Final answer:
The embryological development of the brain and spinal cord starts with the creation of the neural tube, which differentiates into the CNS. The neuraxis establishes the body's nervous system dimensions, with the cephalic flexure orienting this axis. The anterior neural tube forms primary vesicles, which evolve into the brain's major regions and undergo dramatic changes in size from the embryo to the adult.
Step-by-step explanation:
The embryological development of the brain and spinal cord begins with the formation of the neural tube, a structure that arises from the folding of the neural groove present in an early embryo. This neural tube differentiates into the central nervous system (CNS), with its anterior end developing into the brain and the posterior portion forming the spinal cord. As the cells making up the wall of the neural tube proliferate, they differentiate into neurons and glia, with dorsal tissues associated with sensory functions and ventral tissues with motor functions.
The initial formation of the neural tube establishes an axis known as the neuraxis, which provides the structural basis for further development. Humans complicate this standard arrangement due to our bipedal stance, resulting in a neuraxis that superimposes the anterior-posterior dimension of the nervous system with the superior-inferior dimension of the body. Additionally, a significant curve known as the cephalic flexure develops between the brainstem and forebrain, orienting the neuraxis from an inferior position at the end of the spinal cord to an anterior position at the front of the cerebrum.
Throughout embryonic development, the anterior neural tube enlarges into primary vesicles, marking the beginning of the brain's regions: the forebrain, midbrain, and hindbrain. These structures continue to mature and develop into adolescence, contributing to the architecture of the adult brain. The relative sizes of these brain regions significantly change from the early embryonic stage to adulthood, with the forebrain, in particular, experiencing substantial growth.