Paper - The origin of the osteoblast and of the osteoclast (1913)
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Geddes AC. The origin of the osteoblast and of the osteoclast. (1913) J Anat. Physiol. 47(2):159-176. PMID 17232948
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The Origin of the Osteoblast and of the Osteoclast
By Professor A. C. Geddes,
Royal College of Surgeons in Ireland.
This paper records the result of an investigation into the earlier stages of the cytomorphosis of the osteoblast. It includes a description of the origin of the osteoclast.
At the outset of the work it was evident that osteoblasts, if proved to be of mesodermal origin, must arise either from intra-cartilaginous or intra-periosteal inclusions of undifferentiated connective tissue syncytium, from the differentiated cells of cartilage or periosteum, or from the connective tissue syncytium outside the areas transformed into cartilage and periosteum. On the other hand, if found to be not mesodermal in origin, they might come from the entoderm, or, for phylogenetic reasons, more probably from the ectoderm. These possibilities determined the lines of investigation.
Thus, a normal inclusion of undifferentiated elements in differentiated cartilage was considered and excluded. The question of the origin of osteoblasts from cartilage cells was considered and answered in the negative, full weight being given to the facts which determined Retterer’s (1) positive answer. Next, the normal inclusion of undifferentiated elements in periosteum was considered and excluded. Then a site of origin for osteoblasts was sought in the connective tissue syncytium outside the cartilage- and periosteum-forming areas, and was not found. Finally, the osteoblasts were traced migrating as single cells from the ectoderm to the periosteum.
In the course of the search for the origin of the osteoblast, the origin of the osteoclast was observed.
The investigation was carried out chiefly upon the human embryonic series preserved in the Anatomical Department, R.C.S.I, but frequent reference was made to the animal series, more especially the pig, chick, axolotl, and trout.
The stains found most useful were—
1. Hematoxylin and eosin. 2. Hematoxylin and Bismarck brown. 3. Hematoxylin, eosin, and Bismarck brown. 160 Professor A. C. Geddes
. Hematoxylin and safranin (Retterer).
. Hematoxylin and Congo red.
. Hematoxylin and picro-erythrosin.
Alum carmine and Bismarck brown.
Fuchsine, aniline blue, and orange G. (Mallory, principally Sabin’s modification).
9. Methyl blue and eosin.
1. The Normal Absence of Inclusions of Undifferentiated Cell Elements in Cartilage
The connective tissue syncytium from which cartilage is about to arise is a network of foamy protoplasm arranged in strands, « 3-6 broad, bounding meshes « 7-10 in diameter. At places the mesh, instead of containing the usual crystalloid fluid, is “solid.” The islands of protoplasm thus formed may measure as much as uw 25 in diameter. Embedded in the protoplasm, frequently at the nodes of the network, are the “energids,” the cells. Running through the protoplasm, connecting energid to energid, are fibrils which stain deeply with aniline blue. In addition, scattered here and there, are cells which appear to lie, not within the protoplasm, but upon it, or even to be independent of it and to lie in the fluid of the mesh (fig. 1).
Fig. 1. — Microphotograph x 400. Stain: Sabin’s modification of Mallory’s. To show the foamy syncytium with its energids, the precursor of the connective tissues.
The start of differentiation is signalised by a reduction in the size of the meshes of the net and by the fibrils embedded in it becoming less evident. Next the energids increase in number. There is some reason to suppose that these phenomena are the result of a pressure which, continuing to act, ultimately compels the foamy protoplasm to form a continuous, almost homogeneous, matrix within which the cells are embedded (fig. 2, A), for, mechanically, many of the sites of cartilage formation coincide with areas of enhanced pressure.
