Human Embryonic Development

After the sperm reaches the ovum, it deposits its genetic load, the male pronucleus, into the cytoplasm of the egg. After finally completing the meiotic division that had begun years before, the female pronucleus fuses with the male pronucleus to create the diploid nucleus of the zygote.

Human embryology is the study of this development during the first eight weeks after fertilisation. The normal period of gestation (pregnancy) is nine months or 38 weeks.

The acrosome is the part of the sperm containing proteolytic enzymes to digest the zona pellucida. It is exposed by the process of capacitation. Then, under the influence of substances released by corona radiata cells, it releases its proteolytic enzymes and penetrates the oocyte.

The developing embryo has two main populations of cells: an inner cell mass and a trophectoderm. The trophectoderm cells lie at the outer surface of the developing embryo and will become the placenta.

The external orifice of the uterus (or ostium of uterus, or external os) is a small, depressed, somewhat circular aperture on the rounded extremity of the vaginal portion of the cervix. Through this aperture, the cervical cavity communicates with that of the vagina.

The ampulla is the second portion of the fallopian tube. It is an intermediate dilated portion, which curves over the ovary. It is the most common site of mouse and human fertilisation.

The myometrium stretches during pregnancy to allow for the harboring of the pregnancy, and contracts in a coordinated fashion, via a positive feedback effect on the Ferguson reflex, during the process of labor. After delivery, the myometrium contracts to expel the placenta and reduce blood loss; where the crisscrossing fibres of middle layer compress the blood vessels.

The most common site of ectopic implantation is the uterine tube. The growth of the embryo in this site usually causes rupture of the tube and severe hemorrhage in the mother. Abdominally, an ectopic pregnancy often occurs in the rectouterine pouch, an area between the uterus and the rectum.

In the mammalian blastocyst, there are three lineages that give rise to later tissue development. The epiblast gives rise to the fetus itself while the trophoblast develops into part of the placenta and the primitive endoderm becomes the yolk sac.

Around the 9th day, cells from the hypoblast spread around the blastocoele, forming the exocoelomic or Heuser's membrane. When the blastocoele is surrounded by this membrane, it is referred to as the primary yolk sac or exocoelomic cavity.

The trophoblast gives rise to both the syncytiotrophoblast and the cytotrophoblast, as well as the extraembryonic mesoderm. The ectoderm is a derivative of the epiblast, and the secondary yolk sac comes from endoderm cells that line the exocoelomic cavity or the primitive yolk sac.

The cardiogenic region lies cranial to the prochordal plate and its descendant, the oropharyngeal membrane. The heart tubes form on the endoderm side of intraembryonic coelom in the cardiogenic region. Following the head and lateral body folding, heart and its portion of intraembryonic coelom called pericardial cavity lie in the chest.

Chorion frondosum and the decidua basalis make up the placenta. Chorion laeve, or smooth chorion, is covered by decidua capsularis. As the fetus and chorion enlarges, the chorion laeve pushes against the decidua parietalis and the capsularis disappears.

The function of the round ligament is maintenance of the anteversion of the uterus (a position where the fundus of the uterus leans ventrally) during pregnancy. Normally, the cardinal ligament is what supports the uterine angle (angle of anteversion). When the uterus grows during pregnancy, the round ligaments can stretch causing pain.