Morphology of Glial Cells
3.1 Astrocytes
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Figure 3.1 Morphological types of astrocytes; Ia – pial tanycyte; Ib – vascular tanycyte; II – radial astrocyte (Bergmann glial cell); III – marginal astrocyte; IV – protoplasmic astrocyte;
V – velate astrocyte; VI – fibrous astrocyte; VII – perivascular astrocyte; VIII – interlaminar astrocyte; IX – immature astrocyte; X – ependymocyte; XI – choroid plexus cell. (From:
Rechenbach A, Wolburg H (2005) Astrocytes and ependymal glia, In:Neuroglia, Kettenmann H & Ransom BR, Eds, OUP, p. 20.)
Lizard Carp
Golgi stained Muller cells Cajal, 1892
Frog Chicken Ox
Figure 3.2 M¨uller cells from the retina of different species – Golgi stained cells as drawn by S. Ram´on y Cajal
3.1 ASTROCYTES 23 include several types of cells that line the ventricles or the subretinal space, namely ependymocytes,choroid plexus cells andretinal pigment epithelial cells.
1. Protoplasmic astrocytesare present in grey matter. They are endowed with many fine processes (on average ∼50m long), which are extremely elab- orated and complex. The processes of protoplasmic astrocytes contact blood vessels, forming so called ‘perivascular’ endfeet, and form multiple contacts with neurones. Some protoplasmic astrocytes also send processes to the pial surface, where they form ‘subpial’ endfeet. Protoplasmic astrocyte density in the cortex varies between 10 000 and 30 000 per mm3; the surface area of their processes may reach up to 80 000 m2, and cover practically all neuronal membranes within their reach.
2. Fibrous astrocytes are present in white matter. Their processes are long (up to 300m), though much less elaborate compared to protoplasmic astroglia.
The processes of fibrous astrocytes establish several perivascular or subpial endfeet. Fibrous astrocyte processes also send numerous extensions (‘perin- odal’ processes) that contact axons at nodes of Ranvier, the sites of action potential propagation in myelinated axons. The density of fibrous astrocytes is ∼200 000 cell per mm3.
3. The retina contains specialized radial glia called Müller cells, which make extensive contacts with retinal neurones. The majority of Müller glial cells have a characteristic morphology (Figure 3.2), extending longitudinal processes along the line of rods and cones. In certain areas of retina, e.g.
near the optic nerve entry site, Müller cells are very similar to proto- plasmic astrocytes. In human retina, Müller glial cells occupy up to 20 per cent of the overall volume, and the density of these cells approaches 25 000 per mm2 of retinal surface area. Each Müller cell forms contacts with a clearly defined group of neurones organized in a columnar fashion; a single Müller cell supports ∼16 neurones in human retina, and up to 30 in rodents.
4. The cerebellum contains specialized radial glia calledBergmann glia. They have relatively small cell bodies (∼15 m in diameter) and 3–6 processes that extend from the Purkinje cell layer to the pia. Usually several (∼8 in rodents) Bergmann glial cells surround a single Purkinje neurone and their processes form a ‘tunnel’ around the dendritic arborization of Purkinje neurones. The processes of Bergmann glial cells are extremely elaborated, and they form very close contacts with synapses formed by parallel fibres on Purkinje neurone dendrites; each Bergmann glial cell provides coverage for up to 8000 of such synapses.
5. Velate astrocytesare also found in the cerebellum, where they form a sheath surrounding granule neurones; each velate astrocyte enwraps a single granule neurone. A similar type of astrocyte is also present in the olfactory bulb.
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6. Interlaminar astrocytesare specific for the cerebral cortex of higher primates.
Their characteristic peculiarity is a very long single process (up to 1 mm) that extends from the soma located within the supragranular layer to cortical layer IV. The specific function of these cells is unknown, although they may be involved in delineating cortical modules spanning across layers.
7. Tanycytesare specialized astrocytes found in the periventricular organs, the hypophysis and the raphe part of the spinal cord. In the periventricular organs, tanycytes form a blood–brain barrier by forming tight junctions with capil- laries (the blood–brain barrier is normally formed by tight junctions between the endothelial cells, but those in the periventricular organs are ‘leaky’, and the tanycytes form a permeability barrier between neural parenchyma and the CSF).
8. Astroglial cells in the neuro-hypophysis are known aspituicytes; the processes of these cells surround neuro-secretory axons and axonal endings under resting conditions, and retreat from neural processes when increased hormone output is required.
9. Perivascular and marginal astrocytes are localized very close to the pia, where they form numerous endfeet with blood vessels; as a rule they do not form contacts with neurones, and their main function is in forming the pial and perivascular glia limitans barrier, which assists in isolating the brain parenchyma from the vascular and subarachnoid compartments.
10. Ependymocytes,choroid plexus cellsandretinal pigment epithelial cellsline the ventricles or the subretinal space. These are secretory epithelial cells.
They have been considered under the umbrella term glia because they are not neurones. The choroid plexus cells produce the CSF which fills the brain ventricles, spinal canal and the subarachnoid space; the ependymocytes and retinal pigment cells are endowed with numerous very small movable processes (microvilli and kinocilia) which by regular beating produce a stream of CSF and vitreous humour, respectively.