Integumentary System: Epidermis Overview

Integumentary System: Epidermis

• The epidermis is the outermost layer of skin, functioning as a protective barrier.
• It is composed of multiple layers of stratified epithelial cells.
• It is avascular, meaning it does not contain blood vessels.
• Nutrients and oxygen diffuse from the dermis to the epidermis.

Epidermal Cell Types

• Keratinocytes: Are the most abundant cell type (about 90%) in the epidermis. They produce keratin, a tough fibrous protein that gives skin its strength and protection from abrasion, heat, microbes and chemicals. Keratinocytes also produce lamellar granules that secrete a water-repellent sealant and create a barrier to prevent foreign material from entering. As keratinocytes move toward the surface, they undergo apoptosis (programmed cell death) and become increasingly keratinized, eventually becoming the outermost layer of the epidermis.
• Melanocytes: Approximately 8% of epidermal cells, Melanocytes produce melanin, a pigment that gives skin its color. Melanin absorbs harmful UV radiation, protecting DNA from damage. The melanin granules are carried by dendrites and are transferred over keratinocytes to protect against UV radiation damage. Exposure to UV light increases melanocyte activity and in turn induces tanning.
• Intraepidermal Macrophages (Langerhans Cells): Relatively small in the population of epidermal cells. Langerhans cells function as part of the immune system. They recognize and destroy invading microbes.
• Tactile Epithelial Cells (Merkel Cells): The least numerous epidermal cells. Tactile epithelial cells are present in contact with the tactile/Merkel disc of the sensory nerve cells and function in detecting touch sensations.

Dermis

• The dermis is the layer of skin beneath the epidermis.
• It is thicker than the epidermis.
• It contains connective tissue, collagen, and elastic fibers, contributing to its strength and elasticity.
• The dermis contains exocrine glands (sweat and oil glands), blood vessels, nerves, hair follicles, and sensory receptors.

Two regions of the Dermis

• Papillary region: Located directly beneath the epidermis. The papillary region has nipple-like structures (dermal papillae) that project into the epidermis. The surface area of direct contact between the epidermis and the papillary region is increased due to the presence of capillary loops which provide the epidermis with oxygen and nutrients. The papillary region also contains touch receptors (Meissner's corpuscles) and free nerve endings that detect sensations like temperature, pain, and touch.
• Reticular region: Lies beneath the papillary region. The reticular region has thick bundles of collagen fibers and some elastic fibers, providing structure and strength. It contains fibroblasts, macrophages, adipose cells, blood vessels, nerves, sebaceous glands, and sudoriferous glands (sweat). Collagen fibers in the dermis help to resist stretching in the skin.

Hypodermis

• The hypodermis is located beneath the dermis.
• It is not part of the skin.
• Composed of loose connective tissue with collagen and elastic fibers.
• Contains fibroblasts, fat cells and macrophages.
• Houses approximately half of the body's adipose tissue serving as energy storage and insulation.

Accessory Structures

• Hair: Protects the scalp and contributes to touch sensations, composed of a shaft and a root. The hair shaft is composed of dead keratinized cells. The hair root penetrates into the dermis, sometimes extending into the subcutaneous layer.
• Skin Glands:
  • Sebaceous (oil) glands: Secrete sebum, which keeps skin moist and lubricates hair.
  • Sweat glands (sudoriferous): Secrete a dilute salt fluid that cools the body.
  • Apocrine glands: Secrete a potentially odorous substance in response to emotion or stress, typically present in the armpits and groin.
  • Ceruminous glands: Modified sweat glands in the ear that produce earwax (cerumen) to prevent the entrance of foreign bodies and insects.
• Nails: Consist of tightly-packed, hard, dead, keratinized epidermal cells. The nail body/plate is the visible portion, and the nail root is a region of the cells that actively divide to create new nail cells. The lunula is the whitish area at the proximal end of the nail body.

Objectives

• Describe the layers of the epidermis, and the cells that compose them.
• Compare the composition of the papillary and reticular regions of the dermis.
• Contrast the structure, distribution, and functions of hair, skin glands, and nails.

Sample Questions (and Answers)

1. Question: What statement best describes keratin?

• Answer:* Keratin is the fibrous protein produced by keratinocytes.

2. Question: Which component absorbs ultraviolet light?

• Answer:* Melanin

3. Question: What structure prevents water loss?

• Answer:* Lamellar granules

4. Question: What structure plays a role in thermoregulation?

• Answer:* Sweat glands

5. Question: Which statement about skin layers is correct?

• Answer:* Dermal papillae and capillary loops provide oxygen and nutrients to the epidermis.

6. Question: What causes goosebumps?

• Answer:* Contraction of arrector pili muscles

Explore the essential features of the epidermis, the outermost layer of skin that acts as a protective barrier. This quiz covers the composition, cell types, and functions of the epidermis, including keratinocytes and melanocytes. Test your knowledge on how these cells contribute to skin health and protection.