Initial Stages of Development
The development of the nervous system begins with the transformation of the ectoderm. First, a flat neural plate (lamina neuralis) forms. As development proceeds, its lateral edges elevate to form a depression called the neural groove (sulcus neuralis). Subsequently, the edges of this groove fuse with each other, resulting in the formation of a closed neural tube (tubus neuralis).
Simultaneously with the closure of the tube, the neural crest (crista neuralis) is established. Cells of the so-called neural folds form ganglionic plates. The most important elements of the peripheral nervous system develop from these structures:
- spinal ganglia (ganglia spinalia);
- autonomic ganglia (ganglia autonomica).
In the caudal region of the neural tube, the spinal cord is formed. Cellular elements concentrate directly along the walls of the tube, forming the gray matter. The processes of these neurons extend toward the periphery of the anlage and form the white matter. The initial lumen inside the neural tube gradually narrows and transforms into the central canal of the spinal cord, which is normally filled with cerebrospinal fluid.
Brain Vesicle Stage
The cranial portion of the neural tube undergoes significant changes, transforming into the brain. This process proceeds through the stage of brain vesicle formation, where the walls thicken and the structural relief becomes significantly more complex.
At the first stage, three primary brain vesicles are formed:
- Forebrain — prosencephalon;
- Midbrain — mesencephalon;
- Hindbrain — rhombencephalon.
Subsequently, differentiation occurs, and secondary brain vesicles are formed. The forebrain vesicle (prosencephalon) divides into the telencephalon (telencephalon) and diencephalon (diencephalon). The midbrain vesicle (mesencephalon) remains undivided. The hindbrain (rhombencephalon) divides into the metencephalon (metencephalon) and myelencephalon (myelencephalon).
Formation of Brain Structures
The definitive brain structures develop from the brain vesicles, and their cavities take the shape of ventricles, which are filled with cerebrospinal fluid and communicate freely with each other as well as with the central canal of the spinal cord.
Development of the Hindbrain (rhombencephalon) Three major structures form from this region: the cerebellum (cerebellum), the pons (pons), and the medulla oblongata (medulla oblongata). The cavity of the rhombencephalon transforms into the fourth ventricle (ventriculus quartus), which has apertures communicating with the subarachnoid space.
Development of the Midbrain (mesencephalon) Its dorsal wall thickens and forms the roof of the midbrain (tectum mesencephali), also known as the corpora quadrigemina. The ventral part forms the massive cerebral peduncles (pedunculi cerebri). The lumen of the midbrain vesicle narrows and turns into the cerebral aqueduct (aqueductus mesencephali), connecting the cavities of the third and fourth ventricles.
Development of the Diencephalon (diencephalon) This region is characterized by intensive growth of its walls. The thalamus (thalamus) develops from the lateral walls. The ventral wall gives rise to the hypothalamus (hypothalamus). The dorsal wall forms the pineal gland (corpus pineale), although most of it remains undifferentiated, forming a thin epithelial lamina. The cavity of the diencephalon transforms into the third ventricle (ventriculus tertius), a narrow sagittal cleft located between the thalami.