Embryogenesis of the telencephalon
Development of the cerebrum begins at the rostral end of the neural tube from the prosencephalon. During embryogenesis, paired telencephalic vesicles form. They grow so rapidly that they gradually wrap around the brainstem from all sides, covering it like a mantle (pallium).
Due to uneven arched growth, the lumen of the neural tube transforms into semicircular lateral ventricles. The structures growing behind them are forced to follow the same semicircular trajectory: the caudate nucleus (one of the basal ganglia), fornix fibers, corpus callosum, and other commissural pathways.
The telencephalon primordium is divided into two topographical parts:
- Ventral: gives rise to the septum pellucidum area and the ganglionic eminence (from which the amygdala, putamen, and caudate nucleus later form).
- Dorsal: serves as the source for the development of the entire cerebral cortex.
External anatomy and surfaces
The telencephalon is divided into the right and left hemispheres by a deep sagittal longitudinal fissure, at the bottom of which lies a massive commissure — the corpus callosum. Each hemisphere anatomically presents three poles (frontal, temporal, occipital) and three surfaces:
- Superolateral (convex);
- Medial (internal);
- Basal (inferior).
The transition zone between the convex and internal surfaces is called the parasagittal region. Traditionally, each hemisphere is divided into four major lobes: frontal, parietal, temporal, and occipital. Their boundaries are defined by constant sulci; for example, the central sulcus separates the frontal lobe from the parietal lobe.
A special place belongs to the lateral (Sylvian) sulcus. It does not merely separate gyri, but extends deeply inward, forming the Sylvian cistern. Hidden at the bottom of this fissure is the fifth lobe — the insula (insula), which is externally covered by parts of adjacent lobes (the operculum).
Phylogenetic development of the cortex
During evolution, the cerebral cortex formed in stages, which determined its modern topography in humans. The rapid growth of the neocortex caused phylogenetically older areas to be displaced from the outer surface of the hemispheres.
- Paleocortex (ancient cortex). Initially located laterally, but in humans shifted to the base of the brain. It forms the basis for ancient olfactory structures (olfactory bulb, olfactory tract, anterior perforated substance).
- Archicortex (old cortex). Shifted medially under the pressure of growing vesicles. Arching over the corpus callosum, it forms a thin gray covering (indusium griseum), and in the region of the inferior horn of the lateral ventricle, it curls into the hippocampus (Ammon's horn).
- Neocortex (new cortex). The most extensive part. Due to lack of space within the cranial cavity, the neocortex folds, forming a characteristic relief of sulci and gyri.
Histogenesis and neuronal migration
The structure of the neocortex is based on a six-layered cellular organization (cytoarchitecture). The formation of cortical layers follows a strict rule: they are arranged along an "inside-out" vector.
Neurons originate near the ventricles (in the ependymal zone) and begin moving toward the brain surface along guiding radial glial fibers. The very first cells form the so-called preplate, which then splits into the marginal zone and subplate. The main thickness of the cortex is established between them.
The master regulator of this process is the Cajal-Retzius cells located in the marginal zone. They secrete the reelin protein, which serves as a stop signal for migrating neurons. Cells formed earlier remain in the deep layers (V and VI). Younger neurons migrate through them and occupy the superficial tiers. Any genetic or external disruptions in this well-tuned mechanism lead to pathologies grouped under the term "neuronal migration disorders."
Functional organization
Modern neurology rejects the idea that each higher function (e.g., speech) is tied exclusively to a single microscopic area. The entire cortex is divided into two functional types:
- Primary cortical areas — highly specialized regions directly involved in receiving signals or initiating specific actions.
- Association areas — occupy the vast majority of the cortex. Their task is higher-level analysis and complex information processing.
The execution of higher cortical functions always requires the coordinated interaction of multiple diverse fields and association areas of the cerebrum.
Briefly: answers to topical questions
What are the names of the six cellular layers of the neocortex?
The adult human cortex exhibits six cellular layers (laminae). These include:
- Molecular layer (lamina molecularis or plexiformis) — contains the tangential layer and Cajal-Retzius cells.
- External granular layer (lamina granularis externa) — consists of numerous granular cells and a small number of pyramidal cells.
- External pyramidal layer (lamina pyramidalis externa) — contains a huge number of pyramidal cells.
- Internal granular layer (lamina granularis interna) — at this level runs the outer band of Baillarger.
- Internal pyramidal layer (lamina pyramidalis interna or ganglionaris) — here runs the inner band of Baillarger.
- Multiform layer (lamina multiformis).
Which major sulci divide the lobes of the cerebral hemispheres?
The lobes of the cerebral hemispheres are separated by three deep, constant sulci and one additional landmark. These include:
- Central sulcus (sulcus centralis) — separates the frontal lobe from the parietal lobe.
- Lateral sulcus (sulcus cerebri lateralis or Sylvian fissure) — separates the temporal lobe from the frontal and parietal lobes.
- Parieto-occipital sulcus (sulcus parietooccipitalis) — separates the parietal lobe from the occipital lobe.
- Preoccipital notch — serves as a landmark on the inferior margin of the hemisphere and acts as the boundary between the temporal and occipital lobes.
Which basal nuclei (ganglia) lie within the white matter of the hemispheres?
Deep within the white matter of the hemispheres are the following accumulations of gray matter belonging to the basal nuclei:
- Corpus striatum (corpus striatum) — includes the caudate nucleus (nucleus caudatus) and the lentiform nucleus (nucleus lentiformis). The latter consists of the putamen (putamen) and globus pallidus (globus pallidus).
- Claustrum (claustrum).
- Amygdaloid body (corpus amygdaloideum).
The nucleus accumbens (nucleus accumbens) and substantia nigra (substantia nigra), which are functionally connected to the extrapyramidal system, are also classified among the basal ganglia.
Which commissural fibers (commissures) belong to the telencephalon?
The commissural fibers connecting symmetrical regions of both telencephalic hemispheres include three main structures:
- Corpus callosum (corpus callosum) — a densely packed bundle of fibers originating from extensive areas of the cerebral cortex.
- Anterior commissure (commissura anterior).
- Hippocampal commissure (commissura fornicis) — formed where part of the fornix fibers cross the midline and join the fornix of the opposite side.
P-A-N: Paleocortex — base (olfaction), Archicortex — medial (hippocampus), Neocortex — everything else (new cortex).
Class and exam questions
Why is the surface of the cerebral hemispheres folded?
During embryogenesis, neocortical tissues grow extremely rapidly and occupy a massive area. To fit within the limited volume of the skull, the cortex is forced to fold into sulci and gyri, hiding 2/3 of its surface within the folds.
What is the "inside-out" migration principle?
This is the chronological order of cortical layer formation. Neurons formed earlier remain in the deep layers. Later-generated cells migrate along radial glial fibers past the layers of their predecessors and take positions closer to the surface.
Where is the insula located and how to visualize it?
The insula (hidden lobe) is located on the medial wall of the Sylvian cistern, deep within the lateral sulcus. To see it on a gross specimen, it is necessary to separate or remove the overlying parts of the frontal, parietal, and temporal lobes (the operculum).
What else to study on this topic
- Topography of sulci and gyri of the lateral surface
- Limbic system and structures of the medial surface
- Basal ganglia: development of the ganglionic eminence
- Molecular and cellular regulation of neuronal migration
- Detailed layered structure of the cortex (cytoarchitecture)
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