Paper - The Factors Involved in the Excavation of the Cavities in the Cartilaginous Capsule of the Ear in the Human Embryo
Embryology - 18 Apr 2024    Expand to Translate
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Streeter G.L. The Factors Involved in the Excavation of the Cavities in the Cartilaginous Capsule of the Ear in the Human Embryo
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The Factors Involved in the Excavation of the Cavities in the Cartilaginous Capsule of the Ear in the Human Embryo
Department of Embryology, Carnegie Institution of Washington, Baltimore, Maryland
Twelve Figures
The main mass of the cartilaginous capsule of the ear matures into true cartilage when the human embryo reaches a length of 20 to 30 mm., at which time it has acquired what may be considered its adult form with characteristic chambers and openings. From this time on, throughout its whole cartilaginous period, and even after ossification has begun, it undergoes continuous growth, maintaining at the same time, however, its general form and proportions. Such a growth involves both an increase in the surface dimensions of the capsule and a gradual enlargement or excavation of its contained cavities. It is to the manner in which this excavation is accomplished that the writer wishes to call attention and particularly to the factors concerned in its progress whereby a suitable space is always provided for the enlarging membranous labyrinth. The actual amount of increase in size of the labyrinth is graphically pictured in figure 1. The outlines are made so that they show on the same scale of enlargement a series of wax-plate models of the left membranous labyrinth of human embryos having a crown-rump length of 20, 30, 50, 85 and 130 mm., as indicated in the figure. This covers the periodiduring which the otic capsule is in a cartilaginous state. Ossification begins when the fetus has attained a crown-rump length of about 130 mm. The growth from then until the adult condition is reached may be judged by comparing the above with the final stage, labelled adult, which is taken from Schonemann’s reconstruction1 and reproduced here so as to be on the same scale of enlargement as the younger stages. Since the cartilaginous labyrinth corresponds closely in form to the membranous labyrinth, particularly as regards the canals, one can see from figure 1 that there is a progressive increase in the size of the cartilaginous chambers throughout the Whole embryonic period.
Fig. 1 Median views of wax-plate models of the left membranous labyrinth
in human embryos having crown-rump lengths as indicated in the figure. The
largest one is taken from Schonemann (’04) and represents the adult condition.
They are all on the same scale of enlargement (4.4 diameters) and thus comparison of them shows graphically the amount of growth the labyrinth experiences
during this period.
1 Schoenemann, A. Die Topographic des menschlichen Gehiirorganes. Verlag Von Bergmann, Wiesbaden, 1904. Plate 2, figure 20.
In addition to this increase in size, there is a change in the form of the cartilaginous labyrinth. The general proportions
are maintained but there are alterations in the detailed form.
As the canals become larger and longer they describe arcs of lesser curvature. If one compares the superior canal of an 80 mm. fetus with that of a 30 mm. fetus it will be found that in the former it has doubled its diameter and trebled its length. There is, moreover, a constant change in the relative position of the cartilaginous canals. The lateral canal, for instance, progressively recedes from the lateral wall of the vestibule. In studying this canal, therefore, one may know that it is steadily becoming larger by means of a process of excavation, but this is so managed that the canal as a whole moves in a lateral direction through the substance of the cartilaginous capsule. The topography of the cartilaginous labyrinth is so well provided with known landmarks that these changes in its size and form can be accurately followed. It is possible to determine deductively at what points new cartilage is being laid down and at what points it is being removed. On this account the cartilaginous capsule of the ear is a particularly favorable place for determining the histological features of the ‘growth of cartilage.
As has been noted above the growth of the cartilaginous otic
capsule resolves itself into an increase in its external dimensions
with a simultaneous hollowing out and reshaping of its contained
chambers. It at once becomes evident that this cannot be
accounted for on the basis of a simple interstitial increase in the
mass of cartilage together with its passive rearrangement to
allow for the enlarging cavities, due for instance to a mechanical
expansive pressure from the growing membranous labyrinth
with its surrounding tissue and fluid. Such a passive rearrangement could only occur in a tissue that is very plastic, whereas
cartilage is one of the least plastic of the embryonic tissues.
