Exercise 15 Review Sheet Histology Of Nervous Tissue

Exercise 15 Review Sheet: Histology of Nervous Tissue

This comprehensive review sheet covers the key concepts of nervous tissue histology, ideal for students preparing for exams or seeking a deeper understanding of the subject. We'll explore the structural components of neurons and neuroglia, delve into the organization of nervous tissue in the central and peripheral nervous systems, and examine common histological stains used in its study. This guide will equip you with the knowledge to confidently identify and analyze nervous tissue micrographs.

I. Neurons: The Functional Units of the Nervous System

Neurons, the fundamental units of the nervous system, are specialized cells responsible for receiving, processing, and transmitting information. Their unique morphology reflects their function. Let's examine their key components:

A. Cell Body (Soma): The Neuron's Control Center

The cell body, or soma, contains the neuron's nucleus and most of its organelles. It's the neuron's metabolic center, responsible for synthesizing proteins and maintaining the cell's overall function. Key features to identify in histological sections:

• Nucleus: Large, euchromatic nucleus reflecting high levels of transcription activity.
• Nissl bodies (RER): Prominent basophilic structures representing the rough endoplasmic reticulum involved in protein synthesis. These appear as dark-staining clumps in H&E stained sections.
• Golgi apparatus: While not easily visible in routine H&E staining, the Golgi apparatus plays a crucial role in protein processing and packaging.

B. Dendrites: Receiving Signals

Dendrites are branched extensions of the neuron's cell body that receive signals from other neurons. Their extensive branching creates a large surface area for synaptic connections. Histological characteristics:

• Tapering branches: Dendrites progressively narrow as they extend away from the cell body.
• Abundant dendritic spines: Small, spiny projections along the dendrites, sites of synaptic contact, are often visible at higher magnification.
• Nissl bodies: Present in proximal dendrites, gradually decreasing in density distally.

C. Axon: Transmitting Signals

The axon is a long, slender projection that transmits signals away from the cell body. It's typically much longer than dendrites and maintains a relatively uniform diameter. Distinguishing features:

• Uniform diameter: Unlike tapering dendrites, axons maintain a consistent diameter along their length.
• Absence of Nissl bodies: Axons lack Nissl bodies and ribosomes, relying on axonal transport for protein delivery.
• Axon hillock: The region where the axon originates from the cell body; it's the site of action potential initiation.
• Myelin sheath (in some axons): A fatty insulating layer surrounding many axons, significantly increasing signal transmission speed. (See Section III for details on myelin.)
• Terminal boutons (axon terminals): Branches at the axon's end containing neurotransmitters, crucial for communication with other neurons or effector cells.

II. Neuroglia: Supporting Cells of the Nervous System

Neuroglia, also known as glial cells, are non-neuronal cells that provide structural and functional support to neurons. They outnumber neurons and are crucial for maintaining the integrity of the nervous system.

A. Central Nervous System (CNS) Neuroglia

• Astrocytes: Star-shaped cells with numerous processes that contact neurons, blood vessels, and the pia mater. They play crucial roles in regulating the blood-brain barrier, providing metabolic support to neurons, and removing neurotransmitters. Histological features: Their large, branched processes are readily apparent in appropriately stained sections.
• Oligodendrocytes: These cells produce the myelin sheath in the CNS. A single oligodendrocyte can myelinate multiple axons. Histological features: Their smaller cell bodies and fewer processes compared to astrocytes are distinguishing characteristics.
• Microglia: Small, phagocytic cells that act as the immune cells of the CNS, removing debris and pathogens. Histological features: Their small size and elongated, somewhat amoeboid shape helps distinguish them from other glial cells.
• Ependymal cells: Cuboidal or columnar cells lining the ventricles of the brain and the central canal of the spinal cord. They are involved in producing cerebrospinal fluid (CSF). Histological features: Their location lining the ventricles is key to identification.

B. Peripheral Nervous System (PNS) Neuroglia

• Schwann cells: These cells produce the myelin sheath in the PNS. Unlike oligodendrocytes, each Schwann cell myelinates only a single segment of a single axon. Histological features: The myelin sheath they produce is similar in appearance to that produced by oligodendrocytes.
• Satellite cells: These cells surround neuronal cell bodies in ganglia, providing structural support and regulating the microenvironment. Histological features: Their close association with neuronal cell bodies in ganglia is key.

