Nervous System Histology 2024 PDF

CENTRAL NERVOUS SYSTEM HISTOLOGY Lec. Gözde Öğütçü Near East University Department of Histology and Embryology [email protected] BRAIN There are visible indentations and prot...

CENTRAL NERVOUS SYSTEM HISTOLOGY Lec. Gözde Öğütçü Near East University Department of Histology and Embryology [email protected] BRAIN There are visible indentations and protrusions on the brain surface: Protrusions GYRUS Indentations are defined as SULKUS.  There are two different regions in its structure. These, substantia (ore) grisea (gray) and substantia alba (white). Cerebrum -Brain Cerebrum-Brain Cerebellum  Gray matter White matter Outside in the brain and Inside the brain and cerebellum and inside the cerebellum and outside in spinal cord the spinal cord It is gray in color. It is white in color Unmyelinated fibers are Myelinated fibers are abundant abundant Neuron (+) Neuron (-) Neuroglia (+) Neuroglia (+) Capillary (+) Capillary (++) Lymphatic capillary (-) Lymphatic capillary (-) Brain Nerve cell bodies are found only in gray matter. There are clusters of neurons called nuclei in the form of islands in the brain and cerebellum. Neuroglia are found in both substances.  When myelinated fibers in the CNS are stained with Luxol – Fast Blue or Weigert, white matter is well distinguished macroscopically. Myelinated fibers in the white matter are stained blue-green with Luxol-Fast blue and black with Weigert dye. Membranes of the Central Nervous System Meninges Organs are protected by the membranes and bones around them. The membranes consist of fibroblast and collagen fiber bundles. There are three membranes, from outside to inside: ❑Duramater ❑Arachnoid ❑Piamater  Duramater (pakimeninks) Histological structure: connective tissue with many collagen fibrils, few elastic fibrils and fibroblasts located parallel to each other. There are a small amount of blood vessels and sensory nerves. The side facing the arachnoid is lined with flat mesothelial cells. It is separated from the arachnoid Between the two sheets of the membrane by the subdural space, dura are venous sinuses lined with which is thought to potentially exist. endothelium. The brain dura and the spinal dura continue with each other through the foramen magnum. Cranial dura mater: It is tightly adhered to the skull bones. In other words, the periosteum of the bone is tightly connected and forms a 2-layer structure. Therefore, there are no mesothelial cells surrounding the epidural space and the outside of the dura. The dura extends outward around the nerves exiting the skull in the form of a tubular sheath. It fuses with the epineurium, the outermost sheath of the nerve. However, in epidural bleeding, an epidural space similar to the spinal dura is formed. It divides the cranial cavity into cavities by sending septa inwards (falx cerebri, falx cerebelli, tentorium cerebelli, diaphragma sella dura are extensions). Spinal dura mater: There is an epidural space between the bone and the dura. The dura is suspended from the bone by small lateral ligaments. There are many veins in the fat-rich loose connective tissue in the epidural space. The outer and inner surfaces of the dura are lined with mesothelium. Again, the dura extends in a tubular form around the spinal nerves. Septum is not found. Medulla spinalis epidural space Arachnoid membrane It is a thin network-shaped connective tissue without vessels. It is called arachnoid membrane because it resembles a spider web macroscopically. There is a space between the pia surrounding the brain and the arachnoid (subarachnoidal space). On the face of the arachnoid membrane facing the pia, there are trabeculae that connect the two membranes and divide the subarachnoidal space into sections. Trabeculae are also lined with single-layer squamous epithelium. Arachnoid membrane  Cerebrospinal fluid is found in the subarachnoidal space. In certain places, the subarachnoidal space expands to form cisternae. Trabeculae are rare or absent in the cisternae. Piamater The innermost pia mater surrounds all the indentations and protrusions of the CNS. It contains a single or several rows of fibroblasts in its structure. Between fibroblasts, there are collagen fibers. In the inner part, there is the subpial layer (glia limitans) where the extensions (feet) of protoplasmic astrocytes end. Piamater Pia mater is visible under the light microscope as a dark layer located at the outermost part of the brain tissue and looks like a single-layer squamous epithelium. Pia mater also partially surrounds the vessels entering the brain tissue. Additionally, there are nerve fibers in this membrane. Ependymal cell: It has a simple cubic or prismatic shape. The formation of microvilli has greatly expanded the surface and the cytoplasm is rich in mitochondria. It shows that it is suitable for water transport (Golgi complex is not well developed, there are no secretory granules, but there are basal folds). CSF is secreted from here. This process is done through filtration through the pia vessels.  