2. The strong expression of MIB1, an antigen related to Ki-67, which also indicates hyperproliferative activity ;
3. Counts of silver-stained argyrophil nucleolar organiser regions, a technique that likewise displays proliferative activity, showed significantly larger numbers of these structures in the nuclei of acquired cholesteatoma compared with those of the epidermis of the deep external auditory meatal skin ;
4. Acquired cholesteatomatous epithelium shows an abnormally high concentration of IL-1, TGF-alpha, EGF-R and 4F2, all being growth factors  indicating greater growth and differentiating activity than is present in normal epidermis.
Congenital cholesteatoma probably arises due to the continued growth of the epidermoid formation. This structure is derived from external ear epidermis (see above). It seems likely that acquired cholesteatoma is also derived from entry of external ear canal epidermis into the middle ear. This is clearly shown in those cases of acquired cholesteatoma that follow blast injury with perforation of the tympanic membrane at the time of the injury . Acquired cholesteatoma is also known to follow the retraction pocket of the tympanic membrane. This is not due to obstruction of the mouth of a retraction pocket, but rather, it seems, to the ingrowth of a band of stratified squamous epithelium from the fundus of the retraction pocket deeply into the middle ear (Fig. 8.11) . A similar entry of stratified squamous epithelium from the external ear epidermis through the tympanic membrane may sometimes be observed in human temporal bone sections in cases of severe otitis media (Fig. 8.12). The placement of irritants or bacteria into the middle ear cavity of animals
Fig. 8.11. The fundus of a retraction pocket is seen in the top right hand part of the illustration. Emanating from it and passing down to the bottom centre is a band of stratified squamous epithelium, within which is a bluish staining zone of stratum granulosum and keratin - a "mini-cholesteatoma". An ossicle lies to the right of the epidermal band. It is an eroded incus, damage to which has probably been caused by previous otitis media
Fig. 8.12. A thin strand of stratified squamous epithelium is seen passing from the epidermis of the tympanic membrane towards the middle ear epithelium, almost touching it. To the left of this epidermal band the middle collagenous layer of the eardrum is distorted by a deposit of tympanosclerosis, composed of partially calcified hyaline collagen
Fig. 8.12. A thin strand of stratified squamous epithelium is seen passing from the epidermis of the tympanic membrane towards the middle ear epithelium, almost touching it. To the left of this epidermal band the middle collagenous layer of the eardrum is distorted by a deposit of tympanosclerosis, composed of partially calcified hyaline collagen has been known to provoke an otitis media that is associated with epidermal invasion through the tympanic membrane with the subsequent development of choles-teatoma. In chinchillas, destruction of the epithelium of both middle ear and lateral tympanic membrane surfaces takes place in the early stages of such an artificial acute otitis media, induced by insertion of propylene glycol into the middle ear. This is followed by re-epithelialisation with hyperplastic epidermal cells and then penetration of the thickened fibrous layer of the tympanic membrane by the epidermal cells to reach the middle ear cavity and the formation of keratinous masses in the middle ear typical of cholesteatoma [60, 127].
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