Embryology Chapter on Blastocyst Development

Pre-Embryonic Period: Overview

• The pre-embryonic period encompasses the first two weeks after fertilization.
• Fertilization of the egg by sperm initiates this phase.

A single-celled zygote forms, which undergoes rapid cell divisions called cleavage.

• The zygote transforms into a morula, then a blastocyst.

Pre-Embryonic Period: Week 1

• The blastocyst implants in the lining of the uterus.
• This process begins approximately one week after fertilization.
• The blastocyst is a fluid-filled structure with two key components:
  • Trophoblast: The outer layer of cells that forms the chorion.
  • Embryoblast: The inner cell mass that will develop into the embryo.

Pre-Embryonic Period: Formation of the Bilaminar Germinal Disc

• By day 8, cells of the embryoblast separate into two layers:
  • Hypoblast: Forms the inner layer of cells.
  • Epiblast: Forms the outer layer of cells.
• The two layers form a flat disc called the bilaminar disc or embryonic disc.

Pre-Embryonic Period: Formation of the Extraembryonic Membranes

• The bilaminar disc and trophoblast create three extraembryonic membranes:
  • Yolk sac: Critical for early blood cell and vessel development.
  • Amnion: A thin layer that forms above the epiblast, creating a fluid-filled amniotic cavity.
  • Chorion: The outermost membrane that plays a crucial role in placenta formation.

Pre-Embryonic Period: Development of the Placenta

• The placenta develops as a vital interface between the embryo/fetus and the mother.
• The embryonic placenta is formed by the chorion, whereas the maternal portion originates from the uterine endometrium.
• The placenta facilitates nutrient, waste, and respiratory gas exchange between the embryo and the mother.
• It also transmits maternal antibodies to the developing embryo/fetus.
• The placenta produces crucial hormones estrogen and progesterone that maintain the uterine lining.

Learning Outcomes

• The pre-embryonic stage encompasses the first two weeks of development, from fertilization to implantation.
• Key events include cleavage, formation of the blastocyst, implantation, and the development of the bilaminar disc and extraembryonic membranes.
• The placenta, a critical structure for nutrient and gas exchange between the mother and embryo/fetus, also originates during this initial phase of development.