Fig. 2. — Microphotograph x 266. Stain: A to D, Sabin’s modification of Mallory’s; E to I, hematoxylin, eosin, and Bismarck brown. To show: A to F, the evolution of true transitory cartilage ; G to I, its degeneration. For description see text. 162 Professor A. C. Geddes
The next stage of development is marked by an active proliferation of the cells, which, becoming crowded, distort one another (fig. 2,B). Further advance towards differentiation is marked by an apparent loosening of the tissue. The cell crowd begins to disperse. Some cells move off alone, others in groups, still others in chains of from four to ten. As the intercellular spaces open up, lines which come to form the cords of a new net appear in them. The microscopic picture reduced to monochrome recalls to some extent the primitive syncytium (fig. 2, C). There are, however, distinct differences both in staining reaction and in structure (fig. 2, D and E). Sections stained with Bismarck brown show that a muginous substance (chondro-mucin) has appeared and is concentrating upon the strands of the new net, and that the cells, though, for a time, they may lie on or close to the net, soon associate themselves with one mesh each.
At the next stage (fig. 2, F)—the stage of differentiated embryonic cartilage—each cell consists of an approximately central spherical nucleus, surrounded by a cell body of granular protoplasm which fills the whole of its mesh. Bismarck brown stained sections show that this protoplasm is not uniform in composition. Immediately around the nucleus it refuses the stain, but from that to the periphery shows a uniformly increasing avidity for it, though no part of the cell stains so intensely as the intercellular network. Beyond this degree of differentiation the truly transitory cartilages do not advance, but at once begin to show changes leading up to bone formation (fig. 2, G, H, and I).
In summary, (1) cartilage cells are the energids of the connective tissue syncytium transformed. (2) When the connective tissue syncytium is compressed and its meshes obliterated, the energids, either as naked nuclei or as cells with extremely small. cell bodies, free themselves from the common protoplasm—exoplasm—and elaborate for themselves complete coverings of private protoplasm—endoplasm. This increases in amount, and the cell, requiring room, pushes back the exoplasm. Simultaneously it begins to excrete chondromucin into the exoplasm, with the result that the cell comes to lie in a capsule of exoplasm plus chondromucin. Collectively the capsules are the matrix.
This account of the development of cartilage coincides in essentials with that given by Mall (2), who holds that the ground substance of cartilage is deposited directly into the exoplasm of the syncytium, while its nuclei and endoplasm become the cartilage cells. Where the view of Mall differs from that summarised above is in the means of arriving at the same end. To Mall the cartilage cells lie in the meshes of the primitive syncytial net, moving into them out of the substance of the exoplasm, whereas, to the writer, the meshes are obliterated at the time of condensation of the mesenchyme. In the view that after the condensation of the mesenchyme (fig. 2, B) the cells are embedded in a homogeneous matrix, the writer finds himself in accord with Retterer.
This difference matters little. The essential is that under normal circumstances every element of the connective tissue syncytium in the chondrogenetic area is involved in the process of histogenesis, that no visible part of the syncytium remains undifferentiated to form osteoblasts.
2. The Normal Fate of the Cartilage Cell
The fully formed healthy cartilage cell completely fills its capsule and possesses a central spherical nucleus, a granular protoplasm, and a definite margin (fig. 3, A). As the time of its replacement by bone draws near it increases rapidly in size (contrast fig. 2, F and G); the nucleus becomes eccentric; the cell body vacuolated and shrunken (fig. 2, H); calcium droplets appear in the centre of the cell, drift through it, and deposit in the surrounding matrix. Finally, the cell breaks up (fig. 3, B). Asa rule this coincides with the appearance of osteoblasts in the neighbourhood, and to Retierer the osteoblasts are rejuvenated fragments of the shattered cell.
