Welcome to lab classes in basic histology Introducing lab class (in spanish) .

The aim of these lab classes is to provide students with educational resources to acquire basic practical skills of each subject, recognize, locate and describe cell types and basic tissues.

Each of these practical sessions are structured around three main elements:
- Definition of learning objectives.
- Description of basic tissues and their cell types.
- Exercices location of cell types and basic tissues in virtual slides.

LEARNING OBJECTIVES
Identify the neurons in the CNS and SNP.
Differentiate neuroonal cell body, dendrites and axons in CNS and SNP.
Identify the CNS glial cells: ependimal cells, astrocytes, oligodendrocytes and microglial cells.
Identify the SNP neurons.
Identify the SNP glial cells: satellite cells and Schwann cells.
Identify the SNP nerves.
Identify neuron synapses and neuromuscular junctions.

HANDBOOK
The nervous system is the most complex system in the body histologically and physiologically Comparison between Nissl and Golgi techniques . It is formed by a network of millions of cells, called neurons, assisted by further support cells or glial cells. Each neuron has hundreds of interconnections with other neurons, forming a very complex system to process the information and generate responses. Nerve tissue is distributed throughout the body acting as a communications network.
Anatomically, the nervous system can be divided between the central nervous system (brain, spinal cord and the nervous part of the eye) and the peripheral nervous system (peripheral ganglia and nerves that connect the ganglia with central nervous system and with different sensory receptors and muscles responsible for carrying out the body's responses).

CELL TYPES
NEURON General structure of the neuron
The functional unit of nervous system is the neuron, a highly specialized cell type, which comprises three distinct parts, the soma or cell body, dendrites and axon. The soma or cell body often contains rough endoplasmic reticulum highly differentiated and organized in parallel groups of cisternae, with numerous polyribosomes, because the neuron synthesizes larges amounts of structural proteins, transport proteins and secreted proteins. With proper staining and by an light microscope you can see the rough endoplasmic reticulum and the large amount of free proteins. These are called Nissl bodies, accumulations of basophilic material within the neuronal soma Neuronal cell body Neuronal cell body Neuronal cell body .
The dendrites are multiple processes similar to tree branches, which arise from the cell body Dendrite starting point , called dendritic tree. The surface of the dendritic tree is full of small protrusions called dendritic spines Dendrites & spines Dendrites & spines whose mission is increase the usable area of the dendrites to establish numerous synaptic contacts with other neurons Dendritic globet spine Dendritic filiform spines .
Neurons have a single axon General structure of the neuron that rises in the neuronal soma axon hillock and ends in a synaptic terminal arborization or synaptic buttons. The axon has an active bidirectional transport of large and small particles. The organelles and macromolecules synthesized in the neuron cell body anterogradely moving from perikaryon to the synaptic terminals, while for retrograde transport, the opposite direction, other macromolecules and circulating materials from endocytosis, including viruses i toxins.
Note that both the dendrites and the axon has multiple ramifications, but while the axon branches at its end (the telodendron), the dendrites have multiple extensions that originate in the cell soma.
The surface membrane of the soma and dendritic tree are specialized in receiving and integrating information from other neurons to which it is in contact, while the axon is specialized in the trasmission of information as in the form of an action potential or a nerve impulse.

Types of neurons
There are different types of neurons identified by the number of processes that emerge from the soma, and its length. In relation to the number of processes can identify the following types of neurons:
1.- Unipolar neurons. Unipolar neurons are neurons with only a process that splits and acts as an axon and a dendrite. This neuron type transmit the nervous impulse without it passing through the neuronal soma. They are typical of invertebrate ganglia and the retina.
2.- Bipolar neurons Fusiform neuron Fusiform neuron . They have an elongated cell body, where one end portion of a single dendrite and one other with the axon. The nucleus of this cell type is located in the center of the soma. Bipolar neurons are a cell type found in the visual, auditory and vestibular. systems.
3.- Multipolar neurons Multipolar neuron . Multipolar neurons have multiple processes attached to a polygonal morphology soma. Among the multiple processes there is only a single axon, all the others proceses are dendrites organized in different ways. This type of neurons are the most abundant of the nervous system. In multipolar neurons distinguish between those which are Golgi type I Basket cells , with a long axon, and the Golgi type II, with a short axons. Projection neurons are the first type, and local neurons or interneurons in the second type. Note that pyramidal neurons Pyramidal neuron Pyramidal neuron Pyramidal neuron of the cerebral cortex and cerebellar Purkinje cells are two typical examples of such neurons. The pyramidal neuron soma with a pyramidal morphology with several dendritic trees located in each vertices of the pyramid, wich the apical dendritic tree is the most prominent. From the base of the pyramid leaves the unique and thin axon, usually a projection axon. Purkinje cells are a cell type characteristic of the cerebellum, consisting of a large soma with a prominent nucleus and a well developed apical dendritic tree arranged in one plane perpendicluary to the surface of the cerebellar cortex. In the basis of the neuronal soma we can find the axon, as in the case of the pyramidal neuron, is a projection axon.
4.- Pseudounipolar neurons. Pseudounipolar neurons are neurons which has a single dendrite or neurite, which divides in a short distance from the cell body into two branches, one that is directed toward a peripheral structure and one that enters the central nervous system. They are examples of such neurons in the posterior root ganglia of the spinal cord.

