Initial stages of development
The process of nervous system development begins with the transformation of the ectoderm. First, a flat neural plate (lamina neuralis) is formed. As development proceeds, its edges begin to elevate, forming a depression — 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 laid down. The cells of the so-called neural folds form the 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 near the walls of the tube, forming grey matter. The processes of these neurons extend to the periphery of the primordium and form white matter. The original cavity inside the neural tube gradually narrows and turns into the central canal of the spinal cord, which is normally filled with cerebrospinal fluid.
Brain vesicle stage
The cranial part of the neural tube undergoes significant changes, transforming into the brain. This process goes through the stage of brain vesicle formation, where the walls thicken and the relief becomes significantly more complex.
At the first stage, three primary brain vesicles form:
- 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 the diencephalon (diencephalon). The midbrain vesicle (mesencephalon) remains unchanged and does not divide. The rhombencephalon (rhombencephalon) divides into the metencephalon (metencephalon) and the myelencephalon (myelencephalon).
Formation of brain structures
The definitive structures of the brain 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), pons (pons), and 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 called the corpora quadrigemina. The ventral part forms the massive cerebral peduncles (pedunculi cerebri). The cavity 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 various walls. The thalami (thalami) develop 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 plate. The cavity of the diencephalon turns into the third ventricle (ventriculus tertius)—a narrow sagittal cleft located between the thalami.
Briefly: answers to topical questions
Which brain structures develop from the telencephalon?
The cerebrum (cerebrum), represented by the right and left hemispheres (hemispheria cerebri), as well as the lateral ventricles (ventriculi laterales), develop from the telencephalon (telencephalon).
Three systems are distinguished within the hemispheres:
- cerebral cortex (cortex cerebri, pallium);
- basal nuclei (nuclei basales);
- rhinencephalon (rhinencephalon), occupying the most ventral position.
The cavity of the telencephalon forms two lateral ventricles (ventriculi laterales) located within the substance of the hemispheres. The grey matter of these systems is connected via the white matter of the hemispheres.
Into which embryonic zones (layers) does the wall of the neural tube divide during spinal cord formation?
During the formation of the spinal cord, the wall of the neural tube differentiates into three embryonic zones.
- Inner layer (ependymal) gives rise to the ependymal glia, which lines the central canal.
- Middle layer (mantle) is the source of grey matter, which is an accumulation of neuron cell bodies.
- Outer layer (marginal velum) turns into white matter, consisting of myelinated nerve fibers.
The nuclei of which cranial nerves are laid down in the region of the rhombencephalon?
The nuclei of cranial nerves V–XII are described in the pons and medulla oblongata.
The nuclei are arranged in three functional rows:
- Medial row — somatic motor nuclei of the VI and XII pairs.
- Middle row — autonomic nuclei of the VII, IX, and X pairs.
- Lateral row — somatic sensory nucleus of the VIII pair.
The medulla oblongata contains the nuclei of the V pair (partially), VIII pair (partially), IX, X, XI, and XII pairs of cranial nerves.
To easily remember the three primary brain vesicles, use the mnemonic: Forebrain (Prosencephalon), Midbrain (Mesencephalon), Hindbrain (Rhombencephalon).
Class and exam questions
From which germ layer does the nervous system develop?
The entire nervous system, including the brain and spinal cord, as well as peripheral ganglia, develops from the ectoderm.
What forms from the neural tube cavity in the region of the spinal cord?
The cavity of the neural tube in the caudal region turns into the central canal of the spinal cord, filled with cerebrospinal fluid.
Which primary brain vesicle does not divide into secondary ones?
The midbrain vesicle (mesencephalon) does not divide during embryogenesis and remains a single region.
What does the cavity of the diencephalon turn into?
The cavity of the diencephalon turns into the third ventricle (ventriculus tertius), which appears as a narrow sagittal cleft between the thalami.
What else to study on this topic
- Topography and functions of diencephalic nuclei
- Detailed structure of the midbrain roof (corpora quadrigemina)
- Connections of the fourth ventricle with the subarachnoid space
- Development of telencephalic derivatives
- Formation of pathways in the white matter of the spinal cord
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