This belief the writer cannot share, because osteoblasts differ from cartilage cells in chemical composition. With Sabin’s modification of Mallory’s stain the cartilage cells are blue red, the osteoblasts bright red. This alone makes it improbable, though not impossible, that osteoblasts are fragments of cartilage cells. Another argument against Retterer’s view is that not once, provided the tissue studied was serially sectioned, has it been found impossible to trace a chain of osteoblasts back from the regions in which they are just appearing, through regions where they can be seen to possess long pseudopodia—that is, were in active movement at the moment of fixation (fig. 3, C)—back, through tortuous, easily overlooked cell paths, through the matrix (fig. 3, D) to regions where they are present in large numbers. Here, to judge by their bluish-red inclusions, they were fixed in the act of devouring the debris of the cartilage cells. In many hundreds of sections not once has it been impossible to trace the path of the osteoblasts into every cartilaginous cell capsule in which they are present. Retterer states that he has failed to find this, and that closed capsules containing the debris of cartilage cells and osteoblasts do occur. Even so, it would not be conclusive evidence that osteoblasts were fragments of cartilage cells, for it is readily conceivable that a minute cell path through the matrix might be closed by the accidental deposition of a droplet of calcium within it, or even by compression resulting from the terminal hypertrophy of neighbouring cartilage cells. But the writer has modification of Mallory’s. To show the manner of replacement of cartilage cells by osteoblasts. In A, group of four cartilage cells.. The upper left cell is in the height of its development and prosperity ; the upper right is slightly shrunken and its stain reaction altered. The lower pair of cells show shrinkage and commencing vacuolation. In B, complete shrinkage and vacuolation ; an osteoblast is just entering the lower part of the cell’s capsule. In C, an osteoblast is seen pushing a long pseudopodium into a cartilage cell’s capsule immediately above the space in which. it itself is lying. In the lower part of C, three osteoblasts are seen lying in one cartilage cell capsule. The highest of these contains two darkly stained masses which agree in their stain reaction with the debris.of the cartilage cell beside which the . osteoblast is lying. In D, towards the upper right part an almost obliterated cell path through the cartilaginous matrix is shown, while towards the lower left part, a cell path occupied b: pseudopodia of osteoblasts is visible. These cell paths lead into and out of an enlarged cartilage cell capsule which is packed with osteoblasts, invariably found that if at any point an osteoblast be seen, it is possible to trace back its path to some point at which a periosteal bud has grown into the substance of the cartilage (fig. 4, A and B).
Fig. 3, — Microphotograph x 666. Stain: Sabin’s
Fig. 4. — A x 250. Stain: hematoxylin, eosin, and Bismarck brown. To show the bud of osteoblasts preparing to irrupt into the cartilage of the shaft of a human femur. The cartilage is fully calcified. The cartilage cells are shrunken and vacuolated, but no osteoblasts are present.
Bx 250. Stain: Sabin’s modification of Mallory’s. To show the irruption of the osteoblasts. Note especially that the osteoblasts now for the first time appear in the cartilage cell capsules.
Cx150. Stain: Sabin’s modification of Mallory’s. To show the relation of the osteoblasts and the bone which they form to the remnants o: the calcified cartilage. In this photograph the bone shows black and the calcified cartilage grey. (Contrast with A, where the hematoxylin-stained calcified cartilage shows black.)
Dx150. Stain: Congo red. To show the osteoblasts becoming bone cells. Note the ‘*sandwich” formed by the subperiosteal bone, the layer of calcified cartilage (showing light), and the endochondral bone.
3. The Advantage to the Bonne Cells of Succeeding Cartilage
A subsidiary problem obtrudes itself at this point. If cartilage contributes nothing to bone, why are bone and cartilage so frequently related ? Recent work on the cultivation of embryonic tissues seems. to suggest the answer (3) that the advantage to the body of having its long bones.preceded by cartilage is that the calcified cartilaginous matrix forms a scaffolding along which the osteoblasts can clamber and on the surfaces of which they can lay down primitive bone (fig. 4, C and D, and fig. 5).
In summary of sections 2 and 8, there is no conclusive evidence in favour of Retterer’s view that osteoblasts arise from cartilage cells, but there is evidence which makes it extremely probable that in all cases the osteoblasts enter the cartilage as members of one or other of the “periosteal” buds which invade each long bone. Further, bone derives a real advantage from succeeding cartilage in that the calcified matrix provides a scaffold upon which the osteoblasts can move about their business of forming bone.
Fig. 5. — x 1838. Stain: Sabin’s modification of Mallory’s. To show that no bone (black in photograph) is laid down except by the osteoblasts, and that, when it is, it is laid down upon the surface of the calcified cartilage. This is well shown in the lower righthand part of the photograph.
4. The Normal Absence of Inclusions of Undifferentiated Connective Tissue Syncytium in Periosteum
In histogenesis, periosteum is at its beginning an indistinguishable part of the connective tissue syncytium, continuous on the one hand with the chondrogenetic area, on the other with the area forming perimysium, cutis, or whatever it may be (fig. 6, A). Soon the cartilage acquires an edge, and this enables the periosteal part of the syncytium to be recognised: The changes which occur in it are:
- The nuclei multiply and acquire a covering of endoplasm distinct from the exoplasm, which forms the syncytial net and is exactly equivalent to the exoplasm of the chondrogenetic area.
