Overview

This chapter relates the specifics of skin anatomy to its myriad of functions.

Your concept of an organ is broadened to include the skin and its appendages, and what an organ it is!

Our skin reveals our overall health, age, and emotional state.

What an amazing material skin is to resist heat and cold, harsh chemicals, and bacteria.

Knowledge of skin structure also will better enable you to understand damage to skin from burns and diseases, such as cancer.

Use the Complete Anatomy app as well as items under Additional Resources to help you form a firm grasp of the material.

Learning Objectives

At the end of this lecture, you will be able to...

Name the tissue types that compose the epidermis and the dermis, and describe the primary functions of skin.
Name the layers of the epidermis and the dermis, and describe the structure and function of each layer.
Describe the structure and function of the hypodermis.
Describe the factors that contribute to skin color.
Describe the structure of nails.
List the parts of a hair and a hair follicle, and explain the function of each part.
Compare the structure and location of oil and sweat glands.
Compare eccrine and apocrine sweat glands.

Lecture Notes & Slides

Lec.3 Integumentary System (Skin)

Reading — Chapter 5 - The Integumentary System

The Skin and the Hypodermis (pp. 104–111)

The integumentary system (p. 104)
- The largest body organ
- consists of the skin and its appendages (hair, sebaceous glands, sweat glands, and nails). (Figure 5.1)

Figure 5.1 Skin structure.

Five important functions
- Protection — Cushions organs and protects from bumps, chemicals, water loss, ultraviolet (UV) radiation
- Body temperature regulation — Capillary network and sweat glands regulate heat loss
- Excretion — Urea, salts, and water lost through sweat
- Production of vitamin D — Epidermal cells use UV radiation to synthesize vitamin D
- Sensory reception — Contains sense organs associated with nerve endings
The two distinct regions of the skin are the epidermis and dermis. (pp. 105–110) (Figure 5.2)
- The hypodermis, although not part of the integumentary system, is also considered in this lecture.

Figure 5.2 Gross structure of skin and underlying tissues.

Epidermis

keratinized stratified squamous epithelial tissue.
consists of 5 layers in thick skin and 4 layers in thin skin (Figure 5.3 & Figure 5.4)
- Stratum corneum (horny layer)
  - Thick layer of dead keratinocytes;
  - Protects skin against abrasion and penetration
- Stratum lucidum (clear layer)
  - Occurs only in thick skin (i.e. palms and soles)
  - Composed of a few rows of flat, dead keratinocytes
- Stratum granulosum
  - Consists of a few layers of keratinocytes
  - Above stratum granulosum, cells are too far from dermal capillaries to receive nourishment
- Stratum spinosum (spiny layer)
  - “Spiny” appearance caused by artifacts of histological preparation
  - Consist of primarily keratinocytes that produce keratin (a fibrous protein), and is the most abundant cell type in epidermis. Keratinocytes arise from the deepest layer of epidermis and are dead at skin surface.
  - Dendritic cells are also found here. These cells are macrophage-like cells that are part of the immune system.
- Stratum basale
  - Deepest layer of epidermis; Attached to underlying dermis. Cells actively divide here.
  - Contains
    - Tactile epithelial cells—associated with sensory nerve ending
    - Melanocytes—manufacture and secrete the pigment melanin
  - Distinguish each layer of the epidermis by the types of cells present and their specific functions.

Figure 5.3 Epidermal cells and layers of the epidermis.		Figure 5.4 Thick skin.

Dermis

leathery second major layer of skin
Composed of strong, flexible connective tissue proper.
Richly supplied with blood vessels and nerves
Also located in the dermis are skin glands and hair follicles
Has two layers
- Papillary dermis—areolar CT.
  - Includes dermal papillae; abuts the undulating undersurface of the epidermis and is responsible for the epidermal friction ridges that produce fingerprints.
- Reticular dermis — dense irregular CT.
  - Make up 80% of thickness of dermis

Hypodermis

Also known as the subcutaneous layer or superficial fascia
The fatty layer beneath the skin that primarily contains adipose tissue. (p. 110, Figs. 5.1, 5.2)
Functions
- stores fat,
- insulates the body against heat loss,
- absorbs and deflects blows
- Anchors skin to underlying structures
Technically, the hypodermis is not part of the integument, but it does share some of the skin’s functions.
Has different distribution in males and females
- During weight gain, this layer thickens most markedly in the thighs and breasts of women and in the anterior abdominal wall of men.

Skin color

Skin color reflects the amounts of pigments (melanin and/or carotene) in the skin and the oxygenation level of hemoglobin in the dermal blood vessels.
Melanin
- The most important pigment in determining skin color.
- made by melanocytes and transferred to keratinocytes,
- protects skin and folic acid levels in the blood from the damaging effects of UV radiation. The epidermis produces vitamin D under the stimulus of UV radiation. As ancestral populations moved away from the tropics, the need to balance vitamin D synthesis with UV protection resulted in the evolution of a range of skin colors.

Appendages of the Skin (pp. 111–116)

Skin appendages, which develop from the epidermis but project into the dermis, include
- Nails
- hairs and hair follicles,
- sebaceous glands,
- sweat glands

Nails (pp.111-112)

Nails are a scale-like modification of the epidermis made of hard keratin (Figure 5.6)
- Parts of the nail
  - Free edge
  - Body
  - Root
  - Nail folds
  - Eponychium—cuticle
  - Nail matrix — The actively growing region

Figure 5.6 Structure of a nail.

Hair & Hair Follicles (pp. 112–114)

Hair (Figure 5.7)
- Chief parts of hair
  - Root—embedded in the skin
  - Shaft—projects above skin’s surface
- produced by a hair follicle, which are tubular invaginations of epidermis
- consists of heavily keratinized cells.
  - Flexible strand of dead, keratinized cells
  - Hard keratin—tough and durable
- Arrector pili muscles pull the hairs erect and produce goose bumps in response to cold or fear.
- Hair color reflects the amount and variety of melanin present.

Figure 5.7 Structure of a hair and hair follicle.

Sebaceous (oil) Glands

simple alveolar glands that usually empty into hair follicles.(Figure 5.8)
secrete sebum (an oily secretion) and occur over the entire body except palms and soles.
- Sebum lubricates the skin and hair, slows water loss across the skin, and acts as a bactericidal agent.
- increased amounts of sebum secretion at puberty under the influence of androgens.
Blocked and infected sebaceous glands lead to pimples and acne.

Sweat (sudoriferous) glands (pp. 115–116)

coiled simple tubular glands that secrete sweat, a modified filtrate of blood comprised primarily of water with salts and metabolic wastes. (Figure 5.8)
- Evaporation of sweat cools the skin and the body.
located over the entire skin surface except the nipples and parts of the external genitalia.
2 types of sweat glands:
- Eccrine sweat glands are more numerous and produce true sweat.
- Apocrine sweat glands
  - produce pheromones,
  - become active at puberty, secrete a viscous substance
  - localized primarily to the axillary, anal, and genital areas.
  - Their secretion contains proteins and fatty acids;
  - bacterial action on this secretion causes body odor.
  - Sweat from these glands may be used for sexual signaling in mate selection.

Figure 5.8 Skin glands.

Lecture video

Synchronous lecture recording from Summer 2020 available on Zoom website (Optional)

Additional Resources

Additional resources available on 🧪 Lab 3 — Skin

Noneducational Shenanigans...

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