Introduction

Spina Bifida and anencephaly are terms that are used to refer to a series of birth defects that affect the development of the brain and spinal cord, collectively known as neural tube defects (NTDs).

These are a group of disorders that arise early in fetal development, usually around 16-28 days after ovulation, and can potentially cause life-long complications of varying severity. These will occur if there is interference with the closure of the neural tube in human embryos. They are usually described as being malformations of the neuroectoderm and are often linked to abnormalities of surrounding mesodermal structures.

In the developing vertebrae, the neural tube is the embryo's precursor to the central nervous system, which consists of the brain and spinal cord. In the process of neurulation, the neural groove gradually deepens as the neural folds simultaneously become elevated until eventually the folds meet and coalesce in the mid line. This process coverts the groove into a closed tube, called the neural tube.

There are four subdivisions of the neural tube during development that will eventually become distinct regions of the central nervous system:

• Prosencephalon - develops into the telencephalon (forebrain) and the diencephalon (optic vesicles and hypothalamus)
• Mesencepahlon - develops into the midbrain 
• Rhombencephalon - develops into the metencephalon (pons and cerebellum) and the myelencephalon (medulla oblongata)
• Spinal cord

For a short duration, the neural tube is open both cranially and caudally. These so called neuropore openings close during the fourth week in humans and it is the incorrect closure of these that cause NTDs, such as spina bifida and anencephaly.  

The incidence of NTDs is 2.6/1000 total births worldwide and has declined significantly over the past 30 years.

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Neural tube development. Image courtesy of https://upload.wikimedia.org/wikipedia/commons/9/9e/Gray18.png. This image is in the public domain thus free of any copyright restrictions.