The Questions of Developmental Biology

Fig 12.6
Human neural tube closure requires a complex interplay between genetic and
environmental factors. Certain genes, such as Pax3,sonic hedgehog, and openbrain, are essential
for the formation of the mammalian neural tube, but dietary factors, such as cholesterol and folic
acid, also appear to be critical. It has been estimated that 50% of human neural tube defects could
be prevented by a pregnant woman's taking supplemental folic acid (vitamin B 12 ), and the U.S.
Public Health Service recommends that all women of childbearing age take 0.4 mg of folate daily
to reduce the risk of neural tube defects during pregnancy (Milunsky et al. 1989; Czeizel and
Dudas 1992; Centers for Disease Control 1992).
The neural tube eventually forms a closed cylinder that separates from the surface
ectoderm. This separation is thought to be mediated by the expression of different cell adhesion
molecules. Although the cells that will become the neural tube originally express E-cadherin, they
stop producing this protein as the neural tube forms, and instead synthesize N-cadherin and N-
CAM (Figure 12.7). As a result, the surface ectoderm and neural tube tissues no longer adhere to
each other. If the surface ectoderm is experimentally made to express N-cadherin (by injecting N-
cadherin mRNA into one cell of a 2-cell Xenopus embryo), the separation of the neural tube from
the presumptive epidermis is dramatically impeded (Detrick et al. 1990; Fujimori et al. 1990).
Secondary neurulation
Secondary neurulation involves the making of a medullary cord and its subsequent
hollowing into a neural tube (Figure 12.8). Knowledge of the mechanisms of secondary
neurulation may be important in medicine, given the prevalence of human posterior spinal cord
malformations.