One of the most severe developmental defects is called anencephaly, which literally means “without a brain.” However, usually some brain tissue develops normally, but the forebrain and cerebrum is small or absent. This defect is caused by an error in the development of the nervous system and brain, and is thought to be somewhat related to the mother’s intake of folate/folic acid (Vitamin B9). Babies born with anencephaly nearly almost always die either in the womb or shortly after birth, as their compromised nervous system is unable to sustain bodily functions (and certainly not consciousness). In addition, an anencephalic fetus often does not have a skull or scalp covering what neural tissue they have, leaving the brain exposed and susceptible to infection and damage.
How exactly does a fetus develop without a brain?
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Anencephaly is one of many types of “neural tube defects,” and unfortunately one of the most common. As organisms develop during gestation, rudimentary neural tissue, which will further develop into the brain and nervous system, follows a very stereotypical series of events. These events are often conserved through many species and happen in very narrow windows of time. One event of particular importance is the closure of the neural tube.
In humans, neural tube development follows “primary neurulation” meaning that the cells of primitive neural plate form a cup and then fuse to form a tube, and then “secondary neurulation” where cells form a cord structure and migrate to make the tube hollow. For a short time, the neural tube is open at both ends (cranially and caudally; caudally referring to what will become the end of the spinal cord). These openings (neuropores) almost always close during the fourth week of gestation in humans, however sometimes they do not. Failure to close at the cranial end can result in anencephaly while failure to close at the caudal end can cause spina bifida.
The reason that distinct brain regions are affected in anencephalic fetuses has to do with the preliminary segmentation of the neural tube during its open/closing state. The neural tube is roughly segmented into 4 divisions, each which give rise to different CNS regions: the prosencephelon, the mesencephelon, the rhombencephelon, and the spinal cord. The prosencephelon is towards the cranial end of the neural tube, and is therefore most severely affected by a failure to close. The prosencephelon would have given rise to the forebrain and the diencephelon, which are compromised or missing in anencephalic fetuses.
The direct causes of neural tube defects are not crystal clear, but studies have shown that women who have folic acid deficiencies or are taking medication for epilepsy are more likely to have a child with this type of defect. However there also seems to be a genetic component which has not yet been pinpointed. Luckily aminocentresis and ultrasound can often indicate early on if a fetus displays this defect.
Cotran RS, Kumar V, Robbins SL: Robbins Pathologic Basis of Disease. 5th ed. Philadelphia, W.B. Saunders.