Very high power TEM of an early stage 1b embryo in vitro, 3 hours post-insemination showing the developing paternal pronucleus (original magnification x33,810). The sperm chromatin has decondensed in the cytoplasm and a pronuclear envelope is assembled around it by the flattening of vesicular smooth endoplasmic reticulum (S). The pronuclear envelope has been constituted only in certain regions (arrowheads). The vesicular smooth endoplasmic reticulum aligns close to the surface of the chromatin where the envelope is absent. The basal region of the sperm nucleus is still evident and is directed toward the cell membrane. Dense mitochondria (M) with few cristae are also associated with the sperm chromatin. A small region of the fertilization cone is evident where microfilaments (Mf) are seen in cross section.
SS = subzonal space
From: Lopata et al., 1980. Reproduced with permission of the American Society for Reproductive Medicine.
Keywords: basal region of the sperm nucleus , cristae, cytoplasm, dense mitochondria, fertilization cone, microfilament(s), paternal pronucleus, pronuclear envelope, pronucleus, sperm chromatin, vesicular smooth endoplasmic reticulum
Source: The Virtual Human Embryo.