There are 46 human somatic chromosomes. The current estimate of the number human genes is 31,778.Human sex cells divide by meiosis. Growth occurs through mitosis. Human development begins with the union of a spermatozoon and with a secondary oocyte. During week 1 of development, a zygote forms and divides into blastomeres that pass through the 2-, 3-, and 4-cell stages. In the 12- to 16-cell stages, on about day 6, the morula becomes the blastocyst and attaches to the endometrium. During week3, trophoblast differentiation and formation of the amnion and chorion, the two yolk sacs, and two germ layers(epiblast and hypoblast layers) occur. In week3, the mesoderm and endoderm form from the epiblast through the primitive streak, and the ectoderm differentiates. Somites and the neural tube develop at this time. The five brain vesicles appear early in week 4 and gradually differentiate into corresponding brain structures and ventricles. Sulci and gyri appear in approximately week 24. Abn-
ormal development of the central nervous system causes deficits such as anencephaly, cranium bifidum, spina bifida, hydrocephaly, and microcephaly. The peripheral nervous system is derived from a specialized portion of the neural tube, the neural crest. The nervous system continues to develop for many years after birth.
*This chapter is written by K.P Bhatnagar, PhD, University of Louisville School of Medicine,
*本章節由美國路易維耳市醫學大學 K.P Bhatnagar博士所撰寫。