The other major skin region is called
the DERMIS (see fig 5.1). Unlike the epidermis, which was epithelial tissue,
the dermis is a layer of connective
tissue. It has cells in a matrix (like all connective tissue). The
cells are fibroblasts, macrophages, mast cells, and white blood cells.
The matrix is strong but flexible, with collagen, elastin and reticular
fibers. There are many blood vessels and nerves. There are two major layers:
thepapillary layer and the reticular
Papillary Layer - A loose mat of fibers, lots of blood vessels. It is characterized by dermal papillae, little bumps that protrude up into the bottom of the epidermis. In these papillae are capillary loops, and nerve endings (for pain reception) and Meissner's corpuscles (touch receptors). Another feature of some parts of the papillary layer is dermal ridges. These ridges cause ridges in the overlying epidermis, called epidermal ridges. Those ridges and the sweaty marks they leave are what we call fingerprints.
Reticular Layer - Is made of dense irregular connective tissue. The mats of fibers extend in all directions, parallel to the skin surface. There's collagen for strength and elastin for resiliency. The reticular layer has lines of thinner regions calledlines of cleavage or tension lines. These run longitudinally in the limbs and circularly around the trunk and neck. Flexure lines, found in the fingers and toes, etc. are places where the dermis has folds to accommodate joint movement.
Skin coloration comes from three sources:
Melanin from melanocytes, taken up by keratinocytes.
Carotene, a source of vitamin A that is found in plants. It accumulates in the stratum corneum so it's more obvious where the skin is thick, ie palms, soles, calluses.
Hemoglobin is an oxygen carrying pigment of the blood, it turns red when oxygenated and we see that redness through the skin in persons with lighter skin.
The rest of the Integumentary System is made up of what we refer to as the appendages of the skin, the hair and hair follicles, the sebaceous glands, the sweat glands, and the nails.
Hair (figure 5.3) serves for protection, guards against heat loss (mostly on the head in humans), acts as a bug filter in the nose, and helps us detect the presence of insects on our skin. It can also serve to reduce friction in some areas. There are two parts to a hair, the hair shaft is the part that sticks out and the hair root, the part under the skin. A hair has three layers:
Medulla - in the middle. Large
cells and some air pockets.
Cortex - surrounds the medulla, it's the bulky part with layers of flattened cells.
Cuticle - a single layer of highly keratinized cells, arranged like shingles (figure 5.4)
The keratin of hair is hard keratin, more durable and less likely to flake than the keratin of the skin. Hair color comes from melanocytes at the base. Cortex cells take up the melanin (similar to melanin uptake by keratinocytes in the skin).
Hair Follicles are formed from
a pocket of epidermis that extends down into the dermis. The pocket is
wider at the base and this region is called the hair
bulb. Around this bulb is a bundle of nerves called the root
hair plexus. Hairs serve as sensors because of these nerves. A dermal
papilla protrudes up into the base of the bulb. Like the other dermal papillae,
it has lots of blood vessels and supplies nutrients to the growing hair.
The wall around the hair follicle has two parts. The epidermal part is epithelial tissue, and it is called the epithelial root sheath (surprise!). Outside of that is theconnective tissue root sheath, derived from the dermis. Down in the bottom of the hair follicle is the part of the hair bulb that consists of rapidly dividing cells that actually grow the hair. It's called the hair matrix (but it's not "matrix" in the connective tissue sense of matrix vs cells) and it's part of the epidermal part of the hair follicle.
There's a tiny bundle of smooth muscle attached to the follicle called the arrector pili. It raises the hair and it results in "goose bumps" in we humans.
A couple more things about hair...
There's no hair on the lips, nipples,
parts of the external genitalia, palms and soles of the feet.
There are about 100,000 on the scalp and about 30,000 on a man's beard.
Vellus hair is the fuzzy hair that's on children and women. Terminal hair is the longer hair that grows on the scalp, eyebrow, armpits, and pubic area.
Terminal hair grows in response to male hormones, called androgens. Women have these too. There's a section in the book about baldness that may be of interest.
Nails are modifications of the epidermis that are highly keratinized. You can learn more about them in figure 5.4
Sweat Glands or Sudoriferous Glands are found everywhere but the nipples and parts of the external genitalia. > 2.5 million per person. Two types: eccrine and apocrine.
Eccrine Sweat Glands are the most numerous especially on the palms, soles, and forehead, sometimes as many as 3000 per inch2. Eccrine sweat glands produce "true" sweat by exocytosis. Sweat is 99% water with some salt, antibodies, and metabolic wastes.Sebaceous, or Oil, Glands are found all over except on the palms and soles. They are largest on the face and neck. They secrete sebum, an oily substance. They're holocrine glands that secrete into hair follicles. Sebum serves as a lubricant and to prevent drying. It also has bactericidal properties. Blocked ducts result in whiteheads. Oxidated and dried sebum in a duct results in a black head. Active inflammation of sebaceous glands is called acne.
Apocrine Sweat Glands are found mainly in the axillary and ano-genital areas. Their ducts empty into hair follicles. The secretion is like true sweat but with some proteins and fatty substances. It's odorless but as it gets munched on by bacteria it starts to smell a little. This is the basis of body odor. Not used in thermoregulation but it is probably for sex. Within Apocrine glands there are a couple of specialized types:Ceruminous Glands - in the ear, making earwax.
Mammary Glands - making milk.
Functions of the Integumentary System
Protection - 3 types of barriers, chemical, physical, and biological.
Chemical - skin secretions and melanin. Skin secretions' low pH (called the Acid Mantle) retards bacterial growth. Sebum also has antibacterial properties. Melanin is a chemical shield against the damaging effects of UV light.
Physical - Keratin and the fact that skin covers the whole outside. Some things do get in:Lipid soluble molecules, like steroids, oxygen vitamins (A,K,E,D the fat solubles)
Oleoresins from plants, like Poison Ivy
Organic Solvents, like acetone (used to be a big part of nail polish remover)
Salts of heavy metals, lead & mercury for example.
Biological - Langerhanns' cells of the epidermis and macrophages embedded in the dermis.Temperature Regulation - Normally we secrete up to .5 liters of of sweat, calledinsensible perspiration because it goes unnoticed. When it gets hot, dermal blood flow increases and sweat production increases (sensible perspiration). Dermal blood vessels constrict in the cold weather to limit blood flow to the outside.
Cutaneous Sensation via cutaneous sensory receptors. Ex: Meissner's corpuscles (in the dermal papillae) and root hair plexuses. More on this later.
Metabolic Functions - Sunlight stimulates production of a vitamin D precursor necessary for calcium metabolism. Keratinocytes have enzymes that knock out carcinogens that enter the epidermis.
Blood Reservoir - 5% of our blood volume resides in the skin. If blood is needed elsewhere some can be shunted away from the skin.
Excretion - Some nitrogenous wastes
(the waste products of cellular metabolism) are lost through the skin.
This is where we really see the value
of skin - when we lose it.
The immediate problem is water loss and electrolyte imbalance. Water loss can lead to severe loss of blood pressure and circulatory failure. If that is taken care of, say through intravenous addition of fluids, we have to worry about bacteria and fungi.
1st degree - Epidermis only. Redness and swelling. ex: Sunburn
2nd degree - Epidermis and upper dermis. As above but with blisters.
3rd degree - All the way through. Black, ashen, or bright red. Tissue regeneration is difficult. No pain, no nerves.