Haematopoiesis and a new mechanism for the release of mature blood cells from the bone marrow into the circulation in snakes (Ophidia)

This is the first description of haematopoiesis in snakes. Studies were carried out on the following species belonging to Ophidia: Bothrops jararaca, Bothrops jararacusu, Waglerophis merremii, Elaphe taeniura taeniura, Boa constrictor,and Python reticulatus. Smears of the peripheral blood and histological preparations from the vertebrae, ribs, liver, and spleen were studied under a light and electron microscope. Myeloid cells were present in the following locations in the vertebrae: the neural spine, zygoapophysial processes, floor of the neural canal, lacunae in the bodies of vertebrae and also inside the ribs. Although the vascular system was well developed, especially around the ribs, vessels inside the marrow cavities were scarce, both in the ribs and elsewhere where haematopoiesis was found. Venous sinuses were well developed in the vertebrae and in the rib regions from their costal head towards the middle area. They consisted of one layer of fine endothelial cells. Mature cells in the process of migration into the general circulation were only sporadically encountered when venous sinuses were studied on perfusion-fixed specimens. In contrast, almost every sinus venosus contained protrusions directed towards the lumen, filled mostly with mature and immature blood cells. Various stages of their formation were seen in the cross sections of venous sinuses ranging from small, newly formed to large, elongated ones, filled with many fully developed and some maturing blood cells. In many cases the apices of the protrusions were ruptured, and mature blood cells, as well as a few immature ones, were seen in their vicinity. This observation led us to a new hypothesis that blood cells are released from the extravascular space into the lumen of venous sinuses. In snakes, these cells are released into the systemic circulation mainly via the rupture of protrusions filled with mature blood cells and, to a lesser degree, by transcytosis as known in mammals. In the spleens from young specimens, 1-2 foci of haematopoiesis were encountered where lymphopoiesis predominated. Haematopoiesis was not detected in the liver.