HISTOLOGY 1.14.: NERVOUS TISSUE: GLIAL CELLS Neuroglia comprise over 90 % of the cells of the nervous system. Glial cells are relatively small. Their function is to provide structural support form the CNS boundary ensheath and insulate axons maintain the ionic homeostasis of the extracellular space phagocyte cell debris produce scar tissue Classification of neuroglia: in the central nervous system (CNS): ependyma astrocytes oligodendrocytes microglial cells in the peripheral nervous system (PNS): neurolemmocytes: Schwann cells satellite cells

Neuroglia of the CNS: ependymal cells Ependymal cells line ventricular cavities in the brain and the central canal of the spinal cord. They are cuboidal or columnar cells tightly packed together. Their luminal surface is ciliated and microvilliated. The cerebrospinal fluid is produced by modified ependymal cells (choroid plexus) EM LM

Neuroglia of the CNS: astrocytes They have the largest nuclei among glial cells. They are stellate-shaped with numerous processes: in the white matter: long slender processes: fibrous astrocytes in the gray matter: shorter branching processes: protoplasmic astrocytes. They contain glial fibrillary acidic protein (GFAP) forming glial filaments. Astrocyte processes form expanding endfeets which adjoin into limiting membranes: membrana limitans gliae superficialis (at the surface of CNS) membrana limitans gliae perivascularis (blood-brain barrier). Astrocytes provide structural support, form diffusion barriers around synapses, take up extracellular potassium ions are capable of phagocytosis proliferate to form a scar in the case of injury

Schematic drawing of a protoplasmic astrocyte: Nucleus Glial filaments Not found Capillary Endfeet of the astroglia around capillary Basal lamina of the capillary endothelium Endfeet around a synapse Presynaptic terminal Spiny dendrite Myelinated axon Initial segment of an axon Axo-axonic synapse Axospinous synapse Dendritic spine

Neuroglia of the CNS: oligodendrocytes They have small spherical nuclei in routinely stained sections. Their processes are thin and difficult to visualize. Their function in the gray matter is perineuronal satellite-like, in white matter they form the myelin sheaths around axons. LM Oligodendrocyte Tapering process of oligodendrocyte Axon Myelin sheath Synapse at node of Ranvier Soma of a neuron Axon hillock Synaptic boutons

Neuroglia of the CNS: microglia Microglia are cells of mesodermal origin that invade the CNS when it is vascularized. They have antigen presenting and phagocytic abilities. They have small elongated chromophilic nuclei. Other cell types in the CNS with phagocytotic activity: astrocytes, oligodendrocytes, pericytes, hematogenous macrophages Silver impregnation

Neuroglia of the PNS: neurolemmocytes They are glial cells responsible for the myelination of peripheral axons (Schwann cell), or encapsulate neuronal cell bodies as satellite cells. They provide a protected immediate environment for PNS neurons. They can become phagocytic upon nerve damage. Cross-section of a myelinated peripheral axon at the level of a Schwann cell nucleus EM LM Schwann cells (arrows)

Satellite cells ( yellow arrow and no. 3) around a pseudounipolar neuron from the spinal ganglion

NERVE FIBERS: MYELINATED AXONS Node of Ranvier LM EM LM Red arrow: outer mesaxon Blue arrow: inner mesaxon Small red arrow: neuro- filament Small blue arrow: micro- tubulus EM Cross-section Longitudinal section

NERVE FIBERS: NON-MYELINATED AXONS Schwann cell nucleus Mesaxon Cross-section of the axon

Structure of a peripheral nerve 1-6. Layers of epineurium 7. Myelinated fiber with Schwann cell 8. Non-myelinated fibers in a Schwann cell 9. Capillary 10-11. Endoneurium 12. Perineurium 13. Fibrocyte