1 week of human development

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  • Oocytes are produced by the ovaries and expelled from them during ovulation.The fimbriae of the uterine tube sweep the oocyte into yhe ampilla where it may be fertilized.


 

 

  • Sperms are produced in the testes and stored in epididymis, Ejaculation of semen during sexual intercourse results in the deposit of millions of sperms in the vagina . several hundread sperms pass through the uterus and enter the uterine tubes.    

 

        

  • When oocyte contacted by a sperm ,it completes the second meiotic division . As a result a mature oocyte and a second polar body are formed . the nucleus of the mature oocyte constitutes the female pronucleus.

 

 

  • After the sperm enters the oocyte ,the head of the sperm seperates  from the tail and enlarge to become the male pronucleus. Fertilization is complete when the male and female pronuclei unite and maternal and paternal chromosomes intermingle during metaphase  of the frist mitotic division of the zygote.

 

 

  •  As it passes along the uterine tube towards the uterus the zygote undergoes cleavage  into small cells- Blastomeres, Approximately 3 days after fertilization .A ball of 12 or more blastomers - morula - enters the uterus.

 

 

  • A cavity forms in the morula ,converting  it into a blastocyst consisting of the embryoblast , a blastocystic cavity and the trophoblast. the trophoblast encloses the embryoblast  and blastocystic cavity and later forms extraembryonic structure and embryonic part of the placenta.

 

 

  • 4 to 5 days after fertization ,the zona pellucida is shed and the trophoblast adjacent to the embryoblast attaches to endometerial epithelium.

 

 

  • The trophoblast at the embryonic pole differentiate into two layers , an outer syncytiotropoblast and an inner cytotrophoblast. the syncytiotrophoblast invades the endometrial epithelium and underlying connective tissue.A cuboidal layer of hypoblast forms on the deep surface of the embryoblast . by the end of the 1st week , blastocyst is superficially implanted in the endometrium. 

 

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