182).
Abbreviations: Matrix cal., ossein matrix impregnated with calcium salts; Matrix oss., ossein matrix not yet impregnated with calcium salts.
A). When the mesenchymal cells ranged against the fibrous axis of such a strand become active in the secretion of calcareous material they are spoken of as osteoblasts. We should not lose sight of the fact that they are the same cells which formed the fibrous axis of the original strands, given a new name in deference to their further specialization and altered internal chemistry.
In studying the deposition of bone matrix one must bear in mind its dual nature. The matrix consists of an organic fibrous framework which is impregnated by a subsequent deposit of inorganic calcium compounds. We may liken the matrix of bone to reinforced concrete* In the making of a road or a wall, a meshwork of steel is first placed in the forms and concrete is then poured in. The steel gives the finished structure tensile strength and a certain amount of elasticity, the concrete gives form and hardness. So in bone the organic fibers {ossein fibers) impart strength and resilience, while the calcium salts whh which the fibers are impregnated give to the completed matrix body and rigidity.
The two steps in the deposition of bone matrix may be demonstrated readily in areas where active bone forma^tion is going on, owing to the fact that the presence of calcium compounds- in a tissue markedly increases its affinity for stains. Even after most of the calcium salts have been removed from the ossein framework by treatment of the tissue with acids (decalcification) to permit the making of sections, the staining reaction is still apparent. This indicates that the ossein fibers in which calcium has once been deposited are more or less permanently changed chemically even though all the calcium possible is subsequently removed.
If we look at a strand on which the osteoblasts have been active for a time (Fig. 151, B) we see, next to the osteoblasts, a zone of bone matrix which takes very little stain. This is the newly deposited organic portion of the matrix as yet unimpregnated with calcium salts. It consists of a feltwork of minute fibers so delicate and so closely matted together that it is very difficult in ordinary preparations to see the individual fibers at all. Slightly farther from the osteoblasts the matrix is densely stained (Fig. 151, B). This part of the matrix has been impregnated with calcium salts, chiefly phosphates and carbonates, and has thereby been converted into true bone matrix. The calcium utilized by the osteoblasts in this process is brought to them by the blood stream where it is carried in soluble form, probably in organic linkage. It is interesting to note in this connection that the presence of calcium and of phosphates in the blood is not in itself all that is necessary for this process. There must be present also sufficient vitamin D, which in some way facilitates the extraction by the osteoblasts of these raw materials from the blood and their deposition in insoluble form as part of the bone matrix. The absence of vitamin D from the system results in the formation of bone matrix deficient in calcium salts and therefore lacking in rigidity — a cohdition not infrequent in pigs. Stock raisers have miscalled this condition rheumatism but it is really the same condition known medically as rickets.
In the deposition of the matrix, the fibrous core of the original strand serves as a sort of axis on which the first matrix is laid down. When such a strand is completely invested by bone matrix, it is called a trabecula (little beam). As the osteoblasts continue to secrete and thereby thicken the trabecula, the accumulation of their own product forces them farther and farther away from the axial strand about which the first of the matrix was formed. The new matrix added is not laid down uniformly. It is possible to make out in it markings which are suggestive of the growth rings of a tree. Apparently the osteoblasts work more or less in cycles, depositing a succession of thin layers of matrix. Each of these layers of the matrix is called a lamella (Fig. 152). As the row of osteoblasts is forced back with the deposit of each succeeding lamella, not all the cells free themselves from their secretion. Here and there a cell is left behind. As its former fellows continue to pile up new matrix, it becomes completely buried (Fig, 151, B). An osteoblast so caught and buried is called a bone cell {osteocyte)^ and the space in the matrix which it occupies is called a lacuna. The bone cells, thus entrapped, of necessity cease to be active bone formers, but they play a vital part in the maintenance of the bone already formed. They have delicate cytoplasmic processes radiating into the surrounding matrix through minute canalidhli. The processes of one cell come into communication with the processes of its neighbors (Fig. 152). Thus the bone cells nearer to blood vessels absorb and hand on materials to their more remote fellows which in turn utilize these materials in maintaining a healthy condition in the organic part of the bone matrix. It is the senescence of these cells with the consequent lowering of their efficiency and the resultant deterioration of the ossein component of the matrix which is in part responsible for the decreased resiliency of the bones in advanced age.
