The Ganglia-Histology of Nervous System

Ganglia are ovoid structures containing neuronal cell bodies and glial cells supported by connective tissue. Because they serve as relay stations to transmit nerve impulses, one nerve enters and another exits from each ganglion. The direction of the nerve impulse determines whether the ganglion will be a sensory or an autonomic ganglion.

Sensory Ganglia

Sensory ganglia receive afferent impulses that go to the central nervous system. Two types of sensory ganglia exist. Some are associated with cranial nerves (cranial ganglia); others are associated with the dorsal root of the spinal nerves and are called spinal ganglia. A connective tissue framework and capsule support the ganglion cells. The neurons of these ganglia are pseudounipolar and relay information from the ganglion's nerve endings to the gray matter of the spinal cord via synapses with local neurons.

 

Autonomic Ganglia

Autonomic ganglia appear as bulbous dilatations in autonomic nerves. Some are located within certain organs, especially in the walls of the digestive tract, where they constitute the intramural ganglia. These ganglia are devoid of connective tissue capsules, and their cells are supported by the stroma of the organ in which they are found. Autonomic ganglia usually have multipolar neurons. As with craniospinal ganglia, autonomic ganglia have neuronal perikaryons with fine Nissl bodies.A layer of satellite cells frequently envelops the neurons of autonomic ganglia. In intramural ganglia, only a few satellite cells are seen around each neuron.

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The Nerves-Histology of Nervous System

In the peripheral nervous system, the nerve fibers are grouped in bundles to form the nerves. Except for a few very thin nerves made up of unmyelinated fibers, nerves have a whitish, homogeneous, glistening appearance because of their myelin and collagen content.

Nerves have an external fibrous coat of dense connective tissue called epineurium, which also fills the space between the bundles of nerve fibers. Each bundle is surrounded by the perineurium, a sleeve formed by layers of flattened epitheliumlike cells. The cells of each layer of the perineurial sleeve are joined at their edges by tight junctions, an arrangement that makes the perineurium a barrier to the passage of most macromolecules and has the important function of protecting the nerve fibers from aggression. Within the perineurial sheath run the Schwann cell-sheathed axons and their enveloping connective tissue, the endoneurium. The endoneurium consists of a thin layer of reticular fibers, produced by Schwann cells.

The nerves establish communication between brain and spinal cord centers and the sense organs and effectors (muscles, glands, etc). They possess afferent and efferent fibers to and from the central nervous system. Afferent fibers carry the information obtained from the interior of the body and the environment to the central nervous system. Efferent fibers carry impulses from the central nervous system to the effector organs commanded by these centers. Nerves possessing only sensory fibers are called sensory nerves; those composed only of fibers carrying impulses to the effectors are called motor nerves. Most nerves have both sensory and motor fibers and are called mixed nerves; these nerves have both myelinated and unmyelinated axons .

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Peripheral Nervous System-Histology of Nervous System

The main components of the peripheral nervous system are the nerves, ganglia, and nerve endings. Nerves are bundles of nerve fibers surrounded by connective tissue sheaths.

Nerve Fibers

Nerve fibers consist of axons enveloped by a special sheath derived from cells of ectodermal origin. Groups of nerve fibers constitute the tracts of the brain, spinal cord, and peripheral nerves.

Myelinated Fibers

In myelinated fibers of the peripheral nervous system, the plasmalemma of the covering Schwann cell winds and wraps around the axon . The layers of membranes of the sheath cell unite and form myelin. Myelin consists of many layers of modified cell membranes. These membranes have a higher proportion of lipids than do other cell membranes. The myelin sheath shows gaps along its path called the nodes of Ranvier; these represent the spaces between adjacent Schwann cells along the length of the axon. Interdigitating processes of Schwann cells partially cover the node. The distance between two nodes is called an internode and consists of one Schwann cell. The length of the internode varies between 1 and 2 mm. There are no Schwann cells in the central nervous system; there, the processes of the oligodendrocytes form the myelin sheath. Oligodendrocytes differ from Schwann cells in that different branches of one cell can envelop segments of several axons .

Unmyelinated Fibers

In both the central and peripheral nervous systems, not all axons are sheathed in myelin. In the peripheral system, all unmyelinated axons are enveloped within simple clefts of the Schwann cells. Unlike their association with individual myelinated axons, each Schwann cell can sheathe many unmyelinated axons. Unmyelinated nerve fibers do not have nodes of Ranvier, because abutting Schwann cells are united to form a continuous sheath.

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The Central Nervous System-Histology of Nervous System

The central nervous system consists of the cerebrum, cerebellum, and spinal cord. It has almost no connective tissue and is therefore a relatively soft, gel-like organ.

