Paper - The development of the pillar cells, tunnel space, and Nuel's spaces in the organ of Corti (1919)
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Larsell O. Studies on the nervus terminalis: Mammals. (1918) J Comp. Neurol. 30(1): 1-
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The Development of the Pillar Cells, Tunnel Space, and Nuel's Spaces in the Organ of Corti
O. Van Der Stricht
Department of Anatomy, Johns Hopkins Medical School, Baltimore, Maryland
Eighteen Figures
Introduction
In spite of numerous thorough and exliaustive investigations concerning the earhest stages of development of the organ of Corti, our knowledge of the origin of the tunnel space is still very limited and vague. This is true also of the formation of the heads and cephalic appendages of the pillar cells, and almost nothing is known concerning the origin of the so-called spaces of Nuel. This observer (78) described in the organ of Corti in adult mammals a system of intercellular channels, and his findings have been confirmed by Retzius ('84) and other more recent authors. These spaces or channels are situated between the outer rods of Corti and the neighboring outer hair cells, and between the three rows of outer acoustic elements. The}^ contain fluid and are traversed by the phalanx processes of the cells of Deiters, intercommunicating through clefts between the sensory elements, and communicating with the tunnel space through interstices between the outer pillars. The view taken by Nuel, that they communicate with the lumen of the cochlea "par I'entremise de lacunes en fonne de rames de la mambrane reticulaire," must be regarded as erroneous.
The development of the tunnel space between the two spiral rows of rods of Corti appears to be verj^ difficult of observation. Indeed, most authors, referring to its appearance in embryonic material, state that it originates in the form of a narrow cleft between the inner and outer pillars, but give no details concerning the significance of this primitive interstice. Is it intercellular or intracellular? From what source is derived the fluid contained within the cleft? Those observers who clearly specify that the space appears between two neighboring pillars, as do Gottstein (72), Retzius ('84), Vernieuwe ('05), N. Van der Stricht ('08), and Hardesty (.'15), give no explanation as to the origin of its fluid. Alluding to the development of the space in rabbits two daj^s after birth, Retzius states (p. 303) :
Von besonderer Bedeutung ist nun die enge Spalte, welclie zwischen den beiden Pfeillerzellen reichen, ungefahr in der Mitte der Zellenhohe, nach oben vom spiralen Nervenbiindel entstanden ist und den Anfang des Tunnelramiis dai*stellt; diese Spalte ist in der Basalwindung — wo indessen noeh eine geringe Neigiing der Pfeillerzellen nach aiissen hin vorhanden ist — noch viel weiter entwickelt, und man sieht hier deutlieh, dass die durch Einziehung (Verdtinnung) der beiderseitigen Pfeillerzellen entstanden ist. Gleichzeitig ist aber auch die Anlage der Pfeiler in der Zellen als helleglanzende Streifen nunmehr wahrnehmbar. Nach aussen von der ausseren Pfeilerzellenreihe sieht man deutlieh auch die Anlage der Nuelschen Raums.
According to Vernieuwe, the timnel space is produced by the separation of the bases of the two pillar cells, due to elongation of the pillars, increase in size of the nuclei, and chiefly by the extension of the subjacent basilar membrane. Referring to the trend of the spiral organ of Corti towards the axis of the cochlea, Hardesty ('15, p. 52) states: The normal spaces between the elements of the spiral organ, including the large Nuel's space, no doubt result in part from this movement of the organ axisward." Other authors, Rosenburg ('68), Boettcher ('69), and Pritchard ('76) describe two neighboring inner and outer pillars as derived from a single original cell, the nucleus of which divides in two, and by a process of liquefaction of the undivided cytoplasm, the tunnel space is produced within it. This space is originally intracellular and its fluid is a protoplasmic product. Rickenbacher ('01, p. 402) seemingly ascribes a similar origin to the fluid of the space of Nuel in the adult guinea-pig : ' ' Bei der Schnecke des ausgewachsenen Tieres hat der Prozess der Verfltissigung zur Bildung des Nuelschen Intercellularraume und des Leiterepithels gefiihrt." According to Kishi ('02), the tunnel space is due to the spiral course of the nerve fibers after they have passed through interstices between the inner pillar cells. The formation of tunnel and intercellular clefts is considered by Held ('09) to be the result of 'ungleichen Wachstumbewegungen' of different epithelial cells. His so-called 'outer tunnel,' the spaces between the outer hair cells, and the space of Nuel outside the outer pillars are sheer intercellular channels, 'reine Intercellularspalten;' but the tunnel between the pillars is originally intracellular.
Eine reine intrazellular Spalt, da die ersten Nervenfasern, die hier spiralig abbiegen und weiter ziehen, nicht in der Zwischengrenze zwischen Aussenr und Innenpfeiler liegen, sondern im Protoplasma der Innenpfeilerzellen randstandig eingebettet sind, was audi fiir die unten den inneren sowie ausseren Haarzellen resp. zwischen den Deiterschenzellen imd in ihren Intel zellularbriicken gelegenen Formation eines intraepithelialen Nervusplexus gilt.
The development of the tunnel and the pillar cells is closely connected wdth the formation of the pillar heads, the appearance of the 'head-plates' of the inner pillars, the phalanx processes of the outer pillars, and the extension of the membrana reticularis. The superficial structures of the rods of Corti in adult mammals have been exhaustively investigated by many observers: Max Schultze ('58), Koelliker ('59), Boettcher ('59, '72), Deiters ('60), Hensen ('63, 71), Gottstein (70, 72), Nuel (78), Tafani ('84), Retzius ('84), and by most of the more recent authors; but the appearance and extension of these structures and the mechanical factors taking part in their fonnation require more careful study. N. Van der Stricht has shown that the head-plate of the inner pillar is originally represented by a very small square field, the apex of the cell, which becomes fibrillated and extends over the enlarging head of the outer pillar, the foniier undergoing great pressure from the latter The outer pillar cells originally belong to the first spiral row of outer sensory elements. As development advances they are pressed out from this row towards the inner rods of Corti and form a new row of outer rods, the apices of which always remain fixed between those of the outer acoustic elements of the first row. Hence there persists an apical segment of the outer pillar, which runs obliquely from the apex or phalanx of the cell, downward and inward toward the future head of the pillar. This oblique process contains a bundle of fibrils which, issuing from the head, passes between two outer acoustic elements and spreads out upon the phalanx — the head-plate of the outer piljar. By enlargement of the headj the fibrillar bundle 'gradually acquires a more horizontal position. Held ('09, p. 109) seemingly ascribes the head-plafce of the inner pillar not only to the apex, but also to the superficial portion of the cell, der obere Zellteil welche die Faserrohre enthalt," and which is pressed flat from the developing head of the outer pillar. Although he did not recognize the original position of the outer pillar cells within the first row of outer acoustic elements, he nevertheless observes the squeezing of their 'Kopfplatte,' which becomes thinner from compression between two hair cells, and also of the bundle of fibrils, which at first run obliquely, then at right angles to the intermediate piece of the outer pillar, due to pressure from the elongating pillar cell.
In the present paper the appearance of the tunnel space, the development of the heads and cephalic appendages of the pillar cells, and the formation of the Nuel spaces will be dealt with in order.
