Ever wondered where blood (hematopoietic) cells come from? Hematopoietic cell origin has been something of a debate for a long time, mainly because blood cells can migrate so fast (a heartbeat pumps blood cells through the body at an outstanding speed - up to 2 meters per second, or about 4 miles an hour).
A recent article in Nature (Nature 457: 869-900) described how continuous single-cell imaging, over a period of days, showed that blood cells can be generated directly from the haemogenic endothelium (a transient region of the endothelium). The cells were tagged with endothelial and hematopoietic markers and then imaged. The video, below, shows how individual cells shed their endothelial nature and acquire hematopoietic qualities.
Some of the details of the experiment: the group used an optimized wide-field epifluorescence microscope and acquired the images using cell tracking software they wrote.
The video shows cells with clear endothelial morphology shown giving rise to blood cells. Left panel: phase contrast. Right panel: fluorescence detecting Histone 2B-Venues expression. 1st pause in video: single mesodermal cells starting the colony. 2nd pause: all cells in the colony exhibit a clear endothelial sheet morphology. 3rd pause: some cells lose tight integration into the endothelial sheet but keep adhering to endothelial cells. 4th pause: Blood cells detach and are free-floating. The arrow is following the sarting cell and one daughter cell after each cell division until the end of the video. Timescale: days -- hours:minutes:seconds (caption from Eilken et al. (2009) Nature 457: 869-900 paper).