Paper - The ribs in the second month of development (1912)
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The Ribs in the Second Month of Development
By Professor A. C. Geddes,
Royal College of Surgeons in Ireland.
1. At the commencement of the second-month of development each rib is represented by a pyramidal mass of condensed mesenchyme which projects from the posterior third of its segment ventro-laterally for a short distance into the body wall (1) (figs. 1 and 2).
2. In the course of the fifth week the ribs, which are still membranous, become curved with their concavity directed dorso-laterally, a curvature the reverse of the definitive (fig. 8). In the sixth week chondrification commences and proceeds rapidly. The rib increases much in size. Its shape, which is determined by the relatively small size of the lung buds and the great size of the liver, is an open S (figs. 4 and 5). (The peculiar configuration which the thoracic cage as a whole possesses at this stage is well shown in Ch. Miiller’s figures, Morph. Jahrb., 1906, reproduced as figs. 260-263, p. 342, vol. i, Manual of Human Embryology, Keibel and Mall). In the course of the seventh week the cartilaginous rib assumes what is practically its normal curvature (fig. 6).
Fig. 1. — Embryo A, R.C.S I. Collection, C.R. Length 53mm, Prob- 896. x36. To show the shape and relative able age 30-32 days. Selected as size of the ‘‘ membranous” costal process.
Fig. 2. — Microphotograph, Embryo A, Section earliest embryo of the second month in the collection, x 4.
Fig. 3. — Microphotograph, Embryo G 175, Section 407. x15. The embryo belongs to the fifth week. The section passes through the first dorsal segment. The costal process is short and still membranous. On the right side its dorsal concavity is shown.
Fig. 4. — Embryo B, R.C.S.I. Collection, C.R.
Fig. 5. — Microphotograph, Embryo B, Section Length 8mm. Probable age 38-40 days. 817. x15. To show the shape of the The photograph was taken immediately ribs which are undergoing chondrification. Ribs 3, 4, and 5 are cut in the section. Their the membranes were opened, the embryo being unfixed and floating in normal open S-curve is well seen. The ritzer line to saline. x2. the right was made by the camphor smoke
At first it seemed reasonable to suppose that this change was effected by the rib being simply forced outward by the pressure of the growing viscera. The histological evidence is altogether against so simple an explanation. The observed facts and the interpretation put upon them are :—
(a) There is, a short distance lateral to the laterally concave part of the cartilaginous rib, a comparatively loose syncytial mass of undetermined origin. Most probably it is derived from the scleromere, but there is some evidence to suggest that it may belong to the cutis, and through it connect to some extent with the ectoderm (2). Between this and the rib cartilage there is a stratum of condensed mesenchyme, also syncytial, which laterally fades into the looser syncytium and medially into the differentiated cartilage.
Fig. 6. — Microphotograph, Embryo R 49, Fic. 7.—Microphotograph, Embryo B, Section
Section 190. Shoulder Girdle Series. 817 (see fig. 5). Cartilaginous rib on right x12, The embryo belongs to the early side at lateral concavity. x50. The pleura seventh week. Ribs 3, 4, and 5 are cut is below and to the left. The cartilage is exin the section, by contrast with fig. 5, to tending upwards and to the right (see text).
show the rapid rectification of rib shape.
(b) The medial border of the differentiated cartilage is definite and is lined by four or five layers of mesenchymal cells, which are‘separated by an extremely loose syncytium from the pleura (see fig. 7, in which the strata read from the right-hand top to the left-hand bottom corner).
(c) The interpretation of these structural facts, adopted after consideration of all available evidence, is that the rib is reshaped partly by interstitial growth but largely by a process of addition on its lateral side and of subtraction on its medial side; in other words, that the dense mesenchymal syncytium lying lateral to the cartilaginous rib differentiates from the loose syncytium by a process of rapid multiplication of cell units, and that the cartilage differentiates from the dense syncytium by the cell units gradually becoming extruded from the syncytium and separated from one another by the transformation of the denucleated ectoplasm into matrix—the normal mode of formation of embryonic cartilage (3 and 4). (d) The process of subtraction which proceeds on the medial side of the rib is even more interesting. There, there is no sign of cell death or cell removal, but the cartilage cells appear to reverse their specialisation and, escaping from the cartilage, to revert to the mesenchymal type. This is deduced from the following facts: first, there is no sign of cell proliferation along the medial side of the rib; second, large numbers of the cartilage cells lie exposed in cups (half capsules) of large size; and third, because there is no sign of ‘cell disintegration and removal, and yet the median convexity is removed. The four or five layers of cells between the cartilage and tlie subpleural syncytium are apparently the cells which have been cartilage and have reverted. Their later history is discussed below.
