Download 1. Molecular layer

Histology:2nd stage
2015-2016
Dr. Raja Ali
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Nerve Tissue & the Nervous system:
Central Nervous System:
The principal structures of the CNS are the cerebrum, cerebellum,
and spinal cord. It has virtually no connective tissue and is therefore a
relatively soft, gel-like organ. When sectioned ,the cerebrum ,cerebellum,
and spinal cord show regions of white (white matter) and gray (gray matter),
differences caused by the differential distribution of myelin.
Gray matter contains abundant neuronal cell bodies, dendrites, the
initial unmyelinated portions of axons , astrocytes ,and microglial cells.
Gray matter is prevalent at the cortex of the cerebrum and
cerebellum, whereas white matter is present in more central regions.
Cerebral Cortex: fig.(5)
General Features:
Cerebral cortex consists of grey matter that covers the cerebral hemisphere.
The surface area of the cortex is increased due to the presence of many
convolutions or gyri separated by sulci.
The cortex is made of a mixture of nerve cells, fibers, neuroglia and blood
vessels.
Types of Nerve Cells
The nerve cells in cerebral cortex are of five types ,they are:
1. Pyramidal cells
 Are the most common type of neurons found in the cerebral cortex.
 Are pyramidal in shape.
 Their size ranges from 10 μm to 120 μm.
 Giant pyramidal cells (120 μm) in the motor cortex are called Betz
cells.
 The apices of the neurons give rise to dendritic processes which are
directed towards the surface of the cortex, whereas the bases give
origin to axons which forms projection fibers of the white matter.
 They are distributed in layers, 2–5, and progressively increase in size.
2. Stellate/Granule cells
 Small, star-shaped neurons of uniform diameter (8 μm).
 Have short axons terminating in nearby neurons.
3. Fusiform cells
 Spindle-shaped cells placed at right angles to the surface in the deep
layer.
 Dendrites arise from each pole of the cell body and axon arises from
the cell body just above the lower pole and enters the white matter.
4. Horizontal cells of Cajal
 They are also spindle-shaped cells but oriented horizontally, parallel
to the surface in the superficial layer ( molecular).
 Dendrites arise from each pole and axon arises from the cell body and
runs horizontally, parallel to the surface making contact with dendrites
of pyramidal cells.
5. Cells of Martinotti
 Small multipolar cells found in layers 3–6.
 The axons are directed towards the surface of the cortex and generally
end in the molecular layer.
Structure:
The nerve cells and associated fibers of cerebral cortex are so arranged as to
form six layers, which are poorly distinguished.
The layers are named according to the type and density of the cells:
1. Molecular layer (plexiform layer)—is the most superficial, well defined
layer. It consists mainly of nerve fibers and occasional horizontal cells of
Cajal.
2. External granular layer—contains large number of stellate cells and
small pyramidal cells.
3. External pyramidal layer—is mainly made of medium sized pyramidal
cells and also contains few stellate cells and cells of Martinotti.
4. Internal granular layer—is composed of closely packed stellate cells and
horizontally oriented white fi bre band called outer band of Baillarger.
5. Internal pyramidal layer (ganglionic layer)—consists mainly of large
pyramidal cells and few stellate cells and cells of Martinotti. This layer also
contains horizontally arranged fi bres that form the inner band of
Baillarger.
6. Multiform layer (layer of polymorphic cells)—is the deepest layer. It
contains predominantly fusiform cell and also few stellate cells and cells of
Martinotti intermixed with many nerve fibers entering or leaving the
underlying white matter.
Notes:
 The structure of cerebral cortex shows considerable variation from
region to region.
 In sensory areas the granular layers are well developed whereas the
pyramidal layers are poorly developed and is termed as granular
cortex.
 In motor areas it is the other way round, i.e. the pyramidal layers are
well developed and the granular layers are poorly developed and is
known as a granular cortex.
Cerebellar Cortex: fig.(4)
Gneral Cortex:
 The cortex of cerebellum is highly folded. The folds or folia are
separated by closely set parallel transverse fissures.
 Each folium contains a core of white matter covered superficially by
grey matter or cortex.
Structure:
The cerebellar cortex consists of three layers; an external molecular layer, a
middle Purkinje cell layer and an internal granular layer :
1. Molecular layer
 Is the superfi cial thick layer and is usually lightly stained with eosin.
 Mainly made of nerve fi bres and few cells, namely, stellate cells in
the superfi cial part and basket cells in the deeper part.
 The axons of these cells run parallel to the long axis of the folia. The
axons of basket cells form collaterals which arborize around the
Purkinje cells in a ‘basket-like’ manner.
2. Purkinje cell layer
Purkinje cells are large flask-shaped neurons (Golgi type I) and are arranged
in a single row between molecular and granular layers.
The dendrites of these cells pass into molecular layer and arborize profusely
in a plane transverse to the folium.
These dendrites synapse with axons of granular cells
The axons of Purkinje cells pass through the granular layer to end in deeper
nuclei of cerebellum.
3. Granular layer
Is stained deeply with hematoxylin because it is densely packed with very
small granule neurons .
The axons of these granule cells pass into the molecular layer where they
bifurcate in a T-shaped manner and run at to the dendritic processes of the
Purkinje cells and synapse with them.
Few Golgi cells (type II) are also present in the granular layer.
Meninges: fig.(1,2)
The skull and the vertebral column protect the CNS. Between the
bone and nervous tissue are membranes of connective tissue called the
meninges .
Three meningial layers are distinguished:
Dura Mater:
The dura mater is the thick external layer consisting of dense,
fibroelastic connective tissue continuous with the periosteum of the skull.
Around the spinal cord the dura matter is separated from the
periosteum of the vertebrae by the epidural space, which coniains a plexus of
thin-walled veins and areolar connective tissue.
The dura mater is always separated from the arachnoid by the thin
subdural space. The internal surface of all dura mater' as well as its external
surface in the spinal cord, is covered by simple squamous epithelium of
mesenchymal origin .
Arachnoid:fig.(3)
The arachnoid (Gr. = spiderweblike) has two components:
 a sheet of connective tissue in contact with the dura mater .
 a system of loosely arranged trabeculae containing fibroblasts and
collagen ,continuous with the pia matter.Surrounding the trabeculae is
a large cavity ,the subarachnoid space filled with CSF, that
continuous with the ventricles of the brain.
The connective tissue of the arachnoid is said to be avascular ,it lucks
nutritive capillary ,but larger blood vessels run through it. The arachnoid and
pia matter intimately associated and are considered asingle membrane called
pia -arachnoid .
In some areas, the arachnoid peforates the dura matter and protrudes
into blood –filled venous sinuses within the dura mater ,these structures are
covered by vascular endothelial cells ,are called arachnoid villi .Their
function is to
transport CSF from the subarachnoid space into venous
sinuses.
Pia Mater:
The innermost pia mater is lined internally by flattened,
mesenchymally derived closely applied to the entire surface of the CNS
tissue, but this layer does not directly contact nerve cells or fibers. Between
the pia mater and the neural elements is a thin limiting layer of astrocytic
processes. Together the pia matter and glial layer form a physical barrier at
the CNS periphery, separated the CNS from the CSF in the subarachnoid
space.
Figure (1). Spinal cord and meninges
Figure (2). Meninges around the brain.
Figure (3): Choroid plexus.
fig.( 4):cerebellar cortex
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Fig.(5 ):cerebral
cortex
W.M.
fig.(6 ):Spinal cord