STEVE GSCHMEISSNER / SCIENCE PHOTO LIBRARY STEVE GSCHMEISSNER / SCIENCE PHOTO LIBRARY
Placental villi. Coloured scanning electron micrograph (SEM) of a freeze fracture through a placenta showing projections (centre) from the interior of the placenta known as chorionic villi. The villi contain foetal blood vessels and are bathed in maternal blood. A large blood vessel (oval) is at upper right. The placenta surrounds the foetus in the womb. It provides the foetus with nourishment, eliminates waste products and exchanges respiratory gases. Villi provide a large surface area for these exchanges.
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