Fig. 8. — Microphotograph, Embryo R, Section
Fig. 9. — Microphotograph, Embryo R, Section (See figs. 9 and 10.)
1150. x10. The 1150. x133. Cartilage of the fourth rib,
embryo belongs to the middle of the seventh right side. The lateral aspect of the rib is
week, to the observer’s right in the photograph (see text).
3. As noted above, the lateral convexity of the rib is established in the seventh week, and thereafter for a few days growth is extremely rapid. The manner of this growth repays study.
When once the laterally convex cartilaginous rib is established, all further additions of cartilage to it cease so far as its lateral (convex) aspect is concerned. The growth remains in some measure interstitial, but active addition proceeds on the concave side—a reversal of the conditions of the sixth week (figs. 8, 9, and 10). Certain points require to be noted in detail. First, the lateral syncytium undergoes consolidation and tends to withdraw slightly from the cartilage. In the majority of specimens small gaps appear between it and the cartilage. Similar gaps never appear on the medial aspect of the cartilaginous rib. It is not impossible that the gaps are artifacts; their frequent occurrence in otherwise perfect sections makes this improbable, however. But even if they are, they mark a weakness in bonding between the cartilage and the consolidating syneytium, which becomes the periosteum (fig. 9).
Fig. 10. - Microphotograph, Embryo R, Section 1150. x 833. To show the medial edge of the rib cartilage shown in fig. 9 (see text).
In the meantime the medial aspect of the rib is the seat of active cellular addition. The process is shown in fig. 10. Between the subpleural syncytium and the rib cartilage are the layers of cells which in the course of the sixth week were seen to withdraw from the cartilage. These now rapidly proliferate and, establishing protoplasmic connexion with one another, form a syncytium. Next, each cell unit abandons a large part of its protoplasm, the nucleus and its endoplasm withdrawing from the ectoplasm, which becomes the matrix of the cartilage. These stages are well shown in fig. 10. On the right the fully differentiated cartilage is shown; on the left the perichondrial cells which are preparing to add themselves to the cartilage. Between these are two distinct strata. Next to the perichondrium is a broad syncytial band. Betwéen this and the cartilage comes the second stratum. In this the cartilage cells are separating from their own ectoplasm, which is becoming transformed into matrix. This process is well shown in the upper half of the microphotograph where, close to the top of the photograph, there are two cells newly separated from their ectoplasm, which has not yet undergone complete differentiation into cartilage matrix.
Fig. 11. - Microphotograph, Embryo R, Section 1305.
Fig. 12. - Microphotograph, Embryo R, Section x6.3, To show the sixth and seventh ribs (see figs. 12 and 13). 1305. «183. Cartilage of sixth rib, right side. The lateral aspect-of the rib is to the observer's right in the photograph (see text).
Restated, at the middle of the seventh week the rib is cartilaginous throughout its length and is growing chiefly on its medial side. Its continuous adjustment to a larger curve is due partly to interstitial growth, but largely to the increase of substance on its concave aspect, mechanically forcing it out. From these facts it is possible to determine the cellular composition of the rib at this time. First, all the cells on the concave side are younger than those on the convex side, though these are not of equal age. Second, reference to fig. 5 will make it clear that the cells on the convex side in the neighbourhood of the angle have had about ten days or a fortnight’s existence as specialised cartilage cells, those on the middle of
Fig. 13. — Microphotograph, Embryo R, Section 1805. x 383. To show the medial edge of the rib cartilage shown in fig, 12 (see text).
_ the convex side about five or six days; those on the same side near the anterior extremity possibly three or four days; whereas, those on the concave side may only have existed a few hours as specialised cells. It is in relation to this heterogeneously constituted rib that ossification takes place. The processes which lead up to this further advance belong to the last days of the seventh week and to the first four or five days of the eighth week. 4. The six and seventh ribs are usually the -first to ossify and they, during the cartilaginous stage, have to grow more rapidly than the other ribs because of their especially intimate relation to the rapidly growing liver. Figs. 11, 12, and 18 show the condition of a sixth rib in the seventh week. In fig. 12 the lateral convex side of the rib is to the right; the medial concave to the left. Lying a short distance lateral to the lateral aspect of the rib is the periosteum, whose origin from the originally loose syncytium of undetermined origin has already been referred to. This structure has now. commenced to be fibrous. In the space between it and the cartilage, cells have begun to accumulate. These are the osteoblasts.
