Diploë

Diploë is the cancellous, or spongy, bone layer of the skull located between the outer and inner compact bone tables of the cranial bones. This middle layer forms the bulk of the skull’s diploic structure and contains trabeculated bone tissue, marrow spaces, and a network of vessels.

Anatomically, the diploë lies between the outer table (the external cortical layer) and the inner table (the internal cortical layer) of the cranial bones. It houses the diploic veins and arteries, which drain blood from the skull bones into the dural venous sinuses and emissary veins. The thickness and extent of the diploë vary by bone and by age, being more prominent in certain flat bones such as the frontal and parietal bones.

In development, the skull bones largely form through intramembranous ossification, with the diploë forming as a cancellous middle layer between the opposing compact plates. In children the diploë contains more red marrow, which gradually changes to yellow (fat) marrow with age, though red marrow can persist in portions of the skull.

Functionally, the diploë helps reduce the overall weight of the skull while providing structural support. Its vascular channels contribute to intracranial venous drainage, and its marrow spaces can participate in hematopoiesis, particularly in younger individuals.

Clinical relevance includes injury and infection: fractures that traverse the diploë can disrupt the diploic veins, and infections can spread through these vessels to involve the skull bones or the dural venous system. Radiologically, the diploë appears as the middle, cancellous layer between the outer and inner tables on imaging studies.