胚胎学总论

第20章胚胎学总论

——人体胚胎发生CHAPTER 20 GENENRAL EMBRYOLOGY

——The Embryonic Development of Human Body

KYEY POINTS

●Gametogenesis

●Fertilization

●Development in pre-embryonic period

●Development in embryonic period

●Development in fetal period

●Fetal membrane and placenta

●Twins, multiple birth and conjoined twins

一、配子发生和受精

GAMETOGENESIS AND FERTILIZATION

OUTLINE:The fertilization is a process by which the male and female gametes unite to give rise to the zygote. Before fertilization both male and female germ cells undergo a lot of changes involving the chromosome as well as the cytoplasm. The mature process of the germ cells is known as gametogenesis. The gametogenesis is chiefly accomplished by two specialized divisions, known as meiotic or maturation divisions, by means of which the number of chromosome is reduced by half, from 46 to 23. The male germ cell, initially large and round, loses almost all of its cytoplasm and develops a head, neck and tail (a tadpole-like structure). The female germ cell, on the contrary, increases in the amount of cytoplasm. Fertilization is an important biological process by which the zygote restores the normal diploid number of chromosome, determines the genetic sex of the new individual, and initiates cleavage.

(一)配子发生Gametogenesis

（二）受精Fertilization

OVULATION

The process by means of which the oocyte with its cumulus oophorus cells is discharged from the ovary is known as ovulation. In the meantime the first meiotic division is complete, and the secondary oocyte starts its second meiotic division. During ovulation some woman have slight pain called middle pain, because it normally occurs near the middle of the menstrual cycle. Ovulation is also generally accompanied by a rise in basal temperature, which can be monitored to aid in determining when release of the oocyte occurs. Some women fail to ovulate because of a low concentration of gonadotropins. In these cases administration of an agent to stimulate gonadotropin release and hence ovulation can be employed. Although such drugs are effective, they often produce multiple ovulation, so that the risk of multiple births is 10 times as high in these women as in the general pregnancies.

二、胚前期的发育

THE DEVELOPMENT OF EMBRYO IN PREEMBRYONIC PERIOD OUTLINE: The period from fertilized ovum to the end of the eighth week is known as the preembryonic period. During this period the fertilized ovum undergoes a series of mitosis known as cleavage to produce more and more blastomeres. On the third day after fertilization the blastomeres increase to approximately 16. Thus the morula forms. At the end of the third day or the beginning of the fourth day the morula enters the cavity of the uterus and becomes blastocyst. By the sixth day the blastocyst begins to penetrates uterine mucosa, and completely embedded in the endometrial stroma by the 11th to 12th day. This process is called implantation. In the meantime the uterine endometrium becomes deciduas, and the trophoblast differentiates into cytotrophoblast and syncytiotrophoblast. The inner cell mass of the blastocyst also differentiates into two layers, the epiblast and hypoblast. These two layers form the bilaminar germ disc. At the same time a small cavity appears within the epiblast, and soon enlarges to form the amniotic cavity. The epiblast cells adjacent to cytotrophoblast become amnioblast. Both the amnioblast (wall) and the epiblast (floor)