Paper - The Factors Involved in the Excavation of the Cavities in the Cartilaginous Capsule of the Ear in the Human Embryo

Embryology - 6 May 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

George L. Streeter

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

‘Child, O. M. Senescence and rejuvenescence. University of Chicago Press, 1915. Page 293.

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Cite this page: Hill, M.A. (2024, May 6) 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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