(niacin-vitamin B 3 ), vitamin B 6 (pyridoxine), vitamin
B 12 (cobalamin), pantothenic acid (vitamin B 5 ), pyri-
doxal (vitamin B 6 ), phylloquinone (vitamin K), biotin,
folic acid, inositol, choline, and PABA (para amino
benzoic acid). Vitamin supplements are a billion dol-
lar per year industry.
vitamin B 12 SeeCOBALAMIN.
vitiligo A skin condition of unpigmented spots deter-
mined by a dominant gene that destroys special skin
cells (melanocytes). These skin cells produce the pig-
ment melanin in the skin along with tissues (mucus
membranes) that line the inside of the mouth, nose, gen-
ital and rectal areas, and the retina of the eyes. About
40 to 50 million people worldwide have vitiligo, and in
the United States alone, 2 to 5 million people have it.
viviparous Animals that are viviparous are born live
after being nourished by blood from the placenta while
in the uterus, and do not hatch from eggs.voltage-gated channel Ion channels are pores in
cell membranes that allow the passage of ions in and
out of cells. There are two types, voltage-gated and
chemically gated channels. The opening and closing of
the voltage-dependent ion channels is regulated by
voltage, the electrical charge or potential difference
between the inside and outside of the membrane,
while chemical stimuli are responsible for opening and
closing the chemically gated channels. Neurons use
these channels to pass sodium and potassium ions
through them.voltage-gated channel 343