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Histology of Brain Stem, Cerebrum and Cerebellum Brain stem • Brain stem is structurally continuous with the spinal cord. • It consists of; – medulla oblangata – pons – mesencephalon • The regions of gray and white matter are not clearly separated • The nuclei of the cranial nerves appear as islands surrounded by white matter Meninges • Connective tissue coverings of the brain and spinal cord Dura Mater • Dense, collagenous connective tissue • Dura of the brain is composed of two layers; – periosteal dura mater (outer); • attached to the inner surface of the skull • serves as the periosteum • osteoprogenitor cells, fibroblasts, collagen bundles, blood vessels – meningeal dura mater (inner); • fibroblasts, fine collagen fibers, small blood vessels • Dura of the spinal cord – forms a continuous tube around the spinal cord – does not adhere to the walls of the vertebral canal – pierced by the spinal nerves – epidural space; • between the dura mater and the periosteum of the vertebral canal • filled with epidural fat and a venous plexus. Arachnoid Mater • Connective tissue without blood vessels, blood vessels course through it • Composed of two regions: 1) 2) sheet-like membrane in contact with dura arachnoid trabeculae loosely arranged cells (modified fibroblasts) with collagen fibers contact the underlying pia • Subarachnoid space; – cavities between the trabeculae – filled with cerebrospinal fluid (CSF) • Arachnoid villi; – regions where arachnoid perforates the dura for the passage of CSF into the dural venous sinuses • Clinical significance: Subdural space is a “potential space”, it appears only after subdural hemorrhage, when blood forces two layers apart Pia Mater • Innermost layer of the meninges • Follows all the surface irregularities of CNS • Composed of a thin layer of flattened, modified fibroblasts and abundant blood vessels surrounded by fine connective tissue fibers • Completely separated from the underlying neural tissue by basement membrane and neuroglial processes Cerebrospinal Fluid • Bathes, nourishes and protects the brain and spinal cord • Produced by the choroid plexus • %90 water, low protein and low density, rich in ions, few desquamated cells, 2-5 lymphocytes/ ml • Reabsorbed through the thin cells of the arachnoid villi into the superior sagittal venous sinus • Clinical correlations; – Hydrochephalus; reason is a decrease in absorption of the fluid or a blockage within the ventricles which increases intracranial pressure. – Congenital hydrocephalus; leads to enlargement of the head, followed by impairment of mental and muscular functions and death if left untreated. Choroid Plexus • Fenestrated capillaries covered by the modified ependymal cells • Modified ependymal cells – – – – tight junction infolding of the basal plasma membrane, microvilli mitochondrion • Extend into the ventricles of the brain • Produces CSF Ventricles • • • • Cavities in the brain (4th, 3rd, lateral) Contain CSF Ependymal cells Tanycytes; – special ependymal cells located in the floor of the 3rd ventricle. – processes extend deep into the hypothalamus. – Transfer chemical signals from CSF to CNS. Cerebrum • Cortex – gray matter (peripheral) • Medulla – white matter Layers of the Cerebral Cortex I. Molecular layer; parallel nerve fibers, horizontal cells of Cajal, neuroglia II. External granular layer; small pyramidal cells, granule (stellate) cells, neuroglia III. External pyramidal layer; large pyramidal cells, neuroglia IV. Internal granular layer; thin layer composed of closely arranged small granule (stellate) cells, neuroglia. V. Internal pyramidal layer; largest pyramidal cells (Betz cells), neuroglia. This layer has the lowest cell density of the cerebral cortex VI. Multiform layer; cells of various shapes (Martinotti cells, fusiform cells, pyramidal etc.), neuroglia • Isocortex (neocortex) is the outer layer of the cerebral hemispheres made up of typical six layers • Anisocortex (archicortex) (e.g. hippocampus) Hippocampus (sea horse) • hippocampus, dentate gyrus, temporal lobe gyrus • polymorphic layer; nerve fibers, small cell bodies of interneurons • pyramidal cell layer; pyramidal cells • molecular layer; dendrites of the pyramidal cells Dentate gyrus • polymorphic layer; nerve fibers, interneurons • granule cell layer; granule cells • molecular layer; dendrites of the granule cells Hilus • region where the head of hippocampus join the dentate gyrus • contains multipolar neurons Cerebellum • Outer gray matter “cortex” – Molecular layer- Purkinje cell layer – Granular layer • Inner white matter “medulla” • Surrounded by piamater Molecular layer • lies directly below the pia mater • contains – – – – superficially located stellate cells basket cells Purkinje cells and their dendrites unmyelinated axons from the granular layer (parallel fibers) • Purkinje cells – – – – large pear-shaped cells arborized dendrites projecting into the molecular layer myelinated axons project into the white matter only cell of the cerebellar cortex that sends information (always inhibitory) to the outside • Contains Granular layer – small granule cells – Golgi type II cells – glomeruli; synaptic regions between axons entering the cerebellum and the granule cells. • Axons of the granule cells extend to the molecular layer and synapse with the dendrites of Purkinje cells and basket cells (parallel fibers). • Parallel fibers extend parallel to the longitudinal axis of the folium in the molecular layer. Barriers in the CNS • Glia limitans externa- interna • Blood- brain barrier • Blood- CSF barrier Blood-Brain Barrier 1) endothelial cells lining the continuous capillaries linked by tight junctions 2) basal lamina 3) end-feet of astrocytes • This barrier prevents the free passage of selective blood-borne substances into the neural tissue. • O2, CO2, water, small lipid soluble materials can penetrate the barrier. • Transfer of glucose, amino acids, vitamins, nucleosides and ions require transmembrane proteins. References 1. 2. 3. 4. 5. Histology: A Text and Atlas by Michael H. Ross, Wojciech Pawlina (2010). 6th ed. Lippincott Williams & Wilkins, Philadelphia. ISBN: 978-0-78177200-6 Basic Histology: Text & Atlas by Luiz Junqueira, Jose Carneiro (2005). 11th ed. McGraw-Hill, New York. ISBN: 0-07-111888-8 Color Textbook of Histology by Leslie P. Gartner, James L. Hiatt (2001). 2nd ed. W.B. Saunders Company, Philadelphia. ISBN: 0-7216-8806-3 Histology and Cell Biology: An Introduction to Pathology by Abraham L Kierszenbaum, Laura Tres (2011). 3rd ed. Elsevier Saunders, Philadelphia. ISBN: 978-0-323-07842-9 Netter’s Essential Histology by William K. Ovalle, Patrick C. Nahirney (2007). 1st ed. Elsevier Saunders, Philadelphia. ISBN: 978-1-929007-86-8