The nuclear lamina consists of a proteinaceous layer or meshwork situated 
subjacent to the inner nuclear membrane. It is a karyoskeletal structure formed 
by a polymer containing one to three major polypeptides collectively termed the 
lamins. In all cells examined of vertebrates and invertebrates, the lamins 
exhibit very similar Mr ranging from 60 000 to 80 000. In vertebrates, two 
groups of lamins can be distinguished by their isoelectric value, one being 
near-neutral and the other acidic (isoelectric pH values of 5.6 and lower). The 
lamins represent a family of polypeptides with regions highly conserved during 
evolution. In certain species, e.g., the amphibian, Xenopus laevis, they exhibit 
cell type-specific expression during embryonic development, terminal 
differentiation of certain somatic cells, and gametogenesis. The nuclear lamina 
of diverse cell types can be composed of one, two or three different lamin 
polypeptides, without obvious differences in its morphology.