Anatomy Unit 201: Cleavage & Implantation

Implantation Overview

• Implantation refers to the penetration of a blastocyst into the superficial layer of the endometrium, transforming the endometrium into decidua.
• Occurs between the 7th and 11th days post-fertilization.

Normal Site of Implantation

• The typical site for implantation is the endometrium of the uterus, near the upper part of the posterior wall close to the fundus.
• The blastocyst attaches at the embryonic pole and undergoes several changes such as degeneration of the zona pellucida and enlargement.

Blastocyst Development

• Trophoblast exposure differentiates into:
  • Syncytiotrophoblast (outer layer)
  • Cytotrophoblast (inner layer, forms by day 7).
• Adhesive power: allows blastocyst adherence and invasion into the endometrial epithelium.
• Histolytic power: syncytiotrophoblast produces enzymes to lyse endometrial tissues, creating a defect for blastocyst embedding.

Implantation Process

• By day 10, a fibrin plug fills the defect in the endometrial epithelium.
• The defect begins to repair on days 12 and 13, while blood-filled lacunae develop within the syncytiotrophoblast.

Abnormal Sites of Implantation

• Placenta previa: abnormal implantation within the lower uterine segment near the internal os, which can obstruct delivery.
  • Types include:
    • Placenta praevia lateralis: away from the internal os.
    • Placenta praevia centeralis: completely covers the internal os.
    • Placenta praevia marginalis: partially covers the internal os.
• Ectopic pregnancy: implantation occurs outside the uterus, notably in:
  • Fallopian tubes
  • Ovaries
  • Abdominal cavity (peritoneal)

Decidua Definition and Types

• The decidua is the endometrium during pregnancy and consists of three parts:
  • Decidua basalis: lies between the fetus and the uterine wall, contributing to the maternal portion of the placenta.
  • Decidua capsularis: covers the fetus.
  • Decidua parietalis: lines the remaining uterine cavity.
• By the end of the 12th week, the decidua capsularis and parietalis fuse, and by 22-24 weeks, both degenerate due to pressure from the growing embryo.

Case Scenario Insights

• A 20-year-old woman with right-sided abdominal pain, internal bleeding, and missed menstrual period should be evaluated for:
  • Ovarian cancer
  • Appendicitis
  • Normal pregnancy
  • Ectopic tubal pregnancy (key consideration due to symptoms).

Ectopic Pregnancy

• Causes: abnormal implantation leading to potential complications.
• Signs and symptoms can include abdominal pain, missed periods, and internal bleeding.
• Treatment options vary based on disorder extent and location.

This quiz covers key concepts related to cleavage and implantation in human anatomy. Students will explore the stages of blastocyst formation, the process of implantation, and the normal versus abnormal sites of implantation, including ectopic pregnancy. Master these fundamental ideas to excel in your anatomy studies.