called the glial growth factor, induces the growth of
glial cells. A type of brain tumor that forms in the glial
tissue is called glioblastoma multiform.
There are several types of glial cells in the central
nervous system, including oligodendrocytes, astrocytes,
and microglia. It is the oligodendrocytes that produce
the fatty protein myelin that insulates the axons by
wrapping them in layers of myelin. Star-shaped astro-
cytes lay down scar tissue on damaged neurons and
hold the neurons in place as well as supply potassium
and calcium and regulate neurotransmitter levels.
Along with microglia cells, astrocytes remove dead cells
and other matter from the central nervous system.
Similar functionary types of cells in the peripheral
nervous system are the Schwann (provide insulation via
myelin) and satellite cells (support cells).
A debilitating disease caused by the demyelinating
of neurons is multiple sclerosis, and stem-cell research
is being conducted into developing new treatments.
glomerulus A structure, a tiny ball, between the
afferent arterioles and efferent arterioles within the
proximal part of the nephron of the kidney; located
within the Bowman’s capsule. It is composed of a clus-
ter of capillary blood vessels and is involved in the fil-
tration of blood. The glomerulus is a semipermeable
structure that allows water and soluble wastes to pass
through and then discharges them out of the Bowman’s
capsule as urine waste at a rate of about 160 liters or
42.7 gallons per 24-hour period. Most of this is reab-
sorbed back into the blood. The filtered blood then
leaves the glomerulus by way of the efferent arteriole to
the interlobular vein. Each kidney contains about 1
million glomeruli. Changes in the glomerulus may be
responsible for diabetic kidney disease.
Also a nest of nerves in invertebrates found in
invertebrate olfactory processing centers; a discrete,
globular mesh of densely packed dendrites and axons
found in the vertebrate olfactory bulb.
glucagon Aprotein hormone released by the pan-
creas via alpha cells in the islets of Langerhans with the
purpose of breaking down glycogen, in the liver, which
releases glucose and increases blood levels of glucose.
Glucagon works with insulin to maintain normal blood
sugar levels.
glucocorticoid A class of stress-related steroids (hor-
mones) produced by the adrenal glands (cortex) that
respond to the stimulation by adrenocorticotropic hor-
mone (ACTH) that comes from the pituitary gland.
They are involved in carbohydrate, lipid, and protein
metabolism by promoting gluconeogenesis and the for-
mation of glycogen, as well as effects on muscle tone,
circulation, blood pressure, and more. They possess anti-
inflammatory and immunosuppressive properties. Corti-
sol (hydrocortisone) is the major natural glucocorticoid.
Synthetically produced ones—mostly derived from
cortisol, such as cortinsone, prednisone, prednisolone,
methylprednisolone, betamethasone, and dexametha-
sone—regulate metabolism of lipids, carbohydrates,
and protein and work without the use of ACTH.
Glucocorticoids also cause osteoporosis, weight
gain, cataracts, heart disease, diabetes, and psychosis.glucose A formof six carbon sugar (C 6 H 12 O 6 ) that is
the most common energy source and is the usual form
in which carbohydrates are assimilated by animals. It is
carried through the bloodstream and is made not only148 glomerulus
Colored scanning electron micrograph (SEM) of a healthy kidney
glomerulus. The glomerulus is a tight ball of capillaries where
blood passing through the kidneys is filtered. Fluid passes out of
the capillaries into the cavity of the surrounding Bowman’s cap-
sule (not seen) and drains into a long tube, from which essential
substances and some water are reabsorbed. The remaining
unwanted fluid, containing toxins from the blood, drains to the
bladder as urine. Magnification unknown.(Courtesy © Science
Photo Library/Photo Researchers, Inc.)