Korean, Edit

Chapter 7. Neuroembryology

Higher category: 【Biology】 Brain Science Index

1. 1st. The formation of the neural tube

2. 2nd. The cell proliferation of the neural tube

3. 3rd. The formation and differentiation of the brain vesicle

4. 4th. The development of the primitive brain

5. 5th. The change of the wall of the neural tube (medulla oblongata)

6. 6th. The change of the wall of the neural tube (cerebellum, cerebrum)

7. 7th. The partial expansion of the neural tube

8. 8th. The maturation of the nervous system

1. 1^st. The formation of neural tube

⑴ 1^st - 1^st. Zygote becomes embryonic disc which consists of 3 germinal layers(endoderm, mesoderm, ectoderm) through “cleavage”.

⑵ 1^st - 2^nd. In the ectoderm, neurectoderm becomes thicker so that neurectoderm forms special cellular layers called “neural plate”.

⑶ 1^st - 3^rd. Diverticulation : Between neural plate and other ectoderm, there appears a protruded structure called “neural crest”. (A)

① Neural crest : The tissue which differentiates from neurectoderm and doesn’t participate in constructing the neural plate.

○ Examples: cranial meninges, spinal meninges, etc.

⑷ 1^st - 4^th. Both lateral sides of neural plate becomes more crested, and there appears “neural groove”. (B)

⑸ 1^st - 5^th. Both sides of neural crest regions meet and fuse, beginning at the midline and progressing toward the margins; later, all portions—except the rostral (cranial) and caudal ends—unite to form the “neural tube”. (C)

① Anterior neuropore : The entrance of the neural tube at the head

② Posterior neuropore : The entrance of the neural tube at the tail

⑹ 1^st - 6^th. The neural tube soon closes at both openings (the anterior and posterior neuropores); the lumen of this tube is called the central canal.

① Notochord: a structure that, during development, induces neural tube closure.

② If closure fails, anencephaly or spina bifida can result.

⑺ 1^st - 7^th. Cells around the neural crest spread out and some of those cells make spinal ganglion. (D)

2. 2^nd. The cell proliferation of the neural tube

⑴ 2^nd - 1^st. Initially the total wall of the neural tube consists of a few layers of cells.

⑵ 2^nd - 2^nd. Cellular layers at the wall of the neural tube becomes thicker by cell proliferation.

⑶ 2^nd - 3^rd. These cellular layers form the “mantle layer”, and the cells differentiate into neurons later.

⑷ 2^nd - 4^th. The neural fibers of the cells at the mantle layer extend and form the “marginal layer”.

⑸ 2^nd - 5^th. At the innermost portion of the neural tube, adjacent to the central canal, a neuroepithelial layer forms; later it differentiates into the “ependymal layer”.

⑹ 2^nd - 6^th. In the mantle layer, the ventral neurons (in the basal plate) become motor neurons, and the dorsal neurons (in the alar plate) become sensory neurons.

⑺ 2^nd - 7^th. The induced structure of the neural tube

① Spinal Cord

○ Alar Plate : Posterior Horn (Sensory Nuclei)

○ Basal Plate : Anterior Horn (Motor Nuclei)

② Medulla Oblongata

○ Alar Plate : General Somatic Afferent, Special Somatic Afferent, Visceral Afferent

○ Basal Plate : General Somatic Efferent, Special Somatic Efferent, General Visceral Efferent

③ Pons

○ Alar Plate : Tectal Structure

○ Basal Plate : Tegmentum(Central Tegmental Tract, Trapezoid Body), Basis Pontis(Griseum Pontis, Transverse Pontine Fiber, Corticospinal Tract)

④ Midbrain (Mesencephalon)

○ Alar Plate : Tectum

○ Basal Plate : Cerebral Peduncle(Red Nucleus, Substantia Nigra, Crus Cerebri)

⑤ Diencephalon

○ Sulcus Limitans : Hypothalamic Sulcus

○ Alar Plate : Dorsal Thalamus

○ Basal Plate : Hypothalamus, Subthalamus, Zona Incerta

3. 3^rd. The formation and differentiation of the brain vesicle 

⑴ 3^rd - 1^st. The neural tube is divided into 3 brain vesicles: “prosencephalon”, “mesencephalon”, and “rhombencephalon”

⑵ 3^rd - 2^nd. Prosencephalon: It differentiates into “telencephalon”, “diencephalon”, etc.

① Telencephalon : It differentiates into “cerebral hemisphere”, basal ganglia, etc.

○ Basal ganglia : It differentiates into “corpus striatum”, “amygdala”, etc.

② Diencephalon : It differentiates into “thalamus”, etc.

○ Characteristics : It makes up the “limbic system”.

○ Thalamus differentiates into “dorsal thalamus”, “epithalamus”, “hypothalamus”, “subthalamus”, “metathalamus”, etc.

⑶ 3^rd - 3^rd. Mesencephalon: It differentiates into midbrain.

① Characteristics 1. All the animal has it.

② Characteristics 2. The higher intelligence the entity has, the relatively smaller size mesencephalon has.

⑷ 3^rd - 4^th. Rhombencephalon: It differentiates into “metencephalon”, “myelencephalon”, etc.

① Characteristics: The higher intelligence the entity has, the more developed rhombencephalon is.

② Metencephalon: It differentiates into “cerebellum”, “pons”, etc.

③ Myelencephalon: It differentiates into “medulla oblongata”, etc.

⑸ 3^rd - 5^th. Spinal cord: It has very slight change after embryo.