Appearance of the tunnel space
Sections tangential to the surface of the organ of Corti, and always somewhat oblique, affect transversely series of neighboring inner and outer pillars at various and successive levels of their length, from the superficial membrana reticularis toward the basilar membrane (fig. 1). As illustrated in figures 1 and 2, one may distinguish in the prismatic lamv-^Uar pillars three portions, although they are not sharply marked off: a basal or nucleated part, the largest, which is lamellar in shape or flattened out in a radial direction from mutual compression; an intermediate part; and a superficial part, the narrowest, which is compressed between the inner and outer hair cells, and hence more or less flattened out in a spiral direction {ip and op). The basal and intermediate portions are each made up of two cytoplasmic zones, the larger being clear and vacuolated, and occupying the area of the cell body close to the future tunnel; the smaller compact and fibrillated, and occupying the axial side of the inner pillar and the lateral side of the outer pillar. The superficial segment of the two rods of Corti contains no vacuolated protoplasm; it consists of a more homogenous, compact cytoplasm, which in the inner pillar is traversed by a bundle (fig. 2, ip) or a tubule (fig. 3, ip) of fibrils, and in the outer, encloses a bundle of fibrils which pass between neighboring outer hair cells and give rise to a small band, the phalanx process of the outer pillar (figs. 1, 2, and 3, oph), connected with the superficial apex of the cell, the phalanx. In the adult organ a part of this fibrillar bundle is a characteristic constituent of the superficial portion of the head, and thus its early presence in a definite portion of the outer pillar is very important in enabling one to determine, from the earliest stages of development, a very narrow but long superficial portion of the cell (figs. 1, 2, and 3, op), which later enlarges and becomes transformed into a part of the bulky head. It is also obvious that the adjoining portion of the inner pillar, which in figures 1, 2, and 3 is in close contact with this future head of the outer pillar, must be considered as the segment which will become converted into the so-called head of the inner pillar.
The outlines of all the pillars are very sharp, not only between the cells of the same row, but also between the neighboring elements of the two rows. While at the level of the superficial segments this outline is represented by an intercellular material (figs. 1 and 3, tb), which in its staining capacity and chemical constitution agrees with that of the superficial terminal bars, between the two lower segments of the pillars it is composed of a paler, more fluid, or true intercellular cement, in addition to which a very thin superficial cytoplasmic film can be brought into view. This outline and film are lacking along the axial surfaces of the inner pillars and the lateral surfaces of the outer. The spiral nerve bundle (A^") occupies an intercellular position between the nucleated parts of the outer and inner pillars, sometimes encroaching somewhat upon the lower interstice which separates their intermediate portions. The nuclei of the pillar cells are surrounded by vacuolated cytoplasm. The nuclei of the inner pillars are much smaller than those of the outer and much more flattened out radially.
When the tunnel space is about to appear, there occurs a characteristic alteration in the cytoplasm adjacent to this future cleft, the vacuoles running together and thus increasing in size (figs. 1 and 2, t). A common vacuolated mass soon appears (figs. 3 and 4, t) ; at certain places it remains fused with the cell body from which it is derived, at others it is independent, so that one cannot determine to which of the neighboring pillars it belongs. From this moment there exists a narrow intercellular cleft, filled with a small amount of extracellular, vacuolated material, a common mass which doubtless represents the first trace of the intratunnelar fluid, and w^hich gradually increases in quantity by the coalescence of adjoining portions, partly incorporated in the original pillar and partly free or extracellular. Although from the earliest stages of the appearance of the space small extracellular, vacuolated masses can be found between the intermediate segments of the pillars (fig. 4, t), the larger part of the tunnel is generally seen around and close to the spiral nerve bundle, that is to say, between the nucleated portions of the inner and outer rods of Corti (fig. 5, t, T), never ungefahr in der Mitte des Zellenhohe," as Retzius asserts and manyinvestigators illustrate. It is a perinervous space. Later it extends between the intemiediate portions of the pillar cells.
According to this description, the tunnel space must be held to be a true intercellular cleft, the fluid contents of which are developed by a process of secretion from the neighboring parts of the pillars and a simultaneous partial cytolysis of the latter. The space enlarges at the expense of the cell bodies. In the earhest stages of development of the organ of Corti there appears in the pillars not only a fibrillated sustentacular apparatus related to their function of support, but also a large, clear, vacuolated cytoplasm, the bulk, of their cell bodies. This portion of the protoplasm is glandular in nature, and from the blood plasma of the subjacent vas spirale (fig. 1, vs), it derives its nutritive material, which is elaborated and converted into clear vacuoles. The products of secretion are discharged along with a partial liquefaction of the surrounding cytoplasm.
During the extension of the tunnel space the superficial seg-ments of the outer pillars undergo considerable enlargement, and their radial diameters soon correspond to those of the intemiediate portions (fig. 4, ohd). At that time the process of cytolysis obviously extends along these segments (figs. 4 and 5, t), from below upward, involving a rapid reduction of their radial diameters. The intermediate segment of the outer and inner pillars, previously broad and formed of a small fibrillated part and a large vacuolated portion next to the future cleft, becomes gradually converted into a slender band, the so-called 'body' of the pillar. In figure 7 (opb) these bodies are shown cut at successive levels through fifteen outer pillars. In their lower portion, as seen in nine sections, they are reduced to thin cylindrical fibrillar strands, part of their apparatus of support, from around which the clear cytoplasm has disappeared. In their upper part, as seen in the next four sections, the pillar bodies are still composed of the original two zones, the vacuolated portion having been somewhat reduced. Close to the future heads are seen two sections (connected with sections of inner pillars), the structures of which have undergone no change. The process of cytolysis is completed at the level of the first nine elements; it is progressing in the following four and has not yet begun in the last two. On comparing these structures with more advanced stages, and especially with those in the adult cochlea, it is plain that the body of the outer pillars acquires its final form and structure by a process of secretion and cytolysis along with the elongation of the intermediate segment. In young cats, bats, common and white rats it becomes a slender fibrillated strand, destitute of clear cytoplasm (figs. 10, 13^, 14, 15, 17 and 18, oph). Betw^een the pillar bodies, as aheady noted by Nuel, are large clefts through which the fluid of the tumiel space and the neighboring space of Nuel intercommunicate.
The intermediate portions of the inner pillars undergo similar, but never such marked changes. The greater part of their clear cytoplasm disappears, only a very narrow zone of it persisting, so that in young and adult animals the body of the pillar becomes lamellar in shape (fig. 18, ipb) and flattened out in a spiral 'direction. It is composed of a, fibrillar lamella and a thin layer of clear protoplasm (fig. 17, iph). Besides the pores traversed by the nerve fibers, no true intercellular clefts sever the inner pillars.
Along with these alterations and elongation of the pillar bodies, the tunnel space enlarges gradually but considerably, and very soon its radial diameter surpasses that of the two original clear zones belonging to two contiguous pillars. In other words, the fluid accumulated within the cleft exceeds the am.ount of disintegrated protoplasm. Indeed, the cytolysis occurs in a merocrine glandular cell, which, although undergoing partial liquefaction, is able to elaborate new clear secretion products at the expense of material derived from the vas spirale. Hence the tunnel fluid is the result not only of a sheer cytolysis, but also of a true elaboration and subsequent discharge. In the earliest stages of development the process of cytolysis seems to be more prevalent, since the contents of the cleft are seen in the form of a coagulated, vacuolated mass; afterwards the larger space is usually filled with a clear unifomi fluid, which seems to arise from a more active, true secretion. This secretion may continue even in the adult organ, for, according to all the investigators, the nucleus of each pillar cell is surrounded by a clear cytoplasm which extends over the floor of the tunnel. This protoplasm is vacuolated and represents the rest of the original bulky, glandular portion of both sustentacular and secreting cells.
Development of the heads and the cephalic appendages of the pillar cells
According to the results published in a previous paper (now in press) and those obtained by N. Van der Stricht, at the earliest stage of the development of the organ of Corti the outer pillar cells are located within the first spiral row of outer hair cells, and their superficial segments occupy interstices between two neighboring acoustic elements. By the rapid enlargement of the latter, these superficial elements are pressed out of the row and pushed towards the inner pillars, although their apices remain fixed between those of the hair cells. From this tune the inner and outer rods of Corti constitute a scaffolding, which is made up of two spiral rows of sustentacular elements and is triangular in shape on vertical section. The rapidly enlarging base of the triangle abuts against the basilar membrane, and the apex is interpolated within the superficial membrana reticularis, separating the apices of the supporting and sensory elements of the inner spiral row from the apices of those of the first outer spiral row. In a section tangential to the organ of Corti the summit of the scaffolding is represented by a spiral row of very narrow fields, the apices of the inner rods of Corti, separated from one another and from the neighboring fields of the reticular membrane by deeply staining terminal bars, which extend into the d^pth between the superficial portions of the inner and outer pillar cells. Each of these narrow fields contains a diplosome, and will gradually enlarge by a process of compression from the underlying expanding heads of the outer pillars.