When sectioned, the cerebrum, cerebellum, and spinal cord show regions that are white (white matter) and that are gray (gray matter). The differential distribution of myelin in the central nervous system is responsible for these differences: The main component of white matter is myelinated axons and the myelin-producing oligodendrocytes. White matter does not contain neuronal cell bodies.

Gray matter contains neuronal cell bodies, dendrites, and the initial unmyelinated portions of axons and glial cells. This is the region at which synapses occur. Gray matter is prevalent at the surface of the cerebrum and cerebellum, forming the cerebral and cerebellar cortex , whereas white matter is present in more central regions. Aggregates of neuronal cell bodies forming islands of gray matter embedded in the white matter are called nuclei.

In the cerebral cortex, the gray matter has six layers of cells with different forms and sizes. Neurons of some regions of the cerebral cortex register afferent (sensory) impulses; in other regions, efferent (motor) neurons generate motor impulses that control voluntary movements. Cells of the cerebral cortex are related to the integration of sensory information and the initiation of voluntary motor responses.

The cerebellar cortex has three layers : an outer molecular layer, a central layer of large Purkinje cells, and an inner granule layer. The Purkinje cells have a conspicuous cell body and their dendrites are highly developed, assuming the aspect of a fan. These dendrites occupy most of the molecular layer and are the reason for the sparseness of nuclei. The granule layer is formed by very small neurons (the smallest in the body), which are compactly disposed, in contrast to the less cell-dense molecular layer .

In cross sections of the spinal cord, white matter is peripheral and gray matter is central, assuming the shape of an H. In the horizontal bar of this H is an opening, the central canal, which is a remnant of the lumen of the embryonic neural tube. It is lined with ependymal cells. The gray matter of the legs of the H forms the anterior horns. These contain motor neurons whose axons make up the ventral roots of the spinal nerves. Gray matter also forms the posterior horns (the arms of the H), which receive sensory fibers from neurons in the spinal ganglia (dorsal roots).

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Neuroglial Cells-Histology of Nervous System

Glial cells are 10 times more abundant in the mammalian brain than neurons; they surround both cell bodies and their axonal and dendritic processes that occupy the interneuronal spaces.

Nerve tissue has only a very small amount of extracellular matrix, and glial cells furnish a microenvironment suitable for neuronal activity.

1. Oligodendrocytes produce the myelin sheath that provides the electrical insulation of neurons in the central nervous system. These cells have processes that wrap around axons, producing a myelin sheath
2. Schwann cells have the same function as oligodendrocytes but are located around axons in the peripheral nervous system. One Schwann cell forms myelin around a segment of one axon, in contrast to the ability of oligodendrocytes to branch and serve more than one neuron and its processes.
3. Astrocytes are star-shaped cells with multiple radiating processes. Astrocytes bind neurons to capillaries and to the pia mater (a thin connective tissue that covers the central nervous system). Astrocytes with few long processes are called fibrous astrocytes and are located in the white matter; protoplasmic astrocytes, with many short-branched processes, are found in the gray matter. In addition to their supporting function, astrocytes participate in controlling the ionic and chemical environment of neurons. Some astrocytes develop processes with expanded end feet that are linked to endothelial cells. It is believed that through the end feet, astrocytes transfer molecules and ions from the blood to the neurons (Blood-Brain Barrier). Expanded processes are also present at the external surface of the central nervous system, where they make a continuous layer. Furthermore, when the central nervous system is damaged, astrocytes proliferate to form cellular scar tissue. Astrocytes can influence neuronal survival and activity through their ability to regulate constituents of the extracellular environment, absorb local excess of neurotransmitters, and release metabolic and neuroactive molecules. The latter molecules include peptides of the angiotensinogen family, vasoactive endothelins, opioid precursors called enkephalins, and the potentially neurotrophic somatostatin. On the other hand, there is some evidence that astrocytes transport energy-rich compounds from the blood to the neurons and also metabolize glucose to lactate, which is then supplied to the neurons.
4. Ependymal cells are low columnar epithelial cells lining the ventricles of the brain and central canal of the spinal cord. In some locations, ependymal cells are ciliated, which facilitates the movement of cerebrospinal fluid.
5. Microglia are small elongated cells with short irregular processes. Microglia, phagocytic cells that represent the mononuclear phagocytic system in nerve tissue, are derived from precursor cells in the bone marrow. They are involved with inflammation and repair in the adult central nervous system, and they produce and release neutral proteases and oxidative radicals. When activated, microglia retract their processes and assume the morphological characteristics of macrophages, becoming phagocytic and acting as antigen-presenting cells. Microglia secrete a number of immunoregulatory cytokines and dispose of unwanted cellular debris caused by central nervous system lesions.

