Human Embryonic and Fetal Development: Key Stages and Structures

A) Pre-Embryonic Period – First 2 Weeks

1. Zygote

The zygote is the initial cell formed after fertilization, marking the beginning of human development.

• Definition: A single diploid cell (2n) formed in the uterine tube after the fusion of sperm and egg.

• Cleavage Divisions: The zygote undergoes 4-5 rapid mitotic divisions, known as cleavage divisions, which do not increase the overall size but produce smaller cells.

• Morula Formation: These divisions result in a solid ball of cells called the morula.

Example: The zygote divides to form a morula, which is still the same size as the original ovum/zygote.

2. Morula

The morula is a solid ball of cells resulting from cleavage divisions of the zygote.

• Cell Number: Typically consists of 16-32 cells.

• Size: Remains the same as the original zygote due to lack of growth between divisions.

• Function: Prepares for further differentiation into specialized cell types.

3. Blastocyst

The blastocyst is a hollow, fluid-filled structure that forms after the morula stage and is essential for implantation and early embryonic development.

• Features:

  • Development of a fluid-filled cavity (blastocoel/blastocyst cavity).

  • Formation of embryoblast (inner cell mass) and trophoblast (outer cell layer).

  • Embryoblast: Will later divide and differentiate to form the embryo.

  • Trophoblast: Will divide and differentiate to form the chorion, which provides nutrients to the developing embryo.

• Implantation: The blastocyst (at about 5 days) moves from the uterine tube and attaches to the endometrium of the uterine wall.

• After Implantation:

  • Embryoblast differentiates into a bilayered disc (two cell layers):

    • Epiblast (above):

      • Will undergo mitosis to form:

        • Amnion: Forms a fluid-filled amniotic cavity.

        • Three germ layers of the embryo.

    • Hypoblast (below):

      • Will undergo mitosis to form the yolk sac.

  • Extraembryonic Membranes: Start to form, supporting everything around/outside the embryo.

B) Embryonic Period – 3rd Week to End of 8th Week

The embryonic period is characterized by the formation of major organ systems and the establishment of the body plan.

• Gastrulation: Formation of three primary germ layers:

  1. Ectoderm: Forms muscles, cardiovascular system, dermis, and most bones. Example: The ectoderm also gives rise to the nervous system and skin.

  2. Mesoderm: Forms epithelial linings of digestive, respiratory, urinary, and reproductive systems, and their associated glands.

  3. Endoderm: Forms epithelial linings of digestive, respiratory, urinary, and reproductive systems, and their associated glands.

• Organogenesis: Major organ systems almost completely formed.

  • Heart begins to beat.

  • Limb buds differentiate.

• Extraembryonic Membranes:

  1. Chorion: Made from trophoblast cells; surrounds all embryonic membranes.

     • Contains chorionic villi (finger-like projections) that facilitate exchange of nutrients, wastes, oxygen, and carbon dioxide between fetal and maternal blood.

  2. Amnion: Derived from epiblast; surrounds embryo/fetus with amniotic fluid.

     • Function: Shock absorption, temperature regulation.

  3. Yolk Sac: Derived from hypoblast; forms part of digestive tract and is the source of reproductive germ cells and embryonic blood cells.

  4. Allantois: Projection off the yolk sac; collects waste and forms the umbilical cord and urinary bladder.

C) Fetal Period – 9th Week to End of 40th Week

The fetal period is marked by the growth and maturation of organs formed during the embryonic period.

• Growth: Rapid increase in size and mass of the fetus.

• Maturation: Organs and organ systems become fully functional.

Table: Extraembryonic Membranes and Their Functions

Membrane	Origin	Main Function
Chorion	Trophoblast cells	Exchange of nutrients/gases; forms placenta
Amnion	Epiblast cells	Encloses amniotic fluid; shock absorption
Yolk Sac	Hypoblast cells	Forms digestive tract; source of germ cells and blood cells
Allantois	Yolk sac projection	Waste collection; forms umbilical cord and bladder

Key Terms and Definitions

• Zygote: The initial cell formed by the union of sperm and egg.

• Morula: A solid ball of cells resulting from cleavage divisions of the zygote.

• Blastocyst: A hollow, fluid-filled structure that implants into the uterine wall.

• Germ Layers: Ectoderm, mesoderm, and endoderm; primary layers that give rise to all tissues and organs.

• Extraembryonic Membranes: Structures that support the developing embryo/fetus but do not form part of the body itself.

Relevant Equations

• Cell Division: Number of cells after n divisions: $N = 2^n$ where $N$ is the number of cells and $n$ is the number of divisions.