Outer pillars. In the superficial part of the entirely developed outer pillar, as seen in the adult organ of Corti, three different portions are distinguishable: 1) The apex or 'phalanx,' fonning a part of the membrana reticularis. This consists of a lateral, expanded segment (fig. 8, oplv), which constitutes a portion of the roof of a subjacent intercellular interstice, through which course the phalanx processes of the cells of Deiters of the first row (paper in press); and a medial, constricted segment (op/i") lying just between two apices of the outer hair cells of the first row {oh>). 2) A fibrillated band or the phalanx process (fig. 13'", oph\ oph", oph), which runs nearly horizontal and unites the apex to the head. 3) The head proper, or the enlarged superficial part of the pillar, in contact with the inner pillar (figs. 13^, 17, and 18, ohd). This is a cubical segment; in sections tangential to the surface it is square (fig. 13'", ohd) or somewhat lengthened out radially (fig. 18, ohd). Its upper portion is traversed by the fibers of the phalanx process (fig. 17, ohd), and its larger, lower part by a fibrillar bundle belonging to the body of the pillar (fig. 13'^, ohd). Thus two different fibrillated fasciculi spread out, and fade off into the head; there is no direct continuity between the fibrils of the two bundles (figs. 14 and 15, ohd).
In the first stage of development, which may last until the. tunnel space is about to appear and before there is any marked increase in the site of the future head (figs. 1, 2, and 3, op), the three parts of an adult pillar just referred to are recognizable. The apex acquires the appearance illustrated in figure 4, oph. The phalanx process is short and a nearly vertical, dejeply stainmg bundle of fibrils (figs. 1 and 3, oph) which is traceable between the cell bodies of the outer hair cells {oh'), and a little deeper between these sensory elements and the future head. The future head is a thin tapering part of the pillar, composed of a more or less homogeneous cytoplasrh which encloses in its upper two-thirds the rootlets of the phalanx fibrils, and in its lower one-thu'd the summit of the bunch of fibrils of the pillar body (figs. 2 and 3, op). Indeed, in figures 1, 2, and 3, two or three fields, cross-sections of the future head, contain parts of the two fibrillated hands. This rather deep portion of the pillar, situated at the level of the lower poles of the outer hair cells (oh^), doubtless belongs to the developing head. From this it is evident that the superficial, thin, tapering segment of the outer pillar cells, which gives rise to both the phalanx process and the head, attains more than one-third (figs. 1 and 3, op) or about one-half of the entire length of the cell, or about the length of the outer acoustic element {oh^), although no distinct demarcation can be observed between the future head and the pillar body.
Two other features lend support to this view: the existence of an abundant, vacuolated cytoplasm along the intermediate portion of the cell, the future pillar body, which only slightly encroaches upon the lower part of the future head, and the presence of terminal bars or rather true intercellular, obturating partitions. These have been observed and termed 'bandelettes obturantes' by N. Van der Stricht ('08) and 'Kittsubstanz' or 'Kittlinie' by Held ('09). This material stains intensely with iron hematoxylin like the superficial terminal bars with which it is in continuity, and corresponds to them in nature and chemical constitution. It gives rise not to 'lines' or 'bars,' but to true septa, uniting parts of the cells and obturating the subjacent intercellular spaces. These partitions exist not only between contiguous developing and definitive heads of inner and outer pillars, but also between the apical surface (that turned toward the apex of the cochlea) and basal surface (that turned toward the base of the cochlea) of the heads of each spiral row. On the other hand, they are altogether lacking along the medial surfaces of the heads of the inner pillars and the lateral surfaces of those of the outer (figs. 1 and 3, tb).
The second stage of development is characterized by a rapid enlargement of the future head of the outer pillar (fig. 4, ohd), so that it reaches the site of the intermediate portion or even surpasses it, when the process of cytolysis progresses along the tunnel space (fig. 7, op). At first the head remains smaller next to the surface, but soon this portion expands and becomes somewhat larger than the deeper part (fig. 4, ohd) and acquires a cubical or prismatic shape, the larger base of which touches the surface of the organ of Corti, its tapering apex blending with the much smaller pillar body. In cross-sections the prism is square or quadrangular in shape.
During this process of enlargement of the head, many remarkable changes occur. 1) A considerable shortening of the head segment (fig. 7) as if the compact substance of the lower parts had been pushed upward. Moreover, there can be no doubt that, at the same time, the vacuolated cytoplasmic zone of the intermediate portion of the pillar extends upward along the primitive head, so that the pillar body becomes longer at the expense of the latter. 2) A peculiar transformation of the protoplasm of the heads of the outer and inner pillars, close to and through the agency of the obturator septa. Primitively compact, homogeneous, or granular, entirely different from the vacuolated or fibrillated cytoplasm above referred to, the protoplasm of the head becomes converted into a denser material, staining intensely with iron hematoxylin. These changes occur in succession, first within the heads of the outer (figs. 4 and 7, ohd), then within those of the inner pillars (fig. 8, ihd), in proximity of the obturator septa separating their apical from their basal surfaces ; later, along the medial surfaces of the heads of the outer pillars, and finally ^long the lateral surfaces of the heads of the inner pillars, close to the obturator partitions which separate these two elements (fig. 9, ohd and ihd). In sections tangential to the surface of the organ of Corti these altered cytoplasmic portions are seen in the form of deeply staining uniform, planoconvex masses, the planar surface of the clump of one head adjoining that of another mass belonging to a contiguous head (fig. 11). In reality, each planoconvex clump is the section of a vertical band or semicolumn. Thus in each head there appear three semicolumns, which at first are separated from one another, but in more advanced stages Coalesce to fonn a single band or imperfect collar open toward the side of the head where the obturator material is lacking (figs. 7, 8, and 9). What mechanical factors cause these structures to appear is uncertain. It can only be stated that this dense and horny like exoplasmic head-collar develops and extends in close contact with the intercellular septa, as if the material elaborated at the periphery of the cytoplasm to increase the amount of extracellular cement were prevented from leaving the cell and retained within this collar, the staining capacity of which gradually increases, while the more central protoplasm, the endoplasm, becomes clearer and paler- This head-collar has been described in the embryonic pillar cells by N. Van der Stricht as 'plaque cuticulaire;' in the adult organ by Schwalbe ('87) as 'ellipsoider Einschlusskorper,' by Joseph ('00) and v. Spee ('01) as 'Kopfeinschluss,' and by Held ('02) as 'Kopfkorper.' 3) A change in the direction of the phalanx process and the intracephalic rootlets of its fibrils. Previously (fig. 1) inchned almost vertically, this fibrillar bundle gradually takes a more obhque course (fig. 3, oph), becoming in time nearly horizontal (figs. 4 and 6, oph) not only outside the head, but also within it, the fibrils occupying its superficial part. This alteration is caused doubtless by the shortening and considerable enlargement of the head and constitutes a striking evidence that this enlargement is the result not only of a sheer expansion, but also of a process of stretching of its lower parts in a more horizontal and radial direction, as if pushed upward by the strain of the elongating pillar body. At the same tune, this pressure involves a conspicuous shortening of the previous cephahc segment. The peculiar change in the direction of the phalanx process has been observed by N. Van der Stricht and by Held ('09).
