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Sperm/oocyte fusion

High power TEM of an early stage 1a embryo in vitro showing sperm/oocyte fusion, 2 hours post-insemination (original magnification x40,700). The sperm head has undergone the acrosome reaction and its inner acrosome membrane (IA) is exposed. The cell membrane is fused with the post-acrosomal segment of the sperm plasma membrane (small arrowheads). The front two-thirds of the sperm head is engulfed by two villous-like processes (large arrowheads) extending from the oocyte surface. The sperm nuclear envelope (NE) in the posterior region of the sperm nucleus has not yet decondensed. The plasma membrane of the sperm midpiece (Mp) has disappeared but mitochondria, dense fibers, and axoneme are still present. Cortical granules (CG) have released their products into the subzonal space (SS). The embryo is in early anaphase of the second maturation division.

Cy = cytoplasm

From: Sathananthan and Chen, 1986. Reproduced with permission of the publisher, John Wiley and Sons.

Keywords: acrosome reaction, anaphase of the second maturation division, axoneme, cortical granule(s), dense fiber, inner acrosome membrane, mitochondria, nuclear membrane, nucleus, plasma membrane, post-acrosomal segment, sperm / oocyte fusion, sperm head, sperm midpiece, sperm nucleus, stage 1a embryo, subzonal space

Source: The Virtual Human Embryo.