Embryology Series: Introduction and First Week of Development

Lecture Overview

• Introduction to the embryology series.
• Begins with post-fertilization development.
• Subsequent videos will cover:
  • Development to the second week (gastrulation).
  • Development to week three (neurulation and nervous system formation).

Review of Uterine Anatomy

• Vagina: Also known as the sheath.
• Cervix: Part of the uterus leading into the vagina and body of the uterus.
• Fundus: Upper rounded portion of the uterus.
• Fallopian Tubes: Includes the ampulla, where fertilization occurs.
• Fimbrae: Small finger-like structures near the ovaries.
• Ovaries: Site of oocyte (egg) development.

Ovulation and Hormonal Influence

• Occurs around day 14-15 of menstrual cycle.
• Key Hormones: Luteinizing Hormone (LH):
  • Released from the anterior pituitary.
  • Stimulated by Gonadotropin-releasing hormone (GnRH) from the hypothalamus and high estrogen levels.
• Effect of LH:
  • Promotes fluid production in the ovary.
  • Leads to follicle rupture and release of secondary oocyte.

Fertilization Process

• Secondary oocyte is in metaphase II, awaiting sperm.
• Steps of sperm reaching the oocyte:
  • Sperm undergoes capacitation.
  • Sperm binds to ZP3 receptors on oocyte membrane.
  • Enzymes from sperm acrosome facilitate penetration.
  • Fusion of sperm and oocyte nuclei to form zygote (46 chromosomes).

Early Embryonic Development

Cleavage

• Series of mitotic divisions resulting in:
  • Two-cell stage.
  • Four-cell stage.
  • Eight-cell stage.
  • Sixteen-cell stage.
• Morula: Hollow ball of cells termed blastomeres.

Blastulation

• Transition from morula to blastocyst:
  • Formation of a fluid-filled cavity.
  • Inner Cell Mass: Becomes embryoblast.
  • Outer Cell Mass: Becomes trophoblast.

Differentiation

• Trophoblast: Further differentiates into:
  • Cytotrophoblast
  • Syncytiotrophoblast
• Embryoblast: Develops into the bilaminar disc, leading to embryo formation.

Key Steps in First Week

1. Ovulation
2. Fertilization
3. Cleavage
4. Blastulation

Summary

• The first week involves transformation from ovulation to the formation of a blastocyst.
• Prepares for further specialization and embryo development in subsequent weeks.

Next Steps

• Discussion of trophoblast and embryoblast differentiation.
• Development of bilaminar disc and placenta-related structures in next video.