Evolution of the epidermoid formation and its role in the development of the middle ear and tympanic membrane during the first trimester.

In a study of 20 ears from embryos and early fetuses, 16 contained structures recognizable as precursors of the epidermoid formation (EF). The earliest were small, but later they took the form of a long epithelial bud which emanated from the tip of the tubotympanic recess or Eustachian tube. At a later stage still the EF protruded into the lumen of the Eustachian tube. The early formation of the tympanic membrane, the middle ear cavity and the osseous tympanic ring all take place in the vicinity of the EF precursor. It is possible that the EF precursor may act as an organizer, playing a prominent part in the migration of the first pharyngeal pouch towards the first branchial groove and the development of the middle ear and tympanic membrane.