Earl W. Cauldwell

IV Terminal Stages in the Development of the Human Stapes

In the course of its morphogenesis as a cartilaginous, and later as an osseous, skeletal element in vertebrates, the stapes has undergone a remarkable transformation. Some of the more primitive steps in the alteration are repeated in the individual developmental history of the ossicle in man, while others are not only peculiar to the stapes but also unique in the general history of bone formation. The predecessor of the stapes functioned as a support for the respiratory mechanism in aquatic and amphibious vertebrates; in mammals it has been made over to serve as the terminal link in an ossicular chain whose function it is to convert air waves into liquid waves. In response to these new demands, the stapes of the human embryo, appearing first as part of the branchial arch system, later establishes anatomic relationships at one extremity with another ossicle, at the other with an orifice in the capsular shelter for the acoustic endorgans. The stapes develops rapidly, concomitant with a vestibular window whose dimensions are relatively fixed near the middle of fetal life, growth thereupon becoming minimal. The ossicle adapts its histologic structure to an existence within a chamber which is early invaded by pharyngeal mucous membrane, and concurrently sacrifices considerable bulk to reduce inertia. These anatomic objectives and the developmental means employed to attain them will be the subject of the current article.