Embryology Lec 1: Embryogenesis Overview

Embryogenesis Overview

• Human embryogenesis transforms a fertilized egg into an embryo over the first eight weeks of development.
• The embryo progresses from a zygote to a multi-layered, multi-dimensional structure with primitive organs.
• By the ninth week, the embryo is referred to as a fetus, characterized by recognizable features and developing organs.

First Week of Development

• The early developmental events crucial for fertilization and implantation occur, including:
  • Gamete approximation
  • Contact and fusion of gametes
  • Fertilization
  • Mitotic cleavage of the blastomere
  • Morula and blastocyst formation
  • Implantation of the blastocyst

Ovulation

• Begins at puberty, regulated by gonadotropins from the hypothalamus; includes follicle-stimulating hormone (FSH) and luteinizing hormone (LH).
• The pituitary gland controls ovarian cycles, leading to the release of an egg (ovulation).
• Follicles grow, with typically only one reaching maturity while others degenerate.
• After ovulation, the Graafian follicle ruptures, releasing the oocyte and forming the corpus luteum, which produces progesterone and estrogen.

Key Structures

• Cumulus Oophorus: A cluster of cells surrounding the oocyte, with the innermost layer known as the Corona Radiata.
• Zona Pellucida: Glycoprotein layer that protects the oocyte's plasma membrane.

Fertilization Process

• Fusion of mature male and female gametes creates a zygote.
• Sperm undergo capacitation in the female reproductive tract, allowing them to penetrate the oocyte.
• Acrosome Reaction: Release of enzymes necessary to penetrate the zona pellucida after sperm binds with it.

Results of Fertilization

• Restoration of the diploid chromosome number, combining genetic material from both parents.
• Determination of genetic sex based on the sperm type (XX for female, XY for male).
• Initiates cell cleavage.

Morula Formation

• Following fertilization, rapid mitotic cleavage transforms the zygote into a morula, which is a solid sphere of cells.
• Morula remains the same size as cell count increases due to compaction, within the zona pellucida.

Blastulation and Blastocyst Formation

• The blastocyst forms a few days post-fertilization and comprises:
  • An Inner Cell Mass (ICM), later developing into the embryo.
  • An outer Trophoblast layer surrounding the ICM and the blastocoel (first cavity).
• The blastocyst hatches through the zona pellucida as it enlarges.

Implantation Process

• Implantation is a critical biochemical and mechanical process occurring in the first week, extending into the second.
• Subdivided into:
  • Apposition Phase: Weak interactions between blastocyst and uterine wall.
  • Attachment Phase: Stronger binding to the uterine epithelium.
  • Invasion Phase: Blastocyst burrows into the endometrium.

This quiz focuses on the first week of embryogenesis, covering key processes such as ovulation, fertilization, and implantation. Understand the intricate development from a zygote to a complex embryo with functioning organs during the critical first eight weeks. Test your knowledge on this fundamental phase of human development.