Synaptic terminals and synapses Synapse
Synaptic terminals are specializations for neuronal communication as chemical message in response to an action potential. The synapse is the junction between the presynaptic axon and postsynaptic membrane receptor, mainly located in the dendrite.
The prefixes pre-and post- refer to the direction of synaptic transmission, where the presynaptic element is referee to the transmission side, usually an axon, while the postsynaptic element refers to the receiving side, which is usually a dendrite or a neuronal soma, and occasionally, an axon. Presynaptic and postsynaptic elements are covered by a dense material inside the membrane, called the presynaptic and postsynaptic densities; finally noting that both elements are separated by a space called the synaptic cleft. Presynaptic terminals are full of membrane-coated vesicles, the synaptic vesicles, which originate in the neuronal soma and transported along the axon through protein motors. Each vesicle contains a neurotransmissor. Postsynaptic terminals contain mitochondria, some components of smooth endoplasmic reticulum, microtubules and some neurofilaments.
Synapses are classifiable by their location on the postsynaptic neuron according to:
1.- Axodendritic synapse Multiple synapses on a dendrite Synapses on a dendritic spine Synapses on a dendrite in transversal section . Axon terminal contacts with a dendrite (or their dendritic spines).
2.- Axosomatic synapse Synapse on a neuronal cell body . Axon terminal directly contacts with the neuronal soma.
3.- Axoaxonic synapse. Axon terminal contacts with another axon.

NEUROMUSCULAR JUNCTION Motor plate
Neuromuscular junction is a specialized structure formed by the motor nerves and the target muscle. When the motor nerve, which loses the myelin sheath perimysium level when reaches skeletal muscle, it divides into several branches, each innervating a single muscle fiber due to swelling presynaptic button, covered by Scwhann cells. Muscles that need fine control has fewer muscle fibers per motor unit, while very long fibers contain several hundred muscle fibers per motor unit. The buttons contain mitochondria and vesicles presynaptic membrane coated filled with acetylcholine, the neurotransmitter released in the dense regions of the cytoplasmic side of the axonal membrane, the called active zones.
The buttons are located on presynaptic depression of the muscle fiber, called the primary synaptic cleft, where they also found the secondary synaptic clefts, which are deep folds located in the sarcolemma. In the crests of the folds are located acetylcholine receptors while voltage dependent sodium channels are in deep folds.
Basal lamina surrounded muscle and extends into the synaptic cleft and continuous with the basal lamina covering the Schwann cell. Note that in the synaptic cleft, basal lamina contains acetylcholinesterase, the enzyme responsible for inactivating acetylcholine, which makes two molecular precursors, which are the acetyl and the coline.

GLIAL CELLS
Glial cells are 10 times more abundant than neurons. In the central nervous system, glial cells surround much of the neuronal bodies, axons and dendrites, occupying the spaces between neurons. Glial cells are responsible for creating the ideal microenvironment for neuronal activity.
The neuropil is a dense network formed by the processes of both neurons and glial cells, which fills all the spaces of central nervous system.

Astrocytes Fibrous astrocyte
Astrocytes have a high amount of cytoplasmic processes. There are two types of astrocytes based on morphology of the extensions and their location. While fibrous astrocytes have fewer extensions predominate in the white matter, protoplasmic astrocytes with shorter extensions are in the gray matter. Astrocytes develop large number of morphological and structural features and maintained to ensure neuronal survival, one of the most important functions. It should be noted that astrocytes regulate the extracellular environment of neurons, controlling the ionic concentrations of few extracellular space, absorb local excess neurotransmitters, secrete numerous regulatory factors and metabolites of neuronal activity, and finally, it is noteworthy that astrocytes are communicated through gap junctions, allowing the creation of an astroglial network where information flows over long distances through them.
Besides these functions, astrocytes perform other functions related to blood brain barrier. Some astrocytes exhibit cytoplasmic processes called foot vascularthat lining with endothelial cells in the capillaries present in the central nervous system. Vascular feet are important because, among other things, regulate vasodilation and oxygen transfer, ions and other substances from the blood to neurons. Other types of astrocyte extensions that form a layer, the glial limiting membrane that separates the pia mater, the innermost layer of the meningeal layers, and the outer surface of central nervous system. Moreover, astrocytes have an important role when it damages the central nervous system, that astrocytes proliferate to form glial scar, which often interfere with neuronal regeneration.
The processes of all astrocytes are reinforced with intermediate filaments called glial fibrillary acidic protin, or GFAP for its acronym in English.

