Identification of lymphatic vessels in skull periosteum but not bone marrow reveals skull channel heterogeneity

We identified lymphatic vessels at the skull periosteum and found no evidence of lymphatic vessels in the cortical bones or skull bone marrow, where the lymphatic marker VEGFR3 labels blood vessels. Skull periosteum channels to the upper skin are found to occur more frequently in parietal bone than interparietal bone, whereas bone marrow is found more often in interparietal bone than parietal bone. Despite skull bone marrow expansion during aging, skull channels are significantly reduced, suggesting of aging-dependent uncoupling between skull channels and bone marrow. Together, our findings show lymphatic vessels are present in the skull periosteum but absent in bone marrow, with channel and bone marrow heterogeneity varying by skull region and age.