On the development of striped muscular fibre in man, mammalia, and birds

In the domestic fowl, on the fifth day of incubation, the (so-called) voluntary muscular tissue is of a gelatinous consistence, and when examined by means of a sufficient magnifying-power is seen to consist of the following three elementary parts :—1, a granular, semifluid blastema or matrix ; 2, free nuclei ; and 3, nucleated fibres.

1. The granules of the blastema vary considerably in size; the largest (Pl. XI, fig. 1 a) are but little inferior to the smallest nuclei(ó), as if they formed a transition from the one to the other.

2. The free nuclei (fig. 1 b, c, and fig. 3 a) are thickly crowded together in every part of the tissue, and are either globular, oval, triangular, pyriform, fusiform, or somewhat crescentic. Each contains one or a pair of globular nucleoli of variable size, and frequently gives off one or two separate processes from different sides (fig. 3 a). During the development of the muscular fibres these nuclei multiply by subdivision, and undergo considerable alterations in shape.

3. The nucleated fibres at this period, according to their relative state of development, present a considerable variety of appearance. Some of them (fig. 3 b) are but little more than oval or fusiform nuclei, with elongated processes. Amongst these the smaller kind appear to belong to the tendinous and aponeurotic tissues. The majority, however, which are more fully developed have a proportionately different aspect.

The process of development may be divided into two stages—

1. That in which the fibres are formed and isolated from the surrounding substances.

2. That including the changes which they subsequently undergo to complete their development.

At the beginning of the first stage, or the formation of fibres, granular processes of variable breadth and length, resulting apparently from the condensation or coagulation of the surrounding blastema, extend from the opposite sides or ends of a series of nuclei, until they meet and coalesce to form continuous fibres. In these fibres the nuclei are at variable distances from each other, and the processes which grow from them and coalesce are consequently of variable length. In general, the greater the distance between the nuclei the narrower are the processes which unite them. Sometimes they are ranged at considerable intervals in a straight and single series, the processes growing toward each other until they meet, as at fig. 4 a ; or they are placed irregularly and sometimes in close apposition, as at fig. 5 a, b, the processes of one overlying and coalescing with those of another. Occasionally I have seen a number of nuclei arranged in a single linear series, and in contact, but slightly overlapping each other, and held together by a common layer of condensed blastema, formed by the coalescence of their processes (fig. 5 c). Such an arrangement is only a repetition of that which takes place at a and b in the same fig. In some cases it may be seen that granular processes have grown from opposite sides of the nuclei, and thus formed with them bodies which have the shape of fusiform, nucleated cells, but do not appear to be enclosed by a distinct wall. A number of these bodies may become applied to each other at their sides, but in such a way that one is in advance of the other. Fig. 5 d is an exact representation of a fibre so formed, from the anterior extremity of a chick, between the fifth and sixth days of incubation, and magnified 670 diameters.^* After a time the separate bodies coalesce, and, apparently, by a longitudinal growth, the fibre straightens and assumes the appearance of a band, while the distances between the nuclei become increased. At fig. 6 b four such fibres are represented. In the muscles of the trunk, however, the nucleated fusiform bodies from which these fibres are formed are seldom so well defined and arranged with so much uniformity as in the case just described. More frequently a series of nuclei, at variable distances from each other, are partially or wholly surrounded and united by an irregular condensation of blastema, as represented at fig. 4 b. At the same time the column thus produced is isolated on one side by a further layer of material, in the form of a distinct border, which is probably effected under the influence of the nuclei (fig. 4Ô). A similar border is subsequently formed along the opposite side, until the fibre is completely isolated from the surrounding substance.

In some instances, but less frequently, nucleated bodies, having the shape of fusiform cells, taper at their extremities into long, slender fibres, by which they become united in linear series (fig. 6 a). These appear to belong to tendinous tissues.

Besides the muscular fibres above described there are other fibres of a different nature found in the same field, and which belong apparently to the muscular sheath, as they are mostly seen along the borders of dissected portions of muscles. Their nuclei are small, but of uniform size, and are oval, fusiform, and pyriform. In the formation of these fibres (fig. 4/) the oval variety unite nearly end to end by means of short but broad granular processes or tails, which overlap each other and coalesce, as in the case of the muscular fibre represented at fig. 5 d. But those of a more pyriform shape, arranging themselves in a line, with their corresponding ends turned in the same direction, are attached to each other by slender filamentous processes proceeding from their points, each of which rests on the same side against the shoulder of the next succeeding nucleus, so that the entire series appeal’s to be connected on that side by a continuous fibre. This arrangement, however, is less frequently seen than the other.

