128
UNIT 2
Basic Tissues
Figure 7-11A.
Cerebellar folium.
Nissl stain,
3
34
The
cerebellum
(
green structure
in
inset
) is a large, complex struc-
ture that lies beneath the posterior portion of the cerebral hemi-
spheres. Its name means “little brain.” It is critical for smooth,
coordinated movements and participates, to a lesser extent, in many
other functions. The structural organization of the cerebellum is
similar to that of the cerebrum in that it consists of
white matter
,
a
cortex
, and
subcortical nuclei
. However, the organization of the
cortex is very different from that of the cerebral cortex. There are
only three layers, the
granule cell layer
, the
Purkinje cell layer
, and
the
molecular layer
. The granule cell layer contains an enormous
number of very small, tightly packed cells and their dendrites. The
Purkinje cell layer, at the interface of the granule cell and molecular
layers, is only one cell deep. The molecular layer contains primarily
axons and dendrites, with only very few neuron cell bodies. The
red rectangle
indicates the position of the tissue in this F gure.
Purkinje cell
Purkinje cell
Molecular layer
Molecular layer
Granule cell
Granule cell
layer
layer
White matter
White matter
White matter
Molecular layer
Purkinje cell
Granule cell
layer
A
CLINICAL CORRELATION
Figure 7-11C.
Encephalocele.
H&E,
3
17
In some embryos, the
neuroectoderm
does not sepa-
rate from the
surface ectoderm
during early stages of
development. A defect in the
calvarium
may occur as
the bone of the skull is formed, through which CS±,
meninges, and brain tissue may protrude. This case
illustrates immature brain tissue and its meningeal
covering, which have herniated through a bony defect
in the occipital bone. These neuroectodermal tissues
are found directly beneath the dense subcutaneous
connective tissue of overlying skin.
Encephaloceles
are most frequently found in the occipital region. In
their several variations, they may contain (1) only CS±
and meninges, (2) CS±, meninges, and brain substance
(occipital lobe or cerebellum), or (3) CS±, meninges,
brain, and part of the ventricular system. The more
elaborate the encephalocele, the more debilitating and
difF
cult it is to treat.
Skin
Skin
Skin
Abnormal
nervous tissue
Meninges
C
M
3
P
G
Purkinje cell
Purkinje cell
bodies
(P)
bodies (P)
Granule cell
Granule cell
layer
(G)
layer (G)
Molecular layer (M)
Molecular layer (M)
1
Molecular layer (M)
Granule cell
layer
(G)
1
2
Purkinje cell
bodies
(P)
B
Figure 7-11B.
Cerebellar cortex.
Nissl stain,
3
68,
inset 1
,
3
124;
inset 2,
Golgi,
3
74
Purkinje cells
and
granule cells
are the most obvious neurons in
the
cerebellar
cortex
.
Purkinje
cells
, among the largest neurons
in the CNS, lie between the granule cell layer and the molecular
layer (
inset 1
). Purkinje cells have widely branching dendritic
trees that extend through the entire depth of the molecular layer
(
inset 2
). The dendritic tree of a Purkinje cell is shaped like
a paper fan. The wide part of the “fan” is seen when cutting
across the long axis of a folium (
inset 2
). The edge of the fan is
seen when cutting parallel to the long axis of the folium (
inset 3
).
Granule cells
(
blue, inset 3
) send their axons into the molecular
layer in which they divide and run parallel to the long axis of the
folium, making synaptic contact with hundreds or thousands of
Purkinje cell dendrites. Another major element in the basic cer-
ebellar cortex circuitry is the
climbing ± ber
(
red, inset 3
). These
axons originate in the inferior olivary nucleus. Each climbing
F ber encircles the dendrites of a single Purkinje cell. Purkinje cell
axons provide the sole output pathway of the cerebellar cortex.
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