Paper - The Development of the Scala Tympani, Scala Vestibuli and Perioticular Cistern in the Human Embryo

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Streeter G.L. The Development of the Scala Tympani, Scala Vestibuli and Perioticular Cistern in the Human Embryo

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The Development of the Scala Tympani, Scala Vestibuli and Perioticular Cistern in the Human Embryo

George L. Streeter

Department of Embryology, Carnegie Institution of Washington, Johns Hopokins Medical School, Baltimore, Maryland

Nine Figures

The study of the development of the large walled—off connective tissue spaces that surround the membranous labyrinth is particularly interesting in that it shows that they have a very definite morphological individuality. It is evident at least that they are not to be considered as insignificant accessories that merely fill in the waste intervals between the membranous labyrinth and the surrounding cartilage or bone. On the contrary, they have characteristics which are in many respects as definite and constant as ‘those of the ossicles themselves. The individuality of these spaces in all respects is most marked. They make their appearance at a definite stage in the development of the embryo; they are formed at definite places; they pass through a series of definite histogenetic processes; they spread in a definite order and manner and eventually they attain a definite form and structure. The general morphology and relations of these‘ spaces during their developmental period will be described in the following paper, and the opportunity will be taken to point out in the course of the description some of these individualistic features.

Instead of designating the large spaces surrounding the membranous labyrinth as perilymphatic spaces, as has been the general custom since the time of Breschet 1833, they will here be spoken of as perioticular or periotic spaces. The use of the term ‘periotic’ avoids the confusion arising from the incorporation of the Word ‘lymphatic’ in the terminology. The present tendency is to restrict the use of the word ‘lymphatic’ to the lymphatic vascular system and its associated structures, with which these particular spaces have no known connection, either in their origin or in their ultimate relations.1 We shall therefore speak of a periotic connective tissue that everywhere surrounds the epithelial portion of the labyrinth. This connective tissue includes, in part the fine-meshed periotic reticulum, and in part the large walled-off perioticular spaces to which belong the vestibular cistern, the scala Vestibuli and the scala tympani with whose development we are primarily concerned. V

Material and Methods

The observations that are recorded in this paper are all based on human embryos and cover the period included between embryos 35 mm. and 130 mm. CR length, which is approximately equivalent to the period between the ninth and sixteenth week of fetal life.

To facilitate the determination of the form and relations of the spaces, wax-plate models of the membranous labyrinth and the surrounding spaces were reconstructed after -the Born method. Advantage was taken of the improvements in the method recently devised by Lewis 1915.9 The serial sections were photographed at a suitable enlargement on bromide paper. By means of a preliminary model of the membranous labyrinth, the necessary reconstruction lines were established and inscribed on the bromide prints. From these prints then the membranous labyrinth and the perioticular spaces were traced on waxplates. After cutting out from the plates the areas corresponding to these structures, the plates were piled and the resultant cavities were filled with plaster of Paris. The wax was finally melted off and there was left then a permanent plaster cast of the objects desired at a definite enlargement. Views of these models are shown in figures 4 to 9.

‘Sabin, F. R. Harvey Society Address. Science, vol. 44, 1916, p. 145. 9 Lewis, W. H. The use of guide planes and plaster of Paris for reconstructions from serial sections. Anat. Rec., vol. 9, 1915.

In outlining the periotic spaces it was found necessary to make an arbitrary rule as to how much should be included in the model. The smaller spaces of the reticulum that surrounds the ‘main cavities can be seen coalescing to form larger spaces and these in turn coalesce with the main cavity as it advances into new territory. Thus in a given section there is a considerable range in the size and completeness of the spaces. The main spaces and the larger adjacent ones that communicate with them are outlined by a membrane—like border. This characteristic was utilized as the guide for determining which spaces to admit into the model; only those possessing a more or less complete border of this kind were included.

Histogenesis of the Periotic Reticulum

Although this communication is more concerned with the process of conversion of the periotic reticular tissue into the larger walled—off spaces, yet for the purpose of completeness a brief survey will be taken of the earlier history of this tissue and the nature of its histogenesis.

The tissue in which the perioticular spaces develop is derived from the condensed mesenchyme that establishes itself as an encapsulating mass around the otic vesicle in embryos between 4 mm. and 10 mm. long. This condensed mesenchyme is subsequently differentiated into the cartilagenous capsule that completely invests the epithelial labyrinth excepting for the three openings that persist in the adult as the internal auditory meatus, the aqueaductus cochleae and the aquaeductus vestibuli, which openings are present in the very earliest stages.

Originally the cartilagenous capsule abuts directly against the epithelial wall of the labyrinth. In embryos about 14 mm. long, however, the cartilage-forming tissue in the immediate neighborhood of the epithelium undergoes a dedifferentiation, so that an area is established all around the membranous labyrinth, and conforming to it in shape, that is less like cartilage and more like embryonic connective tissue. It is this that constitutes the foundation for the open—meshed periotic reticulum which in embryos 30 mm. long everywhere bridges the space existing between the membranous labyrinth and the surrounding cartilage. The membrana propria that supports the epithelial part of the labyrinth and the perichondrium lining the cartilage are both derived from this periotic reticulum. It i.s also a modification of the meshes of this same reticulum that results in the formation of the perioticular spaces in a manner that will now be outlined.

