Book - Contributions to Embryology Carnegie Institution No.20 part 6
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The changes in size and form which the cartilaginous capsule of the ear undergoes during its development in the human embryo are accomplished in part by a progressive and in part by a retrogressive differentiation of its constituent tissues. Throughout the entire period of growth, as far as material was available for study, it was foimd that the margins of the cartilaginous cavities undergo a process of continual transformation. They exhibit a state of unstable equilibrium in respect to the opposing tendencies toward a deposit of new cartilage on the one hand and toward the excavation of the old on the other. The margins therebj^ are always either advancing or receding, and it is in this way that the progressive alterations in the size, shape, and position of the cavities are produced, due to which a suitable suite of chambers is always provided for the enlarging membranous labyrinth.
The general tissue mass of the otic capsule, during the period represented by embryos from 4 to 30 mm. long, passes through three consecutive histogenetic periods, nameh', the stage of mesenchymal syncytium, the stage of precartilage, and the stage of true cartilage. In the subsequent growth of the capsule it is found that in areas where new cartilage is being deposited the tissues of the areas concerned follow a definite and progressive order of development. In areas, however, where excavation occurs, where cartilage previously laid down is being removed, it is found that the process is reversed. The tissue in such areas returns to an earlier embryonic state â€” that is, it undergoes dedifferentiation. Tissue that has accjuired all the histological characteristics of true cartilage can thus be traced in its reversion to precartilage and from precartilage in turn to a mesenchymal syncytium. In the latter form it redifferentiates into a more specialized tissue, in this case for the most part into a vascular reticulum.
The formation of the periotic reticulum is first indicated by a cluster of deeply staining nuclei that can be seen along the central edge of the semicircular ducts in embryos soon after the ducts are formed, and at about the time the otic capsule begins to change from condensed mesenchyme into precartilage. These nuclei constitute a focus at which the development of the reticulum and its blood-vessels takes origin. Here the tissue of the otic capsule takes on an appearance that is less like that of a cartilage-forming tissue and more like that of an embryonic connective tissue. Spreading from this focus, a narrow area is established which soon encircles the semicircular ducts and becomes the open-meshed vascular reticulum which, in embryos 30 mm. long, everywhere bridges the space existing lietween the epithelial lal)yrinth and the surrounding cartilage. In the earlier stages it could not be definitely shown that the primordium of the i^eriotic reticular tissue is not derived from a few ])rcdestined mesenchyme cells which become inclosed, along with the otic vesicle, by the condensed tissue of the capsule and after a certain latent period undergo proliferation and occup}' the space vacated by the receding precartilage. In the later stages, however, it is cjuite evident that precartilage tissue is actually' converted into a reticulum, and that the replacement of precartilage by a reticular connective tissue is brought about through a process of dedifferentiation.
The perichondrium is a derivative of the periotic reticulum and forms an outei limiting membrane along its cartilaginous margin. During the fetal period the perichondrium does not rest directly against the true cartilage, but is separated from it by a zone of transitional tissue consisting partly of precartilage and partly of reticulum. This transitional zone intervening between the perichondrium and the surrounding cartilage was ob-served in all of the specimens that were studied, which includes fetuses up to 130 mm. crown-rump length. Owing to the fact that the perichondrium is late in making its appearance, being first seen in fetuses about 70 mm. long, it can take no part in the early changes in the cartilaginous capsule, either as regards the deposit of new cartilage or the excavation of cartilage that had been previously laid down.
The periotic tissue-spaces are formed by a modification of the meshes of the periotic reticulum. The latter consists originally of a rather uniform narrow mesh. The essential change which it undergoes in the process of space-formation consists in the gradual disappearance of the traversing trabeculse. The trabeculae consist of the protoplasmic processes of the constituent cells of the reticulum, and their disappearance is apparently due, not to a dissolution or Uquefaction of these cellprocesses, but to an alteration in their form. It apparently is the result of an active motility of the cell protoplasm involving the successive detachment and retraction of the trabeculse. Wlien a trabecula becomes detached it retracts and adapts itself to the formation of the enlarging space, reshaping itself either as a smooth border or as a constituent part of another trabecula.
