Developmental Stages: Germinal and Embryonic Periods

Germinal Period

• Lasting approximately 14 days, this stage begins at conception.
• Fertilization, usually in the fallopian tube, results in a diploid zygote—a new cell with combined parental genetic material.
• The zygote rapidly divides, traveling down the fallopian tube.
• A 100-cell blastocyst forms after five days. This structure has two cell types:
  • The inner cell mass (embryonic disk) develops into the embryo.
  • The trophoblast forms the placenta.
• Trophoblast cells burrow into the uterine wall, establishing contact with the mother's bloodstream.
• The placenta provides nutrients and oxygen to the developing embryo via the umbilical cord.
• Implantation marks the end of the germinal period.
• A significant percentage of conceptions do not progress beyond the first two weeks.

Embryonic Period

• The blastocyst becomes an embryo upon implantation.
• The embryonic period features cell differentiation and development of tissues and organs (organogenesis).
• Three primary germ layers form all adult tissues and organs:
  • Ectoderm ("outer skin"): Forms the skin, nervous system, and parts of sensory organs.
    • Neurulation is a key process in developing the nervous system.
  • Mesoderm ("middle skin"): Develops the circulatory system (blood and lymphatic), bone, cartilage, muscles, and many internal organs (kidneys, spleen, reproductive organs, etc.).
  • Endoderm ("inner skin"): Gives rise to the gastrointestinal and respiratory system along with other organs (thymus, bladder, etc.).
• By week eight, the embryo reaches approximately 1 inch in length and weighs about 8 grams.

This quiz covers the key concepts of the germinal and embryonic periods of development, including fertilization, blastocyst formation, and implantation. Understand the characteristics of cell differentiation and organogenesis that occur in these early stages of life.