  We present a new acquisition method that enables high-resolution, fine-detail
full reconstruction of the three-dimensional movement and structure of
individual human sperm cells swimming freely. We achieve both retrieval of the
three-dimensional refractive-index profile of the sperm head, revealing its
fine internal organelles and time-varying orientation, and the detailed
four-dimensional localization of the thin, highly-dynamic flagellum of the
sperm cell. Live human sperm cells were acquired during free swim using a
high-speed off-axis holographic system that does not require any moving
elements or cell staining. The reconstruction is based solely on the natural
movement of the sperm cell and a novel set of algorithms, enabling the detailed
four-dimensional recovery. Using this refractive-index imaging approach, we
believe we have detected an area in the cell that is attributed to the
centriole. This method has great potential for both biological assays and
clinical use of intact sperm cells.
