Human Embryonic and Fetal Development: Key Stages and Structures

A) Pre-embryonic Period – First 2 Weeks

Zygote Formation and Early Cleavage

The pre-embryonic period encompasses the first two weeks after fertilization, during which the zygote undergoes rapid mitotic divisions and initial differentiation.

• Zygote: A single diploid cell formed in the uterine tube after fertilization.

• Undergoes 4–5 mitotic divisions known as cleavage divisions, resulting in a solid ball of cells called the morula.

Morula

• Consists of a solid ball of 16–32 cells.

• Approximately the same size as the original ovum/zygote.

• Cells continue to divide and begin to differentiate into specific cell types with distinct functions.

Blastocyst Formation

The morula develops into a blastocyst, a structure with a fluid-filled cavity and differentiated cell populations.

• Blastocyst cavity (also called blastocoel): Fluid-filled space within the blastocyst.

• Embryoblast (inner cell mass): Will later divide and differentiate to form the embryo.

• Trophoblast (outer cell mass): Will divide and differentiate to form the chorion, which provides nutrients to the developing embryo.

• The blastocyst moves from the uterine tube and attaches to the endometrium of the uterine wall, a process called implantation.

After Implantation

• The embryoblast differentiates into a bilayered structure (two cell layers):

  • Epiblast (upper layer):

    • Undergoes mitosis to form:

      • Amnion: A fluid-filled amniotic cavity.

      • 3 germ layers of the embryo (ectoderm, mesoderm, endoderm).

  • Hypoblast (lower layer):

    • Will undergo mitosis to form the yolk sac.

• Extra-embryonic membranes begin to form.

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

Germ Layer Formation and Organogenesis

During the embryonic period, the three primary germ layers are established, and the basic body plan and organ systems begin to develop.

• 3 germ layers are now present, forming the embryo (each layer arises from mitosis and differentiation of epiblast cells):

  • Ectoderm: Forms the epidermis, nervous system, and associated glands.

  • Mesoderm: Forms muscle, bone, connective tissue, cardiovascular system, and urogenital organs.

  • Endoderm: Forms the epithelial lining of the digestive, respiratory, urinary, and reproductive tracts, and their associated glands.

• Major organ systems are almost completely formed by the end of this period.

• The heart begins to beat.

• Limb buds differentiate.

Extraembryonic Membranes

Extraembryonic membranes support the developing embryo and facilitate nutrient, gas, and waste exchange.

• Chorion (derived from trophoblast cells):

  • Surrounds all embryonic membranes.

  • Contains chorionic villi, which are projections containing blood vessels that will form the fetal part of the placenta.

  • Functions in nutrient, waste, and gas exchange between fetal and maternal blood.

• Amnion (derived from epiblast):

  • Forms the amniotic cavity, which surrounds the embryo/fetus with amniotic fluid.

  • Functions in shock absorption and temperature regulation.

• Yolk sac (derived from hypoblast):

  • Forms part of the digestive tract.

  • Source of early blood cells and embryonic blood vessels.

• Allantois:

  • Projection off the yolk sac.

  • Forms the umbilical cord and the urinary bladder.

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

Growth and Maturation

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

• Continued development and functional maturation of organ systems.

• Significant increase in size and weight of the fetus.

Summary Table: Extraembryonic Membranes

Membrane	Origin	Main Functions
Chorion	Trophoblast	Forms fetal part of placenta; nutrient/gas exchange
Amnion	Epiblast	Encloses amniotic cavity; shock absorption, temperature regulation
Yolk sac	Hypoblast	Early blood cell formation; forms part of digestive tract
Allantois	Yolk sac projection	Forms umbilical cord, urinary bladder

Additional info: The three germ layers (ectoderm, mesoderm, endoderm) are foundational for all tissues and organs in the body. The process of organ formation from these layers is called organogenesis. The placenta, formed in part by the chorion, is essential for fetal-maternal exchange throughout pregnancy.