Figure 5. The differentiation process of the brain vesicle

The middle image shows primary vesicles and the right shows secondary vesicles.

Table 1. The phylogenesis in brain vesicles during ontogenesis

Most pallium(cortex) of human brain is “neopallium”.

4. 4^th. The development of the primitive brain

⑴ 4^th - 1^st. The flexure formation

⑵ 4^th - 2^nd. The process of changing appearance of brain

⑶ 4^th - 3^rd. The external form of the brain is completed within 8 weeks after fertilization. But it is not more than a water bag.

5. 5^th. The change of the wall of the neural tube (medulla oblongata)

⑴ 5^th - 1^st. The migration of the mantle layer and the marginal layer is observed “only” in medulla oblongata.

⑵ 5^th - 2^nd. In the medulla oblongata, the acellular ventricular wall gapes in a serrated (saw-toothed) pattern.

6 6^th. The change of the wall of the neural tube (cerebellum, cerebrum)

⑴ The explanation focuses on the cerebellar process. This process generates the cortex structure(e.g., gyri, sulci)

⑵ 6^th - 1^st. Proliferation: Granule cells, a type of neuron, proliferate in the mantle layer.

⑶ 6^th - 2^nd. Migration: Half of the proliferated cells migrate toward the cortex.

① In practice, fewer than half migrate because mantle-layer cells must also give rise to glia; therefore, the “half” holds only if mantle-layer cells proliferate exclusively into migratory cells.

② 6^th - 2^nd - 1^st. Radial glial fibers guide the cells so they migrate in a straight path.

③ 6^th - 2^nd - 2^nd. Granule cells move to just beneath the cerebellar cortex, forming a new layer of gray matter called the external granular layer (EGL). (B, C)

④ 6^th–2^nd–3^rd. The granule cells that constituted the external granular layer migrate somewhat inward.

○ Mechanism for the molecular layer: this creates a sparsely cellular layer at the outermost cortex.

⑤ 6^th–2^nd–4^th. At the same time, Purkinje cells are generated and migrate to a outer position superficial to the granule layer (B, C).

⑥ Inside-out phenomenon: During development of the cerebellum and cerebrum, cells that migrate later end up more superficial (outer) than those that migrate earlier; in other words, the outermost cells are the last to be generated.

○ Reason: later-migrating cells pass earlier ones and can form cell–cell connections in the process.

○ Additional note: once all connections are formed, unnecessary ones can be pruned.

○ Approximate proportions: radial migration ~80%, tangential migration ~20%.

⑷ 6^th - 3^rd. Differentiation : The granule cells, the purkinje cells, and the mantle layer cells differentiate in place, and the mantle layer cells become “deep cerebellar nuclei”. (D)

① 6^th - 3^rd - 1^st. The migrating cells are small and lean so that they are optimized in migrating, so the migrated cells are larger than the migrating cells about 10 times.

⑸ 6^th - 4^th. Like above, the structures of both the cerebellum and the cerebrum become ‘basal ganglia’ → ‘medulla’ → ‘cortex’. 

① That is, nerve cell body - neural fiber(axon) - nerve cell body.

② That is, gray matter - white matter - gray matter.

③ In the spinal cord, the mantle (intermediate) zone is gray matter, and the marginal zone is white matter.

Figure 11. Cross-sections of neural tube, spinal cord, and cerebellum during the development of mouse embryos

⑹ The proliferation is ventricle-specific and the migration is CNS-specific.

7. 7^th. The partial expansion of the neural tube

⑴ As the lumen of the neural tube (the central canal) expands locally, it forms the various ventricles in different regions.

① Choroid Plexus : It secretes CSF(Cerebrospinal Fluid).

② CSF : It flows through Lateral Ventricle → 3^rd Ventricle → 4^th Ventricle.

Figure 12. The cross section of the embryonic brain focusing on vesicles

⑵ Spinal cord: The lumen retains its embryonic shape and is called the central canal.

⑶ Medulla oblongata and pons: The lumen is markedly expanded and is called the fourth ventricle.

⑷ Midbrain: The lumen largely retains its original configuration and is called the cerebral aqueduct.

⑸ Diencephalon: The lumen is expanded and singular in the median plane, called the third ventricle.

⑹ Telencephalon: The lumen is expanded, with one in each cerebral hemisphere, called the lateral ventricles.

⑺ Interventricular foramen: The third ventricle and the lateral ventricles are connected via the interventricular foramen.

⑻ Cerebrospinal fluid (CSF): The fluid that flows through the central canal and all ventricles.

Figure 13. The cross section of the adult’s brain focusing on vesicles

Figure 14. Ventricular anatomy

8. The maturation of the nervous system

⑴ Proliferative phase: up to about two-thirds of fetal development

① Cell number increases to roughly 28 billion, with more than 10 billion concentrated in the brain.

⑵ Rapid growth phase: until 2 months after birth

① Rapid development of dendrites and axons; formation of basic synapses

② At birth: about 24% of adult brain size

③ Chimpanzee: 60% of adult size at birth; development ceases within the first year.

⑶ By 2 years after birth

① Basic networks are in place, preparing for learning

② Cell growth and network expansion proceed according to learning and experience.

⑷ Age 6

① About 95% of adult brain size

② Brain-cell growth continues until approximately 16–18 years of age.

⑸ After age 18

① Around 50,000 brain cells are lost per day.

⑹ Senile dementia

① Inability to form new networks; disruption of existing connectivity.

Input: 2018.09.15 23:51