In the heart of the chick, between the third and fourth days of incubation, the fusiform, nucleated bodies are much more numerous and more distinctly marked than in the voluntary muscular tissue. When a portion of either of the ventricles is dissected under the microscope by means of fine needles, it is found to consist—1, of a granular, semifluid blastema; 2, of free nuclei, both round and oval, containing one or two globular nucleoli ; and.3, of the same kind of nuclei, either partially or wholly surrounded each by a granular mass of more or less definite outline and shape (see fig. 7). Sometimes only a small conical projection of granular substance grows from one side of a nucleus, and tapers into a fibre of variable length. By a similar growth on its opposite side a fusiform body begins to make its appearance, and is sometimes prolonged until it assumes, to a certain extent, the form of a nucleated fibre. Pyriform, oval, and circular bodies are produced by the same process, according to the direction which it takes. Sometimes the granular mass enveloping the nucleus is not definitely circumscribed (see fig. 7) ; in many cases its surface is smooth and its outline well defined, while in others it is partially or wholly surrounded by a slightly condensed layer, which presents more or less the appearance of a separate envelope or border, as at a. In their normal position these elementary bodies are arranged side by side with considerable regularity, as represented at fig. 7 b, and by their union and growth constitute bundles of muscular fibres.

Towards the end of the sixth, or at the early part of the seventh, day of incubation, a striking difference or modification may be observed in the process of development of the muscular fibres of the trunk. The free nuclei are still exceedingly abundant, and the nucleated fibres already described are also present in great numbers. But in addition to these a multitude of others, somewhat, different in their form and mode of development, begin to make their appearance. These new structures originate in a fibrillation of the blastema between the densely crowded nuclei, with which they become, as it were, encrusted through a condensation of the surrounding blastema, which also cements them together. Sometimes these masses assume a cylindrical shape, although the elements which compose them have but little regularity of arrangement (fig, 9). Much more frequently, however, the fibres first formed, having increased to a variable degree of thickness, become connected together by the intervening substance, in which new fibres are also developing, to form bodies or masses of different shapes (fig. 9/). Sometimes a number of fine and more or less wavy fibres are cemented side by side, and encrusted with groups of nuclei, which carry on the process of fibrillation in the intervening blastema, as at fig. 10 a and fig. 9 b ; or fibres of different diameters wind their way through groups of nuclei, which surround them like clusters of grapes, as at fig. 10 b. In some instances they resemble the loosened or untwisted fibres of a rope, entangling a number of nuclei (fig. 9 a and ô) ; while in others they are arranged in a kind of plexus of a more or less uniform character, and supported by the intervening and condensed blastema, in which new fibres are forming (fig. 9 c). Occasionally, but not often, at this period of development, I have found some of these fibres distinctly and beautifully striated, as shown at fig 10 c. The large fusiform bodies (fig. 9 e e’) have often a striking resemblance to organic muscular fibre-cells, but enclose a variable number of nuclei. Some of them are comparatively short and broad, and contain two or more nuclei, like those by which they are surrounded. Similar appearances, however, are frequently assumed by portions broken from a longer mass. Many of them are apparently in different stages of transition into long, nucleated, and nearly cylindrical fibres. In the course of this transition their surfaces become plain and smooth, their nuclei multiply by division, and are disposed in more regular series, with their longer axes sometimes transverse. Frequently they are much dilated in the middle, from which they contract into broad but tapering fibres (fig. 9 e). In many cases, however, the transition is more gradual, as at e ; while a great number have assumed the form of large fibres of nearly uniform diameter, and lie side by side in close apposition (A). As incubation proceeds, these fibres increase both in number and development, but at any one period until the thirteenth or fourteenth day they assume a great variety of forms. On the twelfth day many of them have still the appearance of long, fusiform cells, which taper into long and comparatively narrow fibres (fig. II). Their dilatations, in general, according to their length, contain a variable number of nuclei, disposed in regular series, with their longer axes often in a transverse direction. In a few instances, however, as at fig. 11 a, I found only a single and remarkably large” nucleus, which occupied the entire breadth of the dilatation, and contained a well-defined, granular nucleolus, of a correspondingly large size. Sometimes the dilatations were seen to be repeated twice or even thrice in the course of the same fibre, which, at variable intervals between them, contained nuclei of an oval form and of a breadth equal to themselves.