A small amount of CSF absorption occurs in the ependymal cells. Ependymal cells are tightly connected to each other by zonula occludens. Although the capillaries in this region are of the fenestrated type, the entry of substances into the cerebrospinal fluid is prevented by the tight lateral connections of the ependymal cells (Blood-CSF barrier). Blood-CSF barrier At the barrier:, Capillary endothelium of the choroid plexus Basement membrane of capillaries ependymal cells  Circulation of Cerebrospinal Fluid Circulation starting from the lateral ventricles continues with the third and fourth ventricles. It is separated by the foramen of Magendie in the middle and the foramen of Luschka on both sides, and passes into the subarachnoid space and from the subarachnoid region to the superior sagittal sinus with its arachnoid villi. The superior sagittal sinus drains into the internal jugular vein. Thus, CSF passes into the venous system. Any obstruction in the specified pathways leads to enlargement of the lateral ventricles. This condition is called hydrocephalus. This obstruction often occurs in the mesencephalon (aquaductus cerebri). CSF Since CSF fills the subarachnoidal space, the organs float in the fluid and are thus protected against both external mechanical effects and the effect of gravity. The brain, which weighs approximately 1500 grams outside, weighs only 50 grams in the CSF. Another feature is that CSF distributes the current pressure equally to all sides, not to one point. For example, a 100 kg blow to the brain tissue is distributed equally throughout the brain. Since the impact is distributed over a wide surface, damage to the tissue is minimized. Blood-Brain Barrier Capillaries in the CNS consist of flat endothelial cells surrounded by a basement membrane. The endothelium is thin, continuous and not fenestrated.Among the endothelial cells zonula occluding junction present. There is a thin basement membrane under the occludens and endothelium.The perivascular feet of astrocytes extending to this region are seen as a continuous layer. Non-fenestrated capillaries of the CNS are less permeable. Any substance carried to the brain by blood cannot pass directly into the brain tissue. This is called the blood-brain barrier. Astrocytes: The largest cell. They support neurons, regulate their activities, and send footlets around blood vessels. It delivers metabolites to neurons and removes waste products from the neuron. Under the pia, glia limitans form. It participates in the blood-brain barrier. There are protoplasmic (many in gray matter) and fibrous (many in white matter) astrocytes. GFAP (glial fibrillary acidic protein) is distinguished by specific immunostaining.Tumors originating from fibrous astrocytes constitute 80% of adult brain tumors.​ Oligodendrositler Responsible for CNS myelination Their nuclei are smaller and darker in color than astrocytes. Multiple extensions of a single oligodendrocyte can myelinate an axon or several nearby axons. Microglia It normally constitutes 5% of glial cells. It is the smallest cell, it has a small flat nucleus. EM shows that there are many lysosomes in the cytoplasm. Increases in number with damage and disease (reactive microglial cells) Precursors of microglial cells (monocytes) come to the CNS via vessels They play an important role in defense against microorganisms and tumor cells​  CEREBRUM Histologically it consists of white matter or substansia alba and grey matter or substansia grisea. White matter contains the myelinated axon and located in the inner part. Grey matter located in the outer part of the brain, consists of perikaryon or body of neuronal cells. The cortex is highly convoluted, thrown into deep folds which greatly increase the surface area. These convolutions are called gyri and the intervening depressions are sulci.  