Inner pillars
In the adult organ of Corti the superficial portion of the inner pillar can be divided into three parts:
The apex, or 'Kopfplatte' of Held, the ' Innenpfeilerzellenschnabel' of v. Spee and Kolmer ('09), the 'plaque cephahque ou membrane fibrillaire' of N. Van der Stricht. This is a very thin, quadrilateral membrane (fig. 13'), elongated radially and stretched between the apices of the sustentacular cells (originally the outer pillars) and the sensory cells {oh') of the first outer row and the apices of the supporting {is') and acoustic {ih) elements of the inner row of hair cells. It constitutes a part of the membrana reticularis and is fibrillar in structure, the fibrils running parallel to the axis of the plate and in continuity with those of the head.
The so-called 'head' is formed of at least two segments, the smaller superficial one being in close contact with the head of the outer pillar. In the bat, the upper part appears to be reduced, from compression between neighboring elements, to a simple fibrillated lamella (fig. 13"'"'^, ihd), while in the lower part there is a thin cytoplasmic layer lateral to the fibrils (fig. 13^, ihd). In the white rat (fig. 17, ihd), the common rat (fig. 18, ihd), and particularly in the cat (fig. 9, ihd) twelve days after birth, this superficial lamella is obviously thicker, its lateral cytoplasmic layer being larger. In the bat (fig. 13"^, ihd) and other mammals this layer increases in breadth at the level of the lower part of the head, whence, without any demarcation, it blends with a larger, deeper segment. This is not connected with the outer pillar, but is situated below the head of the latter. It is a little shorter than the superficial segment and gradually tapers and continues with the body (ipb) of the pillar.
During the first stage of its development the future head of the inner pillar is a four-sided, somewhat flattened prism (figs. 1, 2, and 3, ip), nearly uniform in diameter, although tapering to its apex. It is composed of a granular or homogeneous cytoplasm and a bundle of fibrils, which occupy the medial side of the lower part of the prism and the central area of its superficial portion where, in the earliest stages of development, the fibrils are arranged in the fonii of a hollow tubule (fig. 3, ip) which later gives rise to a solid bundle. During the second stage of development the future head undergoes no very marked changes. By compression from the outer pillar head its superficial segment becomes somewhat thinner — lamellar in shape (figs. 4 and 6, ip) — while its lower segment maintains its previous size or enlarges slightly in the neighborhood of the pillar body. At the same time the transformations above mentioned are occurring in its cytoplasm in the proximity of the obturator septa. In order to clearly recognize the lamellar shape of the superficial segment of the head, cross-sections are needed. A longitudinal fibrillation as illustrated in figures 7 and 8 {ihd) indicates an oblique or more or less longitudinal section of the pillar, and such preparations are liable to misinterpretation.
The most remarkable changes occur at the level of the free apices of the inner pillars, the summit of the pillar scaffolding. The gradual development of the head of the outer pillar, situated just beneath this summit, produces a radial extension of the latter, and the transformation of a very small square field (figs. 1, 2, and 3, aip) into a long narrow fibrillated membrane or headplate. This gradual extension is clearly shown in figures 4 (aip), 7 (ipl) and 8 (ipl\ ipl'^), whereas no enlargement in a spiral direction is noticeable. On measuring the radial diameters of the fibrillated head-plates in figures 3, 4, 7, and 8, and comparing them with the radial diameters of those portions of the membrana reticularis included between the plates and the outer border of the apices of the third row of acoustic elements, it is found that the former are respectively represented by about 1/11, 1/2.75, 1/2, and 1/1.64 of the latter. This statement gives a rather accurate picture of the rapid enlargement of the head of the outer pillar and the subsequent extension of the superficial inner pillar plate; that is, of the portion of the membrana reticularis formed by the latter during the development of the tunnel space.
From this description it is also evident, according to N. Van der Stricht (p. 610), that the extension of the apex of the inner pillar is due solely to a mechanical factor, a compression by the underlying enlarging head of the outer pillar. This view has been corroborated by Held ('09). He does not mention the description given by N. Van der Stricht but states (p. 212): Je mehr der Kopf des Aussenpfeilers sich bildet und in seiner Masse wachst, um so diinner wird iiber ihm die Kopfplatte des Innenpfeilers." In its extension the head-plate undergoes important structural changes. Originally foniied of a clear cytoplasmic field (figs. 1 and 3, aip) containing a diplosome or two central corpuscles, the elongating plate becomes subdivided into two zones, a lateral, small, clear zone, enclosing the diplosome (fig. 7), and a medial, more extensive, fibrillated one. This continues to lengthen and is composed of several parallel horizontal fibrils, which, close to the apex of the inner hair cells, are continuous with the more vertical fibrils of the subjacent head lamella. Such structures depend upon the extension of the head-plate in a definite direction, i.e., from a fixed point corresponding to the seat of the central corpuscle close to the outer hair cells, towards the inner acoustic elements.
Some sections tangential to the surface of the organ of Corti give pictures which prove that the head-plate is formed of two superposed planes, one deeper and fibrillated (figs. 8 and 9, ipP'-), the other more superficial, destitute of fibrils, and composed only of a clear homogeneous cytoplasm (ipl^), imperfectly enclosed by a part of the above-mentioned firm head-collar (fig. 8, ipl).
Structure of the heads of the inner and outer pillars in adult animals
The ultimate structural changes undergone by the heads of the pillars consist mainly in a broadening and extension of their collars. This band not only becomes thicker, but also extends over the head, to form the roof of the outer and inner pillar head (fig. 10, ohd, ihd). This roof appears to correspond to the 'plaque cultioulaire' of N. Van der Stricht. Due to such transformation, the collar becomes converted into a head cap, a firm exoplasmic zone which circumscribes a clear granular endoplasmic zone, except at the medial side of the inner, and at the lateral side of the outer pillar. In other words, the remainder of the previous cytoplasm having now become much clearer, occupies a cephalic notch (fig. 18,- ohd, ihd) which extends from the head roof towards the pillar body; the bottom and the lips of the groove are represented by the broadened head collar. The clear endoplasm filling up the notch is traversed by the fibrillar bundles of the heads, which in selected preparations stain deeply with iron hematoxylin.
The head of the inner pillar in the white rat (fig. 17, ihd) and in the conmion rat (fig. 18, ihd) is thus fonned of a superficial thinner, and a deeper, enlarged segment, both composed of a medial groove containing a lamella of fibrils and a lateral layer of firm, dark, homogeneous cytoplasm — the walls of the notch.
This lateral layer enlarged rapidly toward the lower pole of the head and tapers downward to blend with a small unifonn protoplasmic zone of the pillar body (fig. 17, ipb). In the cochlea of the adult bat, similar structures are seen (fig. 13'^, ihd), but near the surface of the head (fig. 13'""*^, ihd) only a very thin fibrillated lamella is recognizable. However, in vertical spiral sections showing the longitudinal fibrils throughout the length of the pillars, a part of the head cap is visible (fig. 16, ihd).
