Page 195 - Fundamentals of anatomy physiology

182 Unit 2 Support and Movement

The Role of Epidermal Pigmentation Figure 5–4 Melanocytes. The micrograph and accompanying

The epidermis contains variable quantities of two pigments: drawing indicate the location and orientation of melanocytes in the
carotene and melanin. Carotene (KAR-uh-te. n) is an orange- stratum basale of a dark-skinned person.
yellow pigment that normally accumulates in epidermal cells. It
5 is most apparent in cells of the stratum corneum of light-skinned Melanocytes
individuals, but it also accumulates in fatty tissues in the deep in stratum
dermis and subcutaneous layer. Carotene is found in a variety basale
of orange vegetables, such as carrots and squashes. The skin of
someone who eats lots of carrots can actually turn orange from an Melanin
overabundance of carotene. The color change is very striking in pigment
pale-skinned individuals, but less obvious in people with darker
skin pigmentation. Carotene can be converted to vitamin A, Basement
which is required for both the normal maintenance of epithelia membrane
and the synthesis of photoreceptor pigments in the eye.
Melanocytes LM × 600
Melanin is a pigment produced by melanocytes, pigment-
producing cells, introduced in Chapter 4. There are two types of a This micrograph shows the
melanin, a red-yellow form (pheomelanin) and a brown-black location and orientation of
form (eumelanin). The melanocytes involved are located in melanocytes in the stratum
the stratum basale, squeezed between or deep to the epithelial basale of a dark-skinned person.
cells (Figure 5–4). Melanocytes manufacture both types of mela-
nin from the amino acid tyrosine and package it into intracel- Melanosome
lular vesicles called melanosomes. These vesicles, which contain Keratinocyte
either pheomelanin or eumelanin, travel within the processes
of melanocytes and are transferred intact to about 40 basal ke- Melanin
ratinocytes. The transfer of pigmentation colors the keratino- pigment
cyte temporarily, until the melanosomes are destroyed by fu- Melanocyte
sion with lysosomes. In individuals with pale skin, this transfer
takes place in the stratum basale and stratum spinosum, and Basement
the cells of more superficial layers lose their pigmentation. In membrane
dark-skinned people, the melanosomes are larger and more nu-
merous, and the transfer may occur in the stratum granulosum Dermis
as well, making skin pigmentation darker and more persistent.
b Melanocytes produce and store melanin.
The ratio of melanocytes to basal cells ranges between 1:4
and 1:20, depending on the region of the body. The skin cov- surfaces such as the face, probably due to its greater exposure
ering most areas of the body has about 1000 melanocytes per to the sun. Lentigos are similar to freckles but have regular
square millimeter. The cheeks and forehead, the nipples, and borders and contain abnormal melanocytes. Senile lentigos, or
the genital region (the scrotum of males and the labia majora liver spots, are variably pigmented areas that develop on sun-
of females) have higher concentrations (about 2000 per square exposed skin in older individuals with pale skin.
millimeter). The differences in skin pigmentation among indi-
viduals do not reflect different numbers of melanocytes, but The melanin in keratinocytes protects your epidermis and
rather, different levels of melanin synthesis. A deficiency or dermis from the harmful effects of sunlight, which contains
absence of melanin production leads to a disorder known as significant amounts of ultraviolet (UV) radiation. A small
albinism. Individuals with this condition have a normal distri- amount of UV radiation is beneficial, because it stimulates the
bution of melanocytes, but the cells are incapable of produc-
ing melanin.

There can also be localized differences in the rates of
melanin production by your melanocytes. Freckles are small,
pigmented areas on relatively pale skin. These spots typi-
cally have an irregular border. They represent the areas ser-
viced by melanocytes that are producing larger-than-average
amounts of melanin. Freckles tend to be most abundant on