8
JERALD J. BERNSTEIN
classes of secretory droplets, 0.1 and 1 . 0 ~ and larger (Palay, 1957,
In teleosts bluegill, Lepomis macrochirus, and sand bass, Paralabrax
nebulifer, axosomatic synapses have been observed in the optic tectum
on some undifferentiated cells of the subependymal “matrix” layer, suggesting their identification as neurons (Kruger and Maxwell, 1967).
These observations are in agreement with the work of Kirsche (1967)
at the light microscopic level who has described continuous generation
of neurons by matrix layers of the crucian carp optic tectum. Glycogen
particles appear to be more widespread in lower vertebrates where they
are usually present in endothelial and ependymal cells, and to a variable
extent in neurons (Kruger and Maxwell, 1967). Because of fragility during fixation, neurons appear to lose glycogen granules. Since glycogen
is found in some neurons these cells could be the ultimate consumers of
this source of glucose (Kruger and Maxwell, 1967).
Neuroglial cells can be classified as astrocytes and oligodendrocytes.
These two neuroglial cell types have been observed in the author’s laboratory in the goldfish spinal cord and have been observed in the goldfish
medulla oblongata (Robertson et al., 1963), shark brain (Long et al.,
1968), and bluegill and bass brain (Kruger and Maxwell, 1967). However, two neuroglial types have not been observed in the telencephalon
of the myxinid M!yxine glutinosa (Mugnaini and Walberg, 1965), the
lamprey spinal cord, Petromyzon marinus (Schultz et al., 1956), and
Lampetra planeri, L. zanandreai, and L. fluuiatilis ( Bertolini, 1964, 1966).
The presence of one neuroglial cell type perhaps could have been predicted sincc the oligodendrocyte is the glial element that produces the
myelin sheath of central nervous system axons and the myxinids and
lampreys possess only an unmyelinated nervous system.
In the myxinid telencephalon, Myxine glutinosa L. (Mugnaini and
Walberg, 1965), and lamprey spinal cord, Petromyzon marinus ( Schultz
et al., 1956), glial cells are classified as one class. The processes of the
neuroglial cells intrude between the nervous elements of the neuropil
or encircle neuronal perikarya. The nuclei of neuroglia are often smaller,
more irregular in outline, and contain denser chromatin than the nuclei
of nerve cells. The nucleus and cytoplasm of myxinid neuroglial cells
often contain rodlets ( Mugnaini, 1967). Neuroglial cytoplasm also contains numerous mitochondria, limited ergastoplasm, fine neuroglial filaments, and heterogeneous dense bodies. Desmosomes are often observed
between contiguous cells. Neuroglial cells with their cell bodies and
processes form an extensive layer covering the subpial or ependymal
surface (Mugnaini, 1964) and blood vessels (Mugnaini and Walberg,
1960).
JERALD J. BERNSTEIN
classes of secretory droplets, 0.1 and 1 . 0 ~ and larger (Palay, 1957,
In teleosts bluegill, Lepomis macrochirus, and sand bass, Paralabrax
nebulifer, axosomatic synapses have been observed in the optic tectum
on some undifferentiated cells of the subependymal “matrix” layer, suggesting their identification as neurons (Kruger and Maxwell, 1967).
These observations are in agreement with the work of Kirsche (1967)
at the light microscopic level who has described continuous generation
of neurons by matrix layers of the crucian carp optic tectum. Glycogen
particles appear to be more widespread in lower vertebrates where they
are usually present in endothelial and ependymal cells, and to a variable
extent in neurons (Kruger and Maxwell, 1967). Because of fragility during fixation, neurons appear to lose glycogen granules. Since glycogen
is found in some neurons these cells could be the ultimate consumers of
this source of glucose (Kruger and Maxwell, 1967).
Neuroglial cells can be classified as astrocytes and oligodendrocytes.
These two neuroglial cell types have been observed in the author’s laboratory in the goldfish spinal cord and have been observed in the goldfish
medulla oblongata (Robertson et al., 1963), shark brain (Long et al.,
1968), and bluegill and bass brain (Kruger and Maxwell, 1967). However, two neuroglial types have not been observed in the telencephalon
of the myxinid M!yxine glutinosa (Mugnaini and Walberg, 1965), the
lamprey spinal cord, Petromyzon marinus (Schultz et al., 1956), and
Lampetra planeri, L. zanandreai, and L. fluuiatilis ( Bertolini, 1964, 1966).
The presence of one neuroglial cell type perhaps could have been predicted sincc the oligodendrocyte is the glial element that produces the
myelin sheath of central nervous system axons and the myxinids and
lampreys possess only an unmyelinated nervous system.
In the myxinid telencephalon, Myxine glutinosa L. (Mugnaini and
Walberg, 1965), and lamprey spinal cord, Petromyzon marinus ( Schultz
et al., 1956), glial cells are classified as one class. The processes of the
neuroglial cells intrude between the nervous elements of the neuropil
or encircle neuronal perikarya. The nuclei of neuroglia are often smaller,
more irregular in outline, and contain denser chromatin than the nuclei
of nerve cells. The nucleus and cytoplasm of myxinid neuroglial cells
often contain rodlets ( Mugnaini, 1967). Neuroglial cytoplasm also contains numerous mitochondria, limited ergastoplasm, fine neuroglial filaments, and heterogeneous dense bodies. Desmosomes are often observed
between contiguous cells. Neuroglial cells with their cell bodies and
processes form an extensive layer covering the subpial or ependymal
surface (Mugnaini, 1964) and blood vessels (Mugnaini and Walberg,
1960).