Human Reproduction

Pregnancy and placenta formation

  • Blastocyst is composed of an outer envelope of cells, the trophoblast (or) trophoectoderm and inner cell mass [Embryoblast]. The trophoblast layer then gets attached to the endometrium and embryoblast differentiates into embryo.
  • After attachment, the uterine cells divide rapidly and covers blastocyst. As a result, the blastocyst becomes embedded in the endometrium of uterus. This is called Implantation which leads to pregnancy.
  • The side of the blastocyst to which the inner cell mass is attached is called the embryonic (or) animal pole while the opposite side is the abembryonic pole.
  • Implantation takes place about 7 days after fertilization. The function of zona pellucida is to prevent the implantation of the blastocyst at an abnormal site.
  • After implantation finger like projections appear on the trophoblast called chorionic villi which are surrounded by uterine tissue and maternal blood.
  • The chorionic Villi and uterine tissue become interdigitated with each other and jointly form a structural and functional unit between developing embryo and maternal body called placenta.
  • Placenta acts as a barrier as well as ultra filter between foetus and mother which is connected foetus by a rope like umbilical cord, that helps in the transport of substances to and from the embryo.
  • Placental hormones:
    ⇒ hcg, (human chorionic gonadotrophin)
    ⇒ Chorionic thyrotrophin
    ⇒ human placental lactogen
    ⇒ Oestrogen, progesterone
    ⇒ Chorionic Corticotropin
  • Cells of inner cell mass ⇒ Endoderm [1st germinal layer]
    remaining cells of inner cell mass ⇒ Ectoderm
    cells present in the space between Ectoderm and endoderm arrange to form mesoderm.

Watch this video for the topic from 0:12 to 17:30

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