Chapter 28: Embryology and Human Development

Introduction

• Cell Differentiation: Process where similar cells become specialized in structure and function.
• Human Development Periods:
  • Prenatal Development: From fertilization to childbirth.
  • Postnatal Development: After birth until maturity.
• Main Focus: Prenatal development, divided into:
  • Pre-embryonic Period: First 2 weeks, fertilization to implantation.
  • Embryonic Period: Implantation to end of 8th week, major organ formation.
  • Fetal Period: 9th week to birth, organ growth and maturation.

Fertilization

• Occurs in the ampulla of the uterine tube.
• Haploid Gametes: Sperm and oocyte unite to form a diploid zygote.
• Maternal Contribution: Egg contributes mitochondria and nutrients.
• Sperm Journey: 200 million deposited, only about 0.5% reach the egg.
• Capacitation: Final activation of sperm within the female reproductive tract.

Challenges in Fertilization

• Corona Radiata: Layer surrounding oocyte needs to be penetrated.
• Oocyte Activation: Occurs upon sperm entry, preventing other sperm from entering.
• Meiosis Completion: Oocyte completes meiosis 2 after fertilization.
• Pronuclei Fusion: Male and female pronuclei fuse to form the zygote.

Prenatal Development Stages

• Trimesters: Divided into three 3-month periods.
  • 1st Trimester: Organogenesis, critical and vulnerable period.
  • 2nd Trimester: Organ refinement and rapid growth.
  • 3rd Trimester: Functional maturation of organ systems.

Key Events in First Trimester

• Cleavage: Rapid mitotic divisions without growth, forming a blastocyst.
• Implantation: Blastocyst embeds in the uterine lining.
• Placentation: Formation of the placenta for nutrient exchange.
• Embryogenesis: Basic body plan and major organ systems development.

Cleavage and Blastocyst Formation

• Cleavage: Rapid cell division forming progressively smaller cells.
• Morula: Solid ball of cells.
• Blastocyst: Hollow structure with inner cell mass and trophoblast.

Gastrulation

• Establishes three primary germ layers:
  • Ectoderm: Outer layer.
  • Mesoderm: Middle layer.
  • Endoderm: Innermost layer.

Extra-Embryonic Membranes

• Amnion: Fluid-filled sac protecting the embryo.
• Allantois: Contributes to urinary bladder development.
• Yolk Sac: Early blood cell formation.
• Chorion: Forms blood vessels connecting to maternal tissues.

Placentation and the Placenta

• Develops from maternal and embryonic tissues.
• Chorionic Villi: Establish exchange network with maternal blood.
• Umbilical Cord: Transports nutrients and waste.

Embryogenesis and Organogenesis

• Embryogenesis: Formation of embryo's structure.
• Organogenesis: Formation of organ systems, begins early in gestation.

Second and Third Trimester Developments

• Second Trimester: Rapid fetal growth and organ refinement.
• Third Trimester: Organ systems mature, fetus prepares for birth.

Changes in Female Reproductive System

• Uterus Expansion: Grows significantly to accommodate fetus.
• Organ Displacement: Abdominal organs shift to make room for growing uterus.

Conclusion

• Review and Questions: Open invitation for questions and further exploration of embryology topics.