CEREBRAL CORTEX It is the gray matter overlying white matter Containing billions of cells Neurons of the cerebral cortex are of varying shapes and sizes, but the most obvious are pyramidal cells. As the name implies, the cell body is shaped somewhat like a pyramid Other neurons or to glial cells also present in the cortex. Neuronal Cells In Cortex Neuronal Cells In Cortex Pyramidal neuron: The neuron soma is pyramid-shaped. The apical dendrite (exiting from the top of the pyramid) extends upward and outward. The axon extends deep from the base of the soma, some of it passes into the white matter, and some of it re-enters the cortex. There are small, medium and large types according to diameter. The largest ones are called giant pyramidal cells of Betz(inner pyramidal layer). The apex of all pyramidal neurons is towards the brain surface. The smallest ones are 10 microns and the largest ones are 20 microns. Neuronal Cells In Cortex Granular neuron: They are small neurons (8 microns in diameter) that are star- shaped due to their extensions. Its dendrites make many synapses with other neurons. Its axon is very short, it remains within the cortex and forms a plexus around the dendrites of nearby pyramidal cells, and is found in every layer of the cortex. Neuronal Cells In Cortex Fusiform neurons: They are modified stellate neurons. They are usually found in the deepest layer of the cortex, synapsing with many pyramidal cells. Horizontal neuron: They are superficially located, horizontally located, fusiform neurons. Martinotti neurons: They are small multipolar, forms synapses with pyramidal neurons. Cerebral Cortex Consists of six layers The layers are parallel to the brain surface. Layer thicknesses and cell numbers vary in different parts of the hemisphere. From outside to inside: 1. Molecular layer: (Plexiform layer-lamina zonalis): It contains a small number of horizontal neurons and a large amount of horizontal neuron extensions (pyramidal neuron dendrites in the lower layers, axons of granular and Martinotti neurons).​ 2. Outer granular layer: Small pyramidal neurons and granular neurons are found. Axons and dendrites from neighboring layers and axons and dendrites in this layer form a dense network. 3. Pyramidal layer (Outer pyramidal): There are medium diameter pyramidal cells and small pyramidal neurons. There are also horizontal and vertical fusiform neurons. 4. Inner granular layer: After the molecular layer, the thinnest layer is 4th layers. Mainly stellate (granular) neuron, occasionally small pyramidal neurons are found. The cells are densely located. 5. Ganglionic layer (inner pyramidal): There are the largest pyramidal neurons(Betz cells) and a small number of stellate cells. The size of pyramidal neurons varies in different areas. Betz's pyramidal cells 6. Multiform layer: Consists of different neuron types and extensions. Martinotti neurons are the most prominent. Fusiform, ovoid and triangular somas are seen. The 1st, 2nd and 3rd layers are the layers that show the most advanced differentiation. Cortex of the Cerebellum In the granular layer, Golgi cells are located. In the molecular layer, there are stellate cells and basket cells. Basket cells extend axon branches into Purkinje cells. These extending axon branches surround the cell bodies like a basket. The white matter structure contains myelinated and unmyelinated nerve fibers and glial cells. The cerebellum resembles a branched tree (tree of life – arbor vitae) in sagittal (longitudinal) section. The white matter contains efferent fibers and interneurons, most of which are afferent. PURKINJE LAYER This intermediate layer consists of a single layer of very large Purkinje cells whose cell bodies rest on the innermost granular layer. The Purkinje cell has a large, prominent, flask-shape cell body with a clear vesicular nucleus. It has many tree-like dendritic arbor which project into the molecular layer. WHITE MATTER OF a branched tree(tree of vital-arbor vitae) CEREBELLUM in sagittal (longitudinal) section.  Neurohistology Staining White matter(myelinated fibers) White matter(myelinated fibers) – luxol - fast blue Weigert(black) Central Canal Small, round structure in the center of the spinal cord that contains cerebral spinal fluid. Lined with a simple cuboidal-to-columnar epithelium of ependymal cells.  Grey Matter of Spinal Cord Grey matter has a butterfly like shape, located in the inner part of spinal cord and consists of neuron and unmylinated nerve fibers. The anterior horn contains mostly of multipolar neurons with large polygonal nucleus and large perikaryon and dendrites containing Nissl bodies. Nissl bodies- these regions consist rough endoplasmic reticulum and other polysomes  White Matter of Spinal Cord White matter of spinal cord located in the outer part of spinal cord and consists of bundles of axons having specific functions either motor or sensory for example pain, touch.