Cerebral Cortical Histology
Cerebral Cortical Histology
cerebral cortical classification System
Three distinct histological patterns:
Allocortex
- Allocortex is phylogenetically old, comprises as few as 3 layers (up to 5 layers).
- It comprises the olfactory cortex, uncus of the parahippocampal gyrus (which is its medial thumb) and the hippocampus and associated dentate gyrus.
- The allocortex further subdivides into:
- Paleocortex (eg, olfactory cortex)
- Archicortex (ancient cortex: eg, hippocampus)
Neocortex (aka isocortex)
- Is phylogenetically new, and comprises six histologically distinct layers.
- It constitutes roughly 90% of the cerebral cortex.
Mesocortex
- Represents transitional cortex between neocortex and allocortex.
- Comprises the cingulate gyrus and the remainder of the parahippocampal gyrus.
- Mesocortex further subdivides into:
- Periallocortex (which divides into peripaleocortex and periarchicortex)
- Proisocortex
neurons and glia
Overview
See: Neurons & Glia
- The brain comprises blood vessels and two main cell types: neurons and their supporting cells: glia. It comprises at least 100 billion neurons.
Neurons
See: Neurons
2 Types:
- Pyramidal cells
- 10 to 80 micrometers (?m) in diameter and are shaped like a flask with a single thick, cortically oriented apical dendrite and multiple basal dendrites. One important pyramidal cell type is the Betz cell, which lies within the primary motor cortex.
- Non-pyramidal cells
- 5 to 15 ?m in diameter. The term non-pyramidal cell is used interchangeably with the terms stellate or granule cells. But other non-pyramidal cell types exist, which include basket cells, fusiform cells, Horizontal cells of Cajal, Martinotti cells, and neurogliaform cells.
Glia (aka neuroglia)
4 types:
- Astrocytes
- Microglia
- Oligodendrocytes
- Ependymal cells
Neuron Cell Structure
Cell bodies
- The command center of neuronal signaling; their dendrites which receive signals (typically from other neurons) and send them to the cell body.
- They are the railroad station.
Axons
- Transmits signals to/from the cell body.
- They are the railroad track.
Clinical Correlation: Ganglioglioma & Gangliocytoma
Glia
Astrocytes
Further subdivide, here, into: protoplasmic and fibrous astrocytes.
Protoplasmic astrocytes
- Thick, branched processes that attach to neurons and vessels.
- The term astrocyte refers to the star-like shape of the cell.
Fibrous astrocyte
- Long, thin, unbranched processes.
- Provide structural support
- Help determine capillary permeability
- Guide neuronal migration and brain plasticity
Clinical Correlation: Astrocytoma
Microglia
- Thorny processes, which help monitor neuronal health.
- Can morph into a special type of macrophage that can phagocytosize pathogens or dead neurons.
Oligodendrocyte
- Its processes hold myelin sheaths that wrap around the axons.
- Oligodendrocytes have a similar shape to astrocytes but have fewer processes.
- They create the myelin sheath within the central nervous system.
- A single oligodendrocyte is capable of providing myelin to multiple axons.
- Label the space between myelin segments as a Node of Ranvier.
- Myelin is fundamental to the speed of axonal transmission.
Clinical Correlation: Oligodendroglioma
Ependymal cell
- Cuboidal-shaped, typically (though these cells can have various shapes)
- Cilia on some of the ependymal cells track into the cerebrospinal fluid – depending on the type and location, not all ependymal cells have cilia.
- Ependymal cells form a somewhat permeable barrier between the cerebrospinal fluid filled cavities and the brain and spinal cord tissue.
- Do not confuse this with the blood/brain barrier, which provides a unique barrier between the brain tissue and the blood and requires astrocytes to form.
- Cilia help circulate the CSF.
Clinical Correlation: Ependymoma & Subependymoma
Histology of neurons and glia at high resolution
Neuron
- Large, basophilic cytoplasm.
- Large nucleus with diffuse chromatin.
- Prominent nucleolus.
- Neuronal process.
Oligodendrocyte
- Small, condensed, round nucleus, and clear cytoplasm, which is an artifact of processing: the so-called "fried egg appearance".
Neuropil
- Fibrillary amalgamation of dendritic processes and axons that constitute the bulk of the slide.
The six cytoarchitectural layers of the neocortex
Neocortical Layers
See: Neocortical Layers
From outside to inside (leptomeninges on the outside and white matter on the inside):
Layer I
- The Molecular (or plexiform) Layer
- A nerve fiber layer, meaning that it is cell sparse and predominantly comprises axons and dendritic processes.
Layer II
- The External Granular Layer
- Contains non-pyramidal cells 5-15 micrometers, and relatively few, small pyramidal cells.
Layer III
- The External Pyramidal Layer
- Predominantly contains pyramidal cells of varying sizes: 10 to 80 micrometers. Also, layer III is sparsely populated with non-pyramidal cells.
Layer IV
- The Internal Granular Layer
- Densely packed with non-pyramidal cells.
- Contains the horizontally oriented external band of Baillarger: a prominent thalamocortical nerve fiber layer. In the primary visual cortex, this nerve fiber band is called the line of Gennari.
Layer V
- Internal Pyramidal (or ganglionic) Layer
- Contains the largest pyramidal cells, most notably the Betz cells of the primary motor cortex – the major cortical motor neurons.
- The horizontally-oriented internal band of Baillarger lies deep within this layer.
Layer VI
- The Multiform (or fusiform) Layer
- Contains a wide variety of pyramidal and non-pyramidal cells.
Brodmann Areas
- Although the specific cellular constitution and nerve fiber density of each histologic layer varies across the cerebral cortex, certain areas of similarity exist. In 1909, Korbinian Brodmann published the most widely recognized cytoarchitectural maps of the human brain, which distinguish 52 different cytoarchitectural areas. Each area or group of areas subserves a unique function, which we address elsewhere.
Images
- Histological images modified with permission from http://medsci.indiana.edu/a215/virtualscope/docs/chap6_3.htm