Embryo Development

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•  Implantation of the blastocyst in the endometrium is essential for the continuation of pregnancy

Blastocyst Formation

Following the fusion of an egg and sperm (fertilization), an influx of Ca2+ into the ova prompts the completion of meiosis II

  • The egg and sperm nuclei combine to form a diploid nuclei and the fertilized cell is now called a zygote

The zygote will undergo several mitotic divisions to form a solid ball of cells called a morula

  • As the morula continues to divide, it undergoes differentiation and cavitation (cavity formation) to form a blastocyst 

A blastocyst is comprised of three distinct sections:

  • An inner cell mass (that will develop into the embryo)
  • A surrounding outer layer called the trophoblast (this will develop into the placenta)
  • A fluid filled cavity called the blastocoele 

Early Embryo Development

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Implantation of Blastocyst

The final stage of early embryo development is the implantation of the blastocyst into the endometrial lining of the uterus

  • The blastocyst breaches the jelly coat that was surrounding it and preventing its attachment to the endometrium
  • Digestive enzymes are released which degrade the endometrial lining, while autocrine hormones released from the blastocyst trigger its implantation into the uterine wall

Only once the blastocyst is embedded within the uterine wall can the next stage of embryogenesis occur

  • The growing embryo will gain oxygen and nutrients from the endometrial tissue fluid, ensuring its continued development
  • The entire process (from fertilization to implantation) takes roughly 6 – 8 days

Implantation in the Endometrium