2.1. Microglia
The central nervous system's (CNS) resident immune cells, or microglia, are crucial for preserving brain homeostasis and controlling immunological responses in the brain. Here, we go into some basic facts concerning microglia, such as their anatomy, origin, and purposes (Nayak et al., 2014).
Myeloid progenitor cells, which start off in the yolk sac during embryonic development, give rise to microglia cells. Early in embryogenesis, these progenitor cells go to the CNS where they give birth to progenitors of the microglia. After then, the precursors settle into the brain tissue and develop into adult microglia (Yin et al., 2017). A microglia's structure in the CNS is depicted in Figure 1.
Figure 1. Microglia in central nervous system
Microglia have a characteristic shape with a tiny cell body and many, ramified processes that are both extremely active and numerous. Microglia has a highly branching phenotype while at rest, which aids in their ability to keep an eye on their environment. Continuously scanning the brain microenvironment for indications of damage, infection, or aberrant cellular activity, the processes are active (Morris et al., 2013).
The functions of microglia is summarized in Figure 2 and described briefly below.
Figure 2. Major functions of microglia
As immunological sentinels, microglia keeps an eye out for any changes in the CNS. They are naturally able to identify and react to a variety of signals, including aberrant protein aggregates, damage-associated molecular patterns, and pathogen-associated molecular patterns (PAMPs). When microglia receives these signals, immunological responses are triggered by pattern recognition receptors (PRRs) produced on their surface, such as toll-like receptors (TLRs) (Kigerl et al., 2014).
Highly phagocytic microglia effectively engulfs and removes infections, dead cells, and other detritus from the brain. They are essential for preserving tissue integrity and eliminating noxious materials to stop tissue damage.
By generating and releasing several immune mediators, including cytokines, chemokines, and growth factors, microglia control immunological responses in the CNS. The balance of these chemicals, which can have both pro- and anti-inflammatory actions, is essential for preserving brain homeostasis (Guo et al., 2022).
During development, microglia support synaptic plasticity and refinement by actively interacting with neurons. They take part in the removal of extra synapses, which enables the construction of precise neuronal circuits. Additionally, neurotropic substances that sustain neuronal survival and enhance synaptic function can be released by microglia (Konishi et al., 2019).
Neurotransmitter levels in the brain are regulated by microglia. They have the capacity to absorb and digest neurotransmitters, which has an impact on synaptic transmission and neuronal signalling.