But besides these fibres of later formation there was also a considerable admixture of the kind first described. Many of the latter were slender and delicate, with round and oval nuclei as broad as the fibres themselves, and sometimes raised from their surface (fig. 11 ó). Others were thicker, with sharply defined borders, and enclosed a granular axis, with round or oval nuclei, which in some places were disposed in straight, longitudinal series, but collected in others into small groups (fig. 11 c c). Here and there they were found in the earliest state of development, as represented at d. At e a succession of round nuclei, in contact with the side of a slightly undulating fibre, were each enveloped in a delicate and nearly fusiform mass of condensed blastema, which converted them into what had somewhat the appearance of nucleated cells.

At this period of incubation both transverse and longitudinal striae exist in some of the fibres, but are entirely absent in others. The longitudinal are the first to make their appearance on the surface, and in the form of simple or plain fibrillæ, which subsequently break up into series of dots or granules. These granules are much coarser in some fibres than in others. When they are small, on the same level transversely, and in close proximity, they present, under a moderate power, the appearance of transverse lines, which, however, by the use of higher powers, may be resolved into separate granules that belong to the longitudinal fibrillæ. So long as the fibrillæ remain undivided or plain, the striations, therefore, are only longitudinal. When the fibres are fusiform, these appearances are most conspicuous, or perhaps visible, only in the course of the dilatations (see fig. 11,f,g).

On the thirteenth or fourteenth day of incubation ^* some of the fibres are more or less in the condition of those which first made their appearance, but by far the greater number have now assumed the form of nucleated cylinders of a more peculiar character, and arranged side by side with much regularity (fig. 12a). These fibres differ from each other considerably in diameter, and each of them varies in the same respect at different parts of its course.

At their widest portions, which are nearly uniformly cylindrical, their walls are of considerable thickness, and on each side have the aspect of a broad band or contour, enclosing an axis of delicately granular substance and a series of nuclei. The nuclei assume a variety of shapes and positions. They are round or oval, pyriform or crescentic, and turned with their longer diameters either more or less transverse or parallel to the course of the fibre. Sometimes they are crowded closely together, sometimes the interval between them is variable, while in a great number of instances they lie at moderate and nearly equal distances from each other, and occupy the whole axial breadth of the fibre, which is then constricted between each pair in such a manner as to resemble a jointed or knotted cane (see fig. 12 a). Some of them are quite on the surface, the convexity of which is occasionally embraced by a nucleus of a crescentic form (fig. 12 b).

After a certain course these fibres are frequently seen to change their character, in consequence of a further development. They gradually diminish in diameter to a variable extent, and assume a more cylindrical shape. At the point where this change is taking place the granular axis tapers off ; the fibre, contracting in the same proportion, acquires a uniform structure throughout its entire thickness, and the lateral contours disappear (fig. 12 b). The nuclei also, as if by pressure, become frequently much more elongated longitudinally, and raise the surface in the form of alternate nodes. At the same time some of the fibres exhibit indications of longitudinal fibrillation and even of transverse striæ. Sometimes a fibre tapers for a considerable length without any other alteration of structure, in consequence of the absence of nuclei, which allows the walls to approach each other and the granular substance to form a narrower axis ; and in such instances the walls now and then are seen to have resolved themselves into fibrillae, and these again into granules. (See fig. 12 c.)

By the fifteenth day of incubation the fibres lose entirely their plane structure, and become wholly resolved throughout their thickness into bundles of fibrillæ (fig. 13 a). These fibrillæ, however, are not perfectly parallel, but overlie each other here and there in a loose kind of way, and, in conse-quence, apparently, of this arrangement, the bundles have increased a little in average diameter. Some of them, however, are very small in this respect, and consist of not more than three or four fibrillæ (fig. 13 6). In the majority of cases the fibrils, in turn, have, either wholly or in part, become resolved into a succession of granules, which, under certain conditions, assume the appearance of transverse striæ. Fig. 13 a represents one of the larger fibres, in which each of these appearances is seen at different parts of its course ; and b is one of smaller diameter, in which the fibrillæ are only here and there resolved into granules. Every fibre bears on its surface a variable number of nuclei, which are-frequently disposed alternately and with much regularity around it. From these nuclei granular processes creep along the surface and begin to develop into new fibres.

