Hair that is pale and is found on children and adult females is vellus hair.

A. quantity of melanin produced:Ethnic variations in skin color are due to differences in the amount and type of melanin synthesized and distributed by melanocytes. While all humans have roughly the same number of melanocytes, the activity level of these cells determines pigment intensity.

B. quantity of hemoglobin in the blood:Hemoglobin contributes to skin tone by giving a reddish or pinkish hue, especially in lighter skin. However, it does not account for the baseline differences in ethnic skin color.

C. UV radiation exposure:Sun exposure can increase melanin production temporarily, leading to tanning. However, long-term ethnic differences are genetically determined rather than solely influenced by environmental UV exposure.

D. number of keratinocytes:Keratinocytes are the primary cells of the epidermis and act as recipients of melanin granules from melanocytes. Their number does not vary significantly among ethnic groups and does not explain differences in skin color.

E. number of melanocytes:The number of melanocytes is relatively consistent across all ethnicities. It is the rate of melanin synthesis and the type of melanin produced that create the observed differences in skin pigmentation.

A. sebaceous:Sebaceous glands are exocrine glands that release sebum into hair follicles or onto the skin surface through ducts. They are not involved in direct secretion into the bloodstream.

B. exocrine:Exocrine glands secrete their products through ducts onto epithelial surfaces, such as sweat or salivary glands. They differ from endocrine glands, which release secretions directly into the blood.

C. endocrine:Endocrine glands, including the thyroid, pituitary, and adrenal glands, secrete hormones directly into the bloodstream. These hormones regulate numerous physiological processes throughout the body.

D. ceruminous:Ceruminous glands are specialized exocrine glands in the ear canal that secrete cerumen (earwax). They release their product via ducts, not into the blood.