Embryology Notes

Development and Differentiation

  • Development: Growth and differentiation of the embryo and fetus, directed by inherited genes.
  • Differentiation: Appearance of specialized cells during development.

Fertilization

  • Location: Occurs in the upper 1/3 of the uterine tube.
  • Capacitation: Sperm must undergo activation (capacitation) in the female reproductive tract to fertilize an egg.
  • Sperm Count: Approximately 200 million sperm per ejaculate, but only about 100 reach the egg, and only one fertilizes it (Figure 29-1).

Oocyte at Ovulation

  • Stage: Oocyte is in metaphase II at ovulation.
  • Components:
    • Corona radiata: Follicle cells surrounding the oocyte.
    • Zona pellucida: Gelatinous layer inside the corona radiata. It undergoes structural changes after sperm penetration to prevent polyspermy (entrance of multiple sperm).
  • Egg Activation: Upon sperm entry, the egg activates to prevent other sperm from entering.
  • Meiosis II Completion: The egg completes Meiosis II after sperm entry.

Pronucleus Formation and Amphimixis

  • Pronuclei: Female pronucleus (N) and male pronucleus (N).
  • Amphimixis: Fusion of the female and male pronuclei.
  • Zygote: The resulting fertilized egg is called a zygote (2N), marking the beginning of a new life.

Gestation

  • Definition: The time required for development.
  • Duration: 9 months, divided into 3 trimesters, each 3 months long.

Stages

  • Weeks 1-8: Offspring is an embryo; characterized by specialized cell and tissue development and initial organ formation.
  • Week 9 onwards: Offspring is a fetus; organ systems continue to develop.

Trimesters

  • First Trimester:
    • Embryonic and early fetal development.
    • Major organ systems appear.
  • Second Trimester:
    • Fetal development dominates.
    • Organ systems complete most of their development.
    • The fetus begins to resemble a human.
  • Third Trimester:
    • Rapid fetal growth.
    • Organ systems become functional.

First Trimester Processes

  • Survival Rate: About 40% of embryos survive the first trimester.
  • Major Processes:
    1. Cleavage: Cell division to produce a multicellular stage called a blastocyst.
    2. Implantation: Attachment of the blastocyst to the uterine wall.
    3. Placentation: Formation of the placenta.
    4. Embryogenesis: Formation of the embryo.