The differentiation of the margin of the periotic spaces constitutes the final feature in their maturation. During the period in which the enlargement of an individual space is activeh' going on,. the margins of the main cavity consist of smooth, delicate strands of nucleated protoplasm that resemble the trabeculse between the large reticular spaces. These linear margins are interrupted here and there by openings into adjacent spaces. They tend, however, to form a continuous hne that definitely marks ofT the space from the adjacent reticulum. As the space becomes more mature, the membrane-like border becomes thicker until it reaches a state that will probably not admit of any further opening-up for the coalescence of additional spaces. Any further growth is thereafter limited to a simple distention of the wall of the space, with the consequent adjustment of its constituent cells. In its final form the margin of the space constitutes a mesothehal membrane. Immediately lining the space is a thin membrane with flattened nuclei which is supported underneath by a thin coat of nucleated protoplasm having the form of fibrous connective tissue. The former in its histogenesis differs in no way from the rest of the wall and the difference that exists later seems to be merely the result of its adaptation to the existing physical conditions.
The earliest histological evidence of the formation of the periotic spaces occurs near the stapes, in the reticulum that bridges the interval l)etween the sacculus and the fenestra vestibuh. In embrj'os between 30 mm. and 40 mm. long, it can be seen that the meshes in this region are becoming irregular and larger, due to the disapjiearance of some of the trabeculae and a consequent coalescence of the intertrabecular si)aces. The widening of the mesh at this point constitutes the primordium of the vestibular cistern. It makes its appearance before there is any trace of the scalse, but it is not until the fetus reaches a length of about 50 mm. that the cistern becomes definitely outlined and clearly differentiated from the adjoining reticulum.
Following the appearance of the cistern, the scala tympani is the next space to become established. It can be recognized as a moderate widening of the meshes of the reticulum in the region of the fenestra cochleae in fetuses 43 mm. long, along the basal border of the first turn of the cochlear duct. The scala vestibuli, as can be seen in fetuses 50 mm. long, develops as an extension downward of the cistern along the apical border of the cochlear duct. Starting from these definite foci, these three spaces spread into their destined territory, absorbing as they go the enlarging reticular spaces of the invaded region by a process of space-coalescence, or, in other words, the progressive formation of areas that are free of trabeculse. In fetuses 85 mm. long the two scalse extend downward along the cochlear duct to its last turn, as two separate spaces which do not communicate with each other. When they reach the tip of the duct, which occurs in fetuses about 130 mm. crown-rump length, a free opening is developed between them which represents the hehcotrema. After being completely established along the whole length of the cochlear duct, the scalae continue to enlarge by further coalescence of tissue along their peripheral border, in which the trabecular disappear.
The periotic spaces are analcjgous in their development to the pia-arachnoidal spaces; they are not, however, extensions of them that have invaded the cavity of the cartilaginous labyrinth. They begin at points where there can be no connection with the arachnoidal tissue and their direction of growth is quite independent of it. The communication that is found in the adult between the scala tympani and the subarachnoid sjjace in the neighborhood of the fenestra cochleae, the socalled af|ua'ductus cochleae, is established (juite late. In fetuses 85 mm. crownrump length it exists as a tubular pouch projecting from the subarachnoid spaces along the glossopharyngeal nerve toward the scala tymmpani. In the 13()-mm. fetus, the (jldest examined, this j^ouch is longer and nearly reaches the scala. The communication must be established soon after this.
Similar projections from the subarachnoid spaces at the internal auditory meatus extend as perineural clefts along the trunk and branches of the acoustic nerve. No actual comnnuiications, however, were seen between these spaces and the two scalae.
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Carnegie Institution No.20 Otic Capsule: Introduction | Terminology | Historical | Material and Methods | Development of cartilaginous capsule of ear | Condensation of periotic mesenchyme | Differentiation of precartilage | Differentiation of cartilage | Growth and alteration of form of cartilaginous canals | Development of the periotic reticular connective tissue | Development of the perichondrium | Development of the periotic tissue-spaces | Development of the periotic cistern of the vestibule | Development of the periotic spaces of the semicircular ducts | Development of the scala tympani and scala vestibuli | Communication with subarachnoid spaces | Summary | Bibliography | Explanation of plates | List of Carnegie Monographs
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