Spermatozoa in the uterus. False-colour scanning electron micrograph of a group of spermatozoa (yellow) floating over the internal wall of the uterus. Movements of sperm through the ciliated epithelium (green) cause capacitation. During this phase, which lasts about seven hours, a glycoprotein coat and seminal plasma proteins are removed from the heads of spermatozoa. This enables a spermatozoon, when close to the egg, to release special enzymes which allows it to penetrate the surrounding membranes of the egg. Magnification: x2595 at 6x7cm size. Magnification: x4080 at 4x5 inch size.