Cytology · MU Pleven

Topic 53- Implantation

Explains how the blastocyst embeds in the endometrium, covering the adhesion, invasion and erosion steps and the roles of the cytotrophoblast and syncytiotrophoblast. It lists normal and abnormal implantation sites with approximate incidence percentages, then follows the day-by-day events through to placental connection by day 25. It closes with the second week's bilaminar stage, the differentiation of trophoblast and embryoblast, and a numbered summary of the "week of 2's".

Inside: 5 sections · 6 labelled pictures · 15 flashcards · 4 min

Topic 53 of 59 in the MU Pleven syllabus for Cytology (Medicine, Year 1). The free AceMed note covers it in 5 sections, about 529 words (4 min of reading), with 15 flashcards.

In this note

  1. Topic 53- Implantation
  2. Second Week – Becoming Bilaminar and Fully Implanting
  3. Days 11–12:
  4. Days 13–14:
  5. Summary – Week of 2’s:

Topic 53- Implantation

  • Implantation is the process by which the blastocyst settles into the endometrium.
  • It begins around 6–7 days after fertilisation and lasts approximately 40 hours.

Mechanism of implantation :

  • Adhesion – initial attachment of the blastocyst to the endometrial epithelium.
  • Cytotrophoblast – inner layer of the trophoblast contributing to invasion.
  • Syncytiotrophoblast – outer multinucleated layer that erodes maternal tissues.
  • Invasion – penetration into the endometrium.
  • Erosion of the endometrium – digestion of uterine mucosa to allow embedding.

Sites of implantation:

Normal: uterine cavity, posterior uterine wall, lateral uterine wall, superior cervix.

Abnormal: fallopian tube (tubal pregnancy), cervical canal (cervical pregnancy), ovaries (primary ovarian pregnancy), mesentery, abdominal cavity (abdominal pregnancy).

  • Abnormal implantation incidence (approximate):
  • Abdominal cavity – 1.4%
  • Ampulla of fallopian tube – 80%
  • Tubal implantation – 12%
  • Interstitial implantation – 0.2%
  • Cervix/placenta previa – 0.2%
  • Ovarian implantation – 0.2%
  • Around day 7, the blastocyst attaches to the endometrial epithelium.
  • Trophoblast cells secrete digestive enzymes, creating a hole for the embryo.
  • Embryo moves further into the vascular and connective tissue of the endometrium.
  • By day 10, the trophoblast begins forming the chorion.
  • The opening in the endometrium is sealed with new epithelial cells; development continues inside the endometrium.
  • Maternal blood vessels are opened by enzymes, providing oxygen and nutrients.
  • The amniotic cavity and umbilical vesicle form.
  • By day 25, a direct connection between the chorion and maternal blood vessels is established, forming the future placenta.
  • The embryonic stalk is formed, which will become the umbilical cord.

Additional molecular and cellular points:

  • Preimplantation factor (PIF): secreted by the viable embryo to promote adhesion.
  • Implantation window: period when the uterus is receptive.
  • Hatching: blastocyst emerges from the zona pellucida around day 5.
  • L-selectin and carbohydrate receptors mediate initial attachment.
  • Syncytiotrophoblast actively invades maternal tissue.

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