Embryoblast and Trophoblast Development

Before the developing blastocyst reaches the uterine wall, it needs to undergo several stages of differentiation. After a continuous process of cleavage and compaction, the morula gives rise to the trophoblast and embryoblast, which are the primary components of the blastocyst. Uterine fluid passes through the zona pellucida to form the blastocyst cavity. When the blastocyst reaches the endometrium, implantation begins by the trophoblast dividing into the cytotrophoblast and syncytiotrophoblast, with the syncytiotrophoblast primarily being responsible for invading the endometrium. The embryoblast divides into the epiblast and hypoblast, which are responsible for creating the amniotic cavity and yolk sac, respectively.

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Editorial responsibility: Stanley Oiseth, Lindsay Jones, Evelin Maza

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Formation of the Blastocyst

  • Blastocyst formation begins 5 days after fertilization Fertilization 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. Fertilization and First Week.
  • The blastocyst is approximately 70–100 cells.
  • The blastocyst cavity is formed by uterine fluid, which penetrates the morula after deterioration of the zona pellucida.
  • As fluid increases, the morula differentiates into the trophoblast and embryoblast.
    • Trophoblast (outer cell mass): layer of flattened blastomeres that surrounds the embryoblast
    • Embryoblast (inner cell mass): primordium to the embryo and fetus

Implantation of the Blastocyst

The trophoblast is primarily responsible for implantation of the blastocyst.

  • Implantation occurs 6–10 days after fertilization Fertilization 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. Fertilization and First Week.
  • The trophoblast differentiates into the cytotrophoblast and syncytiotrophoblast.
    • Cytotrophoblast
      • Inner cell layer
      • Essential for placenta Placenta The placenta consists of a fetal side and a maternal side, and it provides a vascular communication between the mother and the fetus. This communication allows the mother to provide nutrients to the fetus and allows for removal of waste products from fetal blood. Placenta, Umbilical Cord, and Amniotic Cavity formation
    • Syncytiotrophoblast
      • Outer cell layer
      • Invades endometrial connective tissue Connective tissue Connective tissues originate from embryonic mesenchyme and are present throughout the body except inside the brain and spinal cord. The main function of connective tissues is to provide structural support to organs. Connective tissues consist of cells and an extracellular matrix. Connective Tissue by producing proteolytic enzymes Enzymes Enzymes are complex protein biocatalysts that accelerate chemical reactions without being consumed by them. Due to the body's constant metabolic needs, the absence of enzymes would make life unsustainable, as reactions would occur too slowly without these molecules. Basics of Enzymes
      • Secretes hCG
  • Decidual cells—polygonal fibroblasts in the endometrium that control depth of blastocyst penetration—are enlarged.
  • Completely implanted 12 days after fertilization Fertilization 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. Fertilization and First Week
Blastocyst penetration guided by syncytiotrophoblasts

Blastocyst penetration guided by syncytiotrophoblasts

Image by Lecturio.

Formation of Bilaminar Disc and Amniotic Cavity

Embryoblast differentiation

The embryoblast gives rise to the bilaminar disc and amniotic cavity. The embryoblast differentiates into the epiblast and hypoblast. Together, they are referred to as the bilaminar disc. The process begins around the 8th day of gestation.

Hypoblast:

  • Located ventrally near the blastocyst cavity
  • Some hypoblast cells will form the anterior visceral endoderm, which becomes the cranial end of the embryo.
  • A cavity emerges within the hypoblast to form the primary yolk sac.
  • On the ventral side of the yolk sac, the hypoblast expands to form the extraembryonic mesoderm.
    • Somatic mesoderm: lines the cytotrophoblast and amnion
    • Splanchnic mesoderm: surrounds the yolk sac

Epiblast:

  • Located dorsally
  • A small cavity will emerge within the epiblast to form the amniotic cavity.

Mnemonic

The 2nd week of development is known as the week of 2s: 

  • The trophoblast differentiates into 2 layers: the cytotrophoblast and syncytiotrophoblast.
  • The embryoblast forms 2 layers: the epiblast and hypoblast.
  • The extraembryonic mesoderm splits into 2 layers: the somatic and splanchnic layers.
  • 2 cavities form: the amniotic and yolk sac cavities.

Clinical Relevance

Spontaneous abortion Spontaneous abortion Spontaneous abortion, also known as miscarriage, is the loss of a pregnancy before 20 weeks' gestation. However, the layperson use of the term "abortion" is often intended to refer to induced termination of a pregnancy, whereas "miscarriage" is preferred for spontaneous loss. Spontaneous Abortion: caused by inadequate production of estrogens and progesterone by the corpus luteum. Other causes include chromosomal abnormalities and physical trauma. The major symptom of early spontaneous abortion is a delayed menstrual period. Treatment of early spontaneous abortion is with supportive care.

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. https://pubmed.ncbi.nlm.nih.gov/30855681/
  2. Rehman, B, Muzio, MR. (2021). Embryology, Week 2–3. StatPearls Publishing. https://www.ncbi.nlm.nih.gov/books/NBK546679/
  3. (2019). irst Week of Development: Ovulation to Implantation. In Sadler, TW. (Ed.), Langman’s Medical Embryology, (14th ed., ch. 3). Wolters Kluwer.

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