In mammalia the development of muscular fibre proceeds on the same plan, and in all essential details is carried out in nearly the same way as in birds. My observations were made chiefly on the fœtal ox and sheep, especially on the latter. In the fœtal sheep of half an inch in length the structure of the muscular tissue has much resembrance to that of the chick near the end of the fifth or at the beginning of the sixth day of incubation. The free nuclei contained in the blastema are rather larger, but similar in all other respects. In the midst of these lie a multitude of fibres, which, when undisturbed, appear to be nearly parallel, and when seen under a power of about 400 diameters resemble pieces of coarse thread (see fig. 14a). Between these, and often in connection with them, finer fibres are formed by a more irregular condensation of the blastema, or coalescence of its granules from the sides or ends of the nuclei, which therefore appear very thickly clustered around them (see fig. 14 6). When the coarser fibres are sufficiently isolated by dissection, their connections with the nuclei, though all on the same plan, present some differences of appearance in different cases. At first a certain amount of blastema apparently condenses all around or at the ends of nuclei into masses of more or less definite shape, and becomes bounded, as already described, first on one side and then on the other, by a distinct fibre or border. Some times the nuclei fall into a single linear series, and if they happen to be pyriform, assume an imbricate arrangement, so that their smaller ends or granular processes, each overlaid by the next, are connected in a continuous line to form a lateral band or fibre (fig. 14 d). If the blastema condense at each end of the nuclei in such a way as to constitute fusiform bodies, and these be arranged in the same imbricate manner, it is easy to perceive that their opposite ends by their coalescence will form a border on their opposite sides. Sometimes a fibre bifurcates here and there to enclose an oval or pyriform nucleus, as shown at d, and sometimes the branches are connected beyond the nucleus by an expansion of condensed blastema in which still finer fibres may be observed.^* Frequently the nuclei are disposed in irregular groups around the fibres, to which they are cemented (fig. 14 e).

As development advances, fibres of different diameters are produced in the way already described. Fig. 16 a represents one of the smaller kind in progress of formation. The majority, however, are of much larger diameter than this, but differ from it only in having a thicker coating of the condensed material, which, when seen on each side, presents the appearance of bands enclosing a granular and nucleated axis (fig. 16 c). That these fibres are more or less cylindrical, and sometimes entirely surrounded by this condensed coating, is rendered probable by the fact that the apparent bands are seen always and only at their sides, just as in nerve-fibres the medullary sheath or white substance is seen only on each side of the axis-cylinder ; but if any doubt exist on the subject it may be set at rest by examining a transverse section of a long muscle, the cut ends of which show that the fibres are more or less cylindrical, with an axis sometimes entirely surrounded by a thick, tubular sheath. Fig. 17 represents a transverse section of fibres from the leg of the fœtal sheep, two inches long. The substance of this coating or sheath is easily injured, broken up, or displaced, even by careful manipulation; and this tendency, under exactly similar circumstances, is greater in mammalia than in the chick. Sometimes it separates into small pieces or coarse granules, which are more or less round, square, cylindrical or discoid, and are very distinctly seen along the sides of the fibres. When these pieces become confusedly heaped together with the nuclei which the sheath contained, without destroying the cylindrical shape of the fibres, these fibres might be supposed to be in the first stage of development by means of an irregular aggregation of their elements. But that this condition is a breaking up, and not a process of development, any one may convince himself by a sufficiently careful examination. The smooth and apparently structureless investing substance, however, by the action of certain reagents, is prone to separate into regular longitudinal fibrillæ, and these again into granules or sarcous elements. Such a separation may be readily produced by a short maceration of the muscles in a weak solution of chromic acid. This change, however, is very different from the irregular breaking up of the fibre to which I have just alluded, and is in every respect identical with the fibrillation and striation which takes place at a little later period in the natural course of development.

These fibres are rather larger in mammalia than in the chick. They differ also from those of the latter in not being so frequently constricted at short intervals between the nuclei. The nuclei, moreover, are rather larger, and in general disposed with greater regularity along the axis.

In a fœtal sheep of inches in length, the fibres were in nearly the same condition as those of the chick on the fourteenth day of incubation. The central granular axis had entirely or almost entirely disappeared, for the tubular substance which invested it now constituted the whole or nearly the whole thickness of the fibre. Now, also, it divided longitudinally into fibrillæ, and these in turn became resolved into granules or sarcous elements, which were so small and close together that at first sight the fibrillæ appeared to be plain, and no indication of transverse striæ was perceived. Moreover, the nuclei had enlarged, had become much more elongated in the direction of the fibre and nearer the surface, but were still as perfect as before. On the surface were a number of others, which were smaller, and round or oval, and evidently engaged in contributing new fibres by the process already described. As the fibres advance in development the internal nuclei disappear, and the sarcous elements and striæ become larger and more distinct.