ANDY WALKER, MIDLAND FERTILITY SERVICES / SCIENCE PHOTO LIBRARY ANDY WALKER, MIDLAND FERTILITY SERVICES / SCIENCE PHOTO LIBRARY
Light micrograph (Kohler illumination) of a human blastocyst, an early stage of embryonic development, photographed 4-5 days after fertilisation. The blastocyst consists of a hollow ball of cells with a localized thickening called the inner cell mass (the small inner circle of cells) which ultimately develops into the actual embryo. The remainder of the blastocyst (the thin part of the ball) is composed of trophoblast tissue. Analysis of trophoblast cells has provided a novel means of pre-natal screening for genetic disorders. Initially unattached, the blastocyst soon becomes implanted in the wall of the uterus.
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