Moreover the histological picture is not that of mechanical pressure. The cartilaginous chambers are always excavated slightly
in advance of the space actually required by the membranous
labyrinth, and there is no evidence of the labyrinth being
cramped or of the creation of pressure grooves in the margins of
the cartilage. Nor is the situation improved by the introduction of the conjectured activity of the perichondrium, either in explanation of the deposit of new cartilage or of the excavation
of the old, since the perichondrium, as will be shown, does not
make its appearance until after a considerable amount of the
growth and hollowing-out of the labyrinth had been already
completed. Therefore there is involved in the development of
the cartilaginous capsule something more than interstitial and
perichondrial growth, in the ordinary sense of the terms. On
account of its bearing upon this problem, it is the purpose of
the present paper to call attention to the occurrence of dedifferentiation of cartilage in the human embryo, and to point out
the important part which this process normally plays in the
hollowing out and reshaping of the otic capsule during its
development.
The term dedifferentiation is applied here in the sense of a
regression of certain areas of cartilaginous tissue to a more
embryonic form, the same areas being subsequently rebuilt or
redifferentiated into quite a different type of tissue. Dedifferentiation is defined by Child as “a process of loss of differentiation, of apparent simplification, of return or approach to
the embryonic or undifferentiated condition.” In his noteworthy review of this subject he makes the assertion that the
wide occurrence and significance of dedifferentiation in the
lower animals and plants “must at least raise the question
whether similar processes do not occur to some extent in higher
forms.”2 From the context it is evident that he refers to man
as well as other mammals. The materialization of his prediction is here at hand in the development of the cartilaginous
capsule of the ear. Before entering into this further it will be
necessary to outline the earlier steps in the histogenesis of this
particular tissue.
The Three Stages in the Development of Cartilage
The cartilage of the otic capsule in its transition from embryonic mesenchyme to true cartilage passes through three fairly definite phases: firstly, the condensation of mesenchyme around the otic vesicle; secondly, the differentiation of the condensed mesenchyme into precartilage; and thirdly, the conversion of precartilage into true cartilage. These three histogenetic stages merge more or less diifusely into one another and one‘ must bear in mind that such a subdivision is necessarily arbitrary and tends to result in an exaggeration of the distinctness of the lines of their demarcation. Their points of difference, however, are here emphasized because the reversal of one state of development into a previous state is the feature to which it is desired to call especial attention.
‘Child, O. M. Senescence and rejuvenescence. University of Chicago Press, 1915. Page 293.
Stage of Condensed Mesenchyme
When a human embryo is 4 to 5 mm. long the mesenchymal tissue surrounding the otic Vesicle differs very little from that in other regions. The nuclei, however, are quite sparse in the regions ventral to the neural tube in the median line, and they become perceptibly more numerous as one explores laterally into the neighborhood of the otic vesicle. This slight increase in the number of nuclei around the vesicle marks the beginning of the mesenchymal condensation that is to form the otic vesicle. A definite layer of such nuclei is not found until the embryo reaches a length of about 9 mm.; it is then possible to recognize a fairly Well outlined zone of mesenchyme which represents the otic capsule in its first stage of development. In figure 2 is shown a sketch indicating the relations which exist at that time. It represents a transverse section through the otic vesicle at the level of the attachment of the endolymphatic appendage. The zone of condensed mesenchyme forming the primordium of the otic capsule abuts directly against the lateral wall of the vesicle and extends from there to a point about one-half the distance between the vesicle and the ectoderm. On the median side of the vesicle this zone is lacking, although there is a considerable number of mesenchyme.cel1s clustered around the vascular plexus ensheathing the central nervous system, and among the nerve rootlets of the acoustic complex. When this zone is analyzed under higher magnification it is found that it still consists essentially of a mesenchymal syncytium. It differs morphologically from the adjacent mesenchyme, with which it is directly continuous, only in its more numerous and more compactly arranged nuclei and its somewhat richer network of internuclear processes. This is shown in figure 3 which is taken from an embryo a little larger than that in figure 2, but which in its general form is apparently in about the same stage of development.
Fig. 2 Section through the region of the otic vesicle in a human embryo 9 mm. long (Carnegie Collection, No. 721) enlarged 66.6 diameters. The primordium of the otic capsule, consisting of condensed mesenchyme, can be seen enclosing the vesicle on its lateral surface.
During the period of growth represented by embryos between 9 mm. and 13 mm. long, that is, up to the time when the semicircular ducts begin to separate from the main labyrinth through the apposition and absorption of the intervening membranous Wall, the zone of condensed mesenchyme around the otic vesicle increases in extent and compactness, thereby forming a sharply defined capsule which completely encases the labyrinth. This capsule of condensed mesenchyme has the same openings and corresponds closely in form to the cartilaginous capsule into which it is destined soon to be converted.
