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SECOND WEEK OF HUMAN DEVELOPMENT

Introduction

  • Completion of Implantation of Blastocyst (39)
  • Formation of Amniotic Cavity, Embryonic Disc, and Umbilical Vesicle (41)
  • Development of Chorionic Sac (42)
  • Implantation Sites of Blastocysts (46)
  • Summary of Implantation (46)
  • Summary of Second Week (48)
  • Clinically Oriented Problems (49)

Completion of Implantation of Blastocyst

  • The implantation of the blastocyst is finalized during the second week.
  • Implantation occurs between 6 to 10 days post-ovulation and fertilization.
  • As the blastocyst implants:
    • Increased contact with endometrium leads to differentiation into two trophoblast layers:
    • Cytotrophoblast:
      • Inner layer, mitotically active (cell division apparent) which forms new cells that migrate into syncytiotrophoblast.
    • Syncytiotrophoblast:
      • Rapidly expands into a multinucleated mass; no clear boundaries between cells.
    • The syncytiotrophoblast invades endometrial connective tissue, embedding the blastocyst.
    • Endometrial cells undergo apoptosis (programmed cell death) aiding the invasion of the blastocyst.
Molecular Mechanisms of Implantation
  • Synchronization of the invading blastocyst and the receptive endometrium is essential.
  • Key components for implantation include:
    • Microvilli of endometrial cells
    • Cell adhesion molecules (integrins)
    • Cytokines, prostaglandins, hormones: hCG (human chorionic gonadotropin) and progesterone.
    • Growth factors and extracellular matrix components: matrix metalloproteinase and protein kinase A.
  • Endometrial cells also modulate the depth of syncytiotrophoblast penetration.
Decidual Reaction
  • At the implantation site, connective tissue cells accumulate glycogen and lipids, becoming polygonal (decidual cells).
  • Decidual cells degenerate near the syncytiotrophoblast and are engulfed, providing nutrients.
  • The syncytiotrophoblast produces hCG, entering maternal blood through lacunae.
  • hCG maintains hormonal activity of the corpus luteum in the ovary during pregnancy.
    • Detected by sensitive radioimmunoassays, forming the basis of pregnancy tests.
  • By the end of the second week, significant levels of hCG are present, often before the woman realizes she is pregnant.

Formation of Amniotic Cavity, Embryonic Disc, and Umbilical Vesicle

  • Amniotic Cavity Formation:
    • As implantation continues, a small space emerges in the embryoblast, leading to the formation of the amniotic cavity.
  • Amnioblasts:
    • Cells that separate from the epiblast to form the amnion, which encloses the amniotic cavity.
Embryonic Disc Formation
  • The embryoblast forms a bilaminar disc consisting of two layers:
    • Epiblast:
    • Thicker layer with high columnar cells connected to the amniotic cavity.
    • Hypoblast:
    • Smaller cuboidal cells adjacent to the exocoelomic cavity (yolk sac).
  • The embryonic disc lies between the amniotic cavity and umbilical vesicle, surrounded by extraembryonic mesoderm, enabling morphogenetic movements.
  • Lacunae appear in the syncytiotrophoblast, which fill with maternal blood and cellular debris, providing nutrient fluid (embryotroph) to the embryonic disc via diffusion.
Establishment of Uteroplacental Circulation
  • Eroded endometrial capillaries communicate with the lacunae, establishing the primordial uteroplacental circulation.
  • Oxygenated maternal blood enters through spiral arteries; poorly oxygenated blood exits through endometrial veins.
  • By Day 10 of development, the conceptus is fully embedded in the uterine endometrium.
  • The endometrial defect is closed by a stimulating fibrin plug, and the epithelial layer regenerates by Day 12.

Development of Chorionic Sac

  • At the second week's end, primary chorionic villi are visible.
  • Primary villi are vascular projections with syncytial coverings that grow into the syncytiotrophoblast, influenced by underlying extraembryonic mesoderm.
  • The extraembryonic coelom splits the extraembryonic mesoderm into:
    • Somatic mesoderm:
    • Lines the trophoblast and covers the amnion.
    • Splanchnic mesoderm:
    • Surrounds the umbilical vesicle.
  • The chorion forms from the extraembryonic mesoderm and trophoblast layers, creating the chorionic sac.
  • The embryo, amniotic sac, and umbilical vesicle remain suspended within the sac through the connecting stalk.
  • Extraembryonic coelom becomes the chorionic cavity.
  • Chorionic Sac Measurement:
    • Utilization of transvaginal ultrasonography to measure chorionic sac diameter provides insights on developmental progress and pregnancy outcomes.
Prechordal Plate Development
  • At 14 days, the embryo retains a bilaminar disc shape with a thickened localized area, forming the prechordal plate, indicating future craniofacial region development.

Implantation Sites of Blastocysts

  • Common implantation occurs in:
    • Superior body of uterus, especially on the posterior wall.
  • Ectopic implantations (95-98% occurring in uterine tubes) can lead to complex cases and require monitoring.

Summary of Implantation

  • The cellular and molecular events include:
    • Degeneration of zona pellucida (Day 5).
    • Blastocyst adheres to endometrial epithelium (Day 6).
    • Trophoblast differentiation into cytotrophoblast and syncytiotrophoblast (Day 7).
    • Embedding of blastocyst into endometrium (Day 8).
    • Formation of blood-filled lacunae (Day 9).
    • Closure of endometrial defect with a fibrin plug (Day 10).
    • Establishment of uteroplacental circulation (Days 11-12).
    • Development of chorionic villi (Days 13-14).

Summary of Second Week

  • Rapid trophoblast proliferation and differentiation ensue alongside the decidual reaction in the endometrium, facilitating the formation of the umbilical vesicle and extraembryonic mesoderm.
  • The primary umbilical vesicle reduces in size as the secondary umbilical vesicle emerges.

Abortion

  • Abortion (miscarriage) refers to the premature termination of a pregnancy.
    • Types of abortion include:
    • Threatened abortion: Bleeding occurs, with possible pregnancy loss.
    • Spontaneous abortion: Natural pregnancy loss before 20 weeks, most common in the 3rd week post-fertilization.
    • Habitual abortion: Recurrent loss across three or more pregnancies.
    • Induced abortion: Medically initiated termination before 20 weeks.
    • Complete abortion: Expulsion of all conception products from the uterus.
    • Missed abortion: Retention of an embryo or fetus post-death.

Clinically Oriented Problems

Spontaneous Abortion
  • Occurs within the first 12 weeks in 10-20% of cases, commonly attributed to chromosomal abnormalities.
  • Higher incidence noted in older women, often related to nondisjunction.
Techniques for Confirmation of Ectopic Pregnancy
  • Key diagnostic techniques include:
    • Ultrasonography
    • hCG assays
  • Likely implantation sites include the isthmus of the uterine tube.
Treatments for Ectopic Pregnancy
  • Surgical removal of the affected tube and conceptus, particularly if hemorrhage or rupture occurs.
  • In cases of heterotopic pregnancies, surgical intervention typically targets the ectopic site without disturbing the intrauterine pregnancy.

Conclusion

  • Understanding the complexities of early pregnancy events—including implantation processes and potential complications—is crucial for optimizing maternal-fetal health outcomes.

Bibliography and Suggested Reading

  • Refer to provided bibliography for further research sources and case studies relevant to developmental human embryology and associated medical practices.