At 18 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
Voyage of Life: Week 5
The emergence of a body plan
- Post-conception week 3
- Days 14-20
- Gestational Week 5
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 fifth week gestation 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.
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