Fio. 152. A small area of bone and adjacent marrow as seen in highly magnified decalcified sections. The drawing has been schematized somewhat to emphasize the relations of the cytoplasmic processes of the osteoblasts* and the bone cells so important in nutrition. In the adjacent marrow developmental stages of various types ofiiiijioQd cells, have been
Fig. 153. Diagrams showing stages in establishing of a characteristic area of primary cancellous bone by extension and coalescence of originally separate trabeculae.
As the various trabeculae in an area of developing bone grow, they inevitably come in contact with each other and fuse. Thus trabeculae, at first isolated, soon come to constitute a continuous system (Fig. 153). Because of its resemblance to a latticework (Latin — cancellus), bone in thb condition, where the trabeculae are slender and the spaces between them extensive, is known as cancellous bone. The spaces between the trabeculae are known as marrpw spaces.
Endochondral Bone Formation
As the term implies, endochondral bone formation goes on within cartilage. It cannot be stated too strongly that cartilage does not, in this process, become converted int# bone. Cartilage is destroyed and bone is formed where the cartilage used to be. The actual bone formation is essentially the same as in the case of membrane bone. The phenomena of special interest in connection with this type of bone development are those involved in the destruction of the cartilage preliminary to the formation of bone.
Cartilage Formation
To trace the process logically we must start back with the formation of cartilage. The first indication of impending chondrogenesis is the aggregation of an exceedingly dense mass of mesenchymal cells. This cell mass gradually takes on the shape of the cartilage to be formed. The histogenetic changes involved are not at first conspicuous. During the period of preliminary massing the cells have been migrating in from surrounding regions and also increasing the local congestion by rapid proliferation. As they are packed in together they lose their processes and become rounded (Fig. 154, A, 1). When it seems as if no more cells could possibly be crowded in, the course of events changes. The cells begin to separate from one another. This is due to the fact that they have become active in
Fig. 154. Photomicrographs of developing cartilage. The areas photographed were from the margins of the paranasal cartilage of pig embryos between 25 and 30 mm. in length. For location of cartilage in head see figure 175.
A, Early stage showing: at (1) the massing of mesenchymal cells which were about to be incorporated in the growing margin of the cartilage; and at (2) an area where matrix formation is already beginning.
B, Slightly more advanced stage of the same cartilage showing: at (1) increase in the amount and density of the matrix in the center of the growing cartilage; at (2) concentration of the surrounding mesenchyme to form tire perichondrium; and at (3) the addition of new cartilage matrix peripherally secreting. It is the accumulation of the secretion of the cells which gradually forces them farther and farther apart unti^ they come to lie isolated from one another in the matrix they have produced (Fig. 1 54, A, 2). Such a method of increase in m^ss, where there are many scattered growth centers contributing independently to the increase in bulk of the whole, is known as interstitial growth. This interstitial growth of young cartilage stands in sharp contrast to the appositional growth of such rigid substances as bone or dentine or enamel where the matrix is laid down in successive layers one upon another. Obviously interstitial growth implies plasticity of the substance produced. Were the substance produced unyielding, the very activity of a number of growth centers within it would soon crowd those growth centers to obliteration.
As the cartilage matrix is increased in amount its affinity for basic stains becomes more marked, due probably to increase in concentration of the characteristic substance in it known chemically as chondrin. At the same time the matrix becomes more rigid with a resultant checking of interstitial growth. The cells continue to secrete to a certain extent, however, as evidenced by the fact that in mature cartilage the matrix immediately surrounding the cells becomes more dense than the rest of the matrix. This area of denser matrix around the lacuna in which the cell lies is known as the capsule. As the cartilage grows older the capsules become more conspicuous and many of them come to contain more than one cell. These nests of cells in a common capsule are the result of cell divisions, following which the daughter cells are held imprisoned in the original capsule of the mother cell — further evidence of the loss of plasticity in the matrix.
The formation of a matrix so rigid that interstitial growth is checked, takes place first centrally in an area of developing cartilage. When the center has become too rigid for interstitial growth to continue, appositional growth begins to take place peripherally. While the cartilage has been increasing in mass it has been acquiring a peripheral investment of compacted mesenchyme. This investing layer of mesenchyme soon becomes specialized into a connectivetissue covering called the perichondrium. The layer of the perichondrium next to the cartilage is less fibrous than the outer layer and the cells in it continue to proliferate rapidly and become active in the secretion of cartilage matrix. For this reason it is known as the chondrogenetic layer of the perichondrium. It is through the activity (rf the chondrogenetic layer that the cartilage continues to grow peripherally, by apposition, long after interstitial growth has ceased in the matrix first formed.
