Fertilization and First Week

To undergo fertilization, the sperm enters the uterus, travels towards the ampulla of the fallopian tube, and encounters the oocyte. The zona pellucida (the outer layer of the oocyte) deteriorates along with the zygote, which travels towards the uterus and eventually forms a blastocyst, allowing for implantation to occur. An ectopic pregnancy occurs if the zygote does not reach the uterus before the zona pellucida degrades.

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Fertilization

Sperm enter the uterus with sexual intercourse, subsequently entering the fallopian tubes and traveling upstream to the ampulla of the fallopian tube. The ampulla is the location of fertilization, which is a multistep process.

Step 1: 1st contact

  • Sperm burrow through the external matrix to touch the oocyte.
  • Facilitated by protein PH-20
Sperm burrows through the external matrix

Sperm burrow through the external matrix of the oocyte with the assistance of protein PH-20.

Image by Lecturio.

Step 2: binding of sperm and oocyte

  • Penetration of the zona pellucida by sperm
  • Sperm receptor binds ZP3 glycoprotein of zona pellucida → digestive enzymes (acrosin) release from the acrosome → sperm moves closer → sperm binds ZP2 glycoprotein → the head moves inside the zona pellucida
Sperm first binds the ZP3 receptor

The sperm binds with the ZP3 receptor and then with the ZP2 receptor to enter the zona pellucida.

Image by Lecturio.

Step 3: release of sperm contents

  • Sperm fuses with the egg membrane by an interaction between integrins (oocytes) and disintegrins (sperm) → DNA released into the ovum
  • Phospholipase C: 
    • Released by sperm into the ovum
    • Breaks down PIP2 into IP3 and DAG → calcium signalling cascade → cortical reaction
    • Cortical reaction makes the egg impenetrable to further insemination.
The fuses with the egg plasma membrane

The sperm fuses with the egg plasma membrane and releases DNA contents with phospholipase C.
The phospholipase C triggers a calcium signaling cascade, which drives the cortical reaction and makes the oocyte impenetrable to further sperm implantation.

Image by Lecturio.

Related videos

1st Week of Human Development

During the 1st week of development, the fertilized egg completes meiosis II within the ampulla of the fallopian tube. After completion of meiosis II, the fertilized oocyte travels from the fallopian tube to the uterus, which is the location of implantation.

  • Days 1–3: 
    • Fertilization of the oocyte
    • Completion of meiosis
    • Early cell division
  • Day 4: 
    • Morula formed: 16 cells
    • Zona pellucida breaks down → fluid moves in
  • Day 5: 
    • Blastocyst formed: 70–100 cells
    • Outer cell mass → trophoblast → supportive tissues of the placenta and umbilical cord
    • Inner cell mass → embryoblast → definitive structures of the fetus
  • Days 6–10:
    • Implantation of the embryo into the uterine wall
    • The blastocyst begins to secrete β-hCG.

Clinical Relevance

Ectopic pregnancy: implantation occurs outside the uterine cavity, most commonly in the ampulla of the fallopian tube. Patients with an ectopic pregnancy present with severe lower abdominal pain, which is often localized to the quadrant where the ectopic pregnancy is located. An ectopic pregnancy is a medical emergency and must be either medically managed (methotrexate) or surgically removed.

References

  1. Coticchio, G., et al. (2019). The enigmatic morula: mechanisms of development, cell fate determination, self-correction and implications for ART. Human Reproduction Update. 25(4), 422–438. Retrieved September 26, 2021, from https://pubmed.ncbi.nlm.nih.gov/30855681/
  2. Khan, Y.S., Ackerman, K.M. (2021). Embryology, Week 1. StatPearls Publishing. Treasure Island (FL). Retrieved September 26, 2021, from https://www.ncbi.nlm.nih.gov/books/NBK554562/
  3. Sadler, T.W. (2019). First week of development: ovulation to implantation. In Langman’s Medical Embryology, 14th edition. Philadelphia: Wolters Kluwer.

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