The notch of the outer pillar head enlarges from the roof towards the pillar body and presents a true asyimnetrical position (figs. 17 and 18. ohd), and so is the structure of the head cap itself. On cross-sections the lips of the groove differ in thickness, the apical (i.e., that turned toward the apex of the cochlea) being obviously thinner than the basal (i.e., that turned toward the base of the cochlea). The clear cytoplasm of the notch is traversed by the nearly horizontal fibrils of the phalanx process (fig. 17, ohd), the rootlets of which merge obliquely into the apical lip. The other bundle of fibrils, running vertically from the pillar body toward the surface of the head, also shows an asymmetrical position. On passing into the head this bundle proves to be bipartite, being formed of a smaller and a larger fasciculus (fig. 13'^-^, ohd). Within the lower and wider portion of the notch (fig. 13'^, ohd) the subdivision into two unequal fasciculi is more evident, and the two bands are more closely connected respectively with the apical and basaJ lip of the groove. At the level of the head roof the horizontal fibrillated bundle courses through the cleft between the two vertical fasciculi, each of which merges into its neighboring lip (figs. 13 and 17, ohd). Most of these asymmetrical structures may be recognized during the development of the head (figs. 4 and 7, op). In vertical spiral sections of the adult organ, the asynometry is very conspicuous. In figure 14 the apical surface (the surface turned toward the apex of the cochlea) of the head is clearly indicated by the course of the apical filament of the cells of Deiters {ap, d^) in the direction of the apex of the cochlea. In such sections (figs. 14 and 15) can be seen the clear, eccentric oval notch, which contains a cross-section of the horizontal bundle (op/i) and is outlined by a thinner apical border or wall, and a larger basal border, the bulk of the head. On penetrating into the head the fibrils of the pillar body (opb) become divided into two fasciculi, a thinner apical, and a broader basal one. The fonner seems to be shorter and its fibrils spread out obliquely through the corresponding lip; the latter is longer and its fibrils spread out fanlike (fig. 15, ohd) through the basal portion of the head, and seem to encroach upon the more homogeneous head roof. When the two systems of fibrils are not stained, the head roof can be more clearly seen to continue into the two borders of the notch. In the cochlea of young animals (fig. 10, ohd) the groove is much larger and its lips may be mistaken for sections through two different separate bodies, the 'ellipsoider Einschlusskorper' of Schwalbe and Joseph. These bodies do exist in earlier stages of development, but later, with the roof, they form one structure — the head cap.
The elongation of the phalanx process of the outer pillar will be dealt with in the next chapter.
The development of the spaces of Nuel
With the exception of very short references, such as those alluded to above, no investigations have been carried out to determine the formation of the spaces of Nuel. Fence the problem appears to be a very knotty one and abiiost insolvable.
In the cochlea of adult animals the largest of these spaces is represented by a spiral cleft between the outer pillars and the, cell bodies of the hair and supporting cells of the first outer row. This space may be termed the first space or the first spiral interstice of Nuel. Another cleft, which may attain considerable size, is the fourth space or spiral interstice of Nuel. This contains the phalanx processes of the cells of Deiters of the third row and is included between the hair cells of the third row and the so-called cells of Hensen. It is the 'external tunnel' of Held ('02). A second and a third space or spiral interstice of Nuel contain the phalanx processes of the cells of Deiters of the first and second rows, respectively, the former situated between the outer acoustic elements of the first and second rows, the latter between those of the second and third rows. The second and third spaces do not extend between the long subjacent cell bodies of the supporting elements.
Appearance of the first spiral space of Nuel. This doubtless develops before the others and before any trace of the tunnel of Corti. The first trace of its appearance may be seen rarely (fig. 1) before the enlargement of the future heads of the outer pillars, in the form of clear, vacuolated, prominent vesicles on the lateral surfaces of the outer rods of Corti. How these vesicles are produced is uncertain; they seem to be only transitory and appear rather abruptly, as though due to pressure within the clear fluid contained in the vacuolated medial zone of the outer pillars, and as though part of this fluid had been driven across the outer fibrillated zone of the cell to give rise to large prominent vacuoles. These are seen along the lateral surfaces of the intermediate, the basal, and occasionally even parts of the superficial portions of the outer pillars. In more advanced stages their outlines and connections with the secreting cells become indistinct, and the vesicles are replaced by a common fluid mass, pervaded by a few delicate trabeculae in process of disintegration or liquefaction (fig. 3, SN). This process is not unHke that of cytolysis by which the fluid of the tunnel is produced. It is noteworthy that a distinct outhne or a superficial membrane is never seen, either on the lateral surface of the outer pillars or on the medial surface of the inner pillars; so that under special conditions of intracellular pressure, fluid may exude and pass into intercellular channels. The cleft, filled up with this fluid, is the first space of Nuel. It enlarges gradually and extends toward the membrana basilaris, from which, even in the adult cochlea, it is separated by the lateral expansions of the feet of the outer pillars.
From this description it would appear that the initial dominant factor in the development of the cleft corresponds to a difference in pressure in two parts of the outer pillars: the large vacuolated medial zone, where clear fluid is being accumulated, and the surface of the fibrillated zone, where a pecuUar structure, the absence of a membrane, and a lower pressure, promote an exudation of fluid. In this respect a second important factor deserves due consideration, i.e., the shifting of the outer pillars. These structures originally are incorporated within the first row of outer hair cells, and although their extremities remain always fixed, their bodies are pushed inward and inside of the acoustic elements, so that at least a virtual, if not a true space appears below the first row of outer hair cells, between the nucleated portions of the cells of Deiters of the first row (fig. 1, d^) and the outer pillars (op) . This virtual cleft contains a spiral bundle of nerve fibers and represents the future space of Nuel.
The enlargement of the space of Nuel is doubtless promoted by a third peculiarity — a change in the shape of the outer pillar. When the shifting of the is latter completed, and before any appearance of a cleft, the lateral surface of the rod of Corti is a plane, represented in a vertical or oblique section by a straight line. Along with the lateral extension of the foot upon the basilar membrane (fig. 3, op) and the appearance of the head (figs. 4 and 7, ohd), and by a considerable elongation of the intermediate portion (the future body, fig. 7, opb), which is only possible by virtue of a curvation, the straight line becomes markedly curved, its concavity being turned towards and embracing the cleft. This may be a more important factor than appears at first sight. Indeed, in previous investigations (in press) it has been noted that the shifting of the cells of Deiters may be " completed (that is to say, the sustentacular elements of the first outer row may be situated beneath their corresponding hair cells) before smy appearance of a tunnel space (fig. 9 of the previous paper), or even of the true space of Nuel. In such figures, the original straight line persists, although the heads of the outer pillars are large, but the lateral extension of the feet is delayed.
Structure and transformation undergone by the phalanx processes of the outer pillars. Should any doubt be entertained as to the process of cytolysis along the lateral lower parts of the outer pillars, the structures and the transformation undergone by their phalanx processes afford striking evidence of such a Uquefaction. In the above description of these apical bands which unite the phalanges to the outer pillar heads, the most distinct constituent, the fibrillated bundle, alone has been mentioned. In the early stages of the development the band is composed of fibrils collected into a fasciculus, which is surrounded by a clear granular cytoplasm. Before the space of Nuel reaches the membrana reticularis this phalanx process proper is very short, being limited to the portion running between two neighboring hair cells (figs. 4, oph; 8 and 11, oph), and the portion lying under the phalanx itself (figs. 8 and 11, opU^). In other words, the enlarged head contains the longest part of the fibrillar bundle (figs. 8, oph'^; fig. 11, oph, oph'^) and covers completely the head plate of the inner pillar. The roof of the developing space of Nuel is made up of two strata, the lateral, thinnest part of the outer pillar head (fig. 11, oph; compare with figs. 6 and 12), and the lateral part of the superficial striated membrane (fig. 8, ipl). When the first interstice of Nuel has attained its entire extent in the adult organ its roof is composed of the lateral part of the head plates of the inner pillars (figs. 13', 17, and 18, ipl) strengthened by equidistant, parallel, fibrillated bundles, portions of the ultimate phalanx processes (figs. 13, 17, and 18, oph), which run in an oblique direction toward the spiral rows of pillars (fig. 13).
