Paper - The Factors Involved in the Excavation of the Cavities in the Cartilaginous Capsule of the Ear in the Human Embryo
Embryology - 23 Apr 2024    Expand to Translate
|
|---|
| Google Translate - select your language from the list shown below (this will open a new external page) |
|
العربية | català | 中文 | 中國傳統的 | français | Deutsche | עִברִית | हिंदी | bahasa Indonesia | italiano | 日本語 | 한국어 | မြန်မာ | Pilipino | Polskie | português | ਪੰਜਾਬੀ ਦੇ | Română | русский | Español | Swahili | Svensk | ไทย | Türkçe | اردو | ייִדיש | Tiếng Việt These external translations are automated and may not be accurate. (More? About Translations) |
Streeter G.L. The Factors Involved in the Excavation of the Cavities in the Cartilaginous Capsule of the Ear in the Human Embryo
| Historic Disclaimer - information about historic embryology pages |
|---|
 |
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
Cite this page: Hill, M.A. (2024, April 23) 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
- © Dr Mark Hill 2024, UNSW Embryology ISBN: 978 0 7334 2609 4 - UNSW CRICOS Provider Code No. 00098G
