Nervous Tissue (II) - Cytology and Histology PDF

Cytology and Histology LESSON 13 NERVOUS TISSUE (II) Neuroglial or glial cells Nervous tissue has 2 classes of cells: neurons, which are the main signalling units of the nervous system, and glial cells, which are support cells. Glial cells, also called neuroglia or simply glia, are much more numer...

Cytology and Histology LESSON 13 NERVOUS TISSUE (II) Neuroglial or glial cells Nervous tissue has 2 classes of cells: neurons, which are the main signalling units of the nervous system, and glial cells, which are support cells. Glial cells, also called neuroglia or simply glia, are much more numerous than neurons: there are between 10 and 50 times more glial cells than neurons in the vertebrate CNS, and their ability to divide remains intact. Glial cells perform support, metabolism and protection functions, since neurons do not come into contact with connective tissue or blood vessels. The name derives from the Greek word for glue, reflecting the 19 th century assumption that the glia held the nervous system together in some way. The neuroglia is divided into two large groups depending on their location: neuroglia of the central nervous system and neuroglia of the peripheral nervous system. II. NEUROGLIAL CELLS IN THE CENTRAL NERVOUS SYSTEM (CNS) Neuroglial cells of the CNS are interstitial cells that are located between the neurons of the brain and the spinal cord. They are divided into two large groups based on their size, but also on their embryonic origin: 1. The cells of the macroglia, larger, present two varieties: the astroglia and the oligodendroglia. They are of neuroectodermal origin, like neurons. 2. The cells of the microglia, of smaller size, present just one single variety, the microglia. They are of mesodermal origin. 1. Macroglia cells: Astroglia Astroglia cells are specifically stained with the Cajal’s gold sublimate histochemical technique and express glial fibrillary acidic protein (GFAP) with immunohistochemical techniques. They are located both in the grey matter and in the white matter. There are two types: astrocytes and ependymocytes (or cells of the ependymal epithelium). Astrocytes They are the most numerous glial cells and owe their name to the fact that their bodies are irregular, more or less star-shaped. In haematoxylin-eosin stained histological slides, astrocytes do not differentiate well from other glial cells because their cytoplasmic processes are not visible. To see the astrocytes in light microscopy, it is necessary to use silver impregnation techniques (Golgi chromato-silver impregnation, Cajal’s gold sublimated, Del Río-Hortega’s impregnation) and immunohistochemical techniques (GFAP). Cytology and Histology Their numerous cytoplasmic processes are evidenced by these techniques. According to the morphology of these processes, two types of astrocytes are distinguished: protoplasmic, located in the grey matter of the CNS and fibrous, which are mainly found in the white matter of the CNS (Figure 1). Protoplasmic astrocytes are stellate cells with abundant cytoplasm, a large nucleus, and numerous short, branched processes. Fibrous or fibrillar astrocytes are elongated, and their cytoplasmic processes are long and mainly unbranched. Some of the cytoplasmic processes of protoplasmic and fibrous astrocytes terminate in pedicels, enlarged club-shaped structures. Pedicels, also called terminal feet, make contact with three different structures: (1) The surface of neurons in the brain and spinal cord (nutrient supply) (neurotropa glia); (2) The blood vessels (capillaries and venules) (helping to form the blood-brain barrier) (angiotropic glia); and (3) The leptomeninges (the pia mater), forming the outer glia limitans or glial limiting membrane. The blood-brain barrier prevents the free circulation of substances between the cerebral capillaries and the nervous tissue. It is made up of the vascular endothelium, the basement membrane, the Virchow-Robin space, and the terminal feet of fibrous and protoplasmic astrocytes (perivascular glia limitans). The endothelium of the capillary is continuous, with tight junctions in between its cells. Below the endothelial cell the basement membrane appears, and immediately the Virchow-Robin space appears. The structure is completed with the terminal feet of astrocytes that adapt to the VirchowRobin space. Protoplasmic astrocyte Microglia Fibrous astrocyte Oligodendrocyte Figure 1. Schemes corresponding to neuroglial cells based on sections prepared by silver impregnations. It can be seen that only astrocytes show vascular feet directed towards the blood capillary. In general, the cytoplasm of astrocytes has an underdeveloped RER and Golgi complex. Mitochondria are large and very abundant. In the cytoplasmic matrix there are accumulations of glycogen. In the branches, but not in the cell body, there are bundles of intermediate filaments or glyofilaments. These glyofilaments are both vimentin and GFAP positive, with only those of GFAP being specific for astroglia. In summary, the functions of astrocytes are: Cytology and Histology 1. Mechanical function of structural support through its dense network of cytoplasmic processes (a function that here in nervous tissue is equivalent to that of the extracellular matrix of connective tissue). 2. Modulation of interneuronal signalling by maintaining the adequate concentration of potassium ions and phagocytosis of neurotransmitters released during synapsis into the extracellular space. 3. Isolation of nervous tissue from other tissues (mesodermal: meninges, blood vessels): This is a protective function through the formation of the outer glial limiting membrane (together with the pia mater) and the blood-brain barrier (together with capillary endothelial cells and venules). 4. Contribution to energy metabolism within the cerebral cortex, since they accumulate glycogen from which glucose can be synthesized when necessary for neurons (through the activation of glycogenolysis by different neurotransmitters, such as norepinephrine and the vasointestinal peptide, VIP). 