Lesson eLearning

Describe the complete process of implantation, from the arrival of the blastocyst in the uterine cavity to the formation of the placenta, highlighting the roles of the trophoblast, decidua, cytokines, and maternal blood vessels.

hard 5 marks Class 12 · Biology · Human Reproduction
Answer
Implantation begins when the blastocyst contacts the decidualised endometrium; the trophoblast differentiates into syncytial and cytotrophoblast layers that adhere to and erode the decidua, cytokines such as IL‑6 and LIF promote trophoblast invasion, maternal spiral arteries remodel to supply blood, and the developing syncytiotrophoblast forms chorionic villi that later become the functional placenta.

Step-by-step solution

  1. The blastocyst reaches the uterine cavity and aligns against the luminal epithelium of the decidua (the hormonally prepared endometrium).
  2. The outer trophoblast layer differentiates into two sub‑layers: an inner cytotrophoblast and an outer syncytial trophoblast.
  3. Adhesion molecules (integrins) on the syncytiotrophoblast attach to the extracellular matrix of the decidua, initiating the first stage called "apposition".
  4. The syncytiotrophoblast releases proteolytic enzymes that digest the decidual epithelium, allowing the blastocyst to penetrate – the "penetration" stage.
  5. Cytokines and growth factors secreted by the decidua (e.g., interleukin‑6, leukemia‑inhibitory factor) and by the trophoblast (e.g., hCG) stimulate further trophoblast proliferation and invasion.
  6. The invasive trophoblast remodels maternal spiral arteries, converting them from high‑resistance to low‑resistance vessels, thereby establishing a rich maternal blood supply.
  7. As invasion proceeds, the syncytiotrophoblast forms finger‑like chorionic villi that interdigitate with maternal blood sinusoids, establishing the fetal‑maternal exchange surface.
  8. These villi, together with the underlying fetal mesoderm, develop into the functional placenta, completing the implantation process.
Common mistake: Omitting the role of cytokines and the remodeling of maternal blood vessels.

Concept tested: Implantation and placental development

Published · How this solution was prepared and checked

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