Normall histologyy of CNS SEMINARRR.pptx

Normal histology of CNS

1. NORMAL HISTOLOGY OF CENTRAL NERVOUS SYSTEM Presented by – Dr Farheen khan ( JR-1) Moderator – Dr Sumaiya Irfan

2. INTRODUCTION The central nervous system (CNS) consists of the brain and spinal cord and is composed of neurones , neuronal processes, supporting cells of the CNS (glial cells) and blood vessels. The CNS is invested with meninges and is suspended in fluid, the cerebrospinal fluid ( CSF ) which is produced by specialised choroid plexus structures. Macroscopically, all parts of the CNS are made up of grey matter and white matter.

3. Grey matter contains most of the neurone cell bodies and their dendritic processes. The white matter contains the axons. The lipid-rich myelin sheaths around the axons accounts for the white appearance of the white matter.

4. Central nervous tissue consists of a vast number of neurones and their processes embedded in a mass of support cells, collectively known as neuroglia , forming half of the total mass of the CNS. Neuroglia- These are highly branched cells that occupy the spaces between neurones ; they have intimate functional relationships with the neurones , providing both mechanical and metabolic support.

5. Four principal types of neuroglia are recognised : Oligodendrocytes - These cells produce myelin, a fatty substance that insulates axons and speeds up the transmission of electrical signals. Astrocytes – Maintains the blood brain barrier Provides nutrients to neurons Regulate the extracellular ion balance Oligodedrocyte : Fig 1 Producing myelin neuron Astrocyte Capillary Fig 2

6. Microglia - Act as the Immune cells of the CNS and have defence and immunological functions. Ependymal cells - make up a specialised epithelium which lines the ventricles and spinal canal.

7. GRAY MATTER Neurones (N) - have large nucleus, prominent nucleoli, and dispersed chromatin.They exhibit extensive basophilic granular cytoplasm with visible processes. Oligodendrocytes (O) - have small, round, condensed nuclei.Their cytoplasm remains unstained by routine methods like H&E. Astrocytes(A) - Round to oval nuclei with finely granular chromatin,and are less intensely stained(pale).Their cytoplasm is typically not well defined under H&E staining. Fig – Gray matter H&E (HP)

8. Neuropil (Np) - A dense network of branching cytoplasmic cell processes of axons , dendrites and glial cells surrounding both neurones and neuroglia. Neuropil appears as fibrillar eosinophilic material on H&E stains.

9. ASTROCYTES Astrocytes are identified by IHC staining for glial fibrillary acidic protein(GFAP). They are the most numerous glial cells in grey matter. Astrocytes have long, branched processes that occupy much of the interneuronal spaces in the neuropil. Their processes terminate on the basement membranes of capillaries, forming perivascular feet that cover most of the capillary basement membranes. These feet are part of the blood-brain barrier and also form the glia limitans between the CNS and the pia mater.

10. All astrocytes contain bundles of intermediate filaments and microtubules. Intermediate filaments are composed of GFAP, characteristic of astrocytes. Astrocytes in grey matter (protoplasmic astrocytes): Have numerous short, highly branched cytoplasmic processes. Astrocytes in white matter (fibrous astrocytes): Have relatively few and straight cytoplasmic processes rich in intermediate filaments. Fig: IHC method for glial fibrillary acidic protein (HP)

11. Fig : 1 (c) EM×12 000 Fig : 2 EM×57 500 Fig (1) shows an astrocyte A lying adjacent to a nerve cell body N in the cerebral cortex. The astrocyte cytoplasm contains many ribosomes, a little RER and a few small mitochondria and lysosomes. The origins of several cytoplasmic extensions C can be identified. Typical of CNS grey matter, the adjacent neuropil Np contains numerous neuronal and glial processes in various planes of section. Fig (2). The cytoplasm appears moderately electron-dense due to its content of intermediate filaments IF, which can be seen at higher magnification .

12. WHITE MATTER White matter consists of nerve fibres (axons) organized in tracts, often myelinated by oligodendrocytes. Supporting components in white matter include astrocytes, microglia, and vessels. Oligodendrocytes were named for their originally observed characteristics under heavy metal impregnation methods: They have a small number of short, branched processes ("oligos" = few, "dendron" = tree).

