Embryogenesis of the Spinal Cord
The development of the spinal cord (medulla spinalis) originates from the trunk portion of the neural tube. During embryogenesis, the wall of this tube thickens and differentiates to form three distinct layers (from the inner lumen outward):
- Inner layer (ependymal layer). The deepest layer adjacent to the central canal. During development, it gives rise to ependymal glia, which form the lining of the spinal canal.
- Middle layer (mantle layer). The intermediate zone of the neural tube. This layer serves as the source for the gray matter of the spinal cord, which histologically represents aggregates of neuronal cell bodies.
- Outer layer (marginal layer). The peripheral portion of the wall. During differentiation, this layer transforms into white matter, formed by myelinated nerve fibers.
Histological Structure of the Meninges
Similar to the brain, the spinal cord is covered by three meninges, which differ in their connective tissue type and anatomical arrangement.
- Pia mater. Composed of loose connective tissue. Its primary feature is its close adherence to the spinal cord tissue. The pia mater precisely follows all complex contours of the organ, dipping into every fissure and sulcus. It houses a rich network of blood vessels that supply nutrients to the neural tissue.
- Arachnoid mater. Like the pia mater, it consists of loose connective tissue. However, unlike the pia, it does not dip into the sulci and fissures of the spinal cord, but instead bridges across them.
- Dura mater. The outermost and toughest layer. In contrast to the two inner meninges, it is formed by dense irregular connective tissue, providing robust mechanical protection for the organ.
Subarachnoid Space
The difference in the anatomical relationship (syntopy) between the pia mater and the arachnoid mater creates an important anatomical space—the subarachnoid space.
Because the pia mater extends to the bottom of the spinal cord sulci while the arachnoid bridges over them, a fluid-filled cavity (space) is created between these two loose connective tissue layers. This topographical feature is critical for understanding fluid circulation within the spinal canal.