From fertilization to implantation.

The process of early human embryo development involves a complex series of remarkable events. Prior to the union of the human gametes, the oocyte first requires timed completion of meiosis. This vital step does not occur throughout a womans life until the one day event of ovulation and subsequent fertilization. Once the sperm enters the egg, its DNA associated proteins are replaced by oocyte histones. The two pronuclei become enveloped with oocyte derived membranes, which fuse and begin the zygotes mitotic cell cycle. Cellular division follows a predictable 12-18 hour cycle resulting in two to sixteen cell preembryos over the first several days. The sperm centrosome controls the first mitotic divisions until day four when genomic activation occurs within the morula stage. The individual blastomeres are totipotent until the morula begins compaction when cells initiate polarization. The outer cells differentiate towards a placental lineage the trophoectoderm, while the inner blastomeres become the inner cell mass or eventually the fetus. The blastocyst forms approximately 24 hours after the morula stage by the development of an inner fluid filled cavity, the blastocele. The blastocyst must first hatch from the thinning zona pellucida by alternating expansion and contraction. Implantation of the hatched blastocyst requires several steps, including apposition, attachment, penetration and trophoblast invasion. The panel members that follow will discuss each of these peri implantation steps in detail.