Lab 3: Tissue of the Nervous System
In this lab you will look at cross sections of the pig embryo and of the
spinal cord.
Two components of the spinal cord can be identified at low magnification: an inner gray
matter, often described as butterfly- or H-shaped, and an outer white matter. The gray
matter consists of nerve cells and the unmyelinated processes (dendrites, axons) of those
cells, as well as neuroglial cells. Neuroglial cells (also known as glial cells) are
support cells in the central nervous system. Generally only the nuclei of the glial cells
are identifiable. The white matter consists of myelinated axons and glial cells. The axons
of the neurons in the gray matter become myelinated upon entering the white matter where
they form the ascending and descending tracts of the spinal cord. (White matter gets its
name from the fact that the lipid-rich myelin it contains makes it a glistening white
color in fresh specimens).
The gray matter has dorsal and ventral horns. The ventral horns are shorter and
thicker. The two sides or "wings" of the gray matter are connected by the gray
commissure inside of which is the central canal lined with ependymal cells. Ependymal
cells secrete the cerebrospinal fluid. The neurons inside the gray matter are variable in
size and shape but are larger than the neuroglial cell nuclei. In many neurons, especially
the larger ones, you may be able to see a nucleus and a nucleolus, as well as Nissl bodies
(accumulations of free ribosomes and rough ER). You should be able to distinguish the cell
bodies of neurons from glial cell nuclei.
The white matter in cross sections appears as dots surrounded by empty circles. The
dots are the axons, the circles surrounding them appear empty because most of the myelin
was removed during histological preparation. |
  Low magnification of the middle of
the spinal cord
The dorsal side of this slide is oriented toward the left, the ventral toward the
right. In this section, the gray matter appears (slightly) darker than the white matter.
The commissure, an isthmus joining the two "wings" of the gray matter
"butterfly", lies between the ventral median sulcus and the dorsal median
sulcus. (The latter, not marked, appears as a very narrow space). The ependymal canal lies
in the middle of the commissure (just to the left of the "c"). Other spaces that
you see in this section are blood vessels. Nerve tissue is well vascularized. |
 High magnification of white matter of spinal cord
Figure 15 shows the white matter of the previous image at a higher magnification. The
myelinated axons that make up the white matter are shown in cross-section. The axons
themselves appear as a small spot of tissue. They are surrounded in life by a myelin
sheath, which is secreted (in the central nervous system or CNS) by glial cells called
oligodendrocytes.. Most of the myelin is removed during routine histological preparation,
leaving an empty space. The profiles of the nerve fibres are variable in size. The fibres
seen here compose the ascending and descending tracts that run the length of the spinal
cord. Some of the circular profiles (those with no axon) belong to blood vessels. |
 Gray matter of the spinal cord
The gray matter of the spinal cord is made up of nerve cells bodies (also known as
somata, sing. soma, or perikarya, sing. perikaryon) and their unmyelinated processes, as
well as glial cells. The nerve cell bodies also vary considerably in size and shape. The
nuclei of the glial cells are much smaller than the nerve cell bodies and are darkly
staining. Several nerve cell bodies can be identified in this section, two are shown
surrounded by asterisks. The cell body highlighted on the right is more obvious than the
one on the left. On the highlighted nerve cell body on the right, one can see a nucleus
that is slightly darker than the cytoplasm, and a small, round, very distinct nucleolus.
Depending on how a nerve cell body is sectioned, one may also see only the nucleus, or
neither the nucleus nor the nucleolus. Large numbers of capillaries and blood vessels are
present. They appear as "holes" in the tissue. |
 High magnification of ependyma
The ependymal canal lies in the commissure of the gray matter. The cells lining it
constitute a simple cuboidal or columnar epithelium. These cells secrete the cerebrospinal
fluid (CSF). Nerve cell bodies and glial cell nuclei can be seen. A capillary is indicated
on the left, a larger blood vessel on the right. At the top left, a small amount of white
matter can be seen. |
 High magnification of the meninges
This picture shows a section of the spinal cord near the ventral median sulcus with its
meninges. The meninges are the protective layers encasing the spinal cord. From innermost
to outermost they are the pia mater, the arachnoid and the dura mater
The delicate pia mater is easiest to distinguish near the sulcus. A strand of the pia
can be seen projecting down into the sulcus. In life, the pia lies directly on the spinal
cord and brain. It is continuous with the connective tssues sheaths of the blood vessels
of the brain and spinal cord. The pia itself also contains blood vessels.
The arachnoid is a delicate sheet of connective tissue that abuts on the inner surface
of the dura mater and extends trabeculae to the pia mater lining the surface of the brain
and spinal cord. The space between these trabeculae is called the subarachnoid space, and
contains cerebrospinal fluid (CSF). The space also contains the large blood vessels that
send branches into the brain. Several such vessels can be seen. Although not visible in
this picture, nerves, (fibres of the dorsal or ventral roots) can also be found in the
arachnoid.
The dura mater is a thick sheet of dense connective tissue. Its outer surface is
continuous with the periosteum of the skull but not of the vertebrae. |
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