Fig. 14. — Microphotograph, Embryo G 210,
Fig. 15. — Microphotograph, Embryo G 210, Section 254, Shoulder Girdle Series. Section 254. x125. Rib cartilage immedix10, To show changes in rib cartilage ately anterior to dark line shown across rib preparatory to ossification, fifth rib. The in fig. 14 (see text).
embryo belongs to the end of the seventh week (see fig. 15).
Within the cartilage itself the cells are rather larger. On the medial aspect, more especially in the region of the angle, cell addition becomes less marked, and finally ceases. The cells which have already passed through a cartilaginous phase and their descendants are much reduced in number. The few remaining which have failed to secure incorporation in the cartilage soon separate from their ectoplasm and undergo, coincidently with it, a partial but abortive differentiation towards the cartilaginous state. Cells of this type are well seen in the upper half of fig. 18.
In the latter part of the seventh week the most striking fact is the reat increase in the size of the cells in the lateral half of the mass of cartilage. This is best seen in the region of the angle of the rib—the region of the oldest cartilage cells. Fig. 14 gives a general view of this stage; fig. 15 shows its details. In this at the right-hand upper part of the circle there is a small portion of the periosteum shown. Between it and the rib are the osteoblasts, quadrupled in number as compared with the stage shown in fig. 12, eroding the margin of the cartilage, the lateral two-thirds of which are composed of very large cells; the medial third of much smaller cells. The inner margin of the cartilage is confused and vague. The pleura crosses the left-hand lower part of the circle. ©
5. The stages of the early eighth week are shown in figs. 16, 17, 18, and
Fig. 16. — Microphotograph, Coronal
Fig. 17. — Microphotograph, Coronal Section, Section, Embryo R 56, Section 147. Embryo R 56, Section 147. x66. Sixth Shoulder Girdle Series. x 6°3. rib (see text).
The embryo belongs to the eighth — week.
19. Fig. 16 gives a general view of the parts. Fig. 17 shows in coronal section the sixth rib in the region of the angle. To the left is the pleura. Next it, the medial margin of the rib is diffuse and vague. The cartilage cells in the neighbourhood have increased considerably in size, but have lost to some extent their outlines, and are commencing to form a symplasma, which appears to be absorbed and thus to disappear. Fig. 18 is a highpower view of the lower part of this aspect of the rib. It shows in detail the collapse of some of the marginal cells and the generally vague limit of the cartilage.
At the same time the cells of the lateral two-thirds of the cartilage have also ceased to be prosperous. There is a rapid deposit of calcium salts in the matrix surrounding them (fig. 17). The Ribs in the Second Month of Development 27
The manner of this deposit is well shown in fig. 19, a high-power view of a cell in the centre of which droplets of calcium are appearing. These drift through the cell substance to be added to its capsule. ~
The fact that the droplets appear in the centre of the cell is not without significance. The old cartilage cells—z.e. cells of the lateral two-thirds of the rib more especially in the neighbourhood of the angle—which show this degeneration are extremely large, and such cells of necessity must get into physiological difficulties of mechanical origin, for their mass increases as the cube, their surface, through which their content is nourished, as the square of their diameter (5). ‘There must, therefore, be a limit of cell size, and. so soon as that limit is reached the centre of the cell must become improperly oxygenated or nourished. It is at the centre that the droplets appear, and this may be regarded as at least suggesting that their appearance is the sign of a commencing necrosis, directly due to the cell’s hypertrophy (6 and 7). The deposit of calcium, whatever its cytological interpretation, appears to stimulate the osteoblasts to form bone. Fig. 17 shows the commencing formation of sub-periosteal bone on the surface of the lateral third of the rib. Outside this are seen the osteoblasts and the periosteum, which can be traced round the lateral two-thirds of the rib, although there is: only an appreciable number of osteoblasts between it and the thin layer of sub-periosteal bone.
Fig. 18, — Microphotograph. 500. Medial border of rib shown in fig. 17 (see text).
Thus at the end of the second month there is a gutter-shaped piece of bone applied to the lateral aspect of the rib in the region of the angle and for a short distance anterior to it. The increasing size of the rib at this time is provided for by an addition of bone on the lateral aspect, and by a subtraction of cartilage on the medial aspect : a condition practically the reverse of that obtaining in the seventh week, and recalling that of the sixth.