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SYNAPSES-Histology of Nervous System

The synapse is responsible for transmission of nerve impulses. Synapses are sites of functional contact between neurons or between neurons and other effector cells (eg, muscle and gland cells). The function of the synapse is to convert an electrical signal (impulse) from the presynaptic cell into a chemical signal that acts on the postsynaptic cell. Most synapses transmit information by releasing neurotransmitters during the signaling process.

The synapse itself is formed by an axon terminal (presynaptic terminal) that delivers the signal, a region on the surface of another cell at which a new signal is generated (postsynaptic terminal), and a thin intercellular space called the synaptic cleft.

If an axon forms a synapse with a cell body, the synapse is called axosomatic; if it forms a synapse with a dendrite, it is called axodendritic; and if it forms a synapse with an axon, it is called axoaxonic .

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Ultrastructure of Neuron-Histology of Nervous System

Cell Body

The cell body, also called perikaryon, is the part of the neuron that contains the nucleus and surrounding cytoplasm. Most nerve cells have a spherical, unusually large, euchromatic (pale-staining) nucleus with a prominent nucleolus. The chromatin is finely dispersed, reflecting the intense synthetic activity of these cells.

The cell body contains a highly developed rough endoplasmic reticulum organized into aggregates of parallel cisternae. In the cytoplasm between the cisternae are numerous polyribosomes, suggesting that these cells synthesize both structural proteins and proteins for transport. When appropriate stains are used, rough endoplasmic reticulum and free ribosomes appear under the light microscope as basophilic granular areas called Nissl bodies . The number of Nissl bodies varies according to neuronal type and functional state. The Golgi complex is located only in the cell body and consists of multiple parallel arrays of smooth cisternae arranged around the periphery of the nucleus. Mitochondria are especially abundant in the axon terminals. They are scattered throughout the cytoplasm of the cell body. Neurofilaments are abundant in perikaryons and cell processes. The neurons also contain microtubules that are identical to those found in many other cells. Nerve cells occasionally contain inclusions of pigments, such as lipofuscin, which is a residue of undigested material by lysosomes.

Dendrites

Dendrites are usually short and divide like the branches of a tree. They receive many synapses and are the principal signal reception and processing sites on neurons. Most nerve cells have numerous dendrites, which considerably increase the receptive area of the cell. Bipolar neurons, with only one dendrite, are uncommon and are found only in special sites. Unlike axons, which maintain a constant diameter from one end to the other, dendrites become thinner as they subdivide into branches. The cytoplasmic composition of the dendrite base, close to the neuron body, is similar to that of the perikaryon but is devoid of Golgi complexes.

Axons

Most neurons have only one axon; a very few have no axon at all. An axon is a cylindrical process that varies in length and diameter according to the type of neuron. Although some neurons have short axons, axons are usually very long processes. For example, axons of the motor cells of the spinal cord that innervate the foot muscles may be up to 100 cm (about 40 inches) in length. All axons originate from a short pyramid-shaped region, the axon hillock, that usually arises from the perikaryon. The plasma membrane of the axon is called the axolemma ; its contents are known as axoplasm.

In neurons that give rise to a myelinated axon, the portion of the axon between the axon hillock and the point at which myelination begins is called the initial segment. This is the site at which various excitatory and inhibitory stimuli impinging on the neuron are algebraically summed, resulting in the decision to propagate—or not to propagate—an action potential, or nerve impulse. It is known that several types of ion channels are localized in the initial segment and that these channels are important in generating the change in electrical potential that constitutes the action potential. In contrast to dendrites, axons have a constant diameter and do not branch profusely. Occasionally, the axon, shortly after its departure from the cell body, gives rise to a branch that returns to the area of the nerve cell body. All axon branches are known as collateral branches. Axonal cytoplasm (axoplasm) possesses mitochondria, microtubules, neurofilaments, and some cisternae of smooth endoplasmic reticulum. The absence of polyribosomes and rough endoplasmic reticulum emphasizes the dependence of the axon on the perikaryon for its maintenance. If an axon is severed, its peripheral parts degenerate and die.

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Structure of Neuron –Histology of Nervous System

Nerve cells, or neurons, are responsible for the reception, transmission, and processing of stimuli; the triggering of certain cell activities; and the release of neurotransmitters and other informational molecules.

Most neurons consist of three parts: the dendrites, which are multiple elongated processes specialized in receiving stimuli from the environment, sensory epithelial cells, or other neurons; the cell body, or perikaryon, which is the trophic center for the whole nerve cell and is also receptive to stimuli; and the axon, which is a single process specialized in generating or conducting nerve impulses to other cells (nerve, muscle, and gland cells). Axons may also receive information from other neurons; this information mainly modifies the transmission of action potentials to other neurons. The distal portion of the axon is usually branched and constitutes the terminal arborization. Each branch of this arborization terminates on the next cell in dilatations called end bulbs (boutons), which interact with other neurons or nonnerve cells, forming structures called synapses. Synapses transmit information to the next cell in the circuit.