Unmodified periotic reticulum is characterized by a rather uniform narrow mesh. The essential change which it undergoes in the process of space formation consists in the disappearance of some of the trabeculae of the mesh followed by the coalescence of the corresponding adjacent spaces. The trabeculae consist of the protoplasmic processes of the constituent cells of the reticulum and their disappearance is probably to be explained, not by a dissolution or liquefaction of these cellprocesses but by 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 trabeculae. When a trabecula becomes detached it gradually retracts and adapts itself to the formation of a larger space, reshaping itself either as a smooth border or as a constituent part of another trabecula. As spaces become larger they require longer trabeculae, and as trabeculae become longer they also tend to become thicker.

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 actively going on, the margins of the main cavity consist of smooth delicate strands of nucleated protoplasm that resemble the trabeculae 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 line that definitely marks off 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 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 roughly resembles an endothelial membrane. Immediately lining the space is a thin membrane with flattened nuclei which is supported underneath by a thin coat of nucleated protoplasm that has the form of fibrous connective tissue. The former, judging only from its final appearance, could be designated as endothelium, thus making a distinction between it and the underlying tissue. In its histogenesis, however, it 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. Its early behavior is entirely different from that of vascular endothelium. Therefore if one uses the term endothelium for its designation this must be done with a considerable amount of reservation.

These phenomena can be particularly well studied in the scalae While they are in the process of spreading and enlarging. As we shall see, the scalae are more mature in their proximal portions and are progressively less mature as one approaches the apex of the cochlea. Thus any one specimen shows several stages in the development. Typical views showing some of the steps in this process are represented in figures 1 to 3. Figure 1 represents a section through the second turn of the cochlea in ahuman fetus 130 mm. CR length (Carnegie Collection, No. 1018). It shows the topography of the cochlear duct and the general character of the perioticular spaces that are developing along its inner margins. The upper one or scala Vestibuli is in a more mature condition. The lower one or scala tympani is less mature and along its peripheral (right) margin, it is in the act of spreading so as to underlie, as it eventually will do, the future basilar membrane. Thescala tympani finally reaches the peripheral margin of the cochlear duct, and it does this by the coalescence of the enlarging reticular spaces which become incorporated with the main cavity of the scala.

This area is a particularly good one for studying the histogenesis of these spaces. It is shown under higher magnification in figure 2, which is a detail of the same section. By comparing this figure with figure 1, the exact location can be readily made out. That portion of the cochlear duct that is to form the organ of Corti can be recognized by the characteristic form and grouping of its cells. A portion of the main cavity of the scala tympani is indicated and to the right of this are a few enlarged reticular spaces that are uniting with each other subjacent to the organ of Corti and the basilar membrane. These will in the end become part of the main space. They are here just in the process of coalescence, the histological features of which procedure are Well illustrated in this figure. The trabeculae are stretched out in long strands and in many cases are detached and project into the spaces as free ends. The detached trabeculae are seen in different degrees of retraction as their constituent protoplasm reshapes itself in adaptation to the new boundaries. It is only at the margins of the larger spaces that the cell-processes exhibit the characteristic flattened appearance, which is the first indication of the formation of the marginal membrane. The residual undifferentiated reticulum that does not enter into the direct formation of the larger spaces constitutes the tissue from which is derived the adventitial coat of the completed scala.

Fig. 1 Section through the second turn of the cochlea in a human fetus 130 mm. CR length (Carnegie Collection, No. 1018). Enlarged 60 diameters. This section shows the topography of the cochlear duct and the general character of the periotic spaces that are developing along its inner margin. Details of this same section as seen under higher magnification are shown in figures 2 and 3.

Fig. 2 Detail of the section shown in figure 1, enlarged 278 diameters. This figure shows the part of the cochlear duct that is to form the organ of Corti, and the adjacent tissue that becomes incorporated in the basilar membrane. Below this is the periotic reticulum whose spaces are in the process of enlarging. By repeated coalescence these spaces finally unite with the large space that constitutes the scala tympani. This figure shows the histological appearance of the reticulum where the formation of tissue spaces is in active operation.

The appearance of the marginal membrane as seen in a more mature space is shown in figure 3, being a detail of the margin of the same scala Vestibuli that is shown in figure 1. Here we have a firm membrane that forms a complete barrier between the periotic reticulum and the lumen of the scala. After reaching this degree of development there is no evidence of any further coalescence of the surrounding reticular spaces with the main cavity. The membrane itself as seen in cross section consists of rather compact nucleated strands of protoplasm, which cannot as yet be separated into the so-called endothelial coat and the supporting fibrous coat. However, a comparison of the coagulated elements of the fluid seen in the reticular spaces with those seen in the scala would indicate a difference between the two and therefore it is probable that the membrane is already partially impervious.

Fig. 3 Detail of the section shown in figure 1, enlarged 400 diameters. It shows the character of the margin of the scala vestibuli in a more mature condition. The scala vestibuli is inclosed by a membrane consisting of the cells that had previously constituted the reticulum occupying this area and which have been modified in form in adaptation to the formation of this large tissue. space, closing it off from the surrounding tissue.

Development of the Periotic Cistern of the Vestibule

Aside from the scala vestibuli and the scala tympani, the largest of the periotic spaces is the large reservoir situated between the tympanic wall of the bony vestibuli with its articulated stapes, and the vestibular chambers of the membraneous labyrinth. This is the spatium perilymphaticum vestibuli (BNA) or the cysterna perilymphatica (Retzius). In order to eliminate the word lymphatic from the terminology it will be designated here as the Cisterna periotica vestibuli or less formally the peri

Cite this page: Hill, M.A. (2021, June 20) Embryology Paper - The Development of the Scala Tympani, Scala Vestibuli and Perioticular Cistern in the Human Embryo. Retrieved from,_Scala_Vestibuli_and_Perioticular_Cistern_in_the_Human_Embryo

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