- The exoplasm increases enormously in amount.
- The cells and their endoplasm are drawn out to form bipolar cells.
- The exoplasm becomes fibrillated. These fibrils are gradually transformed into fibrous tissue, the fibres of which run chiefly in the long axis of the developing bone. As differentiation proceeds the primitive continuity between the developing periosteum and the cartilage disappears (fig. 6, B, C, D, E), a space ultimately developing between the two.
Fig. 6. — x100, Stain: hematoxylin, eosin, and Bismarck brown. To show the evolution of the periosteum, the formation of the sub-periosteal space, the arrival of the osteoblasts, the first formation of sub-periosteal bone, and the formation of the osteoblastic bud.
At this stage in a section stained with modified “Mallory,” the cells of the periosteum stain a blue red, the fibrils an intense blue.
1 This space is always present, even in otherwise perfectly fixed tissue. It may be an artifact, but, even so, indicates weak bonding.
Recapitulated,; the periosteal cell proper begins as an energid of the connective tissue syncytium, develops a cell body (endoplasm), and becomes the cell, first of embryonic fibrous tissue, then of adult fibrous tissue. Simultaneously the fibres develop in the exoplasm.
The essential is, that under normal circumstances every element of the connective tissue syncytium in the area in which periosteum develops is involved in the process of histogenesis, that no visible part of the syncytium remains undifferentiated to form osteoblasts.
5. Osteoblasts do not Arise from the Periosteal Cells
Since the work of Sir William MacEwen (4) there has existed no need for further proof that osteoblasts do not arise from fully differentiated periosteum. It would be indeed surprising if they did. For fully differentiated periosteum is a dense fibrous, almost cell-less, structure (fig. 7), which functions as a limiting membrane for the osteoblasts. It might, however, be argued that osteoblasts arise from developing periosteum. Evidence in favour of this view, apart from the topographical, that osteoblasts appear in the periosteum, is lacking, whereas strong evidence against it exists. When sections are treated with modifications of Sabin’s modification of Mallory’s stain, the modifications being such that the proportions of the ingredients of the second mixture are progressively varied, so that the concentration of the oxalic acid remains constant, the amount of the aniline blue being reduced, the time of action also being varied within narrow limits, specimens are obtained which show that at first all the cells in the periosteal area have similar stain reactions, but that later cells begin to appear in the peripheral part of the periosteum which, compared with the cells present in situ from the commencement of local differentiation, have a reduced affinity for aniline blue and an increased affinity for fuchsine. In other words, there begin to appear in the peripheral parts of the periosteum, cells which have the same kind of difference from periosteal cells that differentiated osteoblasts have from cartilage cells, though the amount of difference is less. When it is recalled that both cartilage cells and periosteal cells arise from the energids of the connective tissue syncytium and are therefore cousins, and when it is observed that these related, though differently specialised, cells have an apparently identical affinity to Mallory’s stain, it becomes evident that it is extremely unlikely that the cells in the peripheral part of the periosteum have arisen from periosteal cells, and extremely likely that they are young osteoblasts which, because of their youth and incomplete differentiation, have an incomplete differential stain affinity.
Cells of this type accumulate in the substance of the. periosteum before the osteoblasts appear in the interval between-it and the cartilage, and the writer believes that it is they which, passing through the periosteum, appear as osteoblasts in the subperiosteal space, where they soon begin to lay down bone upon the surface of the calcified cartilage. In the case of the majority of the bones, sooner or later, depending apparently on the state of vigour of the cartilage cells, they invade the cartilage through numerous breaches in its calcified shell. The essential is that the osteoblast appears in the peripheral part of the periosteum asa partially differentiated cell, with a stain affinity which marks it off from the periosteal cell proper, which, like the cartilage cell, is a transformed energid of the connective tissue syncytium. On account of this, it:is extremely probable that osteoblasts are not natives. of, but immigrants into, or, more strictly, transients through, the periosteum.
