Microscopic guide to the middle ear anatomy in guinea pigs.

OBJECTIVES To reveal microscopic surgical anatomy of the middle ear in guinea pigs (GPs). MATERIALS AND METHODS Twenty ears of GPs were dissected under dissection microscopy via inferior approach. By using digital equipment, the most suitable photograph was taken for each step. Differences and similarities between the ears of GPs and those of human beings are discussed to show the advantages and disadvantages of the GP ear for experimental studies. RESULTS Tympanic membrane of the GP merely consists of pars tensa and above this there is a bony segment called as supratympanic crest. There were two spaces called bulla and hypotympanium in the GP,s middle ear. Cochlea, which is normally found in the inner ear in humans, separates these two spaces. Upper part of this space which is called epitympanium is a slit like cavity having a bony complex called malleoincudal complex. Bulla, the largest cavity in the middle ear, is a hemispherical cavity having a smooth surface. It corresponds to hypotympanum and mesotympanum of human middle ear. Cochlea has 3.5 turns from basal turn to apical turn. While the oval window is placed vertically, the round window is placed horizontally. While the stapes is almost identical to that of the humans, there is a bony bridge between the crura of the stapes called crista stapedius. Stapes can not removed unless this bony bridge is taken. CONCLUSION The middle ear in GP differs from the human middle ear in many aspects. Researchers who are planning to the study with this animal model should be aware of all these differences.