Figures 13', 13, and 13' illustrate the structures of this roof at these successive levels in the adult organ of Corti. Between the apices of the inner hair cells (ih) and the outer sensory elements {oh') they show respectively a superficial plane — the striated head-plates of the inner pillar cells (fig. 13', ipl") — an intermediate plane composed of parts of the preceding plates (fig. 13, ipl) and parts of oblique subjacent fibrillar bundles, and a deeper plane (fig. 13) showing from the axial to the lateral side, the row of fibrillated lamellae, heads of the inner pillars (ikd), the row of outer heads," a gap nearly as large as the preceding row and bridged across by equidistant fibrillar bundles {oph), entirely devoid of clear cytoplasm. The gap is the upper floor of the space of Nuel {SN'), which is covered by the equidistant bundles and the lateral part of the uninterrupted striated membrane, formed of the head plates of the inner pillars. This description is corroborated by vertical spiral sections. In figures 14 and 15, above the heads of the outer pillars (ohd), is seen a dotted, very delicate membrane {ipl"), subdivided into short segments by coarser spots. This is the fibrillated membrane formed of the head-plates of the inner pillars ; the fibrils have been cut transversely and the spots represent the sections of terminal bars which separate the plates. This dotted membrane extends over the neighboring space of Nuel {SN') and covers equidistant coarse granules (op/i'^'), the cross-sections of the phalanx processes of the outer pillars. In figure 15 is seen, above the dotted line, a very fine, pale, uniform covering, which doubtless represents the homogeneous superficial zone {ipl') already mentioned.
The phalanx process of the outer pillar, represented in early stages by two portions, is formed in the adult cochlea of three segments, the original two — a subphalanx (fig. 13, oph') and an intercellular segment (oph) which courses between two hair cells— and a subsequently developed one, the submembranous stalk {oph) which is derived from a part of the original intracephahc bundle (fig. 11, oph). Indeed, transitional stages can be observed. In figure 11 an uninterrupted extracephahc clear protoplasmic layer, a kind of a pale veil, developed from the embryonic heads of the outer pillars (fig. 8, ohd) by a process of differentiation, unites three upper fasciculi (fig. 11, oph) and assumes a festooned appearance around three lower bundles. This festooned appearance has been observed by N. Van der Stricht in the cochlea of a guinea-pig one day after birth. This investigator described (p. 641) each festoon as une sorte de voile triangulaire a sommet dirige vers la rangee des cellules acoustiques externes et a base en continuity avec la tete du piUer externe," and beheves that the veil corresponds to the "Schwelling des Aussenpfeilerschnabels" of v. Spee. This clear protoplasmic sheath of the extracephahc bundle is seen also in vertical spiral sections (fig. 10, oph). In the cochlea of a dog about four or five months of age it seems to persist, but unquestionably disappears by a process of cytolysis in the adult bat (fig. 13 ^ 14 and 15, oph') and the white rat (fig. 17). Evidences of such disintegration are shown in figures 9 and 10 (cy). The result is that a part of the clear cytoplasm which belongs to the originally enlarged heads of the outer pillars (fig. 8, ohd) undergoes a process of liquefaction. A portion of the intracephalic horizontal fibrillar band becomes free or at least partially destitute of protoplasm, so that the fluid of the large space of Nuel, in direct communication with that of the tunnel space through the wide interpillar clefts, comes in close contact with the very fine, superficial, fibrillated membrane of the head of the inner pillars. This membrane separates the fluid in question from that of the cochlea duct.
The physiological importance of these structures is evident, for the vibratory waves may be readily transmitted from one to another fluid through the intermedium of the striated membrane. As regards the transmission of vibrations from thefibrillated basement membrane of the membrana basilaris to the contents of the first interstice of Nuel, it is noteworthy that the fonner, at the level of the floors of the Nuel and tunnel's spaces, is separated from the latter by only a very thin cytoplasmic covering which belongs to the laterally expanded feet of the outer pillars and to the feet of the outer and inner pillars. Hence the transmission can be readily carried out.
Development of the second, third, and fourth spaces of Nuel. As shown in previous investigations, the phalanx processes of the cells of Deiters of the first and second rows are represented, in the earliest stage of development, by long apical segments of the cell bodies, which segments are included, respectively, in the second and third row of outer hair cells, within which they run between two neighboring acoustic elements. In length these apical segments agree with the hair cells. Due to the rapid enlargement of the latter, the apical segments are pressed out from their original row and shifted into interspaces; those of the first row of Deiters cells reaching the second future interstice of Nuel and those of the second row of Deiters cells reaching the third interval. All around the phalanx processes of the cells of Deiters of the third row, which remain in situ between the third row of sensory elements and the cells of Hensen, will appear the large fourth interstice.
Before the appearance of any space the phalanx process is composed of a clear cytoplasmic sheath, enclosing a darker, axial, mitochondrial strand, which by juxtaposition and fusion of the chondrioconts becomes gradually transformed into an axial, fibrillated filament. The process is larger at its base, which issues from the nucleated cell body, and tapers to the superficial membrana reticularis.
In a kitten nine days old, at the level of the apical spiral turn of the cochlea, the second, third, and fourth sustentacular interstices are still filled up with the unmodified phalanx processes, so that intercellular spaces are absent. In the- second turn, narrow channels appear and are somewhat larger near the surface of the epithelium than towards the base of the processes. Inversely the processes have become reduced in diameter at the expense of their clear protoplasm. At the level of the basal or third turn the enlargement of the spaces of Nuel and the reduction in size of the cytoplasmic sheath of the phalanx processes are much more marked. It must be noted that the thinning out of the latter is not the result of a sheer concomitant elongation, for these alterations are accompanied by a considerable elongation of the nucleated cell bodies of the sustentacular elements, involving a subsequent shortening of the supported hair cells, hence of the neighboring phalanx processes. These become more slender on account of a process of elaboration and secretion and a subsequent extrusion of clear fluid from the protoplasmic sheath. Whether, as many preparations seem to prove, this discharge is accompanied by a process of true cytolysis is uncertain, for these structures are very delicate and the shrinkage caused by the reagents might give rise to artefacts liable to misinterpretation.
The fourth space of Nuel de^'elops in the same manner as the second and third, and when it appears, is but little larger than the others. It is occupied by the apical processes of the ceUs of Deiters of the third row. Originally as long as the neighboring sensory elements, these processes are more numerous (previous, paper) and larger than those of the first and second supporting rows. Situated outside the hair cells of the third row, they are not squeezed and impeded in their lateral expansion like the others. It is not to be wondered at that the products of secretion or cytoplasmic disintegration around the apical fibrillated bundles are more abundant and result in an expansion of the fourth interstice. Nevertheless, the process of development is identical with that of the two preceding spaces and therefore the application of a special term, external tunnel, to designate this formation is unnecessary. However, there can be no doubt that phalanx processes of the cells of Deiters of the third row retain their cytoplasmic sheath much longer than the others, and may show parts of it in the adult cochlea, as pointed out by Held ('02) for the 'apical type' of these cells in guinea pig, cat, dog, and even the mouse. In such cases these processes are in closer connection with the outer wall of the space than with the medial.
In the basal spiral turn of the cochlea of a kitten twelve days after birth (fig. 12), the floor of the second, third, and fourth spaces of Nuel is fonned by parts of segments of the cel^s of Deiters {d\ d^\ d) supporting their corresponding sensory elements {oh\ o}v\ oh). The medial and lateral boundaries of the second interstice are represented, respectively, by the lateral surfaces of the hair cells of the first row and the medial surfaces of those of the second. The medial and lateral boundaries of the third interstice are represented, respectively, by the lateral surfaces of the hair cells of the second row and the medial surfaces of those of the third. The adjoining surfaces of the acoustic elements of each sensory row are separated by narrow clefts, through which all of the spaces of Nuel intercommunicate. These channels, originally occupied by the phalanx processes of the sustentacular cells (those of the first sensory row being the superficial segments of the outer pillars), are liberated after the shifting of the phalanx processes. At first very narrow and virtually obliterated by the process of enlargement of the hair cells, these intercellular clefts become wider by the reduction in size of the acoustic elements. The medial and lateral boundaries of the fourth interstice of Nuel are represented, respectively, by the lateral surfaces of the sensory elements of the third row and the medial surfaces of the so-called cells of Hensen, which, according to previous investigations, should be held as atrophied hair cells {aoh).
