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is responsible for variations in skin pigmentation. All races
have the same number of melanocytes, but the different
races have specific genes that determine the amount of
melanin produced by the melanocytes. Darker-skinned
individuals have more active melano-cytes that produce
more melanin. Melanocytes are ac-tivated to produce
melanin by exposure to sunlight. We darken when we
expose ourselves to the sun. All races get darker after
exposure to the sun over a period of time. We call this
getting a suntan.
Based on the discoveries and research done in
anthropology by the Leakey family in Olduvai Gorge,
Tanzania,- scientists believe humans evolved in Africa-.
The first humanoids were probably very dark to pro-tect
themselves from the harmful ultraviolet rays of the sun.
They had very active melanocytes like today’s Africans.
Over time, some humans migrated away from the equator
and genetic recombinations and mutations governing the
activity of their melanocytes occurred. Over long periods,
this led to the evolution of the different races, whose
variations in skin color are determined by the amount of
melanin produced and its distribution. The strongest factor
in increasing pigmentation in the skin is the sun’s
stimulating effect- on melanocytes. Melanin cross-links
with protein to form a tough resistant compound. Hence,
heavily pig-mented skin is more resistant to external
irritation. People who live closer to the equator, where
there is maximum exposure to sunlight, will be darker than
people who live in the north like the Baltic- States of
Norway, Sweden, Finland, and Denmark. This varia-tion in
melanin content is the principal -factor respon-sible for the
color differences among races. Individuals of darker-
skinned races have more active melano-cytes, while
individuals of lighter-skinned races have less active
melanocytes.
Larger amounts of melanin can occur in certain ar-eas
of the body, producing the darkened areola area of the
nipples, freckles, and moles, although other areas of the
body’s skin have less melanin, like the palms of the hands
and the soles of the feet. Even though many genes are
responsible for skin color, one mutation can cause the
absence of skin color by preventing the pro-duction of
melanin. This condition is called albinism (AL-bih-nizm)
and results from a recessive gene that causes the absence of
melanin. Albinos have no pig-ment in their skin and
appendages of the skin. Their hair is white, their eyes pink,
and their skin very fair. These individuals must be very
careful to avoid overex-posure to the sun.
Chapter 6The Dermis
The dermis is also known as the corium (KOH-ree-um). It
lies directly beneath the epidermis and is -often referred- to
as the true skin. It is composed of dense connective tissue
with tough white collagenous fibers and yellow elastin
fibers. Blood vessels, nerves, lymph vessels,- smooth
muscles, sweat glands, hair follicles, and sebaceous- glands
are all embedded in the dermis.
The dermis can be divided into two portions (see
Figure 6-2). The papillary portion is the area adjacent to
the epidermis, and the reticular portion is found between
the papillary portion and the fatty subcutaneous tissue
beneath. A sheet of areolar tissue, usually containing fat
(adipose tissue), is known as the subcutaneous tissue or
superficial fascia and attaches the dermis to underlying
structures like muscle or bone. This subcutaneous -tissue is
sometimes referred to as the hypodermis. It is into this
area that hypodermic injections are given. The pink tint of
light-skinned individuals is due to blood vessels in the
dermis. There are no blood vessels in the epidermis. When
an individual is embarrassed, blood vessels in the dermis
dilate. This causes “blushing” or the reddish tint seen in the
facial area.
When a light-skinned individual suffocates or drowns,
carbon dioxide in the blood causes the blood to take on a
bluish tinge. This results in the bluish discol-oration of skin
or cyanosis (sigh-ah-NOH-sis) caused by lack of oxygen
in the blood. When a dark-skinned indi-vidual suffocates or
drowns, the same condition occurs but results in a grayish
or ashy tinge to the skin rather than a bluish tinge.The Accessory Structures
Of The Skin
The structures associated with the skin include hair, nails,
sebaceous glands, ceruminous glands, or wax glands in the
ear canal and sweat glands.Hair
Hair, in addition to mammary glands, is a main charac-
teristic of all mammals. When the hair is very thick and
covers most of the surface of the body, as on a dog or cat, it
is called fur. Even on humans, hair covers the entire body
except the palms of the hands, the soles of the feet, and
certain portions of the external genitalia (e.g., the head of
the penis). In some parts of the body, the hair is so small
that it appears invisible, yet in other places it is