Fig. 7.— x 833. A. To show young periosteum prior to the osteoblastic irruption into the cartilage.
B. To show the absence of cells from fully formed periosteum (from the femur of a child of five). In B, the bong is directly below the cartilage in A; the adult periosteum directly below the embryonic periosteum.
6. Search for the Site of Origin of the Osteoblast
Because the young osteoblasts at their first appearance in the peripheral layers of the periosteum possess an incompletely developed power of differential staining, it seemed from the first extremely improbable that they could be recognised before they reached the periosteum, when they were still younger and presumably still less differentiated, and, in fact, it was found impossible to obtain any clearly visible difference between the various cells just outside the periosteum.
Fig. 8. — A. To show, with approximate accuracy, the absorption curves of aniline blue and of fuchsine in strengths of 1/150-1/2500 and the Hight actually employed in photographing specimens.
B. To show superimposition of curves shown-in A.
After study of the optical problem involved it was thought that a method of making the invisible difference appreciable to a photographic plate might be devised. The light absorption curve of fuchsine in concentrations of 1 in 150 to 1 in 2500 is shown in fig. 8, A. Above this is shown the curve of absorption of aniline blue in corresponding strengths. Beneath is shown a light curve which will be uninfluenced by the presence of aniline blue in its path, but will be affected by all the strengths of fuchsine.
The problem, therefore, was to obtain a cone of light rays with a wave length between 5000 and 5800, with a maximum transmission about 5350. This was readily effected by the interposition of a subtraction screen in the transmission path of an electric arc. Fig. 8, B, in which the curves are superimposed, shows that such a light will entirely fail to penetrate tissue containing fuchsine in any higher concentration than (approximately) 1 in 1600; that it will partially penetrate tissue containing fuchsine in less concentration, and that it will be entirely unaffected by any probable concentration of aniline blue in the tissues.
Fig. 9. — x 1000. To show three cells believed to be primitive osteoblasts in process of migration through the general connectivetissue syncytium.
Fig, 10. — x100. To show, in contrast with fig 9, the photographic _ similarity existing between ectodermal cells and those believed to be ’ primitive osteoblasts.
If, now, any cells anywhere in the tissues possess a greater fuchsineholding power than others, however slight the difference, it must be possible by careful decolourisation to obtain specimens: in which the fuchsine-holding cells will, when exposed to light with a wave length of about 5350, absorb all the light rays reaching them, and be therefore photographically inactive, whereas all the other cells, whether holding aniline blue or fuchsine in reduced quantities, will fail to absorb all the light and will be photographically active. Further, unless it can be shown that the fuchsineholding power of the osteoblasts arises by saltation and not by gradation, it is legitimate, provisionally, to regard the cells with the greater fuchsineholding power as osteoblasts in process of development, as precursors of the osteoblasts, or as their near relatives.
More by good luck, perhaps, than by good guidance, specially stained sections of 10-12 mm. human embryos were obtained, which, when photographed by the selected light upon Ilford process plates, showed, on the negatives, some cells as clear glass. In other words, some cells present in the tissues absorbed all the rays striking them: that is, some cells contained fuchsine in greater concentration than 1 in 1600. The rest of the tissue structure was practically ignored by the camera. In other words, it failed . to absorb an appreciable proportion of the rays striking it: that is, the rest of the tissue, protoplasm, energids, and what not, contained fuchsine in less concentration than 1 in 1600.
The cells which held the fuchsine in the greater strengths were found as singletons, or in groups in the syncytium and in the ectoderm, from which they appeared to arise (figs. 9 and 10).
Putting the whole statement of results into other words, cells exist in the ectoderm and in the connective tissue syncytium which are marked off from the energids of the syncytium by a difference of the same kind as, but of a less degree than, that which marks off the differentiated osteoblast from such differentiated energids as the cells of cartilage and periosteum.
Further, the cells with the fuchsine-holding power discovered in syncytial areas were found, on examination, to be examples of the cells which appear to lie on, or within the meshes of, the primitive syncytium (section 1, fig. 1).
