- Inner ear – The inner ear is responsible for hearing and balance and is fully formed at birth. The complex structures in the inner ear, including the cochlea, vestibule, and semicircular canals, do not grow or regenerate throughout a person’s life.
- Number of fat cells – The number of fat cells in the body remains relatively constant after early adulthood. While the size of fat cells can increase or decrease depending on a person’s diet and lifestyle, the number of fat cells does not increase.
- Brain size – While the brain continues to develop and change throughout a person’s life, the overall size of the brain remains relatively constant after early adulthood.
- Number of neurons in the brain – The number of neurons in the brain is established early in fetal development and remains relatively constant throughout a person’s life.
- Lens of the eye – The lens of the eye is fully formed at birth and remains the same size and shape throughout a person’s life, although its flexibility decreases with age.
- Eye muscles – The muscles that control eye movement do not grow significantly after birth, but they can weaken over time.
- Number of hair follicles on the scalp – The number of hair follicles on the scalp is established before birth and does not increase significantly after birth.
- Number of sweat glands in the body – The number of sweat glands in the body is established before birth and does not increase significantly after birth.
- Fingernails and toenails – While fingernails and toenails do grow in length, they do not increase in size or number after they have fully developed.
- Cartilage in joints – While cartilage in joints does not grow significantly after birth, it can degenerate over time due to wear and tear or injury.