Fig. 19. — Microphotograph. 1000. Lateral border of rib shown in fig 17. At the right of the photograph is the lateral margin of the rib (calcified cartilage showing black extending from top to bottom of the field). Extending from near the centre of the field upwards and to the left is a large cell surrounded by a calcified capsule. The droplets of calcium may be seen appearing in the centre of the cell and drifting through it to its margin, where they add themselves to the matrix, Partially incorporated droplets are visible at short intervals all round the cell.
6. The shape of the rib at this period of development is not without interest. On section it is oval, with the long axis of the oval running obliquely from within, outwards and somewhat downwards, in marked contrast to the direction of the long axis of the oval of the adult sixth rib. The Ribs in the Second Month of Development 29
The reason for this, indicated by the available evidence, is that the whole body is increasing in circumference, and that in this increase the tissues of the body wall, by their interstitial growth, play an active part. The periosteum belongs definitely to the tissues of the body wall, and as they tend to adopt a larger curvature it, moving with them, tends to adopt a larger curvature also, and therefore tends always to move away from its rib, with the result that the bone has to increase extremely rapidly on its convex lateral aspect. At the same time, with the unexpanded lungs, the need for intra-thoracic space is comparatively slight, and the removal of the medial aspect of the rib tends to be less rapid than the addition to the lateral aspect, with the result that the medio-lateral diameter of the rib becomes disproportionately great.
(a) The rib chondrifies in the sixth week and is at that time S-shaped.
(b) The change from this shape to the definitive curvature is effected by means of lateral cartilaginous addition and medial cartilaginous subtraction. This appears to be due to an escape of the cartilage cells, which revert to the undifferentiated mesenchymal type; that, to a process exactly comparable to what is found at all growing cartilaginous tips.
(c) Once the curvature is approximately definitive, the most active cell addition takes place on the medial aspect of the rib, though there is also some interstitial growth.
(d) The periosteum begins to appear on the lateral aspect in the course of the seventh week. The osteoblasts rapidly accumulate; the cartilage cells undergo hypertrophy. This affects chiefly the oldest cells, which are situated in the lateral two-thirds of the substance of the rib in the neighbourhood of the angle. (e) In the eighth week the medial aspect of the rib commences to undergo removal, passing through a stage closely resembling a colliquative necrosis. The matrix of the lateral two-thirds of the mass of the cartilaginous rib becomes calcified. This is a result of the degeneration of the cells which, in turn, is probably the result of their hypertrophy. The commencement of calcification is the signal for the deposition of the first bone, which takes the form of a gutter applied to the surface of the lateral third of the cartilaginous rib.
8. The processes described and summarised above are those leading up to and initiating ossification in the ribs. It is contemplated in a future communication to bring forward evidence to show that the differences existing between the manner of their ossification and that of other long bones are due not to any essential difference in the process, but to growth needs which are accidents of position. 30 The Ribs in the Second Month of Development
I am indebted to Dr N. C. Rutherford for the preparation of much of the histological material, and to Mr Wm. Gill for assistance in the work of microphotography.
(1) Barpegn, C. R., and Lewis, W. H., ‘“ Development of the Limbs, Body Wall, and Back in Man,” Amer. Jour. of Anatomy, vol. i. pp. 16-18, 1901.
(2) Sztuy, A. von, “ Uber das Enstehen eines fibrillaren Stutzgewebes in Embryo uud desen Verhialtnis zur Glaskoperfrage,” Anat. Hefte, Bd. xxxv., S. 649-758, 1908.
(3) Manual of Human Embryology, Keibel and Mall, vol. i. C. R. Bardeen’s article, ‘ Histogenesis of the Connective Tissues.”
(4) Matt, F. P., ‘On the Development of the Connective Tissues from the Connective Tissue Syncytium,” Amer. Jour. of Anatomy, vol. i. pp. 329-365.
(5) Geppgs, P., and Thomson, J. A., The Evolution of Sex, Walter Scott.
(6) Geppzs, A. C., “The Surface Details of the Bones of the Skeleton as an Index of Nutrition,” Proc. Roy. Phys. Soc. of Edinburgh, vol. xvii. pp. 201-216, 1908.
(7) Guppes, A. C., “Abnormal Bone Growth in the Absence of Functioning Testicles,” Proc. Roy. Soc. of Edinburgh, vol. xxxi., part 1, pp. 100-150, 1910.
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