Fig. 155. Drawing showing periosteal bud and an area of endochondral bone formation from the radius of a 125 mm. sheep embryo. The small sketch indicates the location of the area drawn in detail.
Abbreviations: Cart, eros., area from which cartilage has recently been eroded; Cart, pre-eros., area with cartilage cells enlarged and arranged in rows presaging erosion; Cart, trab,, remnant of cartilage matrix which has become calcified and serves as an axis or core about which bone lamellae are deposited to form a bone trabecula; Mes., mesenchymal cell.
Cartilage Erosion. When a mass of cartilage is about to be replaced by bone, very striking changes in its structure take place. The cells which have hitherto been secreting cartilage matrix begin to destroy the matrix. The lacunae become enlarged and a curious arrangement of the cartilage cells becomes evident. The cells erode the cartilage in such a manner that they become lined up in rows (Fig. 155). This process of destruction continues until the cartilage is extensively honeycombed. Meanwhile the tissue of the perichondrium overlying the area of cartilage erosion becomes exceedingly active. There is rapid cell proliferation and the new cells, carrying blood vessels with them, begin to invade the honeycombed cartilage (Fig. 155).
The Deposition of Bone
It is a striking fact that during its growth cartilage is devoid of blood vessels, the nearest vessels to it being those in the perichondrium. The invasion of cartilage by blood vessels definitely determines its disintegration as cartilage, and at the same time is the initial step in the formation of bone. For this reason the enveloping layer of connective tissue, up to this time called perichondrium because of its relation to the cartilage, is now called periosteum because of the relations it will directly acquire to the bone about to be formed. This change will not be confusing if we stop to think that both these terms are merely ones of relation, which translated mean, respectively, that tissue which surrounds cartilage, and that tissue which surrounds bone. The important fact to bear in mind is that this enveloping layer of tissue is of mesenchymal origin and therefore contains cells of the stock that may develop into any of the connectivetissue family to which bone as well as cartilage belongs. When, therefore, a mass of periosteal tissue {periosteal bud, Fig. 155) grows into an area of honeycombed cartilage it carries in potentially boneforming cells. These cells come to lie along the strand-like remnants of cartilage, just as in membrane bone formation osteoblasts ranged themselves along fibrous strands. The actual deposition of bone proceeds in the same manner endochondrally as it does intramembranously. The only difference is that in one case a strand-like remnant of cartilage serves as an axis for the trabecula, whereas in the other case deposition begins on a fibrous strand. Extensions and fusions of the growing trabeculae soon result in the establishment of typical cancellous bone similar to that formed intramembranously.
The Formation of Compact Bone from Primary Cancellous Bone
The difference between cancellous bone and compact bone is architectural rather than histological. The fundamental composition of the bone matrix, its lamellation, and the relations of the bone cells to the matrix, are the same in both cases. It is the way in which the lamellae are arranged that distinguishes these two types of bone from each other. In cancellous bone the disposition of lamellae is such that it leaves large marrow spaces between the trabeculae. In compact bone there has been a secondary deposit of concentrically arranged lamellae in the marrow spaces which greatly increases the density of the bone as a whole.
The essential differences between the two, and the way in which cancellous bone may become converted into compact bone, can be illustrated by a simple schematic diagram. Figure 156, 1, shows the arrangement of lamellae and marrow spaces in primary cancellous bone. The osteoblasts which have formed the trabeculae still lie along them on the surface toward the marrow cavity. If such an area is to IxTome compact, these osteoblasts enter on a period of renewed activity and deposit a series of concentric lamellae in the marrow cavity. Frequently if the marrow spaces are irregular there is a preliminary rounding out of them by local resorption of the bone already formed (Fig. 156, 2). This is then followed by the deposition
Fig. 156. Diagram showing transformation of cancellous to compact bone. The solid lines indicate the lamellae of primary cancellous bone; the dotted lines show the subsequently added concentric (Haversian) lamellae which nearly obliterate the marrow spaces of cancellous bone. The sequence of events is indicated by the numbers. Note that irregularly shaped spaces in the cancellous bone may be rounded out by absorption before the concentric lamellae are laid down.
