Regional variations in proliferation rate and degree of differentiation in B16 melanomas.

The relationship between proliferation rate and degree of differentiation has been studied in different regions of transplantable B16 melanomas. The regions studied were peripheral parts of the tumours and the inner (perivascular) and outer (external) zones of so-called corded structures. The highest proliferation rates and the poorest cellular differentiation (estimated by volume densities of premelanosomes and melanosomes) occurred in the perivascular zones of corded structures. Conversely, the lowest proliferation rates and the best cellular differentiation was encountered in the external zones of corded structures. Proliferation rates and cellular differentiation in the peripheral parts of the tumours were intermediate between the values obtained in the perivascular or external regions of corded structures. The results in corded structures indicate an inverse relationship between proliferation rate and degree of differentiation, and a positive correlation between proliferative capacity and blood vessel proximity. The intermediate values in the peripheral parts of tumours may reflect the presence of a mixed population of cells which vary in their proximity to the nearest blood vessel.