5. Cleaning and repair of lesions of the nervous tissue (along with microglial cells). They undergo morphological and functional transformations (reactive astrocytes). The ependymal epithelium Also called simply the ependyma, it is a simple epithelium that lines the wall of the cerebral ventricles and the central canal of the spinal cord (or ependymal canal) (Figure 2). The cells are cubic or low columnar and usually have cilia at the apical border, through which they facilitate the movement of cerebrospinal fluid. They have a central spherical to oval and vesicular nucleus with uniformly distributed chromatin and cytoplasm with abundant mitochondria and bundles of intermediate GFAP filaments (for which this epithelium is recognized as of astroglial lineage). Cell varieties of the ependymal epithelium: 1. Ependymocytes, properly speaking, lining the lumen of the lateral ventricles and the central canal of the spinal cord. 2. Tanicytes: They are ependymal cells located next to the third ventricle, whose processes extend through the hypothalamus and end on blood vessels and neurons. They are cells closely related to the neuroendocrine system (release of neurohormones and / or transport of cerebrospinal fluid). 3. Choroid plexus cells: They line the choroid plexuses (pia mater with blood vessels) responsible for secreting cerebrospinal fluid (CSF). They are cubic in shape, with microvilli and some cilium in the apical border, lateral tight junctions, near the apical border, and in the basal border, invaginations and mitochondria (active transport). Note: During the development of the central nervous system, some ependymal cells present long cytoplasmic processes that reach the surface of the nervous parenchyma; They are the cells of the Radial Glial cells, which serve as a guide for the migration of new neurons. After the development of the CNS, these cells of the radial glia retract their processes and many of them transform into free astrocytes. In the cerebellum, the cells equivalent to the radial glia are the Bergmann glial cells. 2. Macroglia cells: Oligodendroglia The oligodendrocytes were described by Del Rio Hortega using the Golgi’s method. They resemble astrocytes but are smaller and have fewer cytoplasmic processes which, in turn, are less branched. In fact, etymologically oligodendrocyte means "cell with few processes". They are related to the soma and neuronal axons and are therefore considered neurotropic glia. Cytology and Histology They have a spherical cell body, with short and thin branches and a spherical nucleus with heterochromatin. The Golgi and the RER are highly developed. They have microtubules in the branches but there are no glyofilaments (they do not express GFAP), and their cytoplasm is electron dense. Different enzymes such as esterase and carbonic anhydrase (histochemical techniques) and the proteins Olig1 and Olig2 (immunohistochemical techniques) are specific oligodendroglial markers. They are located in both the grey matter and the white matter. In the grey matter they are located next to the neuronal bodies and are called satellite oligodendrocytes. Its function is not clear, but they seem to monitor the extracellular fluid around neurons and they may also act as reserve cells for interfascicular oligodendrocytes. In the white matter they are located in a row on the sides of the axon bundles and are called interfascicular oligodendrocytes. Its function is to form the myelin sheath for the axon. Each interfascicular oligodendrocyte can envelop several internodes from different axons with myelin segments (difference from Schwann cells: a single Schwann cell envelops a single myelinated axon) (Figure 3). Figure 2. Ependymal cells lining the central canal of the spinal cord. Figure 3. Electron microscope oligodendrocyte forming the myelin sheath to several axons (red arrows). 3. Microglia cells: Microglia Microglial cells were described by Pío del Rio Hortega in 1919 using the silver carbonate technique. They have a mesodermal origin, they come from blood monocytes, which is evidenced by their morphology, as well as their proliferative and cytochemical capacity that show that it is a class of brain mononuclear phagocyte. They are found throughout the nervous tissue of the CNS, and they contact different parts of neurons, blood vessels and other glial cells. They are small, stellate or spindle-shaped, with scant, dense and dark cytoplasm with 2 or more spiny processes. They present a small, oval, nucleus with dense chromatin and numerous primary lysosomes in the cytoplasm. Specific markers: macrophage antigens (MAC-1, MAC-3, F4/80) and MHC class I and class II antigens after nerve tissue injury. Its function is phagocytosis to eliminate waste and damaged structures in the central nervous system. In addition, it secretes astrocytes growth factors. Cytology and Histology III. NEUROGLIAL CELLS IN THE PERIPHERAL NERVOUS SYSTEM (PNS) There are three types: 1) Neuroglia of peripheral nerves: Schwann cells or lemnocytes. 2) Neuroglia of the spinal ganglia: satellite cell or ganglion cells that peripherally cover the soma of the ganglion cells, although they can also surround the axonal processes of these cells. They are small cells with heterochromatic nucleus. 3) Teloglial cells: ganglion cells that surround certain sensitive nerve endings. 1. Schwann cells These cells wrap around axons in the peripheral nervous system and can form two types of sheaths: non-myelinated and myelinated. Axons without myelin sheath are called unmyelinated fibres, and axons with myelin sheaths are called myelinated fibres or peripheral nerves. Each Schwann cell can envelop more than one unmyelinated fibre, but when it comes to peripheral nerves, a single Schwann cell produces the myelin sheath of a single axon (Figures 4 and 5). * B Figure 4. (Upper figure) Schwann cells forming the myelin sheath to an axon. (Lower figure) Diagram of a section of a myelinated nerve fibre showing myelin. Figure 5. Schematic of a Schwann cell involving several axons without formation of the myelin sheath (unmyelinated nerve fibre).