Embryology Lecture Week 1

Embryology Lecture Notes: First Week

• Learning Objectives: By the end of this lecture, students should be able to:
  • Identify the steps of capacitation.
  • Identify the steps of acrosomal reaction.
  • Identify the steps (phases) of fertilization.
  • Define normal and abnormal fertilization.
  • Know the steps of blastocyst formation.

Capacitation

• Definition: A process occurring mostly in the cervix.
• Duration: Approximately 7 hours.
• Process: Removal of glycoprotein coats and seminal plasma proteins from the sperm's acrosomal region.
• Result: Increased sperm activity and motility.
• Importance: Essential for sperm to penetrate the corona cells and undergo acrosomal reaction.

Acrosomal Reaction

• Definition: Release of acrosomal contents by the sperm.
• Trigger: Contact with corona radiata.
• Importance: This process is essential for sperm penetration.
• Mechanism: Release of enzymes like hyaluronidase, trypsin-like substances, and zona lysin.

Fertilization

• Location: Ampulla of the uterine tube.
• Timing: Around midcycle (ovulation).
• Phases:
  • Penetration of the corona radiata.
  • Penetration of the zona pellucida.
  • Fusion of the oocyte and sperm cell membranes.
• Outcome: Formation of a zygote.

Results of Fertilization

• Resumption of the 2nd meiotic division of the oocyte.
• Restoration of the diploid number of chromosomes.
• Sex determination.
• Zygote formation and beginning of cell division.

Cortical and Zona Reaction

• Zona pellucida: Glycoprotein layer around the oocyte.
• Process: Release of enzymes from cortical granules in the oocyte plasma membrane, altering zona pellucida structure and its permeability to prevent polyspermy (multiple sperm fertilizations).

Abnormal Fertilization

• Superfecundation: Two or more ova are fertilized during the same cycle by sperms from different events of intercourse (coitus).
• Parthenogenesis (virgin birth): Embryo development without fertilization by a male sperm. Occurs naturally in some plants, invertebrates, and fish.

Cleavage

• Definition: A series of mitotic divisions of the zygote.
• Result: Rapid increase in cell number (blastomeres).
• Continuing cleavage results in blastomeres that shrink in size and become smaller with each division. This division takes place as the zygote travels along the fallopian tube toward the uterus, and is still surrounded by the zona pellucida.

Morula

• Stage: About three days after fertilization.
• Characterization: 16 to 32 blastomeres. Blastomeres of the morula are undergoing repeated mitotic divisions within the zona pellucida. The zona pellucida keeps the cells together during cleavage.
• Function: The zona pellucida prevents adhesion of the cells to the uterine tubes.

Blastocyst

• Formation: Fluid-filled cavity forms inside the morula.
• Components:
  • Inner cell mass (embryoblast): Forms the embryo.
  • Outer cell mass (trophoblast): Forms part of the placenta.
• Outcome: The blastocyst enters the uterus.

This quiz covers the foundational concepts introduced in the first week of Embryology. Students will explore key processes such as capacitation, acrosomal reaction, and the stages of fertilization. Understanding these concepts is essential for grasping the complexities of reproductive biology.