Development of the Cochlear Aqueduct and the Round Window Membrane in the Human Embryo

Abstract

The physiologic importance of the cochlear aqueduct and the question of its patency still belong to the moot chapters of otology. Streeter 1 described the aqueduct in 85 and 130 mm. fetuses as a tubular pouch projecting from the subarachnoid spaces along the glossopharyngeal nerve toward the scala tympani and came to the conclusion that the communication had to be established soon after this stage. Karlefors2 identified the first anlage of the aqueduct in a 53.3 and 65 mm. embryo as a more or less dense connective tissue which connected the scala tympani with the glossopharyngeal nerve. Later the connective tissue becomes loose to the extent that a lumen is formed in that portion of the aqueduct which is close to the posterior cerebral fossa. The connective tissue of the aqueduct continues with the lining of the scala tympani on one side and with the dura and the arachnoidea or the posterior fossa on the other side. Finally Karlefors stated that in most instances a loose and a dense con¬ nective tissue lining run parallel to each other in the aqueduct and together form a subdural and a subarachnoid space.The present studies were undertaken with the hope of throwing some light on the significance of several anatomic structures and possibly on the physiology of the cochlear aqueduct. The close spatial relationship of the round window with the aqueduct made the study of the former imperative. An answer to the peculiar role of the membrane of the round window in infections of the inner ear was sought. Hardly any data, however, were found in the literature on the development of the round window membrane.The specimens marked C.C. (Carnegie Collection) were lent by the METHOD This investigation is based on a study of serial sec¬ tions of specimens ranging from embryos measuring 12 mm. from crown to rump to infants 4 weeks of age.This method was preferred to that in which liquids are injected through the cochlear aqueduct because routes of communication are usually formed as artefacts with the latter procedure and the question of patency could not be answered by this mode of experimentation. In order to preserve the original relationship between the arachnoidea and the cochlear aqueduct, temporal bones were in most instances decalcified and embedded with the contents of the posterior cerebral fossa in situ. Tearing and pulling of some of the soft tissues of the cochlear aqueduct in adults during autopsies are hardly avoidable. Both ears were examined in all specimens. OBSERVATIONS 1. In a 15.8 mm. embryo (CC. [Carnegie Collection] no. 4405) the primordium of the cochlear aqueduct was as yet not recognizable in the precartilage which sur¬ rounded the otic vesicle. The glossopharyngeal nerve was lined by a loose network of mesenchymal tissue.2. In an 18 mm. embryo (CC. no. 4501) a rarefaction of the precartilage between the ninth nerve and the otic epithelium was recognizable. The area of rarefac¬ tion corresponded to that portion of the otic capsule which was traversed by the cochlear aqueduct in older embryos. The increa...

Normal Development of the Ear and Its Mechanics

“…crown-rump length) the round window niche becomes separated from the aqueduct' by a thin pillar of periosteal bone. The round window membrane develops by con¬ densation of the loose syncytium of the niche (Waltner, 1945).…”