Myeloid cells and lymphangiogenesis.

The lymphatic vascular system and the hematopoietic system are intimately connected in ontogeny and in physiology. During embryonic development, mammalian species derive a first lymphatic vascular plexus from the previously formed anterior cardinal vein, whereas birds and amphibians have a lymphatic vascular system of dual origin, composed of lymphatic endothelial cells (LECs) of venous origin combined with LECs derived from mesenchymal lymphangioblasts. The contribution of hematopoietic cells as building blocks of nascent lymphatic structures in mammals is still under debate. In contrast, the importance of myeloid cells to direct lymphatic vessel growth and function postnatally has been experimentally shown. For example, myeloid cells communicate with LECs via paracrine factors or cell-cell contacts, and they also can acquire lymphatic endothelial morphology and marker gene expression, a process reminiscent of developmental vasculogenesis. Here, we present an overview of the current understanding of how lymphatic vessels and the hematopoietic system, in particular myeloid cells, interact during embryonic development, in normal organ physiology, and in disease.