Hyaluronic Acid-based Scaffolds for Brain Tissue Engineering - Hyaluronic Acid for Biomedical and Pharmaceutical Applications - page 121

The brain is composed primarily of two classes of cells, neurons and
glial cells, as shown in Figure 6.1 . The neuron is the most important
cell in the brain that consists of a cell body (soma), dendrites and a
long axon. Its function is to generate, gather, or transmit electrical
impulses away from the CNS towards the body or from the body
toward the CNS over long distances via specific chemical contacts
called synapses, which release a chemical called a neurotransmitter to
bind the receptors in the membrane of the target cell. There are over
150 billion neurons in the brain, which form an extensive network
through synaptic connections between axon and soma or dendrite.
Glial cells, such as oligodendrocytes, astrocytes and microglia,
protect, support and nourish the neurons. Oligodendrocytes
form myelin sheaths around axons that help to insulate the axons
and propagate electrical signals more efficiently. Myelin actually
comprises cytoplasmic extensions containing about 40% water, 50%
lipids and 10% proteins. Astrocytes are the most abundant type
of glial cell in the brain, from which numerous projections extend
anchoring neurons and blood vessels. They perform many important
functions, including promoting oligodendrocyte myelinating activity,
forming the blood-brain barrier, modulating neurotransmitter
uptake and release, providing nutrients to the neurons, maintaining
the extracellular ion balance, and leading to glial scar formation
following brain injuries.
Neurons and glial cells reside in the brain extracellular matrix (ECM)
which is composed of a ternary network of HA, proteoglycans
of the lectican family, and link proteins (tenascins) ( Figure 6.2 )
[10, 13]. HA is an anionic, highly hydrated glycosaminoglycan
distributed ubiquitously within almost all the ECM throughout
mammalian connective, neural, and epithelial tissues, which consists
of repeating disaccharides of β(1,4)-glucuronic acid and β(1,3)-
N -acetyl- d -glucosamine [14, 15]. In adult brain tissue, as shown
in Figure 6.2 , HA forms a filamentous network to maintain the
ECM structure. The proteoglycans of the lectican family include
brevican, aggrecan, neurocan, and versican, which contain a lectin
domain and a HA-binding domain, that can regulate cell adhesion
and neurite outgrowth. Beside HA and lecticans, tenascins are also