of the concentrically arranged lamellae, sometimes called Haversian lamellae after the man who first described them in detail (Fig. 156, 3 ). In this process the original marrow spaces are reduced to small canals {Haversian canals^ into which have been crowded the blood vessels which formerly lay in the marrow cavities (Fig. 156, 4 ). These canals maintain intercommunication with each other in the substance of the bone, constituting a network of pathways over which the bone receives its vascular supply. As compared with the marrow spaces of cancellous bone, however, they are very small; and the gross appearance of a bone which has undergone this secondary deposit of concentric lamellae amply justifies characterizing it as ‘'compact.’’
II. The Development of the Skeletal System
In dealing with the development of the skeletal system we must recognize at the outset that the subject is far too extensive to be covered here with anything like completeness. It is not difficult, however, to become acquainted with the outstanding features in the development of two or three characteristic bones, as, for example: the sequence of events in the formation of a flat bone; the steps involved in the establishment and growth of a long bone; the way separate ossification centers appear in a common primordial cartilage mass and give rise to the various parts of a vertebra. Familiarity with such type processes gives one an understanding of the factors operative in the development of the skeleton as a whole and a background sufficient to permit ready and intelligent following up of developmental details in specific bones in which one may become interested.
Development of Flat Bones. The flat bones, such as the bones of the cranium and face, are for the most part of intramembranous origin. We are, therefore, already familiar with the early steps in their development from our study of the histogenesis of membrane bone (Figs. 151 and 153). After a mass of primary cancellous bone has been laid down in a configuration which suggests that of the adult bone being formed, there appears about this mass a peripheral concentration of mesenchyme (Fig. 157, A). This periosteal concentration of mesenchymal tissue contains potentially bone-forming cells which soon become active and lay down a dense layer of parallel lamellae about the spongy center of the growing bone (Fig. 157, B). Anatomically this dense peripheral portion is known as the outer table of the bone. The inner portion, which in the flat bones usually remains cancellous, is called the diploii. The original mesenchymal tissue which remains in the marrow spaces of the diploe develops into characteristic “red bone marrow†rich in blood-forming elements (Fig. 152).
Fig. 157. Diagrams showing the manner in which the dense peripheral layer of a flat bone is formed by the deposition of subperiosteal lamellae about an area of primary cancellous bone.
The story of the growth of the mandible, a membrane bone which starts after the manner of flat bones but which later takes on a very elaborate shape and finally becomes largely compact, can be gleaned by a comparative study of figures 178, 180, and 184.
Development of Long Bones. The long bones are characteristically of endochondral origin. The cartilage in which they are performed is a tempiorary miniature of the adult bone. Ordinarily there are several ossification centers involved in the formation of long bones. The first one to appiear is that in the shaft or diaphysis. The location of this center is shown schematically in figure 158, A, Such details as the cartilage erosion which preceded its appearance and the manner in which the deposit of bone was initiated have already been considered (Fig. 155). Our interest now is in the relation of such an endochondral ossification center to other centers, and to the bone as a whole.
Fig. 158, Diagrams showing liie progress of ossification in a long bone. The stippled areas represent cartilage; the black areas indicate bone.
A, Primary ossification center in shaft. B, Primary center plus shell of subperiosteal bone. C, Entire shaft ossified. D, Ossification centers have appeared in the epiphyses. E, Entire bone ossified except for the epiphyseal cartilage plates.
Almost coincidently with the beginning of bone formation within the cartilage the overlying periosteum begins to add bone externally (Fig. 158, B). In view of the fact that the bone-forming tissue carried into the eroded cartilage arose from the periosteum, this activity of the periosteum itself is not surprising. Moreover we have already encountered this same phenomenon of periosteal bone formation in the outer table of flat bones.
The formation of bone which starts at about the middle of the shaft soon extends toward either end until the entire shaft is involved (Fig. 158, C), leaving the two ends {epiphyses) still cartilage. Toward the end of fetal life ossification centers appear in the epiphyses. The number and location of these epiphyseal centers vary in different long bones. There is always at least one center in each epiphysis and there may be two or more. Not uncommonly there are two centers in one epiphysis and one in the other, as illustrated in figure 158, D.