Fig. 3 Camera lucida drawing of a portion of the otic capsule while it is the
state of condensed mesenchyme. It is taken from a human embryo 13.5 mm.
long (Carnegie Collection, No. 695). The section is 10 microns thick and is
enlarged 950 diameters. The syncytial character of the capsule can be seen and also its relation to the epithelial wall of the otic vesicle and to the surrounding mesenchyme.
Stage of Precartilage
The histogenetic changes which initiate the conversion of the capsule of condensed mesenchyme into a cartilage-like tissue make their first appearance just after the separation of the semicircular ducts from the main Vestibular pouch. This occurs when the embryo is about 14 mm. long. The conversion of the capsule into a true cartilage with a characteristic tinctorial reaction of its matrix is not completed until the embryo attains a length of 30 mm. Thus in embryos between 14 and 30 mm. long the otic capsule consists of a tissue in an intermediate condition between condensed mesenchyme and cartilage. This intermediate form is known as precartilage. It constitutes the second of our three stages of cartilaginous growth.
Fig. 4 Section through the region of the otic capsule in a human embryo 15
mm. long, (Carnegie ‘Collection, No. 719). Enlarged 66.6 diameters. The
epithelial portions of the labyrinth are shown in solid black and it will be noted
that they are in direct contact with the substance of the capsule; there is as yet
no periotic reticular tissue. The section passes through the superior and posterior semicircular ducts and through the utricle near its junction with the
crus commune.
The general form and relations of the otic capsule at the beginning of its conversion from condensed mesenchyme into precartilage is shown in figure 4, which represents a horizontal section through this region in a human embryo 15 mm. long (Carnegie Collection, No. 719). It will be noted that the capsule abuts directly against the epithelial Wall on the labyrinth. Around the margins of the capsule there is a Vascular network the branches of which, however, do not penetrate into its substance. In its form it is essentially the same as its antecedent capsule of condensed mesenchyme, but in structure it can be seen to be undergoing certain characteristic alterations. These do not occur uniformly throughout its substance but appear earlier in some areas than in others. They consist of an increase in distance between the nuclei, together With an alteration in the internuclear protoplasmic network and its spaces. Whereas the capsule, as seen in prepared sections, has previously consisted of a mesenchymal syncytium, it now gradually loses its syncytial appearance. Most of the branching processes disappear and are replaced by a homogenous mass. Some of the processes, on the other hand, persist, and become thicker‘ and more sharply outlined. These persisting larger processes usually exhibit a characteristic relation to the nuclei. Two or more of them unite in the formation of a loop at one side or at one or both ends of a nucleus, thereby creating a perinuclear space which soon takes on a more transparent appearance than the surrounding homogeneous material that accumulates in the place of the disappearing processes. These changes can be seen in the sketches shown in figure 5, which represent characteristic areas in the otic capsule while in the precartilage stage in human embryos 17 and 18 mm. long. In the two sketches marked A the contrast beween the permanent and disappearing protoplasmic processes is already noticeable. In the sketch marked B the transition is more advanced although one can still recognize in the homogeneous matrix remnants of branching processes which have not yet disappeared. The persisting processes enclose characteristic capsular or perinuclear spaces. Similar spaces are shown in figure 6 which presents aiseries of isolated nuclei with their associated permanent processes such as are found in sections of maturing precartilage. In some of these (figure 6, C and figure 5, B,) there is a beginning accumulation of granular protoplasm at the margin of the nucleus which constitutes the so-called endoplasm and becomes enclosed with the nucleus in the capsule. After the formation of the spaces the endoplasm gradually accumulates and forms the cell body of the encapsulated nucleus. Thus in precartilage we find all stages in the transition, from a mesenchymal syncytium to a tissue consisting of partially encapsulated cell-islands separated from each other by a homogenous matrix.
Fig. 5 Camera lucida sketches showing characteristic fields in sections of the otic capsule while it is in the precartilage state. Enlarged 950 diameters. The groups labelled A are taken from an embryo 17 mm. long (Carnegie Collection, No. 576). Group B is taken from an embryo 18 mm. long (Carnegie Collection, No. 409).
