Development of the hypotympanum in the human fetus and neonate.

The human hypotympanum develops over a 10-week period in utero (22-32 weeks) as a triphasic osseous mosaic. The tympanic bone ossifies as a membranous bone (8-36 weeks in utero). The canalicular otic capsule ossifies in cartilage by 26 weeks in utero. A petrosal ledge of bone ossifies as periosteal bone at 24-29 weeks in utero. The fusion of these three bony structures closes the hypotympanum and forms two persistent hypotympanic fissures. The medial hypotympanic fissure forms between the canalicular otic capsule and petrosa at 24 weeks in utero. It transmits the medial hypotympanic artery and vein (from the posterior CNS circulation) and Jacobson's nerve (from the ninth nerve). The lateral hypotympanum fissure forms between the tympanic bone and the petrosa at 29 weeks in utero. It transmits the lateral hypotympanic artery and vein from the external carotid circulation. The two vascular supplies form a rich hypotympanic plexus on the floor of the middle ear. The fibrous annulus and middle layer of the tympanic membrane form an avascular plane.