Curves from One to Four
A fetus's spine looks much different from an adult's spine. While in the womb, a fetus's spine has one C-shaped curve. Once the baby is born and starts walking, the spine settles into four curves that hold the body up while distributing weight.
If you've read How Pregnancy Works, you already know about how a baby develops in a woman's body. A man's sperm joins with a woman's egg, creating a one-celled zygote. That one cell divides into two, which divide into four, over and over until there are about 100 cells. At this point, a few days after conception, the zygote becomes a blastocyst, and that's when the earliest beginnings of spinal development start to happen.
A blastocyst starts off as a collection of similar cells, but it doesn't stay that way for long. It develops three cell layers, called germ layers, in a process called gastrulation. The layers are the ectoderm, mesoderm and endoderm. Most of the body's innermost organs are formed from the endoderm, while most of the external features, like skin and hair, come from the ectoderm. The spine, part of the middle of the body, comes from the mesoderm. These layers are distinct within 12 days of the egg's fertilization.
The road from undifferentiated cells to a whole body is complex, so here's a rundown of what happens just in terms of the spine:
- The layers of the blastocyst move to where they're needed, arranging themselves to build a body.
- Cells from the mesoderm get together to form a structure called the notochord. This structure provides some support for the developing embryo.
- About 25 days after fertilization, the ectoderm above the notochord folds. The folds form a canal called the neural tube, which will become the central nervous system.
- Mesenchymal cells from the mesoderm form groups called somites on either side of the neural tube. These are like tiny building blocks that will become the vertebrae. Since an embryo develops a tail that disappears as it grows, there are more somites than vertebrae.
- When the embryo is 6 or 7 weeks old, ossification, or bone formation, begins. The somites harden, eventually becoming vertebrae. How Bones Work explains exactly what's happening during ossification. The notochord becomes part of the discs that lubricate the connections between the vertebrae.
In order for the spine to grow correctly, everything has to happen without a hitch, from migration of the blastocyst's layers to ossification. If the spinal column doesn't close correctly, the result can be one of a number of birth defects. Among the most common are neural tube defects, which include spina bifida and anencephaly, or a lack of brain development. Getting enough folate and folic acid during the earliest days of pregnancy reduces the risk of these defects.
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