Between the bone fojcmed in the diaphysis and that formed in the epiphysis there persists a mass of cartilage known as the epiphyseal plaie which is of vital importance in the growth in length of the bone.
We should expect from the rigidity of bone matrix that interstitial growth could not account for its increase in length. This was long ago demonstrated experimentally by exposing a developing bone and driving into it three small silver pegs, two in the shaft and one in the epiphysis. The distance between the pegs being recorded, the incision was closed and development allowed to proceed until a marked increase had occurred in the length of the bone. On again exposing the pegs, the two in the shaft were found to be exactly the same distance apart as when they were driven in, but the distance between the pins in the shaft and that in the epiphysis had increased by an amount corresponding to the increase in length of the bone. This indicates clearly that the epiphyseal plates constitute a sort of temporary, plastic union between the parts of the growing bone. Continued increase in the length of the shaft is accomplished by the addition of new bone at the cartilage plate. These epiphyseal plates persist during the entire postnatal growth period. Only when the skeleton has acquired its adult size do they finally become eroded and replaced by bone which joins the epiphyses permanently to the diaphysis.
As the bone increases in length there is a corresponding increase in its diameter. The manner in which this takes place is also susceptible of experimental demonstration. If madder leaves, or some of the alizarin compounds extracted from them, be fed to a growing animal, the bone formed during the time the feeding is continued is colored red. If the madder is discontinued, bone of normal color is again formed ; but the color still remains in the bone laid down while madder was being added to the diet. Thus it is possible, by keeping a record of alternate f>eriods of feeding and withholding madder and comparing these records with the resulting zones of coloration in a bone, to obtain very accurate information on the progress of bone growth and resorption. Applied to the development of long bones this method shows their increase in diameter to be due to continued appositional growth beneath the periosteum. As the bone is added to peripherally there is a corresponding resorption centrally. This central resorption results in the formation of a cavity in the axis of the long bone which is called the marrow canal (Fig. 158, C). With the further increase in the diameter of a bone, its marrow canal becomes correspondingly enlarged. A significant mechanical fact might be cited in this connection. 'Engineers have determined that the strongest rod which can be made from a given weight of steel is obtained by molding it into tubular form. The development of an essentially tubular shaft by progressive increase in the size of the marrow cavity gjivcs a long bone maximum strength with minimum weight.
The Formation of the Vertebrae. The development of the vertebrae is of interest to the student primarily because it exemplifies so excellently a fundamental embryological phenomenon — the origin of separate parts from an undifferentiated primordial tissue mass, and the subsequent association of these parts to form an organized structure. In studying young embryos we traced the history of the mesodermic somites through their early differentiation. It will be recalled that from the ventro-mesial face of each somite there arises a group of mesenchymal cells called collectively a sclerotome (Fig. 42). These cells migrate from either side toward the mid-line and become aggregated about the notochord. From these masses of cells the entire vertebral column is destined to arise.
The first significant change which takes place in these primordial masses is the clustering of sclerotomal cells derived in part from each of the two adjacent somites into groups which are located opposite the intervals between the myotomes. In studying series of transverse sections this arrangement is easy to overlook unless the density of the cells about the notochord is carefully noted in passing from section to section. It shows very clearly, however, in frontal sections (Fig. 159). Each of these cell clusters is the primordium of the centrum of a vertebra. Once formed they rapidly become more dense and more definitely circumscribed (Fig. 160). Soon after the centrum takes shape, paired mesenchymal concentrations extending dorsally and laterally from the centrum establish the primordia of the neural arches and of the ribs (Fig. 161).
Fig. 159. Semi-schematic coronal sections through the dorsal region of young embryos to show how the vertebrae became intermyotomal in position. Note that the primordium of a centrum is formed by cells originating from the sclerotomes of both the adjacent pairs of somites.
Fig. 160. Transverse section from pig cm])ryo of 17 mm. cut at the level of the lungs to show the structures in the dorsal body-wall. (After Minot.)
Fig. 161 . Transverse section of 20 mm. pig embryo cut at the level of the lungs to show the developing vertebra and ribs. (After Minot.)
Fig. 162. Transverse section from 40 mm. pig embryo cut at the level of the lungs to show the developing vertebra and ribs.