It was tentatively concluded, therefore, that the osteoblast arises in the ectoderm and moves from that through the mesodermal syncytium, as a singleton, or as one of a scattered group, to the periosteum.
7. Reconstruction of the Life-History of an Osteoblast
Osteoblasts appear to detach themselves as single cells from the under surface of the ectoderm, and to migrate alone or in groups of three or four to the nearest area of periosteal formation. Into this they enter, The Origin of the Osteoblast and of the Osteoclast 173
and in it they accumulate. Passing through the periosteum they crowd together in great numbers between it and the cartilage, in which the matrix is becoming filled with calcium and the cells are dying. Asa rule the osteoblasts soon begin to lay down a rind of bone around the calcifying cartilage, then, gathering together here and there to form buds, they break into the cartilaginous shaft of the bone that is to be. Here they find in the calcified cartilaginous matrix a scaffolding upon which they can lay down their primitive bone.
The later stages of bone formation are well known; it would be tedious to repeat them, but one or two points may be referred to. In their definitive state, osteoblasts are bone corpuscles, and lie in the lacunae, sending out from their capsule long processes which fill the canaliculi. These do not connect cell with cell, but extend from their own cell to an Haversian canal (specimens stained with Retterer’s stain). If they come from one of the outlying cells of an Haversian system they pass on their way through one or two lacune occupied by other osteoblasts, but they do not connect with these cells, for on careful focussing the hematoxylinholding cell processes can be traced round the outside of the bodies of. the osteoblasts occupying the lacune through which they pass (fig. 11). In other words, adult osteoblasts appear not to form a syneytium, as has been supposed.
Fig. 11.— x 500. Stain: Retterer’s (hematoxylin and saffranin). Longitudinal section ef the femur of a child of five. The right of the figure extends into the lumen of a Haversian canal. In the left half of the field a bone corpuscle is seen. It fills its lacuna and possesses numerous processes extending out into the canaliculi. About the centre of the field a vertical dark area marks the position of another bone corpuscle, slightly out of the focal plane. To show that the cell processes extend from the first corpuscle through, not to, the lacuna of the second corpuscle and ultimately reach the Haversian canal.
8. Origin of the Osteoclast
In the process of the work the origin of the osteoclast was stumbled upon by an accident. It may at once be stated that an osteoclast is a composite mass consisting of the fused bodies of two, three, or more cartilage cells containing any number of osteoblasts—in short, it is a mesoectodermal syncytium.
The evidence upon which this statement is based is, that in specimens of embryos about the tenth week stained with Mallory, photographed upon a Gem “Tricol,” colour-sensitive, plate by the light of an electric arc, the beam of which has been passed through a subtraction filter to remove all rays with a wave length about 4500, the blue-red protoplasm of the cartilage cells is rendered photographically inactive; the bright red of the osteoblasts, photographically active.
Fig. 12. — x666. To illustrate the origin of the osteoclast. Stain: Sabin’s modification of Mallory’s. An osteoclast is-seen near the centre of the oval. The main part of its mass lies in a large cartilage cell capsule ; from this a process of the cell extends to the right into a neighbouring cartilage cell capsule. It is especially to be noted that two of the nuclei of the osteoclast which are in the focal plane agree in staining reaction with an osteoblast present in the upper part of the space in which the osteoclast lies, whereas the body of the osteoclast agrees in stain reaction with the undoubted debris of a cartilage cell seen low down on the right of the oval. Beside this is an osteoblast which looks dark, but, on close examination, its darkness is seen to be due to the presence of lumps of dark matter within it. These agree in stain reaction with the debris of the cartilage cell which the osteoblast is touching. (Evidence of ingestion of the debris of cartilage cells by osteoblasts. Compare also fig. 3.)
This specimen is from the second metatarsal of a foetus of the 8th-9th week. At this stage of development there is no piece of bone in, or in connection with, the metatarsals so large as this osteoclast. This is of some importance as negativing the recent suggestion that osteoclasts are merely pieces of bone from which the lime salts have been removed through the action of some enzyme.
Some osteoblasts which contain large numbers of fragments of dead cartilage cells show dark in the photograph, and possess in low-power views the same tone value as cartilage cells. Under high magnification these dark osteoblasts are seen to consist of typical osteoblastic protoplasm containing large coarse granules.