The roofs of the spaces of Nuel are made up of parts of contiguous apices of the supporting elements (fig. 9). The roof of the second space is formed of alternating lateral and medial segments of phalanges of the outer pillar {aop), and the Deiters cells of the first row {d^). The roof of the third space is composed of alternating lateral and medial segments of the phalanges of Deiters cells of the first (d') and second row (rf'Ol the roof of the fourth space is represented by the lateral segments of the phalanges of Deiters cells of the second row and the apices of those of the third (c?'"). The roofs of the intercellular clefts between the hair cells of the first, second and third sensory rows are formed, respectively, of the medial or constricted part of the phalanges of the outer pillars (fig. 9, aop), the middle part of those of Deiters cells of the first row (d'), and the middle pari of those of Deiters cells of the second row (d^'). The fluid contents of the first, second, third, and fourth spaces of Nuel intercommunicate through the intercellular clefts. Like the fluid of the first interstice, that of the others is separated from the contents of the cochlear duct only by very thin membranes, partially fibrillated, since the fibrillar bundles of the phalanx processes of the sustentacular elements spread over the under surface of their corresponding phalanx, according to the investigations of Held ('02) and of N. Van der Stricht. Such structures doubtless are able to promote the propagation of vibratory waves from the membrana basilaris to the fluid and the membrana tectoria within the cochlear canal.
Summary
- The tunnel space is developed around the spiral nerve bundle, which runs between the nucleated portions of the inner and outer pillar cells. It is originally an intercellular cleft, the fluid contents of which are elaborated in the vacuolated cytoplasm of the pillar cells and discharged into the adjoining space. Parts of this secreting protoplasm undergo a process of cytolysis or liquefaction, so that the cleft enlarges and the fluid contents increase in quantity at their expense.
- The tunnel extends upward along the vacuolated zones of the intermediate portions of the pillars. This clear cytoplasm also disappears by a similar process of secretion and cytolysis. Ultimately the intermediate segments of the pillars become transfonned into pillar bodies, reduced almost to their fibrillar apparatus of support, the outer being represented by thin cyhndrical strands which are separated by large intercellular clefts, and the inner, lamellar in shape, being composed of a medial, thin, fibrillar lamella and a lateral narrow zone of clear cytoplasm.
- In the earhest stage of development the heads of the outer rods of Corti are represented by thin tapering segments, nearly as long as the neighboring outer hair cells. They are characterized by the presence of two fibrillated bundles, the intracephalic rootlets of the fibrils of the phalanx process and the apical intracephalic extremities of those of the future pillar body. Moreover, pecuhar structures, a more homogeneous cytoplasm and a system of obturator septa between the two rows of the future outer and inner heads and between the contiguous surfaces of the heads of each row, constitute important features, which enable one to recognize the *nature of these apical segments.
- In a more advanced stage the long superficial segment of the outer pillar enlarges rapidly into a shorter prismatic mass, the head proper, within which develop, in contact with the obturator septa, a firmer exoplasmic collar, and ulthnately a head cap, enclosing imperfectly a clearer endoplasmic mass, which is traversed by the nearly horizontal and the vertical fibrillated bundles, the rootlets of the fibrils of the phalanx process and the apical ends of those of the pillar body, respectively.
- The original head of the inner pillar is represented by a small, four-sided, somewhat flattened prism. Later by compression from the outer pillar's head, the superficial part of the prism assumes a more distinctly lamellar shape; the lower part enlarges and acquires its largest size at the level of the lower pole of the outer pillar's head, whence it tapers upward and downward. The so-called 'head' of the inner pillar extends down beyond that of the outer and furnishes a pad of support to the latter. The prismatic head is composed of a lateral uniform cytoplasm, which enlarges at the level of the lower, broader portion, and a medial fibrillated lamella. In contact with the obturator septa and within the homogeneous protoplasm the head collar develops.
- By compression from the underlying and enlarging head of the outer pillar, the free apex of the inner pillar undergoes a gradual and extensive elongation, becoming converted into a fibrillated, long head-plate. The constant position of the central corpuscle within this niembrane, close to the apices of the outer hair cells of the first row, proves that this elongation occurs in a definite direction, from the seat of the diplosome toward the apices of the inner hair cells.
- The heads of the outer pillars are asymmetrical in structure. The cephalic notch, imperfectly surrounded by the head cap, is traversed by two fibrillated bundles and has a parapical position; it is situated nearer to the apical surface of the head (i.e., the surface turned toward the apex of the cochlea) than to the basal, since the apical lip of the groove is thinner than the basal. The horizontal rootlets of the fibrillated phalanx bundle are also parapical and merge obliquely into the apical border of the notch. The vertical fibrillar cephalic bundle, arising from the pillar body, divides into two unequal fasciculi, a smaller and a larger, circumscribing a cleft through which run the horizontal fibrils. The smaller fasciculus merges into the apical lip of the notch and the larger one into the basal lip, the bulk of the head cap.
- The first space of Nuel, which, in the adult organ of Corti, is situated between the outer pillars and the outer hair cells and the cell bodies of the sustentacular elements of the first row, appears as an intercellular cleft, within which is accumulated a fluid discharge from the outer pillars. At the lateral surfaces of the latter project clear secretion vesicles, which undergo a process of cytolysis and liquefaction. The exudation of this fluid seems to be due to a difference of pressure, on the one hand within the medial vacuolated cytoplasmic zone of the outer rods of Corti, and on the other, at their lateral surface. It is promoted by the shifting of the embyronic pillars from the first spiral row of outer hair cells towards the inner rods of Corti, by the development of laterally enlarged segTiients at the tw^o extremities of the outer pillars, the foot and the head, and an elongation of the outer pillar bodies, only possible by virtue of an incurvation, the concavity being turned towards the cleft of Nuel.
- This process of cytolysis is manifest at the level of the heads of the outer pillars. Indeed the enlarged embryonic head is more bulky than the adult head, and the phalanx process, shorter in the earlier stages of development, later becomes longer. Originally the phalanx process is represented by two segments, a subphalangeal and an intercellular segment, the latter running between two hair cells. Later on, a submembranous segment appears. This lies beneath the lateral portion of the head plates of the inner pillars, and is developed from a lateral part of the enlarged outer head by a process of disintegration of clear cytoplasm, which encloses the horizontal intracephalic fibrillated bundle.
- The roof of the first space of Nuel in the earliest stages of development of this interstice is composed of two uninterrupted coverings, one superficial and very thin, the lateral portions of the head-plates of the inner pillars, the other deeper and much thicker, the lateral parts of the outer heads. In the adult cochlea this roof is made up of the same parts of the superficial head-plates and a largely interrupted covering, the equidistant submembranous segments of the phalanx processes.
- The second, third, and fourth spaces of Nuel are located, respectively, between the first and second, the second and third, and the third row of hair cells and the cells of Hensen (atrophied hair cells of a fourth row). These spaces do not extend do^^^l between the sustentacular elements, but communicate with each other and with the first space through clefts between the hair cells. These intercellular channels, originally occupied by the phalanx processes of the sustentacular elements, become free after the shifting of the latter into the neighboring medial spaces, the phalanx processes of the cells of Deiters of the third row remaining in situ.
- Each of these phalanx processes is composed of an axial, fibrillar filament and a peripheral, clear, cytoplasmic sheath. In the course of development this sheath becomes thinner and may disappear by a process of secretion, which gives rise to the fluid contents of the primitive second, third and fourth spaces of Nuel.
- The roofs of the second, third, and fourth spaces of Nuel and of the intercellular clefts between two neighboring hair cells of each sensory row are made up of delicate membranes, partially fibrillated, which betong to various parts of the phalanges of the sustentacular elements.