The Stage in which the various parts of the vertebrae arc sketched in mesenchymal concentrations, is frequently spoken of as the blastemal stage. It is rapidly followed by the cartilage stage. Conversion to cartilage begins in the blastemal mass first in the region of the centrum and then chondrification centers appear in each neural and each costal process (Fig. 161). These spread rapidly until all the centers fuse and the entire mass is involved (Fig. 162). The cartilage miniature of the vertebra thus formed is at first a single piece showing no lines of demarcation where the original centers of cartilage formation became confluent, and no foreshadowing of the separate parts of which it will be made up after the cartilage has been replaced by bone. Shortly before ossification begins the rib cartilage becomes separated from the vertebra, but the vertebra itself remains in one piece throughout the cartilage stage (Fig. 162).
The locations of the endochondral ossification centers which appear in a vertebral cartilage are indicated schematically in figure 163. It readily can be seen how the spreading of these centers of bone formation will establish the conditions which exist in an adult vertebra. The median ossification center gives rise to the centrum. The centers in the neural processes extend dorsally to complete the neural arch. The spinous process in most of the vertebrae is formed by a prolongation of these same centers to meet dorsal to the neural canal.
Fig. 164. Diagram of four types of vertebrae indicating the parts derived from the different ossification centers shown in figure 163. The part formed by the median center in centrum is cdncentrically ringed; the parts arising from the costal centers are stippled; parts derived from the lateral centers in the neural arches are indicated in line-shading,
In forms such as the pig the spinous processes of tl>e more anterior thoracic vertebrae are very long. In these vertebrae additional ossification centers appear in the spinous process and fuse with those in the neural processes. The transverse processes with which the tubercles of the ribs articulate are formed by the lateral extension of the primary ossification centers in the neural processes. These same centers extend ventrally also, and meet the centrum (cf. Figs. 163 and 164).
Fig. 165. Photograph (X 1/^ showing the ossification centers which have appeared in pig embryos of 35 mm. This and the two following figures were made by photographing in transmitted light embryos in which all the uncalcified tissues had been rendered transparent by treatment with potassium hydroxide and glycerine.
Fic. 166. Photograph (X IM) showing the progress of ossification in the skeleton of a 65 mm. pig embryo.
Fig. 167. Photograph {X showing the extent of ossification in the skeleton of a 90 nun. pig embryo.
The shaft of the rib is formed by extension of its primary ossification center (Fig. 163). After birth, secondary epiphyseal centers appear in the tubercle and head of the rib. These centers are separated from the shaft by persistent cartilage plates in the manner described in discussing the development of long bones. Fusion of the secondary epiphyseal centers with the shaft of the rib does not take place until the skeleton has acquired its adult dimensions.
The foregoing discussion has been based on a thoracic vertebra in which the relations of the rib to the vertebra show most clearly. All the vertebrae have the costal element represented, although it is greatly reduced and modified in other regions than the thoracic. A study of figure 164, in which the components of vertebrae from the cervical, thoracic, lumbar, and sacral regions are schematically indicated, will make the homologies apparent. With these homologies in mind it is sufficiently evident, without going into further detail, how all these vertebrae arise by a process similar to that described for the thoracic vertebrae.
The Progress of Ossification in the Skeleton as a Whole. It would carry us beyond the scope of this book to take up the development of specific bones. Each has its own story involving the formation of the connective tissue or the cartilage mass which precedes it ; local erosion centers if it be preformed in cartilage; number, location, and time of appearance of ossification centers; growth in length and diameter; development of epiphyses; time of fusion of epiphyses and diaphysis; and finally the development of muscle ridges and articular facets. Without entering into a discussion of details of this sort, it is possible nevertheless to follow the general progress of ossification in the skeletal system as a whole. Embryos which have been treated with potassium hydroxide and then cleared in glycerine clearly show the various ossification centers. In such preparations the areas where calcium salts have been deposited stand out white in reflected light and opaque in transmitted light. ^Figures 165-167, which are photographs of preparations of this type, can be used to trace the history of the more important bones. It should perhaps be stated explicitly that these figures arc included primarily to give a general view of the progress of ossification and secondarily to afford a readily available source of reference for following up points of interest that may arise. It is not a profitable use of the student’s time to attempt to memorize the ossification centers which have appeared in embryos of any given age.
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