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.
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.
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.
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.
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.
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.
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