Three-dimensional reconstruction from serial sections ($V=\int A(z)dz$)

Aligned two-dimensional sections constrain the three-dimensional structure of a specimen. For parallel sections of known spacing $\Delta z$, the Riemann sum ${\sum }_i{A}_i\Delta z$ approximates volume. Distortion, missing sections and alignment errors constrain the reconstruction; the volume estimate alone does not determine the identity or developmental fate of each visible structure.