Integumentary System

Integumentary System : The Skin

As an organ system, the integumentary system is made up of several organs, which in turn are made up of different tissues

The epithelial sheets of stratified squamous make up the cutaneous membrane, which is more commonly called the skin, the largest organ of the body.
The skin plays an important role in regulating homeostasis

Protection, reduction of water loss, sensory reception, synthesis of some compounds, excretion of wastes

General layour of the skin : deepest layer is the hypodermis, or subcutaneous layer. This is mostly adipose tissue, with some blood vessels. The middle layer is the dermis, where the sensory receptors are found, where sweat glands are located and where hair originates. Dermis includes fibrous (dense) tissue, smooth muscle and nerve and blood tissues. The epidermis is the most superficial layer. It is composed of the avascular stratified squamous layers, which are of the keratinized variety.
Stretch marks are caused by rapid growth which tears the dermis. It will repair itself, but the resulting scar tissue appears white through the epidermis.
Subcutaneous injections are administered to the hypodermis.

Cells of the Epidermis

Melanocytes : produce melanin; found in deepest layers of the epidermis. Melanin protects the skin from UV ray damage.

Langerhans’ cells : arise from bone marrow. They are part of the immune system.

Merkel cells : found at the junction of the dermis and epidermis, these cells are associated with sensory reception (touch).

Layers of the Epidermis

Stratum basale : composed of stem cells (young keratinocytes) and melanocytes (10-25%). ~1 cell layer thick.

Stratum spinosum : several layers thick, and cells contain tonofilaments (prekeratin). Langerhans’ cells are found here.

Stratum granulosum : 3-5 layers of flattened keratinocytes. Tonofilaments are abundant. Last layer of living cells.

Stratum lucidum : present only in thick skin. It appears as a thin translucent layer; has a few rows of dead keratinocytes.

Stratum corneum : 20-30 layers thick, ¾ of the epidermal thickness. The dead keratinocytes of this layer are little more than keratin and cell membranes.

Layers of the Dermis

Papillary : contains the dermal papillae, which form the fingerprints. Meisner’s corpuscles are found here – they are light touch receptors.

Reticular : accounts for 80% of the dermal thickness. Reticular tissue and dense irregular tissue compose it. Cleavage lines in this layer are analogous to grain in wood.

Melanin : ranges in color from yellow to red-brown to black. It is made in melanocytes, then is passed on to basal keratinocytes. Skin coloration reflects the kind and amount of melanin made.

Carotene : ranges in color from yellow to orange; it is a source of vitamin A. Because it accumulates in the stratum corneum and the hypodermis, it can affect skin color.

Hemoglobin : in the blood, shows through the epidermis, especially in fair-skinned people. Cyanosis is a condition where the hemoglobin is poorly oxygenated. The skin turns faintly blue.
What do different colors mean? Redness? Whiteness? Yellow cast? Bronzing? Black/blue?

Vestigial hairs warn us of impending insect attacks
Hair is made of hard keratin, which does not flake and is tougher and more durable than the soft keratin of the skin.
Hair pigment is due to melanocytes at the base of the hair follicle.
The hair follicle extends through the epidermis into the dermis.

Sebaceous, or oil glands are often associated with hair follicles. A sebaceous gland is a simple exocrine gland found everywhere except the palms and soles. Sebum (the oil secreted) is a bactericide.

Eccrine sweat glands : found everywhere, but are more abundant on the palms, soles and forehead. They are coiled, tubular glands. The sweat secreted is 99% water, and trace amounts of salts (NaCl and others), antibodies, vitamin C, lactic acid, and metabolic wastes (uric acid)

Apocrine sweat glands : found primarily in the axillary and anogenital areas. The apocrine glands are bigger than eccrine glands, and they empty their sweat into hair follicles. The sweat secreted is basically the same as that from eccrine glands except for the presence of lipids and some proteins. When secreted, these break down (due to bacterial action), and get nasty smelling.
While eccrine glands function in thermoregulation, apocrine glands have no known useful function

Ceruminous glands : modified apocrine glands that secrete cerum, or ear wax.

Homeostatic Imbalances