Chapter 6 (2)

• Keratinocytes are produced by mitosis of stem cells in stratum basale or mitosis of keratinocytes in the deepest part of stratum spinosum. Mitosis requires abundant oxygen and nutrients, so once cells migrate away from the blood vessels of the dermis, mitosis cannot occur.
• New keratinocytes push older ones toward the surface. Over time, keratinocytes flatten, and produce more keratin and membrane-coating vesicles. In 30 to 40 days a keratinocyte makes its way to the skin surface and flakes off (exfoliates) as dander. This process is slower in old age but is faster in injured or stressed skin. Calluses or corns are thick accumulations of dead keratinocytes on hands or feet.
• Four important events occur in stratum granulosum: The Keratohyalin granules release filaggrin (a protein that binds keratin into tough bundles). Cells produce tough envelope proteins beneath theirmembranes. Membrane-coating vesicles release a lipid mixture thatspreads out over the cell surface and waterproofs it. Keratinocytes’ organelles degenerate and the cells die.
• The epidermal water barrier is formed when water retention is fostered by tight junctions between skin cells and the waterproofing that occurs in the stratum granulosum. It helps prevent dehydration but does NOT prevent the absorption of water by the stratum corneum when we soak in a bath (“prune fingers”).
• The Dermis is the connective tissue layer beneath the epidermis. It ranges from 0.2 mm (eyelids) to 4 mm (palms, soles). It is composed mainly of collagen and is well supplied with blood vessels, sweat glands, sebaceous glands, and nerve endings. It houses hair follicles and nail roots. It is the tissue of the facial skin to which skeletal musclesattach and cause facial expressions of emotion. And it has a wavy, conspicuous boundary with the superficial epidermis.
• The wavy, conspicuous boundary with the superficial epidermis are: Dermal papillae which are upward, finger-like extensions of the dermis. And Epidermal ridges which are downward waves of the epidermis. The prominent waves on fingers produce friction ridges of fingerprints.
• The papillary layer is the superficial zone of the dermis. There is a thin zone of areolar tissue in and near the dermal papilla. It allows for the mobility of leukocytes and other defense cells and it is rich in small blood vessels.
• The reticular layer is the deeper and thicker layer of the dermis. It consists of dense, irregular connective tissue. Stretch marks (striae) are tears in the collagen fibers caused by stretching of the skin due to pregnancy or obesity.
• The hypodermis is subcutaneous tissue that has more areolar andadipose than the dermis has. It pads the body and binds the skin to underlying tissues. It is the common site of drug injection since it has many blood vessels.
• Subcutaneous fat is an energy reservoir. Is there for thermal insulation. Is thicker in women and thinner in infants and the elderly.
• Melanin is the most significant factor in skin color. It is produced by melanocytes and accumulates in keratinocytes. It has two forms of pigment: Eumelanin, the brownish black, and Pheomelanin, the reddish yellow (meaning it is sulfur-containing).
• People of different skin colors have THE SAME NUMBER of melanocytes.
• Darker-skinned people produce greater quantities of melanin. The melanin breaks down more slowly. The melanin granules are more spread out in keratinocytes. And the melanized cells are seen throughout the epidermis. For lighter-skinned people, melanin is clumped near the keratinocyte nucleus and little melanin is seen beyond the stratum basale.
• Exposure to UV light stimulates melanin secretion and darkens skin. This color fades as melanin is degraded and old cells are exfoliated. Other pigments can influence skin color such as Hemoglobin the pigment in red blood cells that adds reddish to pinkish hue to skin. And Carotene the yellow pigment acquired from egg yolks and yellow/orange vegetables and is concentrated in the stratum corneum and subcutaneous fat.
• Colors of the skin as diagnostics: Cyanosis (blue skin), Erythema (red skin), Pallor (just paler than normal skin), Albinism (no melanin so milky white skin with blue-grey eyes), Jaundice (yellow skin and eyes), Hematoma (black and blue bruising/blood clot under the skin).
• Some Skin Markings include: Friction ridges, Flexion lines (flexion creases), Freckles and moles, and Hemangiomas (birthmarks)
• Friction ridges are markings on the fingertips that leave oily fingerprints on surfaces we touch. Everyone has a unique pattern formed during fetal development that remains unchanged throughout life. Not even identical twins have identical fingerprints. These ridges allow the manipulation of small objects.
• Flexion lines (flexion creases)are lines on the flexor surfaces of the digits, palms, wrists, and elbows. They mark sites where skin folds during the flexion of joints. Skin is bound to deeper tissues along these lines.
• Freckles and moles are tan to black aggregations of melanocytes. Freckles are flat, melanized patches. Moles (the medical term is nevi) are elevated, melanized patches often with hair. Moles should be watched for changes in color, diameter, or contour that may suggest cancer.
• Hemangiomas (birthmarks)are patches of discolored skin caused by benign tumors of dermal capillaries. Some disappear in childhood while others last for life. There are capillary hemangiomas, cavernous hemangiomas, and port-wine stains.