Development Of The Human Zygote Essay -- essays research papers fc

Development of the Human Zygote     Hundreds of thousands of times a year a single-celled zygote, smallerthan a grain of sand, transforms into an amazingly complex meshing of cells, anewborn infant. Through cellular specialisation and growth, this process iscompleted with precision time and time again, but very rarely a luxate in the"blueprint" of growth and development does occur. Following is a description ofhow the pathways of this intricate web are followed and the mistakes whichhappen when they are not.     The impressive process of differentiation changes a single-cell into acomplicated system of cells as distinct as bold and bone. Although embryonicdevelopment takes approximately nine months, the greatest amount of cellulardifferentiation takes place during the first eight weeks of pregnancy. Thisperiod is called embryogenesis.     During the first week after fertilization, which takes place in theFallopia n tube, the embryo starts to cleave once every twenty-four hours (Fig.1). Until the eight or sixteen cell stage, the individual cells, or blastomeres,are thought to have the potential to form any part of the fetus (Leese, Conaghan,Martin, and Hardy, April 1993). As the blastomeres continue to divide, a solidball of cells develops to form the morula (Fig. 1). The accumulation of fluidinside the morula, transforms it into a hollow sphere called a blastula, whichimplants itself into the inward lining of the uterus, the endometrium (Fig. 1).The inner mass of the blastula will produce the embryo, while the outer layer ofcells will form the trophoblast, which eventually will provide nourishment tothe ovum (Pritchard, MacDonald, and Gant, 1985). bode 1Implantation process and development duringembryogenesis (Pritchard, MacDonald andGant, 1985)     During the second week of development, gastrulation, the process bywhich the germ layers are formed, begins to occur. The i nner cell mass, nowcalled the embryonic disc, differentiates into a thick plate of ectoderm and anunderlying layer of endoderm. This cellular multiplication in the embryonicdisc marks the beginning of a thickening in the midline that is called theprimitive streak. Cells spread out laterally from the primiti... ...e anormal infant. When whatsoeverthing does go wrong, the embryo or fetus willunfortunately have some type of defect. The amazing accuracy with which asingle cell can become something as complex as a newborn infant is a truleyincredible featWorks CitedBaker, David A. "Danger of Varicella-Zoster Virus Infection." ContemporaryOB/GYN April 1990 52.Carlson, Bruce M. Pattens Foundations of Embryology. McGraw-Hill Inc. 1981.Cunningham, MacDonald, and Gant. Williams Obstetrics, Supplement no. 10. 18thed, Prentice-Hall, Inc. Februay/March 1991 2,3."Folic Acid for the Prevetion of Recurrent Neural tubing Defect." Medicine March1993.Harrison, Ross G. Organization a nd Develpment of the Embryo. Yale UniversityPress. 1969.Leese, Conaghan, Martin, and Hardy. "Early Human Embryo Metabolism." BioEssays vol. 15, No. 4 April 1993 259.Pritchard, MacDonald, and Gant. Williams Obstetrics. 17th ed, Prentice-Hall,Inc. 1985 139-142, 800.Pritchard, MacDonald, and Gant. Williams Obstetrics, Supplement no. 13. 17thed, Prentice-Hall, Inc. July/August 1987 2."Teratology." ACOG Technical air February 1985.