Although
breast carcinomas are considered to originate from glandular epithelial cells, some exhibit 'squamoid features', comprising stratification with a gradient in the nuclear-cytoplasmic ratio within individual
cancer cell nests on microscopy. In parallel with a histological review of squamoid features, we immunohistochemically investigated the expression of
involucrin, a marker of terminal squamous differentiation, in 223
breast carcinomas with one to three regional nodal
metastases but no distant
metastases and analysed their association with other clinicopathological parameters to explore their clinical and
biological implications. Squamoid features and
involucrin expression, detected in 22% and 27% of cases respectively, correlated with each other and were associated with high-grade atypia, a solid-nest pattern,
cancer cell
necrosis on histology and negative oestrogen receptor status. The incidence of regional recurrences was higher in patients with
involucrin expression, whereas bone
metastases were less frequent in groups with squamoid features or with diffuse (> or = 10%)
involucrin expression. Both squamoid features and
involucrin expression, which were considered to be derived either from differentiation into keratinocytes or from some kind of cellular degeneration caused by high turnover rate, are suggested to influence the
biological behaviour of
breast cancer cells in vivo, and they may be effective in predicting the most likely recurrence sites.