Nuclear pockets and clefts in the lymphoid cell population of bone marrow and blood of children with acute lymphoblastic leukemia.

Ultrastructural investigation of the nuclei of the lymphoid cell population of bone marrow and blood of children with acute lymphoblastic leukemia regularly shows the presence of two types of nuclear pockets and nuclear clefts. The incidences of these nuclear features decrease significantly during cytostatic therapy. The pockets consist of either a cytoplasmic segment enclosed by a nuclear heterochromatin bridge or a nuclear segment enclosed by an intranuclear cleft. One type of nuclear cleft is, for its greater part of whole length, situated just under the nuclear surface. The other type of cleft is oriented more or less perpendicular to the nuclear surface. It is proposed that these four types be designated as nuclear pockets and nuclear clefts "Type 1" and "Type 2."