Astrocyte and glial ratios Paper

The glial ratio increases during evolution depending on the different bodies of species. The human brain contains a glial ratio of one-to-one and the cerebral cortex of 1.4. During evolution, large mammals like savannah elephants increase the glial ratio by 80%. The change in the glia-neurons ratio can result from the correlation between the number of glial and the density of neuronal density. Astrocytes tend to differ due to the glial population of different species (Vasile, 2017).

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Astrocyte Subpopulations within Human Brain

Interlaminar Astroglia –The anatomical location of this type of astrocyte is in layers I and II of the cortex. This type of astroglia has a multilamellar structure and mitochondria. The role of interlaminar astroglia in Alzheimer’s disease and Down syndrome provides neuronal support to the brain.

Protoplasmic Astroglia -The anatomical location of protoplasmic astroglia is in layers III and IV of the cortex. The protoplasmic astroglia is characterized as bulbous because of the diolistic labeling process and GFAP processes. Protoplasmic astroglia is used to modulate and integrate inter-neuronal communication among several synapses. Also, it provides exceptional computational power.

Varicose Projections Astroglia -The anatomical location of varicose projection astrocyte is in layers V and VI of the cortex. This type of astrocytes has long fibers structures with varicosities. Varicose projection astroglia plays a role in the cognitive functions of a human. The varicosities in varicose projection astrocytes ensure compartmentalization and communication among subcellular sections and cortical layers.

Fibrous Astroglia – The anatomical location of fibrous astroglia is in the white matter in the cerebral cortex. They are large, have straight processes, are less branched, and have few fine processes. Fibrous astroglia plays a role in supporting metabolic processes in the vasculature. Fibrous astroglia tend to be distant from each other so as to provide full support to the axon tract.

Functions for Human Astrocytes

Neuronal Survival and Synaptogenesis

Human astrocytes provide neuronal survival, where the astrocytes contribute to synapses and engulfment of synaptosomes. The secretion of the growth factor in human astrocytes promotes synapses through the process of D-serine mediation (Vasile, 2017). Also, human astrocytes promote neuronal survival.

Electrophysiology

Human astrocytes have a larger capacitance which helps in increasing the size of soma and proximal processes. Unlike rodent astrocytes, the human astrocyte is highly increased for membrane resistance. The membrane resistance has a constant length to allow further transmission of current within the brain.

Intercellular Coupling

Intercellular coupling allows the exchange of neuromodulators, metabolites, and ions. Also, the human astrocyte mediates signal in-network upscale. The availability of connexins in human astrocytes is used for the modulation of protein expression. The availability of gap junction (GJ) is the major astroglia function as a sub-unit of astrocytes.

Neurotransmitter Recycling and Uptake

The human astrocyte contains glutamine transporters that help the human brain colocalize with astrocytes to avoid excitotoxicity. The glutamate transporters are expressed and functional for transient inward currents. Neurotransmitters are used in support of energy production.

Calcium Signaling and Gliotransmission.

The presence of signaling-related proteins and calcium transients in human astrocytes helps in gliotransmission. The low nature of human astrocytes helps detect synaptic activities within the brain. Increased speed of propagation is a result of the enlargement of astrocytes. The availability of glutamate in mature astrocytes lead to an increase in intracellular calcium through the glutamate receptor 5.

Reference

Vasile, F., Dossi, E., & Rouach, N. (2017). Human astrocytes: structure and functions in the healthy brain. Brain Structure and Function, 222(5).