Monday, July 2, 2012

Biological Science 12 (Skin System)

A. Skin System

Day 21:

Also know as the integumentary system, the skin protects against solar radiation by making melanin to absorb UV light, the skin also makes vitamin D (when exposed to sunlight), intact skin is the bodies best defense against disease, the skin offers protection from physical danger and contains receptors to sense heat, cold, pressure, vibration and pain.

The Integumentary System

1. Functions in Homeostasis and Osmoregulation
The skin regulates temperature by sweating (to cool the body) and shivering (to heat the body). The skin prevents water loss, due to its oily coat. Sweat also aids osmoregulation, adjusting urea and salt balance.

2. Functions in Thermoregulation
Beside shivering, hair also traps heat. 

A. Hair, Erectile Musculature
Arrector pili muscles can change the angle of hair respective to the body. Hair stands up to trap heat. If more heat is needed contraction of the many arrector pili muscles occurs.

B. Fat Layer for Insulation
The hypodermis of the skin is a large layer of fat, which acts like a wet suit to keep heat in the body.

C. Sweat Glands, Location in Dermis
Exocrine glads producing sweat (sudoriferous glands).

Located in the subcutaneous tissue.

D. Vasoconstriction and Vasodilation in Surface Capillaries
The surface capillaries give off the most heat to the environment, therefore to cool the body vasodilation of the surface capillaries occurs. To retain heat, vasoconstriction of the surface capillaries occurs.

3. Physical Protection
The skin gets worn down by contact with other surfaces. The outer layer is stratified (multiple layers of dead cells are squished together). 

A. Nails, Calluses, Hair
Skin, hair and nails contain keratin protein (like a rhino horn), that hardens the outer layer of the body to protect it. Calluses are extra layers of skin/keratin in areas that have been worn off repeatedly.

B. Protection Against Abrasion, Disease Organisms
The keratin protein guards somewhat against abrasion, however having extra layers of dead cells that can be sloughed off is essential because abrasion is inevitable. The oily coat (sebum)  of skin keeps many disease organisms out of the skin, as well as the thick layer of tightly packed dead cells (keratinized stratified squamous epithelium). Sweat and normal skin are both acidic to through off disease causing organisms. The healthy residents of skin (flora) take up room, helping keep bad things away.

4. Structure
The skin is mostly epithelial tissue (epidermis: different in the various layers) and a little connective tissue on the bottom (dermis) side to allow blood flow to the cells of the skin. Below the dermis is a fat layer (the hypodermis), a natural cushion and insulator (subcutaneous fat, healthier than adipose fat).

Layers of the Epidermis

A. Layer Differentiation, Cell Types, Tissue Types (Epithelial, Connective)
Epithelial cells form the epidermis, connective cells form the dermis and hypodermis. The epidermis has many different cell types in the various layers. 

Cell Types:
Keratinocytes, make keratin, the most common cell in the epidermis, they die and flatten. ED
Melanocytes, make melainin, the skin pigment that absorbs UV radiation. ED
Langerhans cells, are phagocytes that kill pathogens. ED
Fibroblasts, make fibers and matrix. D (all)
Hair follicles, grow hair. D (bottom)
Sweat glands (sudoriferous), make sweat. D (bottom)
Oil gland (sebaceous), make sebum. D (top)
Adiposcyte (fat), insulation and shock absorber. HD

Layer Differentiation of the Epidermis:

  • Stratum Corneum: this layers comes off (1300 cells/square cm/hr on the forearm alone) and forms house dust. Made of dead keratinocytes (called corneocytes). Old name, horned layer. 15-20 layers of dead cells. 10 and 40 μm thick.
  • Stratum Lucidum: this layer contains melanin. 3-5 layers of dead cells.
  • Stratum Granulosum: in this layer keratinocytes are called granular cells due to lamellar and keratohyalin granules. The granules are nucleus that have been ejected from keratinocytes.
  • Stratum Spinosum: this layer starts waterproofing the skin with bipolar lipids. Keratinocytes are connected with desmosomes, here they produce lamellar bodies (in the Golgi apparatus). Langerhans cells are in the middle of this layer.
  • Stratum Basale: this layer consists of stem cells that create migrating daughter cells. This is the layer that undergoes mitosis continually. Melanocytes are located here. 1-3 cells thick. Connected to the dermis by hemidesmosomes.

B. Relative Impermeability to Water
Keratin, sebum and glycolipids make skin significantly waterproof. Keeping the cellular contents of the human body from diffusing out, when we swim or bathe.

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