Oligodendrocytes Oligodendrocyte Oligodendrocyte
Oligodendrocytes produce the myelin sheaths that provide the necessary electrical isolation of the central nervous system neurons through oligodendrocytes that extend several processes involving different parts of several axons. They are the predominant cell type in the white substance of central nervous system. The oligodendrocyte processes are not observed with the help of light microscope and a routine histological staining, although they can be differentiated as small rounded cells with a highly condensed nucleus.

Schwann cells Schwann cell and myelinated axons
Schwann cells are located in the peripheral nervous system where they interact with the axons forming the myelin sheath similarly to how they do the oligodendorocytes in the central nervous system. A Scwann cell forms a segment of myelin sheath around a single axon, compared with the ability of oligodendrocytes to branch out and involve different axons.

Microglial cells
Slightly less numerous than oligodendrocytes or astrocytes, but more evenly distributed between gray matter and white matter, microglial cells are small cells with very irregulary cell processes. Unlike other glial cells, microglia can migrate through the neuropil, tracking the tissue to detect damaged cells or invading microorganisms. Secrete a lots of immunoregulatory cytokines and constitute the largest immune defense mechanism of central nervous tissue.
Microglial cells, unlike other glial cells do not originate in the embryonic neural tube, come from circulating blood monocytes, and belong to the same family of macrophages and other antigen-presenting cells.
The nucleus of microglial cells can be differentiated in histological preparations with routine stains such as hematoxylin-eosin, thanks to its elongated structure, which contrasts with the rounded shape of other glial cells. Immunohistological, using antibodies against immune cell surface antigens can be visualized the microglial cell processes.
When microglial cells are activated, retract their processes and recover their rounded morphology characteristic of macrophages, becoming antigen-presenting cells with phagocytic capacity.

Ependymal cells
Ependymal cells are cuboidal or prismatic cells limiting the ventricles of the brain and spinal cord. In several locations the central nervous system, ependymal cells have cilia in their apical surface to facilitate the movement of cerebrospinal fluid, or long microvilli, which are involved in absorption.
The ependymal cells have junctional complexes similar to those found in epithelial cells. However, unlike the true epithelial cells, ependymal cells have no basal lamina.

Satellite cells of the ganglia
Like Schwann cells, satellite cells of the ganglia derived from the neural crest. Form a cover around large nerve cell bodies in the ganglia of the peripheral nervous system. Satellite cells play the role of trophic support on ganglion neurons, but the molecular basis of this function are very poorly studied.

PERIPHERAL NERVES
Myelinated fibers Myelinated nerve Myelinated nerve Myelinated axon
Large caliber axons that grow in the peripheral nervous system are enclosed in undifferentiated Schwann cells, which become myelinated nerve fibers through the membranes of Schwann cells are fused around the axon and wrap around the axon. The multiple layers of membrane wrapped become the myelin sheath. The membranes of Schwann cells have a high lipid content axons to protect and maintain constant ionic microenvironment necessary for action potential transmission. The length of an axon covered by a Schwann cell is called internodes can measure more than 1 milíletro. Separating the internodes, we find the nodes of Ranvier, small gaps coated interdigitated processes, but not myelinated, which are found throughout the length of the axon.

Unmyelinated fibers
In the peripheral nervous system unmyelinated axons are all surrounded by simple folds of Schwann cells. Schwann cells do not form a myelin sheath, which may each include portions of the cells of various small-caliber axons.

EXERCICES

- Locate the neuron cell nucleus in the virtual slide Neuronal somata (neuronal cell bodies) Klüver-Barrera 7 um .
- Observe the localization of nuclei in the virtual slide Neuronal somata (neuronal cell bodies) Klüver-Barrera 7 um .
- Observe the small nuclei corresponding to glial cell nucleus in the virtual slide Neuronal somata (neuronal cell bodies) Klüver-Barrera 7 um .
- Locate the pyramidal neuron in the virtual slide Neurons Golgi's technique 150 um .
- Distinsh between the pyramidal neuronal soma, the axon and the dendrites in the virtual slide Neurons Golgi's technique 150 um .
- Note the presence of other neurons in the surface of the virtual slide Neurons Golgi's technique 150 um .
- Locate dendritic spines in the virtual slide a major augments Neurons Golgi's technique 150 um .
- Localize the myelinated nerve in the virtual slide Myelinated axons (nerve fibers) H-E 1,5 um .
- Locate a single nervous fiber. Observe the myelin sheat and the central axon Myelinated axons (nerve fibers) H-E 1,5 um .
- Locate the perineurium in the virtaul slide Myelinated axons (nerve fibers) H-E 1,5 um .
- Locate the neuronal cell nuclei in the virtual slide Neuronal somata and nerve fibers Silver Nitrate 1,5 um .
- Localize nervous fibers in longitudinal section Neuronal somata and nerve fibers Silver Nitrate 1,5 um .
- Localize nervous fibers in transverse section Neuronal somata and nerve fibers Silver Nitrate 1,5 um .