Brain Development
The brain grows at an amazing rate during development. At times during brain development, 250,000 neurons are added every minute!! At birth, almost all the neurons that the brain will ever have are present. However, the brain continues to grow for a few years after birth. By the age of 2 years old, the brain is about 80% of the adult size.

You may wonder, "How does the brain continue to grow, if the brain has most of the neurons it will get when you are born?". The answer is in glial cells. Glia continues to divide and multiply. Glia carries out many important functions for normal brain function including insulating nerve cells with myelin. The neurons in the brain also make many new connections after birth.

The Brain During Development

The nervous system develops from embryonic tissue called the ectoderm. The first sign of the developing nervous system is the neural plate that can be seen at about the 16th day of development. Over the next few days, a "trench" is formed in the neural plate - this creates a neural groove. By the 21st day of development, a neural tube is formed when the edges of the neural groove meet. The rostral (front) part of the neural tubes goes on to develop into the brain and the rest of the neural tube develops into the spinal cord. Neural crest cells become the peripheral nervous system.

At the front end of the neural tube, three major brain areas are formed: the prosencephalon (forebrain), mesencepalon (midbrain) and rhombencephalon (hindbrain). By the 7th week of development, these three areas divide again. This process is called encephalization.

Average brain weights at different times of development:

       AGE               BRAIN WEIGHT (grams)
20 weeks of gestation       100 
Birth              	    400 
18 months old      	    800 
3 years old        	    1100 
Adult              	    1300-1400

Brain Weight
The top graph on the left shows the brain weights of males and females at different ages. The bottom graph shows the brain weight to total body weight ratio (expressed as a percentage). The adult brain makes up about 2% of the total body weight.

(Data from Dekaban, A.S. and Sadowsky, D., Changes in brain weights during the span of human life: relation of brain weights to body heights and body weights, Ann. Neurology, 4:345-356, 1978)

More about Brain Development

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