Bio 361. Chapter 12 Outline A

12.1 Neurulation as an example of organogenesis

Neurulation is the sequence of morphogenetic events that form the central nervous system. The process begins in the area of dorsal ectoderm, which is transformed into the neural plate. The neural plate then closes into a hollow tube called a neural tube, which forms the brain and spinal cord (fig 12.3).

Neural tube defects
The process of neurulation is of major scientific interest because of the prevalence of a congenital malformation known as spina bifida. In this defect, closure of the neural tube is delayed, resulting in abnormal development of bone, muscle and skin around the brain and spinal cord. The defect ranges from very mild where a few vertebrae are absent and the spinal cord bulges into a cyst, to a severe form where the cephalic part of the neural tube fails to close and the infant is born without a forebrain (anencephaly). This is fatal. See Fig 12.4. Folic acid in the diet reduces the incidence of neural tube defects.

Neurulation in amphibians
This occurs in two phases. The first phase is formation of the neural plate. This ends with the keyhole stage. In this stage, the neural ectoderm cells on the dorsal surface of the embryo move to the dorsal and anterior midline. The cells become columnar in shape and form the neural plate. This contains a depression called the neural groove. Ridges of cells called neural folds surround the neural groove. The remaining ectoderm cells assume a flattened shape and eventually become epidermal ectoderm (fig 12.5).

In the second phase of neurulation, the neural folds fuse along the dorsal midline to enclose the neural plate into a tube. The epidermal ectoderm fuses above the neural tube in the dorsal midline. A group of intervening cells become the neural crest. These eventually migrate throughout the body to give rise to many different cell types.

Neurulation in birds
In bird embryos, the neural plate forms as Hensen's node regresses from anterior to posterior. Soon afterwards the neural plate undergoes convergent extension. Cells of the dorsolateral region form the dorsolateral hinge points, and cells of the median region with the underlying notochord form the median hinge point (fig 12.6). All three hinge points facilitate bending and closure of the anterior neural plate into a tube that will form the brain. In the posterior region of the neural plate, only the median hinge point is used, and this region folds into the spinal cord. Cells also undergo intercalation to increase the anteroposterior dimension and reduce the lateral dimension.

Neurulation in humans
Neurulation is similar to that in birds except that the anterior portion of the neural plate is delayed in closure (fig 12.7). The shape of the neural plate results from coordinated shape changes and cell intercalations. Closure begins in the neck region of the human embryo and proceeds anteriorly and posteriorly. The unclosed openings are known as the anterior neuropore and posterior neuropore. These remain connected to the amniotic cavity until closure.

Neurulation in fish
The neural plate forms on the dorsal side of the epiblast (fig 10.21). Instead of curling and closing into a tube, cells on both sides of the median hinge point grow together to form a longitudinal structure called the neural keel (fig 12.8). The neural keel first closes into a cylindrical rod, then the halves of the neural plate separate again to form of a lumen. This type of neurulation is known as secondary neurulation. Amphibians, humans, and birds undergo primary neurulation.

12.2 Mechanisms of neurulation in amphibians

Tissues adjacent to the neural plate influence the process of neurulation. The epidermal ectoderm and the axial mesoderm (future notochord) are both important for the process of neurulation. During neurulation, cells of the neural plate undergo major shape changes. These changes include columnarization, convergent extension, and cell intercalations.
In the second phase of neurulation, the neural plate folds into a tube due to a combination of apical constriction and anteroposterior elongation.