13. These processes connect the cell body to myelin sheaths. The process of myelin sheath formation by oligodendrocytes is similar to that of Schwann cells in peripheral nerves. A single oligodendrocyte can myelinate up to 50 axons from the same or different fibre tracts. Oligodendrocytes are the predominant type of neuroglia in white matter and are also abundant in grey matter.

14. Fig 1 TS, solochrome cyanin(HP) Fig 2 LS, solochrome cyanin (HP)

15. CHOROID PLEXUS The choroid plexus is a vascular structure located in the walls of each of the four ventricles of the brain. It is responsible for the production of cerebrospinal fluid (CSF). CSF drains from the interconnected ventricular cavities via three channels that connect the fourth ventricle with the subarachnoid space surrounding the central nervous system (CNS). CSF is produced at a constant rate and is reabsorbed into the superior sagittal venous sinus through finger-like projections called arachnoid villi.

16. CSF serves as a circulating fluid medium that supports and cushions the CNS, acting as a shock absorber. Each choroid plexus consists of a branching system of blood vessels that form fronds(highly folded and tuft like) composed of collagenous tissue. These fronds are covered by a cuboidal or columnar epithelium, giving the choroid plexus a villous (villus-like) structure.

17. Fig 1 - shows the choroid plexus CP within a ventricle of the brain V. Fig 2 - shows detail of one of the choroid plexus processes

18. EPENDYMA Ependymal cells line ventricles of the brain and spinal canal. They are cuboidal or low columnar in shape, tightly bound at luminal surfaces. Unlike typical epithelia, they lack a basement membrane. Bases of cells taper and branch into fine processes, intertwining with astrocytic processes. Luminal surface features variable cilia and microvilli. Microvilli likely involved in absorption and secretion. Fig : Ependyma H&E (HP)

19. MENINGES The brain and spinal cord are invested by three layers of supporting tissue, collectively called the meninges. These layers include the delicate pia mater , which covers the surface of the nervous tissue and contains collagen fibers, fine elastin fibers, and occasional fibroblasts. The pia mater is separated from the astrocytic processes of the underlying CNS parenchyma by a basement membrane, which is completely invested by astrocytic processes, forming the impermeable glia limitans.

20. Overlying the pia mater is the arachnoid mater , a thicker fibrous layer named for its cobweb-like strands that connect it to the pia. The pia and arachnoid are continuous and together form the pia-arachnoid or leptomeninges. The space between the pia and arachnoid is the subarachnoid space, which contains cerebrospinal fluid (CSF) and forms large cisterns in some places. CSF circulates continuously from the ventricles into the subarachnoid space through foramina in the fourth ventricle.

21. The subarachnoid space is lined by flattened arachnoidal cells, with arteries and veins passing through it loosely attached to the pia mater and invested by subarachnoid meningothelium . Larger vessels extending into nervous tissue are surrounded by a delicate sleeve of pia mater and have a perivascular space between them and the pia. The pia blends with the vessel's adventitia as it penetrates the brain, but this component is not present around CNS capillaries. External to the arachnoid mater is the dense fibroelastic layer called the dura mater . Fig : (b) Dura mater, H&E (MP ) A arachnoid mater D dura mater P pia mater S sulcus D

22. SPINAL CORD The spinal cord structure is consistent throughout its length, categorized into four regions in transverse sections. The grey matter centrally resembles a butterfly shape, with prominent ventral horns (V) containing cell bodies of large alpha lower motor neurons. Dorsal horns (D) house cell bodies of small second-order sensory neurons, relaying sensory information on pain, temperature, and participating in spinal reflexes. Small lateral horns (L) are found in thoracic and upper lumbar regions, housing cell bodies of sympathetic nervous system efferent neurons. Grey matter volume is greater in cervical and lumbar regions, reflecting limb innervation.

23. The central canal within the central commissure (C) of grey matter is lined by ependymal cells and contains cerebrospinal fluid (CSF). White matter consists of ascending sensory and descending motor tracts, increasing in volume from sacral to cervical regions due to additional fibers entering and leaving. A deep ventral (anterior) median fissure (F) is present, contrasting with a shallow dorsal midline sulcus dorsally. Dorsolateral sulci (S) mark the dorsal nerve roots (R) entry points on each side. The white matter between dorsal horns represents ascending dorsal columns (DC) conveying fibers for senses of vibration, proprioception, and fine touch to the medulla. In the cervical region, dorsal columns split into medial fasciculus gracilis (FG) for lower limbs and lateral fasciculus cuneatus (FC) for upper limbs.

