Neural tube defects: Nursing

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Neural tube defects, or NTD for short, refer to developmental defects of the central nervous system secondary to failed closure of the neural tube during the fetus's embryonic development.

Now, let’s quickly look at the physiology of neural tube development. Let’s zoom in on a cross section of the early embryo. Here we can see it is made up of three main layers: the ectoderm, the mesoderm, and the endoderm.

The ectoderm then goes on and differentiates into three populations of cells. The first one, located internally, gives rise to the neural tube; the second one, located externally, gives rise to skin cells and the epidermis; and the third population, called the neural crest cells, develop in between the neural tube and epidermis, and give rise to various structures throughout the body.

The mesoderm, on the other hand, develops into a transient midline structure, called the notochord, which produces various signals that guide the development of other embryonic structures with respect to the midline.

During the third and fourth weeks of development, the notochord signals for the formation of the neural tube, through a process called neurulation, which is highly dependent on adequate folic acid levels. Neurulation progresses in two stages.

Primary neurulation begins when the ectoderm right above the notochord thickens and gives rise to the neural plate.

At both ends of the neural plate, there are neural crest cells, and beyond them, the ectoderm that gives rise to the epidermis.

Soon after, the edges of the neural plate thicken and tilt upward, forming the so-called neural folds. This allows for an u-shaped neural groove to form, which sets the limit between the right and left sides of the embryo.

During secondary neurulation, the neural folds eventually fuse at the midline, forming the hollow neural tube, separate from the epidermis, above it. Now let's look at a different view of the developing embryo, this time from above.

At this point, like any hollow tube, the neural tube still has openings at both ends: a large opening at the top end called the cranial neuropore, and a smaller opening at the bottom end called the caudal neuropore. The cranial neuropore seals up around day 25, while the caudal neuropore seals up a few days later, around day 28. The neural tube eventually develops into the brain and spinal cord, while the notochord gives rise to a part of the intervertebral disc called the nucleus pulposus.