Resources for Communication Problems

Sunday, March 9, 2008




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, Louisville, KY.


人類有46條體染色體。最近估計人類基因的數目為317778。人類的生殖細胞藉由減數分裂來分離。發育的發生透過有絲分裂。人類的發展凱始於精子和第二卵母細胞(成熟卵)的結合。當第一周的發展,一個合子經過234細胞階段組成且分裂成胚葉細胞。在1216 細胞階段大約第六天,桑葚卵變成胚囊且附著在子宮內膜。當第三週時,滋養層變異且組成羊膜及絨毛膜、兩個卵黃囊和兩個胚層(內胚葉層和外胚葉層)出現。在第三週,中胚葉和內胚葉從內胚葉層透過原線組成和外胚層的變異,體節和神經管發展於此時。在第四周五個原始的腦泡出現很早最後變異成相映的腦部結構及腦室。皮膚及腦回出現大約在第24週。中樞神經系統不正常的發展導致不足例如像無腦畸形、顱骨裂、脊柱裂、腦積水、小頭畸形。週邊神經系統從神經管特殊的位置導出,就是神經脊。神經系統在出生之後幾輾持續發育。

*本章節由美國路易維耳市醫學大學 K.P Bhatnagar博士所撰寫。

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