Cholesteatoma is the presence of stratified squamous epithelium in considerable quantities in the middle ear. The common acquired form of cholesteatoma is associated with severe otitis media.
Stratified squamous epithelium in the normal foetal middle ear: small colonies of cells being epidermoid in nature as confirmed by immunohistochemistry are found near the tympanic membrane on the lateral anterior superior surface of the middle ear in every temporal bone after 15 weeks' gestation. These epidermal colonies, which are known as "epidermoid formations", increase significantly in size with increasing age and pari passu undergo increasing epidermoid differentiation . During the first postpartum year these epider-moid formations disappear. It is possible that the entry and growth of epidermoid formations in the foetal middle ear may lead to a local cellular immunity as a defence mechanism against the entry of keratinocytes into the middle ear. This could cause the eventual dissolution of all epidermoid formations. If immunity is delayed or defective epidermis could continue to grow and lead to congenital cholesteatoma.
Stratified squamous epithelium in the middle ear of a young child (congenital cholesteatoma): congenital cho-lesteatoma is seen, in most cases as a spherical whitish object in the anterosuperior part of the tympanic cavity behind an intact tympanic membrane (Fig. 8.8). In some cases the lesion may fill most of the tympanic cavity. At operation the cholesteatoma is reported usually to be a cyst in the anterosuperior part of the middle ear. Bone erosion is not present when the cholesteatoma is small. In larger lesions some degree of this change is present  and eventually it may enlarge to involve the mastoid, cause a perforation of the tympanic membrane and even grow into the middle cranial fossa  so that it becomes indistinguishable from acquired cholesteato-ma (see below). Indeed, it is possible that many cases of acquired cholesteatoma originated from congenital cho-lesteatoma.
In approximately 10% of cases the cholesteatoma is not cystic, but open and shows layers of squames and a matrix (living basal and malpighian layers of epidermis) that is plastered to the wall of the tympanic cavity . The microscopic appearances of the matrix of congenital cholesteatoma are those of skin epidermis, comprising a single row of basal cells, several rows of malpighian cells and a thin granular layer. The surface of dead, kera-tinous squames merges with the keratinous contents of the cyst, or lamellae in the case of the open type. When the histological appearance of these cases is compared with that of acquired cholesteatoma, little difference can be seen.
Stratified squamous epithelium in the middle ear of an older child or adult, acquired cholesteatoma: typically in acquired cholesteatoma a lesion far more common than that of congenital cholesteatoma, the patient presents with a foul-smelling aural discharge and conductive hearing loss. On examination of the tympanic membrane there is, in most cases, a perforation of the superior or posterosuperior margin. The cholesteatoma appears as a pearly grey structure in the middle ear cavity. The wall of the cyst may often be seen as a thin membrane.
The cholesteatoma is usually situated in the upper posterior part of the middle ear cleft and discharges usually through a perforation of the pars flaccida of the tympanic membrane, sometimes through a perforation located at the edge of the tympanic membrane near the annulus. The cholesteatoma may extend through the aditus into the mastoid antrum and mas-toid air cells. Frequently, the outline of the cholestea-tomatous sac is adapted to that of normal structures such as ossicles. Chronic inflammatory changes are always present. In most cases at least one ossicle is seriously damaged, thus interrupting the continuity of the ossicular chain. The scutum, the upper part of the bony ring of the tympanic opening, is eroded in most cho-lesteatomas.
Under the microscope acquired cholesteatoma is usually "open" rather than "closed" or cystic. The pearly
material of the cholesteatoma consists of dead, fully differentiated anucleate keratin squames. This is the corneal layer of the squamous cell epithelium. As in any normal stratified epithelium there are one to three basal layers of cells above which is a prickle (malpighian or spi-nous) layer composed of five or six rows of cells with intercellular bridges (Fig. 8.9).
The deeper layers of the epithelium of the cholestea-toma matrix frequently show evidence of activity in the form of downgrowths into the underlying connective tissue (Fig. 8.10). Such excessive activity has been confirmed by:
1. The strong expression of cytokeratin 16, a marker for hyperproliferative keratinocytes, by cholesteatoma, but its absence in middle ear and external ear epithelium, except in the annulus region of the external tympanic membrane epithelium ;
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