III. Myelin: Insulating Axons for Faster Signal Transmission

Myelin is a lipid-rich sheath that surrounds many axons, increasing the speed of signal transmission. It's formed by oligodendrocytes in the CNS and Schwann cells in the PNS. Histological appearance:

• Nodes of Ranvier: Gaps in the myelin sheath where the axon is exposed. These are crucial for saltatory conduction, allowing for faster signal propagation.
• Internodes: Myelinated segments between the nodes of Ranvier. They appear as brightly stained, relatively homogenous regions in H&E stained sections.

IV. Histological Stains for Nervous Tissue

Various stains are used to visualize the different components of nervous tissue.

• H&E (Hematoxylin and Eosin): A routine stain that differentiates the nucleus (basophilic, stained blue/purple by hematoxylin) from the cytoplasm (acidophilic, stained pink/red by eosin). It's useful for visualizing general tissue architecture and identifying Nissl bodies.
• Silver stains: These stains are used to visualize the fine processes of neurons and glial cells, providing a more detailed picture of neuronal morphology. They are particularly helpful in tracing neuronal pathways.
• Myelin stains (e.g., Luxol fast blue): These stains specifically highlight myelin, making it easy to distinguish myelinated from unmyelinated axons.

V. Organization of Nervous Tissue

Nervous tissue is organized into two major divisions: the central nervous system (CNS) and the peripheral nervous system (PNS).

A. Central Nervous System (CNS)

The CNS consists of the brain and spinal cord. Histological features:

• Gray matter: Primarily composed of neuronal cell bodies, dendrites, and unmyelinated axons. It appears darker in H&E stained sections.
• White matter: Primarily composed of myelinated axons. It appears lighter in H&E stained sections due to the high lipid content of myelin. The white matter tracts are organized in bundles.
• Cerebral cortex: The outermost layer of the cerebrum, characterized by a layered structure with distinct neuronal populations in each layer.
• Cerebellum: Characterized by a highly organized structure consisting of the cerebellar cortex and the white matter deep to it. The cerebellar cortex displays a unique arrangement of neurons in layers.
• Spinal cord: Contains a central gray matter region shaped like a butterfly, surrounded by white matter.

B. Peripheral Nervous System (PNS)

The PNS consists of nerves, ganglia, and sensory receptors. Histological features:

• Nerves: Bundles of axons (myelinated and unmyelinated) surrounded by connective tissue sheaths (epineurium, perineurium, endoneurium).
• Ganglia: Clusters of neuronal cell bodies outside the CNS. They are surrounded by satellite cells and connective tissue capsules. Ganglia may be sensory or autonomic.
• Sensory receptors: Specialized structures that detect stimuli from the external or internal environment.

VI. Common Histological Artifacts and Considerations

It's crucial to be aware of potential artifacts when analyzing nervous tissue histology slides. These can include:

• Shrinkage artifacts: Tissue processing can cause shrinkage, leading to distorted morphology.
• Staining inconsistencies: Variations in staining intensity can make interpretation challenging.
• Post-mortem changes: The time elapsed between death and tissue fixation can affect tissue preservation and appearance.

VII. Clinical Correlations

Understanding the histology of nervous tissue is fundamental to diagnosing neurological disorders. Abnormal histological findings can indicate a range of conditions, including:

• Neurodegenerative diseases: Conditions like Alzheimer's disease and Parkinson's disease are characterized by specific neuronal loss and changes in glial cell populations.
• Demyelinating diseases: Multiple sclerosis, for example, is characterized by damage to the myelin sheath.
• Neoplasms: Tumors of the nervous system can affect both neurons and glial cells.

This comprehensive review sheet provides a solid foundation for understanding the histology of nervous tissue. By mastering the identification of key cellular components, understanding tissue organization, and appreciating the role of histological stains, you'll build a strong foundation for further study in neurobiology and related fields. Remember to practice identifying structures in histological slides, comparing images with descriptions, and relating microscopic observations to the overall function of the nervous system. Consistent review and active learning are key to success.