Embryogenesis: formation of the neural tube
The development of the nervous system begins with neurulation. On the dorsal side of the embryo, the ectoderm thickens to form the neural plate. This plate then invaginates to form the neural groove, flanked by neural folds.
The folds gradually fuse (starting in the cervical region and extending cranially and caudally), creating the neural tube. Finally, the tube sinks inward, separating from the surface ectoderm, while elements of the neural crest form the ganglion plate.
Disorders at this stage lead to severe dysraphisms:
- Complete failure of the neural tube and soft tissues to close — rachischisis.
- Cleft spine (spina bifida), which can be occult (occulta) or accompanied by cyst formation (cystica).
- Failure of the anterior end of the tube to close causes anencephaly (absence of the cerebral hemispheres).
Stages of brain vesicles
The brain forms through the sequential division of vesicles at the anterior end of the neural tube:
- 3-vesicle stage: the forebrain (Prosencephalon), midbrain (Mesencephalon), and hindbrain (Rhombencephalon) vesicles are formed.
- 5-vesicle stage:
- Prosencephalon divides into the telencephalon (Telencephalon) and diencephalon (Diencephalon).
- Mesencephalon remains unchanged.
- Rhombencephalon gives rise to the hindbrain (Metencephalon — cerebellum and pons) and the medulla oblongata (Myelencephalon).
If the anterior vesicle fails to divide, holoprosencephaly occurs (its severe form is accompanied by cyclopia).
Types of vertebrate brains
In the course of evolution, three principal types of brain organization have succeeded one another:
- Ichthyopsid (fish, amphibians): The leading center is the midbrain (Mesencephalon). The forebrain is poorly developed, acts as an olfactory analyzer, and contains only elements of the paleocortex (paleocortex).
- Sauropsid (reptiles, birds): The main integrative hub becomes the floor of the forebrain — the striatum (striatum). The first primordia of the neocortex (neocortex) appear, while the midbrain takes a backseat, retaining its visual function.
- Mammalian (mammals): The neocortex of the forebrain dominates. Massive commissures (corpus callosum) form to connect the hemispheres. In higher mammals, the brain surface becomes covered with sulci and gyri.
Evolution of the forebrain
The development of the anterior division progressed from a simple thickening to a complex multilayered structure:
- In cyclostomes and fish, the hemispheres are barely expressed, the roof is formed by epithelial tissue, and the floor by nervous tissue (striatum).
- In amphibians, a clear division into two hemispheres occurs, and neural tissue appears in their roof — the archipallium (archipallium).
- In reptiles, the forebrain increases significantly, with neocortical primordia forming laterally.
- In mammals, an explosive growth of the neocortex is observed, acquiring a 6-layer cytoarchitecture and covering the other structures.
Ontophylogenetic malformations (atavisms)
Certain anomalies of human brain development represent an arrest at stages characteristic of evolutionary ancestors:
- Microcephaly — small brain volume with a simplified gyral pattern, resembling the brain of anthropoid apes.
- Lissencephaly (agyria) — an absolutely smooth brain. This condition is normal for lower mammals, rodents, and human fetuses up to the 20th week of development.
- Pachygyria — the presence of thickened, wide, and sparse gyri.
- Agenesis of the corpus callosum — absence of connections between the hemispheres, characteristic of lower mammals (e.g., marsupials).
- Prosencephaly — incomplete division of the brain into hemispheres (corresponds to the stage in fish and amphibians).
Briefly: answers to topical questions
Which structures of the peripheral nervous system are formed from the neural crest?
The neural crest (ganglion plate) gives rise to structures of the peripheral nervous system as well as several other tissues. The main derivatives include:
- Spinal ganglia (ganglia spinalia) — sensory ganglia.
- Autonomic ganglia (ganglia autonomica) — autonomic nerve ganglia.
- Cranial ganglia — sensory ganglia of the head.
In addition, migrating neural crest cells form the adrenal medulla, skin pigment cells (melanocytes), and cells of the neuroendocrine system.
What anatomical structures are derivatives of the telencephalon (Telencephalon)?
Derivatives of the telencephalon (telencephalon) are the right and left cerebral hemispheres (hemispheria cerebri) and their cavities. The hemispheres include the following structures:
- Cerebral cortex (cortex cerebri, pallium) — the superficial layer of gray matter (pallium or mantle).
- Basal nuclei (nuclei basales) — subcortical clusters of gray matter.
- Rhinencephalon (rhinencephalon) — the most ventral part of the hemispheres.
- White matter — conduction pathways of the hemispheres.
- Lateral ventricles (ventriculi laterales) — derivatives of the cavity of the forebrain vesicle.
What anatomical structures develop from the diencephalon (Diencephalon)?
The diencephalon (diencephalon) gives rise to structures forming its walls and cavity, as well as the highest centers of endocrine regulation. These include:
- Thalami (thalami) — optic thalami, developing from the lateral walls.
- Hypothalamus (hypothalamus) — formed from the ventral wall.
- Pineal body (corpus pineale) — epiphysis, developing from the dorsal wall.
- Pituitary gland (hypophysis) — a neurogenic endocrine gland.
- Third ventricle (ventriculus tertius) — the slit-like cavity of the diencephalon.
Most of the dorsal wall remains undifferentiated, forming the epithelial plate.
What cell layers make up the mammalian neocortex (neocortex)?
The mammalian neocortex (neocortex or isocortex) has a typical six-layered neuronal structure. It comprises the following cellular layers:
- Molecular layer — the most superficial layer, containing few neurons and tangential nerve fibers.
- External granular layer — composed of small pyramidal, stellate, and inhibitory cells.
- Pyramidal layer — formed predominantly by medium-sized pyramidal neurons.
- Internal granular layer — composed of small excitatory stellate neurons.
- Ganglionic layer — contains the largest pyramidal cells.
- Polymorphic layer — the deepest layer, bordering the white matter.
To remember brain types in order of increasing complexity, look at the origin of their names: Ichthyopsid brain (from Greek "ichthys" — fish), Sauropsid brain ("sauros" — lizard, reptile), and Mammalian brain ("mamma" — mammary gland, mammals).
Class and exam questions
Which structure dominates in the ichthyopsid brain type?
In the ichthyopsid brain, the main integrative center is the midbrain (Mesencephalon).
What is the main difference between the sauropsid and mammalian brains?
In the sauropsid brain, the highest control center is the corpus striatum (striatum) on the floor of the forebrain, whereas in the mammalian brain, this role shifts to the neocortex (neocortex).
What malformation results from failure of the anterior neuropore to close?
Failure of the anterior end of the neural tube to close causes anencephaly, an incompatible-with-life malformation characterized by the absence of the cerebral hemispheres.
What else to study on this topic
- Genetic programs and factors controlling neural tube induction.
- Neural crest cell migration and formation of the peripheral nervous system.
- Histological structure and functions of the paleocortex (paleocortex) and archipallium (archipallium).
- Clinical manifestations and forms of Spina bifida (occulta and cystica).
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