Main Stages of Human Embryonic Development and the Formation and Role of the Placenta - Coursework Example

Main stages of human embryonic development and the formation and role of the placenta Human embryonic development undergoes a few general stages. The first stage is fertilization, followed by implantation in the uterus, and embryonic and placental development. After ovulation, the egg or ovum is swept by fimbria from the peritoneal cavity of the ovary into the fallopian tube. The ovum is viable for 18-24 hours after ovulation. In the fallopian tube, the ovum remains in the ampulla where it awaits fertilization. A sperm comes into the fallopian tube, makes contact, and penetrates the ovum’s cumulus oophorus mass of cells through chemical and mechanical means. The sperm also binds the glycoprotein membrane of the ovum or the zona pellucida. This binding is necessary for fertilization. Upon binding, the sperm penetrates the zona pellucida, upon which enzymes are released that activate glycoproteins in the zona pellucida to form cross-linkages. The cross-linking hardens the matrix, and makes the zona pellucida impenetrable to other sperm. In the egg’s cytoplasm, the oocyte undergoes meiotic division to release a polar body and a haploid ovum. The sperm nucleus fuses with the haploid ovum re-establishing the diploid chromosome number and the progression of mitotic cell division. Cell differentiation and uterine implantation follow fertilization (reviewed in (Norwitz, Schust, and Fisher, 2001). As it passes through the fallopian tubes, the fertilized ovum or zygote undergoes cell division without increase in size to form a compact mass of 12-16 cells called the morula. The passage towards the uterus takes from 48 to 72 hours. At this time, the zygote is still encased in the zona pellucida. After entering the uterine cavity, the morula transforms into a blastocyst. This transition is characterized by the appearance of a fluid-filled inner cavity and further cellular differentiation. Surface cells become the trophoblasts, where the extraembryonic structures like the placenta arise. The inner cells of the blastocyst become the embryo. Seventy-two hours after entry into the uterine cavity, the embryo emerges from the zona pellucida, exposing multinucleate syncytial trophoblasts or the syncytiotrophoblast. The chorion develops from the syncytiotrophoblast and it surrounds the embryo and other extraembryonic membranes. The chorion is made up of two layers: the outer layer formed by the trophoblast, and an inner layer formed from the mesoderm. The amnion is in contact with the inner layer of the chorion. Within the amnion is the embryo, and the amniotic sac. External to the amniotic sac are connections to allantois, and the placenta. The allantois is involved in gas exchange between maternal tissues and the fetus, and the disposal of liquid wastes. The roles of the placenta will be discussed in more detail later in this essay. Chorionic villi develop after rapid proliferation of the chorion. Chorionic villi are initially small and non-vascular, but these later increase in size, and are vascularised when the maternal mesoderm grows into them. The mesoderm carries some branches of umbilical arteries, and it is through the mesoderm that blood is transported to the embryo. Implantation of the embryo occurs 6 or 7 days after fertilization. In humans, implantation occurs in three stages. Initial adhesion (apposition) of blastocysts to the uterine wall is unstable, and is stabilized when microvilli on the surface of syncytiotrophoblasts interact with the microprotrusions found on the surface of the uterine epithelium. Stable adhesion, the next stage, occurs with increased interaction between blastocyst and uterine epithelium. Invasion is the last stage; this is characterized by the penetration of the syncytiotrophoblasts into the uterine epithelium. At the final stage, the blastocyst is oriented with its embryonic pole toward the uterine epithelium. Ten days after fertilization, the blastocyst should be embedded completely in the stromal tissue of the uterus, the uterine epithelium has covered the site of implantation, and mononuclear cytotrophoblasts are streaming out of the trophoblast layer. Cytotrophoblasts are cells that can be found in the inner layer of the trophoblast. Eventually, the cytotrophoblasts invade the maternal tissues’ endometrium, myometrium, and uterine vasculature. The invasion of uterine vasculature establishes uteroplacental circulation, and contact with maternal blood. The placenta starts to develop after the implantation of the conceptus (embryonic and extraembryonic structures) (reviewed in (Jauniaux, Poston, and Burton, 2006). The basic structure of the early placenta is the chorionic villi, which form at 4-5 weeks after last menstruation. The chorionic villi surround the gestational sac until the 9th week of gestation. Before the full development of the placenta, there is no arterial supply to the placenta. It is believed that the growing placenta and embryo obtain their nutrients from carbohydrate- and lipid- rich secretions of the endometrial glands from the first trimester to the start of the second trimester (Burton, Jauniaux, and Charnock-Jones, 2007). The endometrial glands also secrete an array of growth factors that may regulate the morphogenesis of the placenta. Receptors for these hormones can be found on villous, villous endothelial cells, and extravillous trophoblast. Components of the secretions may also be involved in modulating immune responses. From the 3rd to the 4th month of gestation, the villi at the site of implantation elaborates to form the placenta, while at the opposite pole, the villi degenerate to form the placental membranes. During placenta development, the spiral arteries in the embryo also lose their smooth muscle and elastic lamina, affecting their response to certain circulating compounds in the blood. Extravillous trophoblast cells also form a continuous shell, which not only anchor the placenta to the maternal tissue, but also form plugs at tips of the placental arteries. The plugs filter maternal blood allowing only plasma to enter the intervillous space. This occurs for at least a trimester, after which the plugs are dislocated to allow maternal blood to flow freely to the foetal tissues. Uterine relaxation during pregnancy is facilitated by the release of substances from the chorion (Carvajal, Buhimsch, Thompson, Aguan, and Weiner, 2001). In the development of the fetus, the placenta has the most significant functions (Maccani and Marsit, 2009). It provides the fetus with nutrients. Placental supply of nutrients to the fetus is a major determinant of growth. Nutrient supply occurs through diffusion and transporter-mediated transport (Sibley, et al., 2004), which is dependent on blood flow, transporter abundance, size and morphology of the placenta (Fowden, Ward, Wooding, Forhead, and Constancia, 2006). Thus foetal body weight is positively correlated with placental weight, especially during late gestation. The placenta also allows the transfer of waste materials, which are secreted maternally. Furthermore, the placenta protects the fetus from autoimmune attacks by immune system of the mother. The placenta also shows endocrine and metabolic activities because it is involved in the secretion of hormones that are responsible for the maintenance and regulation of the different stages of pregnancy. Studies have shown that some toxic compounds from the maternal blood are able to pass the placental barrier. Some of these compounds are nicotine (Bush, Mayhew, Abramovich, Aggett, Burke, and Page, 2000), cocaine (Pastrakuljic, Derewlany, and Koren, 1999), and lead (Rastogi, Nandlike, and Fenster, 2007). Whenever possible, the placenta regulates and protects the fetus from being exposed to toxic and harmful compounds (Sood, Zehnder, Druzin, and Brown, 2006). After implantation, the embryo develops rapidly into the fetus. Twenty days after fertilization, the foundations for the major systems are established. On the third week of gestation, the heart starts to beat, and at the 8th week, the fetus, still an inch long is fully complete with the organs and system that are present in an adult. Full-term babies are ready to be delivered after 38 weeks gestation period. References Burton, G., Jauniaux, E., and Charnock-Jones, D. (2007) Human early placental development: potential roles of the endometrial glands. Placenta, 28, pp. S64-S69. Bush, P., Mayhew, T., Abramovich, D., Aggett, P., Burke, M., and Page, K. (2000). Maternal cigarette smoking and oxygen diffusion across the placenta. Placenta, 21, pp.824–833. Carvajal, J., Buhimsch, I., Thompson, L., Aguan, K., and Weiner, K. (2001) Chorion releases a factor that inhibits oxytocin-stimulated myometrial contractilitiy in the pregnant guinea pig. Human Reproduction, 16(4), pp.638-643. Fowden, A., Ward, J., Wooding, F., Forhead, A., and Constancia, M. (2006) Programming placental nutrient transport capacity. Journal of Physiology, 572, pp. 5-15. Jauniaux, E., Poston, L., and Burton, G. (2006) Placental-related diseases of pregnancy: involvement of oxidative stress and implications in human evolution. Human Reproduction Update, 12(6), pp.747-755. Maccani, M., and Marsit, C. (2009). Epigenetics in the placenta. American Journal of Reproductive Immunology, 62(2), p.78 Norwitz, E., Schust, D., and Fisher, S. (2001). Implantation and the survival of early pregnancy. The New England Journal of Medicine, 345(19), pp.1400-1408. Pastrakuljic, A., Derewlany, L., and Koren, G. (1999). Maternal cocaine use and cigarette smoking in pregnancy in relation to amino acid transport and fetal growth. Placenta, 20, pp.499–512. Rastogi, S., Nandlike, K., and Fenster, W. (2007). Elevated blood lead levels in pregnant women: identification of a high-risk population and interventions. Journal of Perinatal Medicine, 35, pp. 492–496. Sibley, C., Coan, P., Ferguson-Smith, A., Dean, W., Hughes, J., Smith, P., et al. (2004). Placental-specific insulin-like growth factor 2 (Igf2) regulates the diffusional characteristics of the mouse placenta. Proceedings of the National Academiy of Sciences USA, 101, pp. 8204-8208. Sood, R., Zehnder, J., Druzin, M., and Brown, P. (2006). Gene expression patterns in human placenta. Proceedings of the National Academy of Sciences USA, 103, pp. 5478-5483. Read More