The bodies of developing osteoclasts agree with cartilage cells in their The Origin of the Osteoblast and of the Osteoclast 175
power of transmission and absorption of light—that is, have a similar affinity for the dyes of Mallory’s stain. This is equivalent to saying that the protoplasms of osteoclasts and cartilage cells are of similar chemical composition. Similarly, the majority of the nuclei of osteoclasts agree with the nuclei of osteoblasts in their power of transmission and absorption of light—that is, have a similar affinity for the dyes of Mallory’s stain—in other words, are of similar chemical composition. In the case of the minority of the nuclei, two or three in each osteoclast, the light transmission and absorption corresponds with that of the nuclei of cartilage cells.
It is legitimate, therefore, to conclude that osteoclasts are hybrid syncytial masses composed of fused cartilage cells containing osteoblasts.
It appears probable that the explanation of the formation of the osteoclast is that sometimes osteoblasts force their way into spaces occupied by cartilage cells which are not dead, or at least not sufficiently devitalised to fall an easy prey to their attack, with the result that instead of devouring the cartilage cells, the osteoblasts are engulfed, though not destroyed, and, persisting within their new environment, are the multiple nuclei characteristic of the osteoclast. It is important to add that osteoclasts are present in ossifying cartilage at a time when bone formation is just commencing, and long before any areas of bone as large as an osteoclast have been formed.
9. General Summary and Conclusions
There is evidence which makes it probable, though by no means certain, that—
- Bone derives from the ectoderm and not from mesoderm.
- Osteoblasts arise from the cells of the ectoderm, and migrate as individuals to the sites of bone formation, passing through the periosteum en route.
- Adult osteoblasts do not form a syncytium. The cell processes which are lodged in the canaliculi, though frequently passing through lacunze occupied by other cells, extend uninterruptedly to the Haversian canal of their system.
- Periosteum, far from being an osteogenetic membrane, is a limiter of bone formation.
- Cartilage, a mesodermal tissue, has, when it precedes bone, the function of providing a scaffolding upon which the osteoblasts can move.
- Osteoclasts are composed of the fused bodies of one or more cartilage cells, with numerous osteoblasts living within the protoplasmic mass as cell inclusions (multiple nuclei).
In conclusion, I desire to acknowledge the assistance I have received 176 The Origin of the Osteoblast and of the Osteoclast
from Mr William Gill, both in the preparation of histological material and in the intricate work of microphotography.
(1) Rerrerer, Ep., Journ. del’ Anat. et dela Physiologie :—Année 28, 1892, p. 211, ‘‘ Les découvertes relatives au développement du tissu conjunctif”; année 36, 1900, p. 358, ‘“ Similitude du processus histogénétique chez l’embryon et Vadulte”; année 36, 1900, p. 467, “Evolution du cartilage transitoire”; année 41, 1905, p. 561, “Structure et histogénése de l’os”; année 42, 1906, p. 193, “Evolution du tissu osseux.” Also papers, C. R. Soc. Biol., 1895-1908.
(2) Matt, F., Amer. Journ. Anat., vol. i., 1901-2, p. 329, “ On the Development of the Connective Tissues from the Connective-Tissue Syncytium.” Also paper, Abhandl. d. K. Sachs, Geselisch. d. Wiss,, Bd. xvii., 1891.
(3) Harrison, Ross G., Anat. Record, vol. vi., 1912, p. 181, “The Cultivation of Tissues in Extraneous Media as a Method of Morphogenetic Studies.” Also references.
(4) MacEwen, Sir Wm., Proc. Roy. Soc., B, 524, 1906, p. 237, ‘ Regeneration of Bone.” Proc. Roy. Soc., B, 533, 1907, p. 397, ‘‘The Réle of the Various Elements in the Development and Regeneration of Bone” (full paper, Phil. Trans, Roy. Soc., vol. excix., 1908, p. 253). Annals of Surgery, vol. 1., 1909, p. 957, ‘‘ Intrahuman Bone-grafting and Reimplantation of Bone.”
For fuller literature list see Manual of Human Embryology, Keibel and Mall, vol. i. pp. 312-316.
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