24. Ventrolateral sulci (VS) mark ventral nerve roots exit points; ventrolateral white matter contains various tracts including spinothalamic and corticospinal. The spinal cord is surrounded by meninges; the dura mater loosely connects to the vertebral canal's periosteum via denticulate ligaments. The epidural space contains adipose tissue and a venous plexus. During development, the vertebral column lengthens more than the spinal cord, causing lower cord segments to lie above corresponding intervertebral foramina. Below the cervical region, nerve roots (NR) take an oblique course in the subarachnoid space before passing through intervertebral foramina, particularly notable in lumbar and sacral regions.

25. Fig : Spinal cord transverse Sections Luxol fast blue, H&E photomontage, (LP) Cervical (b) Thoracic (c) Lumbar (d) Sacral FG- fasciculus gracilis FC- fasciculus cuneatus (S) - Dorsolateral sulci (R) - Dorsal nerve roots (F) - median fissure (DC) - Dorsal columns (VS ) - Ventrolateral sulci (V) - Ventral horns (D) - Dorsal horns (NR) - Nerve roots

26. Fig : Spinal cord and central canal Luxol fast blue, H&E Ventral spinal cord (MP) Commissue and spinal canal (HP) Grey matter anterior horn(HP) (d) White matter (HP)

27. BRAINSTEM Consists of 3 main parts Midbrain Pons Medulla oblangata . The medulla oblongata is the most caudal part of the brainstem, connecting the brain to the spinal cord. It extends from the lower border of the pons to the foramen magnum

28. Pons 1. Location and Structure: - The pons is in the middle of the brainstem, between the midbrain and medulla. - It consists of a ventral (basal pons) and a dorsal (tegmental) region. - Basal pons has crisscrossed bundles of fibers and pontine nuclei. 2. Fibers and Tracts: - Longitudinal fibers include corticospinal tract axons for motor signals. - Other cortical fibers synapse in pontine nuclei, then cross into the cerebellum. 3 Dorsal Tegmentum: - Contains ascending sensory tracts (spinothalamic). - Houses nuclei of cranial nerves V, VI, and VII. - Medial lemniscus carries proprioceptive and sensory signals.

29. Fig (b) Basal pons , H&E (MP) L longitudinal fibre bundles T – Transverse fibre bundles PN- pontine nucleus Microscopic Structure : Transverse fibres (T) - rostral-caudal fiber bundles.(Horizontal fibres ) Longitudinal fibres bundles (L) - between cerebellar hemispheres. (Vertical fibres ) Pontine nuclei (PN) - Grey matter contains pontine motor nuclei (PN).

30. CEREBELLUM 1. Function and Structure: - Coordinates muscular activity, maintains posture, and equilibrium. - Grey matter cortex with a central white matter core. - Contains four pairs of nuclei. 2. Peduncles and Connections: - Afferent and efferent fibers connect cerebellum to brainstem via inferior, middle, and superior cerebellar peduncles. - Links medulla, pons, and midbrain.

31. 3. Microscopic Structure : -Cerebellar cortex is deeply folded into folia, supported by central white matter. - Cortex has three layers: molecular layer (ML), granular cell layer (GL), and Purkinje cell layer (PL). - Purkinje cells (P) have large bodies, fine axons extending through GL, and elaborate dendritic systems in ML. - Granular cell axons bifurcate in ML to synapse with Purkinje cell dendrites. Fig : Cerebellum (b) H&E (LP) (c) H&E (MP)

32. 4. Neuronal Types: - Other cerebellar cortex neurons include stellate cells and basket cells in ML, and Golgi cells in superficial GL. - Basket cell axons surround Purkinje cell bodies. 5. Fiber Pathways: - Afferent fibers enter cerebellum from brainstem, connect with granular cells, and then synapse with Purkinje cell dendrites via basket cells. - Efferent fibers from Purkinje cells traverse GL to synapse in central nuclei of cerebellum.

