The development of vertebral bone marrow of human fetuses

The development of the bone marrow of the thoracic vertebrae in seven human fetuses ranging from 95 to 150 mm in crown-rump length (CRL) was studied using light and electron microscopy. In the 95-mm CRL, hypertrophy of the chondrocytes occurred in the central region of the vertebrae, and blood vessels penetrated there from dorsal and ventral sides of the vertebral body. The primary marrow was represented by liberated cartilage lacunnae, occupied by the thin-walled blood vessels and a few mesenchymal cells and mononuclear cells containing granules or vacuoles (GMC). In the 99-mm CRL, chondroclasts were active in removing the cartilage near the central region of the vertebrae. Consequently, a large cavity was formed and occupied by a dilated sinus. GMC were numerous. Osteoblasts and osteocytes were increased in number. Reticular cells with long processes containing large amounts of glycogen began to appear in the extravascular space and formed the loosely arranged cellular meshwork of the hematopoietic compartment. Bundles of collagen fibrils were scattered in the meshwork. Hematopoietic cells were recognizable only in the 105-mm-CRL fetus and increased in number in the 120-mm-CRL fetus. The sinus endothelium was very thin and continuous without apertures except where blood cells crossed the wall. The developing blood cells lying against the outside of the sinus endothelium indented it. At points, collagen fibrils attached to the outside of endothelial cells and appeared to function as the anchoring filaments of lymphatics. The physiologic implications of the association of stromal cells, vascular sinuses, and hematopoietic cells are discussed in relationship to the microhematopoietic environment of the bone marrow.