Book - Contributions to Embryology Carnegie Institution No.21
By O. Van der Stricht.
The membrana tectoria belongs to a group of organs produced at the surface of the epithelium and termed superficial cuticles or superficial cuticular formations. Once developed, the generating epithelium persists in its entirety beneath the cuticle or exceptionally may disappear, as in the case of the ameloblasts, which atrophy after forming the enamel at their bases.
One may subdivide these structures into three groups: In the first the process of development can not be doubted or denied. It occurs just within the superficial layer of the cytoplasm and the cuticle produced remains in close contact, even continuity, with the generating cells. Examples are the striated borders of the columnar epithelium of the intestine, of the crypts of Lieberklihn, of the convoluted tubules of the kidney, of the syncytial layer of chorionic villi in the human placenta, and of osteoclasts.
The second is represented by the series of reticulares or fenestrated membranes covering the surface of sensorial epithelia â€” for example, the reticular membrane of the cristae and macula acustica and the organ of Corti, the membrana limitans externa of the retina, the membrana limitans olfactoria. The openings of the membrane are traversed by the apices of sensorial cells, the hairs of the acoustic cells, the rods and cones of the retina, the ciliated vesicles of the olfactory cells. These membranes in adult sensorial organs are in close contact wdth the surface of the epitheUum, but are completely separated from the generating substratum. Hence their origin must be studied -during the embryonic period of their development. Many authors regard them as a real cuticle derived from the free surfaces of the subjacent, that is to say, sustentacular cells. N. Van der Stricht (1908) has demonstrated that the reticular membrane of the acoustic epithelium is formed by a system of terminal bars closing the intercellular spaces between the embryonic epithelial cells. G. Leboucq (1909) proved that the membrana limitans externa of the retina is not formed at all by the Muller cells, but by the closing bars separating the apices of the rods and cones and Muller cells. The present writer (1909) found the membrana limitans olfactoria to have a similar origin. The zona pellucida surrounding the ovarian ovum in mammals and traversed by the prolongations of follicular cells which reach the surface of the egg must be considered as a fenestrated membrane of the same nature. According to the investigations of Dubreuil and Regaud (1908), it is derived from exoplasmic fibers produced within the intercellular spaces of the follicular cells. My own preparations of ovaries of bats and dogs show that it is fornied by the terminal })ars, and AHcc Thing (1917) considers that the very thick zona pellucid of the ovum of the turtle is produced by the terminal bars of the surrounding epithelium. They extend over the free surfaces of these cells, where appears a delicate network of the same nature as that of the bars. This network, together with the bars, gives rise to the cuticular fundamental substance of the zona pellucida.
Enamel and the membrana Corti or membrana tectoria are included in a third group. In both cases the adult organ becomes comi)letely detached from its generating substratum, the first from the bases of the ameloblasts, each of which produces a kind of cuticular prism (the enamel prism). These elements are separated by the calcified cement substance which is considered to be a kind of intercellular i)roduct, although its origin has not been clearly described. The second, the membrana tectoria, becomes detached from the surface of the greater and the lesser epithelial ridges in the cochlear duct and remains fixed to only the least active portion of its generating substratum, the crista spirahs. Held (1909), discussing the nature of the membrana Corti, thinks that the membrane should not be considered cuticular, not because its layer first formed is not homogeneous, but because its constituent elements, its fibrils, as they become more and more elongated, proceed from the cytoplasm as different plasmic products and not as cell prolongations. A cuticle, he states, is not represented by flagella, by cilia of a ciliated epithelium, or by sensorial hairs. In addition a cuticle always remains attached to the surface of the cell. Hence Held regards the membrana tectoria as a specific product of the free surface of the greater and lesser epithelial ridges, the sensorial cells of which do not take part in its development. Therefore the fibrils of the membrana Corti can not be termed cuticular. Held seems to forget the recognized fact that enamel prisms are real cuticular elements, although they become completely detached from their anatomical substratum.
The object of my research is the study of the development and structure of the membrana tectoria. Although this problem has received the attention of many investigators, it seems to me that the results obtained have been rather contradictory and give for the most part no satisfactory interpretation because of differences between the morphological substratum and the real structure of the membrane derived from it. Recent investigators have more or less neglected the structure of the crista spiralis. I intend to devote to it special attention.
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Cite this page: Hill, M.A. (2019, November 21) Embryology Book - Contributions to Embryology Carnegie Institution No.21. Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/Book_-_Contributions_to_Embryology_Carnegie_Institution_No.21
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