Leech brains6/16/2023 (b) The dorsal view of the ganglion presents packet glial cells enveloping neuron cell bodies. (a) Diagram of leech nervous system containing a head ganglion, 21 body ganglia, and 7 fused tail ganglia joined by connectives. While the importance of ‘‘bone marrow-derived microglia” is highly questioned in neurodegenerative diseases, the understanding of the functional differences of infiltrating macrophages is a necessary prerequisite to elucidate successive steps, involving activated microglia, occurring in CNS pathologies. Importantly, despite the in vitro studies, morphological and histological in vivo analyses do not allow reliable discrimination of these two cell types because of common changes in form and marker staining. In mammals, the CNS is supported and defended by two different macrophage populations: resident microglia and CNS-infiltrating macrophages. Thus the microglial functions appear to be complex as they exhibit both neuroprotective and neurotoxic effects. The complexity of microglia researches is also increased when we consider that the blood brain barrier is variably broken in these pathologies and that infiltrated cells might subsequently play a critical role in the disease. In addition, phagocytes with morphological features of endogenous microglia might be derived from bone-marrow (BM) cells or from circulating monocytes during CNS diseases. In addition to the endogenous microglia which result from invasion processes in brain during early embryogenesis, studies showed that myeloid progenitors can penetrate into the brain even in normal adult mice to replace decaying microglial cells. The complexity of microglial responses is reinforced by the cell origin which is still controversial. Microglial cells with this particular form are generally referred to as “activated microglia” or “reactive microglia”. When the brain is injured or affected by diseases (e.g., degenerative, infectious, or autoimmune diseases), the resident ramified microglia morphologically transform into cells with retracted processes and enlarged cell bodies and increase in number at the affected site. That constitutes a very early stage in response to injury. Microglia constitute the first line of cellular defense mechanisms against central nervous system diseases, participating in the regulation of nonspecific inflammation as well as adaptive immune response. In vertebrates, microglia are currently considered to be a kind of sensor in the brain because they respond to alterations in the brain and are activated by such changes. Microglial cells are regulators of tissue homeostasis in the adult central nervous system and readily participate in pathological processes, orchestrating tissue remodeling. The study of this dialog is necessary to elucidate the balance of the inflammation leading to the leech CNS repair.Īlthough long underestimated, microglia nowadays comprise an attractive target for accessing the diseased CNS. Those questions aim to better understand the mechanisms of microglial cell recruitment and their crosstalk with damaged neurons. They will discuss the implication of leech factors in the microglial accumulation, the identification of nerve cells producing these molecules, and the study of different microglial subsets. ![]() The present review will describe the questions which are addressed to understand the nerve repair. ![]() The microglia recruitment is known to be essential for the usual sprouting of injured axons and does not require any other glial cells. Considering the low infiltration of blood cells in this process, the leech CNS is studied to specify the activation mechanisms of only resident microglial cells. The medicinal leech Hirudo medicinalis is a well-known model in neurobiology due to its ability to naturally repair its CNS following injury. Functions of microglia appear to be complex as they exhibit both neuroprotective and neurotoxic effects during neuropathological conditions in vivo and in vitro. During pathologies in mammals, inflammatory processes implicate the resident microglia and the infiltration of blood cells including macrophages. Microglia are intrinsic components of the central nervous system (CNS).
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