Märghåligheter

Märghåligheter, or medullary cavities, are the internal spaces within bones that house bone marrow. They run primarily through the diaphyses of long bones and are present in some flat bones as well. The cavities are lined by the endosteum and are surrounded by cortical bone, connected to other marrow sites via small vascular channels.

There are two main types of marrow that occupy these cavities: red marrow and yellow marrow. Red marrow is hematopoietic, producing red blood cells, white blood cells, and platelets. Yellow marrow is largely adipose tissue and serves as an energy reservoir. In children, red marrow is more widespread; with age, some red marrow is gradually replaced by yellow marrow, especially in the medullary cavities of long bones.

Distribution and development: At birth, most marrow is red and distributed throughout the skeleton. As growth continues, red marrow is gradually replaced by yellow marrow in many long bones. In adults, red marrow persists mainly in the axial skeleton—vertebrae, sternum, ribs, pelvis, and skull—as well as in the proximal ends of the femur and humerus. The medullary cavity functions as a hematopoietic niche and contributes to mineral homeostasis within bone tissue.

Clinical relevance: The medullary cavities are central to several medical concepts. They can be sites of hematologic diseases such as leukemia and marrow metastases. They are the target for bone marrow biopsy and aspiration and may participate in extramedullary hematopoiesis when marrow function is impaired. Imaging modalities such as MRI and CT can assess marrow composition and cavity size, providing diagnostic information in various conditions.