33. SUBSTANTIA NIGRA 1. Location and Structure: - Located in the midbrain, divides cerebral peduncles into dorsal and ventral parts. - Easily identified by darkly pigmented neurons containing neuromelanin. 2. Connections and Function: - Extensively connected with cortex, spinal cord, corpus striatum, and reticular formation. - Plays a crucial role in fine motor control.

34. 3. Neurons and Neuromelanin: - Neurons are multipolar and contain neuromelanin pigment. - Neuromelanin contains dopamine, acting as a neurotransmitter with inhibitory effects, especially on corpus striatum neurons. 4. Neuromelanin Accumulation: - Amount of neuromelanin increases with age, starting from childhood. - Origin of neuromelanin debated; possibly enzymatically generated or a byproduct of dopamine oxidation. 5. Functional Role of Neuromelanin: - May sequester metals like iron and toxic organic compounds, potentially protecting neurons from oxidative stress.

35. Fig : Substantia nigra (a) H&E (LP) neurones containing dark pigment (b) H&E shows the cytoplasm containing numerous granules of neuromelanin pigment

36. THALAMUS 1. Location and Composition: - Located on each side of the third ventricle, forming a major part of the diencephalon within the brain. - Includes various nuclei, such as reticular, motor, and specific sensory nuclei. 2. Function: - Acts as a relay and integration center for sensory and motor information from throughout the central nervous system (CNS).

37. 3. Structure and Histology: -Fig shows typical basal nucleus structure with neuron cell bodies (N) surrounded by neuropil (nerve fibers and glial cells). - Neurons in the thalamus accumulate light brown lipofuscin pigment with age, which consists of indigestible lipid byproducts. Fig : Thalamus H&E (HP)

38. CEREBRAL CORTEX Structure of Cerebral Hemispheres: - Consist of folded cortex of grey matter overlying white matter. - White matter conveys fibers between different parts of cortex and other parts of CNS. Neuronal Types in Cerebral Cortex: - Divided into five morphological types, arranged in layers. - Oldest part (olfactory cortex) has three layers; neocortex (in mammals) has six layers. - In humans, neocortex comprises about 90% of cortex.

39. Basic Principles of Cortical Organization: - Functional units arranged vertically, corresponding to axon and dendrite orientations. - Afferent fibers synapse high in cortex with dendrites of efferent neurons in deeper layers. - Efferent pathways (from pyramidal cells) often branch back into superficial layers.

40. Types of Cortical Neurons: Pyramidal Cells : Pyramid-shaped with apex toward cortical surface, large Betz cells in motor cortex. Stellate (Granule) Cells : Small neurons with star-shaped bodies, granular appearance histologically. Martinotti Cells : Polygonal cells with horizontal axons in superficial layers. Fusiform Cells : Spindle-shaped, perpendicular to cortical surface. Horizontal Cells of Cajal : Spindle-shaped, parallel to surface, found in superficial layer. Supporting Cells in Cortex : includes astrocytes, oligodendroglia, and microglia, in addition to neurons.

41. Fig : (a) Neurone cell types diagram

42. Layers of Neocortex: Plexiform (Molecular) Layer: Superficial layer with dendrites and axons making synapses. Outer Granular Layer: Dense with small pyramidal and stellate cells. Pyramidal Cell Layer: Moderate to large pyramidal cells, Martinotti cells present. Inner Granular Layer: Densely packed stellate cells. Ganglionic Layer: Large pyramidal cells (like Betz cells) and others. Multiform Cell Layer: Contains various morphological forms of cells.

43. Fig : Cerebral cortex (caption opposite) ((b) Methylene blue (MP) (c) H&E (MP)

44. CORTEX Fig (a): - Part of layer V. - Thick section stained with heavy metal impregnation technique. - Several identifiable pyramidal cells (P). - Principal dendrites of pyramidal cells rise towards cortical surface. - Axons of pyramidal cells not visible in this section plane. - Cell of Martinotti (M) identified by polygonal shape. Fig : Cortex (a) Golgi method (MP)

45. Fig (b): - Also shows layer V. - Section is thinner. - Stained with routine histological method. - Less morphological detail visible at this magnification. - Most cells identifiable as pyramidal cells. - Pyramidal cells increase in size in deeper part. - Includes several very large cells. Fig Cortex (b) Nissl method (MP)