Cartilage Stage
The transition from precartilage into cartilage gradually takes place in the otic capsule when the embryo is between 25 and 30 mm. long. This maturation is characterized by an increase in the amount of matrix combined with a more complete encapsulation of the nuclei, or cartilage-cells, as they may now be designated. With the increase in the amount of the matrix there is also a change in its chemical composition, so that it becomes possible to stain it differentially. This tinctorial reaction constitutes an arbitrary point at which it may be said that the precartilage becomes cartilage. In embryos 30 mm. long the greater portion of the otic capsule reacts tinctorially and has the histological character of young cartilage. With this stage we reach the third and final phase of the process with which we are dealing. The further changes from younger cartilage to
Fig. 6 Characteristic precartilage cells showingvthe manner in which spaces become enclosed around them, eventually becoming encapsulated cells of true cartilage. Enlarged 950 diameters. Group A is from the otic capsule of an embryo 17 mm. long (Carnegie Collection, No. 296); Group B is from an embryo 24 mm. long (Carnegie Collection, No. 455); and Group C is from an embryo 23 mm. long (Carnegie Collection, No. 453).
older cartilage, and the conversion of cartilage into bone, are doubtless a continuation of the same general process but in the present paper they will not be taken into consideration.
Periotic Reticulum
It has been pointed out elsewhere by the Writer‘° that there is derived from the condensed mesenchyme surrounding the otic capsule not only the cartilaginous capsule but also the periotic
3 Streeter, G. L. The development of the scala tympani, scala vestibuli and perioticular cistern in the human embryo. Am. Jour. Anat., vol. 21, 1917.
12 GEORGE L. STREETER
reticulum which eventually intervenes between the capsule and the epithelial labyrinth. The relation existing between this reticulum and the three stages of cartilage that have just been defined must therefore now be referred to. The formation of the periotic reticulum is first indicated by a cluster of deeply stained 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 capsule gradually takes on an appearance less like a cartilage-forming tissue and more like 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 between the epithelial labyrinth and the surrounding cartilage.
While in the stage of condensed mesenchyme and in the earlier part of its precartilage period the tissue of the otic capsule to all appearances abuts directly against the epithelial wall of the labyrinth as shown in figures 2, 3 and 4. It is possible, however, that some of the cells directly adjacent to the epithelium do not properly belong to the tissue of the otic capsule. It is conceivable that such cells may represent indifferent mesenchyme and perhaps angioblasts which were originally enclosed, along with the otic vesicle, by the condensed tissue of the capsule where they remain in contact with the epithelial wall in a resting condition until the embryo attains a length of 20 m. We might regard as an indication of their resumed activity the formation of the deeply stained foci along the central margins of the canals which have been described above. It might thus be maintained that the periotic reticulum is derived from a few predestined mesenchyme cells which after a latent period undergo proliferation and occupy the space vacated by the receding precartilage. On the other hand one may also maintain that the reticulum is derived from carti1age—forming tissue; that it is not a predetermined tissue but is simply precartilage that has undergone dedifferentiation. In the early stages when only a few cells are concerned this matter cannot be so well determined, the histological difference between early precartilage and indifferent mesenchyme cells not being sufficiently great for their certain recognition. In the later stages, however, it is quite evident that precartilage tissue is actually converted into a reticulum, and that the replacement of precartilage by a reticular connective tissue is accomplished by a process of dedifferentiation. By identifying a special area through its relation to a particular canal, and comparing this selected area in a series of stages, it is possible to observe the conversion of precartilage into reticulum, and to trace histologically step by step the manner in which a space occupied by precartilage in a younger stage is replaced by a reticulum in an older stage. This is the same procedure which occurs in the conversion of cartilage into pre~ cartilage and in the latter case, on account of the more highly specialized structure of the tissues, the picture is even more striking, as will be seen in the following outline in which the main features of the process will be pointed out.
Dedifferentiation of Cartilage
It has been noted that in embryos 30 mm. long the main capsular mass consists of true cartilage possessing encapsulated cartilage cells and an intervening matrix that is differentially stainable. A section passing transversely through the lateral semicircular canal of an otic capsule of this age is shown in figure 7. This, and figures 8 and 9, form a series showing at the same enlargement the same canal, i.e., lateral, cut in the same plane at three successive stages in its development. A direct comparison of these figures can thus be made and there is thereby seen the histological changes that occur with the growth of the canal. The successive figures may be superimposed upon each other and in this way the relative amount and position of the constituent tissues be determined. When this is done it is found that in the process of enlargement the true cartilage around the margin of the canal becomes replaced by precartilage
Cite this page: Hill, M.A. (2024, April 18) Embryology Paper - The Factors Involved in the Excavation of the Cavities in the Cartilaginous Capsule of the Ear in the Human Embryo. Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/Paper_-_The_Factors_Involved_in_the_Excavation_of_the_Cavities_in_the_Cartilaginous_Capsule_of_the_Ear_in_the_Human_Embryo
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