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Leukopoiesis Powerpoint 2012

Powerpoint untuk blok Hematology semester III tahun 2012 dengan topik Leukopoiesis. Presentasi ini membahas tentang pembentukan leukosit yang dibagi menjadi : granulocytopoiesis yaitu pembentukan granulosit (neutrofil, basofil dan eosinofil), monocytopoiesis yaitu pembentukan monosit dan lymphopoiesis yaitu pembentukan limfosit.



LINK DOWNLOAD : LEUKOPOIESIS

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Posted in 2012 , hematology , hematopoiesis , power point

LEUKOPOIESIS

Granulopoiesis

The maturation process of granulocytes takes place with cytoplasmic changes characterized by the synthesis of a number of proteins that are packed in two organelles: the azurophilic and specific granules. These proteins are produced in the rough endoplasmic reticulum and the Golgi complex in two successive stages. The first stage results in the production of the azurophilic granules, which stain with basic dyes in the Wright or Giemsa methods and contain enzymes of the lysosomal system. In the second stage, a change in synthetic activity takes place with the production of several proteins that are packed in the specific granules. These granules contain different proteins in each of the three types of granulocytes and are utilized for the various activities of each type of granulocyte.

 

Maturation of Granulocytes

The myeloblast is the most immature recognizable cell in the myeloid series. It has a finely dispersed chromatin, and nucleoli can be seen. In the next stage, the promyelocyte is characterized by its basophilic cytoplasm and azurophilic granules. These granules contain lysosomal enzymes and myeloperoxidase. The promyelocyte gives rise to the three known types of granulocyte. The first sign of differentiation appears in the myelocytes, in which specific granules gradually increase in quantity and eventually occupy most of the cytoplasm. These neutrophilic, basophilic, and eosinophilic  myelocytes mature with further condensation of the nucleus and a considerable increase in their specific granule content. Before its complete maturation, the neutrophilic granulocyte passes through an intermediate stage in which its nucleus has the form of a curved rod (band cell).

 

Maturation of Lymphocytes & Monocytes

Study of the precursor cells of lymphocytes and monocytes is difficult, because these cells do not contain specific cytoplasmic granules or nuclear lobulation, both of which facilitate the distinction between young and mature forms of granulocytes. Lymphocytes and monocytes are distinguished mainly on the basis of size, chromatin structure, and the presence of nucleoli in smear preparations. As lymphocyte cells mature, their chromatin becomes more compact, nucleoli become less visible, and the cells decrease in size. In addition, subsets of the lymphocyte series acquire distinctive cell-surface receptors during differentiation that can be detected by immunocytochemical techniques.

Lymphocytes

Circulating lymphocytes originate mainly in the thymus and the peripheral lymphoid organs (eg, spleen, lymph nodes, tonsils). However, all lymphocyte progenitor cells originate in the bone marrow. Some of these lymphocytes migrate to the thymus, where they acquire the full attributes of T lymphocytes. Subsequently, T lymphocytes populate specific regions of peripheral lymphoid organs. Other bone marrow lymphocytes differentiate into B lymphocytes in the bone marrow and then migrate to peripheral lymphoid organs, where they inhabit and multiply in their own special compartments.

The first identifiable progenitor of lymphoid cells is the lymphoblast, a large cell and dividing two or three times to form prolymphocytes. Prolymphocytes are smaller and have relatively more condensed chromatin but none of the cell-surface antigens that mark prolymphocytes as T or B lymphocytes. In the bone marrow and in the thymus, these cells synthesize cell-surface receptors characteristic of their lineage, but they are not recognizable as distinct B or T lymphocytes in routine histological procedures. Using immunocytochemical techniques makes the distinction.

 

Monocytes

The monoblast is a committed progenitor cell that is almost identical to the myeloblast in its morphological characteristics. Further differentiation leads to the promonocyte, a large cell (up to 18 um in diameter) with a basophilic cytoplasm and a large, slightly indented nucleus. The chromatin is lacy, and nucleoli are evident. Promonocytes divide twice in the course of their development into monocytes. A large amount of rough endoplasmic reticulum is present, as is an extensive Golgi complex in which granule condensation can be seen to be taking place. These granules are primary lysosomes, which are observed as fine azurophilic granules in blood monocytes. Mature monocytes enter the bloodstream, circulate for about 8 h, and then enter the connective tissues, where they mature into macrophages and function for several months.

 

source : Basic Histology

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Posted in 2012 , hematology , materi kuliah