Week 2 to 3

The emergence of a body plan

Human Prenatal Age
  • PCW 3
    i
  • Days 14-20
  • Gestational Week 5
Highlights
  • If a woman has regular menstrual cycles, she likely realizes that her period is late.

  • The inner cells of the embryo now form three layers: The top layer becomes the skin, nervous system, eyes and ears. The middle layer contributes to the muscles, bones, kidneys and reproductive system. The inner layer contributes to the baby’s lungs and intestines.

  • Brain tissue and heart tissue start to develop.

This is no ball of cells! The complexity of the embryo after the third week of development gives the developing human a well-defined blueprint. Cells form distinct layers and the entire body plan begins to emerge. Each cell’s position will give it a specific destiny based on the chemical messages surrounding it.

Cell layers

Remember how the zygote and the early embryonic cells could become almost any cell in the human body? Now the cells start to specialize based on their position in the embryo. As can be seen in the picture, the embryo becomes wide and flat. A cross section of the embryo reveals three different layers: endoderm, mesoderm, and ectoderm.1

Different body systems form from each layer. Cells from the ectoderm will become the skin, hair, and fingernails. A specialized layer of ectoderm, called neuroectoderm, becomes the nerves and brain. Cells from the middle layer, called mesoderm, will become the heart, kidneys, bones, blood vessels and muscles. Cells from the endoderm, near the yolk sac, become major parts of the intestines, pancreas, liver, and lungs.2

(Image Credit: Nik Spencer/Nature)
The early nervous system

At two weeks and four days after conception, scientists observe the first sign of the developing brain. Neuroectoderm tissue thickens and forms the neural plate. The neural plate will fold together to become the neural tube, which later becomes the brain and spinal cord.3

The early circulatory system

The circulatory system also starts developing remarkably early. Early blood cells start forming in the yolk sac, early blood vessels start to form a network throughout the embryo, and the embryo’s heart cells appear by the end of the third week.4

Dive Deeper
The placenta and umbilical cord form as maternal and embryonic tissues interact...