- The fluid contents of the tunnel and the first space of Nuel are separated from the fibrillated basement membrane of the membrana basilaris by a thin protoplasmic covering, beonging to the feet of the inner and outer pillar cells. They ntercommunicate through clefts between the outer pillars and communicate with those of the second, third, and fourth spaces of Nuel. The fluid of all the spaces of Nuel is separated from the endolymph of the cochlea duct by the roofs of these inter .stices, very thin membranes, entirely or partially fibrillated. Such structures doubtless promote the propagation of vibratory waves from the basilar membrane to the membrana tectoria, contained in the cochlear canal.
All the material and reagents necessary for the present investigations were suppUed by Dr. T. Wingate Todd, Director of the Anatomical Laboratory of the Medical School, Western Reserve University, Cleveland, Ohio. It affords the author great pleasure to express his deep gratitude to Dr. Todd.
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Description Of Plates
All figures were outlined with a Zeiss camera lucida, at the level of the stage of the microscope, with the aid of a Zeiss ocular no. 3, and 2-mm., homog. immersion. Apert. 1.30, except figures 1, 2, 3, 4, 5, and 7, which were outlined with Zeiss ocular no. 1.
General Abbreviation
aip, apices of embryonic inner pillar cells
aoh^^, apices or bodies of atrophied hair cells of an outer fourth spiral row
aop, apices or phalanges of the outer pillar cells
cy, cytoplasm in process of cytolysis
c?\ d", d"', apices or bodies of cells of Deiters, respectively, of the first, second, and third rows
H, apex of the cell of Hensen
ih, apices or cell bodies of the inner hair cells
ihd, heads of the inner pillar cells
ij), inner pillars
iph, bodies of the inner pillars
ipl, head-plates of the inner pillars
ipl^, ipV\ respectively, the superficial homogeneous zone and the deeper fibrillated zone of the head plates of the inner pillars
t's', is", apices or cell bodies of supporting cells, respectively, of the first and second inner rows
A, Nerve bundle passing through the foramen nervinum
A'", spiral nerve bundle running between the inner and outer pillar cells or within the tunnel space
iV"', spiral nerve bundle running between the outer pillars and the cells of Deiters of the first row
nd, apices of non-differentiated cells of the greater epithelial ridge
m, nuclei of non-differentiated cells of the greater epithelial ridge
nih, nuclei of inner hair cells
niji, nuclei of inner pillar cells
nis^, nis", nuclei of inner supporting cells, respectively, of the first and second rows
nop, nucleus of an outer pillar cell, seated near the head (abnormality)
oh^, oh", oh}", apices or cell bodies of outer hair cells, respectively, of the first, second, and third rows
ohd, heads of outer pillars
op, outer pillar cells
oph, bodies of outer pillars
oph, phalanx processes of outer pillars
oph^, opd", opd"^, op<^^, respectively, the subphalanx, intercellular, submembraneous segments and the intracephalic roots of the phalanx process of the outer pillar
pd^, pd", pd"^, phalanx processes or apical filaments of cells of Deiters, respectively, of the first, second, and third rows
SN, SN\ the first space of Nuel
t, developing tunnel space
T, tunnel space
tb, terminal bars or obturator septa
VS, vas spirale
Plate 1
EXPLANATION OF FIGURES
1 Section tangential (and somewhat oblique) to the surface of the organ of Corti, through the second (middle) turn of the cochlea. New-born kitten. Fixation: osmic acid, 1 per cent aqueous solution for about one hour, followed by immersion in Zenker's fluid. Stain: Iron hematoxjdin, Congo red, light green.
2 Section tangential to the surface of the organ of Corti, through the second turn of the cochlea. Kitten 3 days, 12 hours after birth. Exposure of the cochlea, the bony wall of which had previously been provided, with two small openings, to vapors from a 2 per cent aqueous solution of osmic acid for approximately one hour, and subsequent treatment of the piece by trichloracetic acid 5 per cent in water. Iron hematoxylin, Congo red.
3 Section tangential to the surface of the organ of Corti, through the basal portion of the second turn of the cochlea. Dog 3 days, 18 hours after birth. Zenker's fluid. Iron hematoxylin, Congo red.
4 and 5 Sections tangential to the surface of the organ of Corti, through the basal portion of the second turn of the cochlea. Kitten 3 days after birth. Solution of trichloracetic acid 5 per cent in water. Iron hematoxylin, Congo red, light green.
6 Radial vertical section of the organ of Corti through the third (basal) turn of the cochlea. Kitten 3 days, 12 hours after birth. Exposure of the cochlea to vapors from a 2 per cent aqueous solution of osmic acid and subsequent treatment of the piece by trichloracetic acid 5 per cent in water. Iron hematoxylin, Congo red.
Plate 2
EXPLANATION OF FIGURES
7 Section tangential to the surface of the organ of Corti, through the third turn of the cochlea. Dog 3 days, 18 hours after birth. Solution of trichloracetic acid 5 per cent in water. Iron hematoxylin, Congo red.
8 Section tangential to the surface of the organ of Corti, through the third turn of the cochlea. Kitten 5 daj'^s, 12 hours after birth. Solution of trichloracetic acid 5 per cent in water. Iron hematoxylin, Congo red, light green.
9 Section tangential to the surface of the organ of Corti, through the basal portion of the first (apical) turn of the cochlea. Kitten 12 days after birth. Osn.ic acid 1 per cent aqueous solution for about one hour, followed by immersion in a 5 per cent aqueous solution of trichloracetic acid. Iron hem.atoxylin, Congo red.
10 Vertical spiral (parallel with the spiral rows) section of the organ of Corti, through the second turn of the cochlea. Kitten 11 days after birth. Osmie acid 1 per cent aqueous solution for a^bout one hour, followed by immersion in a 5 per cent aqueous solution of trichloracetic acid. Iron hematoxylin, Congo red.
11 Section tangential to the surface of the organ of Corti, through the basal portion of the second turn of the cochlea. Kitten 12 days after birth. Osmic acid 1 per cent aqueous solution for about half an hour, followed by immersion in a 5 per cent aqueous solution of trichloracetic acid. Iron hematoxylin, Congo red, light green.
12 Section tangential to the surface of the organ of Corti, through the third turn of the cochlea. Kitten 12 days after birth. Osmic acid 1 per cent aqucd»us solution for about half an hour, followed by immersion in a 5 per cent aqueous solution of trichloracetic acid. Iron hematoxylin, Congo red.
Plate 3
EXPLANATION OF FIGURES
13 Sections tangential to the surface of the organ of Corti through the third turn of the cochlea. Adult bat (Vespertilio fuscus). Zenker's fluid. Iron hematoxylin, Congo red. The figures illustrate structures at five successive levels of the organ of Corti.
14 and 15 Vertical spiral sections of the organ of Corti, through the second turn of the cochlea. Adult bat (Pipistrellus subflavus). Bouin's fluid. Iron hematoxylin, Congo red, light green.
16 Vertical spiral section of the organ of Corti through the second turn of the cochlea. Adult bat (Pipistrellus 'subflavus). Trichloracetic acid. Iron hematoxylin, Congo red, light green.
17 Section tangential to the surface of the organ of Corti. through the second turn of the cochlea. Adult white rat. Trichloracetic acid. Iron hematoxylin, Congo red, light green.
18 Section tangential to the surface of the organ of Corti, through the third turn of the cochlea. Adult rat (Mus decumanus). Bouin's fluid. Iron hematoxylin, Congo red, light green.
Cite this page: Hill, M.A. (2024, April 19) Embryology Paper - The development of the pillar cells, tunnel space, and Nuel's spaces in the organ of Corti (1919). Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/Paper_-_The_development_of_the_pillar_cells,_tunnel_space,_and_Nuel%27s_spaces_in_the_organ_of_Corti_(1919)
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