The neural plate is a foundational structure in early embryonic development, serving as the initial blueprint for the entire nervous system. It emerges during a precise stage of embryonic growth, laying the groundwork for complex neural structures. This structure is fundamental for the proper formation and function of the brain and spinal cord. Without its precise formation, the intricate network that controls all bodily functions would not be able to develop.
Formation of the Neural Plate
The neural plate begins to form around the third week of human embryonic development, originating from a specialized region of the embryonic ectoderm. This ectodermal area, on the dorsal side of the embryo, undergoes thickening and flattening. Cells within this region elongate, forming a distinct, paddle-shaped structure. This transformation is initiated by molecular signals emanating from the underlying notochord, a rod-like structure that forms along the embryo’s dorsal midline.
The notochord releases specific signaling molecules, primarily Sonic Hedgehog (Shh) and bone morphogenetic protein (BMP) inhibitors. These signals induce the overlying ectoderm to differentiate into neuroectoderm. The precise concentration gradients of these molecules guide the patterning of the neural plate, influencing its patterning into brain and spinal cord regions. This intricate signaling ensures the correct spatial organization of the developing nervous system.
From Plate to Tube
Following its formation, the flat neural plate undergoes a transformation, folding inward during neurulation. The edges of the neural plate, neural folds, elevate and move towards each other. This inward folding creates a central depression, the neural groove, along the midline of the embryo. This dynamic process is driven by changes in cell shape and cell-to-cell adhesion within the neuroectoderm.
As the neural folds continue to elevate, they eventually meet and fuse along the dorsal midline. This fusion begins in the middle and extends both cranially and caudally. Closure of these folds forms the hollow neural tube, now fully internalized beneath the surface ectoderm. This tube is the precursor to the central nervous system.
What the Neural Tube Becomes
The newly formed neural tube undergoes differentiation, forming the central nervous system. The anterior (cranial) portion of the neural tube expands and subdivides into three primary brain vesicles: the prosencephalon (forebrain), mesencephalon (midbrain), and rhombencephalon (hindbrain). These vesicles differentiate into the major structures of the adult brain. The posterior, or caudal, part of the neural tube develops into the spinal cord.
As the neural tube forms, cells at the crests of the neural folds detach and migrate throughout the embryo. These are the neural crest cells, multipotent cells. Neural crest cells give rise to various tissues, including parts of the peripheral nervous system (e.g., sensory neurons, autonomic ganglia). They also form structures outside the nervous system, such as melanocytes and parts of the craniofacial skeleton.
When Development Goes Awry
Disruptions during the formation and closure of the neural plate can have serious consequences, leading to birth anomalies called neural tube defects (NTDs). These conditions arise when the neural tube fails to close completely. The severity of the defect depends on the location and extent of the incomplete closure.
Two common NTDs are spina bifida and anencephaly. Spina bifida results from incomplete closure of the neural tube in the spinal region, causing varying degrees of spinal cord and nerve damage. Anencephaly, a more severe condition, occurs when the cranial end of the neural tube fails to close, resulting in the absence of much of the brain and skull. These errors highlight the precision required during early neural development.