Early Human Development Flashcards

Learning Objectives

• Describe the compaction process in mammalian embryos and explain its importance.
• Diagram a blastocyst stage embryo. Label the blastocoel, trophoblast, and inner cell mass. What is the fate of each of these structures?
• When does "hatching" occur? How does the zona pellucida protect against ectopic pregnancy?
• Explain why an ectopic pregnancy is not a viable pregnancy.
• Diagram an implantation stage embryo. Label the syncytiotrophoblast, cytotrophoblast, epiblast, hypoblast, and bilaminar germ disc.
• Define the following structures and identify what cells created them: epiblast, hypoblast, blastocoel, primary yolk sac, extraembryonic mesoderm, chorion, connecting stalk, definitive/secondary yolk sac.
• For each structure listed in 5, create a diagram or flowchart showing how the structures change over time.

Timeline of Early Development

Week 1

• Day 6-7: Trophoblast differentiates into cytotrophoblast and syncytiotrophoblast and begins to implant into the uterine endometrium; the embryonic disc becomes bilaminar.

• Day 8: Amniotic cavity forms; syncytiotrophoblast expands.

Week 2

• Day 10: Cells migrate from the hypoblast to form the primary yolk sac; lacunae form within the syncytiotrophoblast; implantation is complete; syncytiotrophoblast surrounds the embryo.

• Day 11: Extraembryonic mesoderm forms and splits to form the chorionic cavity; trophoblastic lacunae anastomose with maternal blood sinusoids.

Week 3

• Day 12-13: Cells migrate from the hypoblast to form the secondary yolk sac; the primary yolk sac is pushed aside and begins to degenerate.

• Day 14-15: The primary yolk sac is reduced to a remnant at the abembryonic pole of the chorionic cavity; the embryo proper is attached to the chorion by the connecting stalk.

Early Human Development

• Human cleavage begins 24 hours after fertilization, with each division taking 12-24 hours.

• Cleavage is rotational, meaning the first and second divisions occur at perpendicular angles.

• Each cell in the cleavage stage embryo is called a blastomere.

• Cleavage is asynchronous, meaning not all blastomeres divide at the same time, so the embryo often has an odd number of cells.

• Cleavage does not change the size of the embryo. The fertilized egg is divided into smaller and smaller cells without increasing its overall size.

• The embryo does not get larger during cleavage because it is dividing the existing cytoplasm into smaller cells rather than synthesizing new material.

Compaction

• Around the 8-16 cell stage, the embryo undergoes compaction, when the blastomeres flatten and adhere to one another tightly.

• Blastomeres develop an inside-outside polarity derived from asymmetrical division of the apical versus basal halves of each blastomere.

• Certain blastomeres are found on the inside or outside of the embryo.

• Inner blastomeres become the inner cell mass; outer cells become the trophoblast ("inside-outside hypothesis").

  • Inner cell mass → embryo
  • Trophoblast → placenta

• This is the first differentiation event in the embryo: after compaction, cells on the inside and outside of the embryo have different fates.

• Cells are totipotent before compaction; after compaction, their potential fates are limited to two pathways.

• Compaction is important because it allows for the formation of distinct cell lineages with different fates (inner cell mass and trophoblast), which is essential for proper development.

Morula and Blastocyst Formation

• Within 4 days, the compacted embryo is composed of 16-32 cells and is called a morula (from Latin, for mulberry).

• The morula reaches the uterus around day 5 of development and begins to form a fluid-filled cavity, the blastocoel.

• The embryo is now a blastocyst (blastula) and consists of two types of cells:

  • The inner cell mass (ICM) is a cluster of cells on one side of the blastocoel.
    • Also called the embryoblast.
    • It will become the embryo; cells are pluripotent but no longer totipotent.
  • Surrounding the blastocyst is a single layer of epithelial cells called the trophoblast.
    • It will form extraembryonic tissues (placenta).
    • Bipotential: can become two types of trophoblast cells.

• On day 5 of development, the blastocyst "hatches" from the zona pellucida.

• The blastocyst can now attach to the uterine endometrium.

Hatching

• Hatching from the zona pellucida occurs when the embryo reaches the uterus on day 5-6.

• The zona pellucida prevents the embryo from prematurely adhering to the oviduct wall and implanting too soon.

Ectopic Pregnancy

• Implantation in the wrong place can result in an ectopic pregnancy.

• The embryo implants elsewhere than in the uterus (usually in the oviduct: approximately 90%).

• The pregnancy can appear normal until 6-8 weeks.

• Then it becomes an emergency; the growing embryo will rupture the oviduct, causing severe hemorrhaging.

• The embryo is almost always incapable of survival except in a handful of cases; it cannot be relocated to the uterus.

• The risk of ectopic pregnancy is increased by prior STDs, smoking, pelvic inflammatory disease, endometriosis, premature hatching from the zona pellucida, and ciliary defects in uterine tubes.

Second Week of Development

• In the second week of human development, the embryo changes rapidly.

• The inner cell mass forms two layers:

  • The epiblast or primary ectoderm: a layer of external cells.
  • The hypoblast or primary endoderm: a layer of internal cells.
  • This two-layered embryoblast is called the bilaminar germ disc.

• The trophoblast cells differentiate into two cell types:

  • The syncytiotrophoblast actively invades the endometrium.
  • The cytotrophoblast divides to create more syncytiotrophoblast cells.

Amniotic Cavity Formation

• Fluid accumulates between the epiblast cells, forming a cavity.

• A new layer of epiblast cells forms to separate this cavity from the cytotrophoblast.

• These cells are the amnioblasts, and they become the amniotic membrane.

• The cavity is the amnion or amniotic cavity.

Primary Yolk Sac Formation

• Hypoblast cells migrate out to cover the inner surface of the cytotrophoblast.

• They form the extraembryonic endoderm that lines the blastocoel cavity.

• This tissue is also called Heuser’s membrane.

• The blastocoel cavity is now called the primary yolk sac.

Extraembryonic Mesoderm Formation

• Another set of cells migrates from the epiblast between Heuser’s membrane and the cytotrophoblast.

• They form the extraembryonic mesoderm.

• The extraembryonic mesoderm continues growing and migrates around to surround the entire embryo and separate the amnion from the cytotrophoblast.

Chorionic Cavity Formation

• The extraembryonic mesodermal cells split to form two layers, and a cavity develops between them - the chorionic cavity or extraembryonic coelom.

• The chorionic cavity continues expanding until it surrounds the embryo, leaving a portion of extraembryonic mesoderm remaining as the connecting stalk.

Secondary Yolk Sac Formation

• The hypoblast cells now expand again and push the primary yolk sac away from the embryo, forming the definitive or secondary yolk sac.

• Eventually, the primary yolk sac is connected by a thin stalk and then degenerates.

End of Second Week

• The extraembryonic mesoderm surrounding the definitive yolk sac is the first site of hematopoiesis (blood cell formation).

• Primitive endothelial and blood cells migrate from the primary ectoderm into the yolk sac, where they form the blood islands.

• By the end of the second week, the embryo is suspended in the chorionic cavity by the connecting stalk. The connecting stalk will form the future umbilical cord.