(a) "Mucosal" reaction pattern of CHC in a skin flap. The epidermis (E) shows absence of orthokeratinisation, which is difficult to observe at this magnification, variably desquamating parakeratinising layers (area in rectangle) and irregular hyperplasia (acanthosis). Nodular/perivascular chronic inflammation and fibrosis expand the reticular dermis (asterisk). Subcutaneous fat and sweat glands (S) are seen, which allows confident identification of the flap. The area in rectangle is magnified in Fig. 4b. (b) The absence of orthokeratinisation and desquamating parakeratinised layers are better appreciated here. (c) Adjacent section stained with PAS shows individual hyphae (arrowheads) between desquamating keratinocytes.

(a) "Mucosal" reaction pattern of CHC in a skin flap. Here, the loss of orthokeratinisation and overall appearances of epidermis (E) and papillary dermis could be described as "psoriasiform mucosalisation". Deep dermal and subcutaneous features as in Fig. 4a. (b) The "psoriasiform features" are better appreciated on higher magnification. Note the tube-like epidermal rete, attenuated suprapapillary plates (arrowhead) and elongated dermal papillae that contain hyperaemic capillary-sized vessels (arrow). The area in rectangle corresponds to that shown in Fig. 5c. (c) This is higher magnification and from an adjacent section stained with PAS. The arrowheads indicate individual hyphae infiltrating parakeratinised layers. The densely stained elements in the lower part of the photomicrograph are emigrating neutrophils.

(a) Part of an intra-orally transferred skin flap includes two "widely separate"' groups of "shrunk" pilosebaceous units. The randomly arranged collagen bundles (asterisk) of the reticular dermis are seen between the groups. Sweat glands are not present here. (b) The pilosebaceous group in the right of Fig. 2a comprises sebaceous glands (arrowhead), arrector pili muscle (P) and hair follicles (arrows) cross-sectioned through their upper (left) or lower (right) segments. Despite the "wide separation" and "shrunk" appearance, the preservation of lower segment suggests that the follicles have not entered the involutionary catagenous stage that would be expected during "mucosalisation".

(a) "Cutaneous" reaction pattern of CHC in a skin flap. Thick orthokeratotic plaque (asterisk) covers epidermis (E) of increased thickness. Bacterial colonies (arrowhead) are seen on the surface of the plaque. Although the plaque is largely compact, superficial (vertical arrows) and deep (horizontal arrow) areas of variably decreased/lost intercellular cohesion (dyscohesion) are seen. While the superficial dyscohesion leads to desquamation of individual cornified cells, lateral expansion/"burrowing" of deep dyscohesion could eventually effect the loss of the overlying non-dyscohesive cornified heap. The processes would result in formation of surface ditches. (b) Even without special staining, candidal hyphae deeply "entombed" within the plaque, are discernible. Note the nuclear "ghosts" of the polygonal cornified keratinocytes. (c) The septate and grouped hyphae (arrowheads) are highlighted on PAS-staining. Compare the unstained nuclear ghosts (arrow) with the viable haematoxyphilic nuclei in the right lower corner of the photomicrograph.

(a) Scanning view of a "proliferative" carcinoma in-situ arising in an intra-orally transferred skin flap. The continuous linear segment indicates the surface extent of the lesion. The interrupted linear segment indicates the junction between the flap and native mucosa. Note that the carcinoma in-situ does not extend to the mucosa. The epidermal area between the arrows is not affected by in-situ carcinoma. This is attributable to irregular margins of a single lesion, being not included in the plane of the section, rather than separate lesions. The areas in rectangles labelled as b-d are magnified in Figs 6b-d respectively, so that particular features are appreciated. PA, panniculus adiposus; asterisk, submucosal minor salivary gland. (b) The linear segment indicates the junction between carcinoma in-situ and epidermis of the flap. While the surface of the carcinoma in-situ (right part of the photomicrograph) is gently mammillated and non-keratinised, the epidermis is mildly orthokeratotic (arrowhead) and shows changes similar to those in Fig. 1h. (c) The linear segment indicates the junction between carcinoma in-situ and native, parakeratinised oral mucosa. The arrow indicates artefactual crease of the section. (d) The affected epidermis shows absence of keratinisation, full-thickness severe dysplasia and elongated, variously bulbous or fused rete. Although the lower epidermal border is asymmetrical and at different levels, it is smooth and frank invasion is not seen. The increased epidermal thickness accounts for the characterisation of the carcinoma in-situ as "proliferative". Dermal inflammation is inconspicuous. The structure indicated by the arrowhead is magnified in Fig. 6e. (e) Carcinoma in-situ spreads along the lining of an eccrine sweat duct ("eccrinotropism"). The paler, polymorphic and variably dyscohesive malignant keratinocytes, one of which undergoes mitosis (arrow), are easily distinguished from the more compact and densely stained normal ductal lining (arrowhead). The lumen and adluminal cuticle of the duct can be seen.

(a) Scanning view of a skin flap (asterisk) which has been transferred adjacent to lingual foliate papillae (FP). The panniculus adiposus (subcutaneous fat, PA), submucosal skeletal muscle (M) and the mammillated surface of the papillae are easily seen. The linear segment indicates the junction between the flap and native mucosa. The areas in rectangles labelled as b-i are magnified in Figs 1b-i respectively, so that particular features are appreciated. (b) The linear segment indicates the junction between epidermis (left part of the photomicrograph) and oral epithelium (right part of the photomicrograph). The stratum granulosum (arrowheads) of the epidermis is discernible. The lower border of both epidermis and oral epithelium is irregular. This indicates hyperplasia of rete in response to the presence of non-specific chronic inflammation (asterisk) in underlying stroma. (c) Cross-sectioned profile of a crypt (C) of the lingual tonsil, which is surrounded by hyperplastic lymphoid tissue with germinal centres (arrows). Emigrant lymphoid cells and debris are present in the crypt lumen. (d) Cross-sectioned profile of a metaplastic collecting duct of the posterior superficial lingual salivary gland (Weber's). Lymphoid tissue with three germinal centres is present at the left of the photomicrograph. It is difficult to distinguish the surface oral epithelium at this magnification. (e) Two lobules of the posterior superficial lingual salivary gland, which are variably affected by parenchymal atrophy and non-specific chronic inflammation. Inflammation (asterisk) is prominent in the upper lobule, whereas mucous acini and tubules are preserved in the lower. The arrows indicate ectatic atrophic duct-like structures. (f) Randomly mixed fat (F), skeletal muscle (M), nerve fascicles (N) and vessels (V) characterise the submucosal soft tissues and contrast with the uniform and orderly arrangements of panniculus adiposus. (g) Two eccrine sweat-gland lobules. Paler secretory segments (S) and darker ducts (D) are distinguished. Adipocytes are mixed with the secretory segments. (h) Retiform downward growth of epidermis. The changes resemble senile (solar) lentigo or inchoate seborrhoeic keratosis. The structure indicated by the arrowhead is probably a cross-sectioned plugged infundibulum. Elsewhere, the epidermis of the flap shown in Fig. 1a is attenuated or ulcerated. (i) Area of ulceration covered by fibrinous membrane (F). There is underlying non-specific chronic inflammation (asterisk). E, preserved epidermis. Other small ulcerated areas are discernible in Fig. 1a, but these are not shown. Note: Unless otherwise specified, the photomicrographs in this article are from sections of routinely processed tissue, which were stained with haematoxylin and eosin (HE); it was not deemed necessary to give objective magnifications.