3.3: Embryonic Development of the Brain and Spinal Cord

The very first system to differentiate in the embryo is the nervous system, with its formation initiating as early as three weeks into development. As the nervous system starts to take shape, ectoderm, the outer layer of the embryo, engages in differentiation, giving rise to the skin and the foundation of the nervous system. Within this layer, a specialized region known as the neural plate emerges, marking the inception of neural development. This initial step sets in motion the formation of the neural tube, a structure that ultimately evolves into the brain and spinal cord (Figure 1).

From its earliest stages, the neural tube undergoes intricate transformations, eventually expanding and diversifying into distinct regions that define the complexity of the mature brain. Segmentation into three primary vesicles—prosencephalon, mesencephalon, and rhombencephalon—lays the groundwork for subsequent specialization. Later in development, two of these vesicles further specialize, resulting in five distinct vesicles. The prosencephalon differentiates into the telencephalon and diencephalon, while the rhombencephalon becomes the metencephalon and myelencephalon (Figure 2). These five vesicles correspond to the major regions of the adult brain. The cerebrum originates directly from the telencephalon, while the diencephalon encompasses the hypothalamus, thalamus, and epithalamus. The metencephalon gives rise to the pons and cerebellum, whereas the myelencephalon forms the medulla oblongata. The mesencephalon remains as the midbrain. 

Concurrently, the spinal cord emerges from the remainder of the neural tube, maintaining its tubular structure while undergoing essential modifications. Nervous tissue thickens within the spinal cord, while the once-hollow center diminishes to form a small central canal. Remarkably, the original hollow center of the neural tube transforms into the ventricles within the brain, serving as reservoirs for cerebrospinal fluid.¹

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