The development of the canaliculus and fenestra cochleae in human embryo

Abstract

The material consists of 7 human embryos which lengths range from 20mm to 16cm, inserting 22, 2 7, 35, 40 and 55mm long specimens between them. The youngest 20mm long embryo belongs to the Streeter's horizon XVIII developmental stage, in which three semicircular ducts and all the peripheral nerves to the sensory epithelium of the membranous labyrinth are distinct. Its otic capsule is cartilageous only in its vestibular part, its cochlear part remains precartilageous.
In the next 22mm long embryo the cochlear part becomes also cartilageous, though not so much advanced as the vestibular part, and the fenestra perilymphatica becomes distinct which is in the previous stage of 20mm long specimens already only as a less dense area from the surrounding precartilageous tissue discernible (Fig. 1, 2, 3, Pl. 1)
In this 22 and next 27mm long specimens the histological appearance of the medial and lateral margin of the fenestra perilymphatica is not the same. The surface of the lateral margin shows a dense mesenchymal layer, while that of the medial not.(Fig 4-5 Pl. 1)
In the next 35mm long specimen appears a little fibrous characteristic in this dense mesenchymal layer of the lateral margin so that two fibrous layers in the opening radiating from the lateral margin medialward becomes distinct. These two layers in the opening have a less dense layer between them. The inner layer among the two is direct under the cochlear duct, the outer one continues medialward to that area, where canaliculus cochleae appears. These structures in 35mm long become more distinct in 40mm long one.(Fig. 9, Pl. 2, Fig. 15, Pl. 3)
Streeter's periotic space begins to form already in 35mm long specimen and it becomes spacious neighbering fenestra vestibuli in the next 40mm long. But the space of the scala tympani is not yet to be seen, its spacious structure appears only in the next 55mm long specimen. In the latter not only the scala tympani, but also the scala vestibuli is very spacious in comparison with the cochlear duct.
In the region of the fenestra perilymphatica is formed the cartilageous canaliculus cochleae, although very short, in this 55mm long embryo.Such formation of cartilageous canal begins already in 35mm long one, in which only as a cartilageous protrusion (Fig. 6-9, Pl 2*), in the next 40mm long one in the form of a cartilageous process (Fig. 11, Pl. 2, Fig. 11-14, Pl. 3*) under the mesenchymal anlage of the canaliculus (Fig. 9, Pl. 2, Fig. 15, Pl. 3°) from behind foreward. This cartilageous process may fuse with the cochlear part and make a canal.(Fig. 18-23, Pl. 4, Fig. 24-26, Pl. 5)
This cartilageous canal of the canaliculus becomes long and covers almost all length of the mesenchymal aquaeductus canal in 16cm long embryo (Fig. 27-35, Pl. 5-6) As the cartilageous tube of this canal reaches near the scala tympani in this specimen, there appears a cartilageous process protruding to this end of the tube, This cartilageous process makes a new hinder rand of the fenestra cochleae and coming near the tympanic end of the canaliculus tube it does not fuse with the latter so that there remains a very narrow mesenchymal part which connects the two part canaliculus and fenestra cochleae. The same structure is also in the osseous capsule observable.
The mesenchymal canaliculus begins to be formed. in 40mm long one as a continuation from the endocranial dural layer before the gangl. sup. of the glossopharyngeal nerve to the outer fibrous layer of the opening of the fenestra perilymphatica. This fibrous strand becomes in the next stage of 55mm long one a tube with fibrous wall, in which lumen there is some reticular structure. Such structure does not change in 16cm long one, its wall becomes thicker, its surrounding mesenchym is not so loose as that of the fenestra cochleae.
The existence of the Waltner's barrier membrane is doubtful.