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4: The Integumentary System

  • Page ID
    12524
  • Skills to Develop

    By the end of this section, you will be able to:

    • Identify the components of the integumentary system
    • Describe the layers of the skin and the functions of each layer
    • Identify and describe the hypodermis and deep fascia

    Layers of the Skin

    Although you may not typically think of the skin as an organ, it is in fact made of tissues that work together as a single structure to perform unique and critical functions. The skin and its accessory structures make up the integumentary system, which provides the body with overall protection. The skin is made of multiple layers of cells and tissues, which are held to underlying structures by connective tissue (Figure 4.1). The deeper layer of skin is well vascularized (has numerous blood vessels). It also has numerous sensory, and autonomic and sympathetic nerve fibers ensuring communication to and from the brain.

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    Figure 4.1 Layers of Skin The skin is composed of two main layers: the epidermis, made of closely packed epithelial cells, and the dermis, made of dense, irregular connective tissue that houses blood vessels, hair follicles, sweat glands, and other structures. Beneath the dermis lies the hypodermis, which is composed mainly of loose connective and fatty tissues.

    The Epidermis

    The epidermis is composed of keratinized, stratified squamous epithelium. It is made of four or five layers of epithelial cells, depending on its location in the body. It does not have any blood vessels within it (i.e., it is avascular). Skin that has four layers of cells is referred to as “thin skin.” From deep to superficial, these layers are the stratum basale, stratum spinosum, stratum granulosum, and stratum corneum. Most of the skin can be classified as thin skin. “Thick skin” is found only on the palms of the hands and the soles of the feet. It has a fifth layer, called the stratum lucidum, located between the stratum corneum and the stratum granulosum.

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    Figure 4.2 Layers of Epidermis

    Dermis

    The dermis might be considered the “core” of the integumentary system (derma- = “skin”), as distinct from the epidermis (epi- = “upon” or “over”) and hypodermis (hypo- = “below”). It contains blood and lymph vessels, nerves, and other structures, such as hair follicles and sweat glands. The dermis is made of two layers of connective tissue that compose an interconnected mesh of elastin and collagenous fibers, produced by fibroblasts.

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    Figure 4.3 Layers of the Dermis This stained slide shows the two components of the dermis—the papillary layer and the reticular layer. Both are made of connective tissue with fibers of collagen extending from one to the other, making the border between the two somewhat indistinct. The dermal papillae extending into the epidermis belong to the papillary layer, whereas the dense collagen fiber bundles below belong to the reticular layer. LM × 10. (credit: modification of work by “kilbad”/Wikimedia Commons)

    Hair

    Hair is a keratinous filament growing out of the epidermis. It is primarily made of dead, keratinized cells. Strands of hair originate in an epidermal penetration of the dermis called the hair follicle. The hair shaft is the part of the hair not anchored to the follicle, and much of this is exposed at the skin’s surface. The rest of the hair, which is anchored in the follicle, lies below the surface of the skin and is referred to as the hair root. The hair root ends deep in the dermis at the hair bulb, and includes a layer of mitotically active basal cells called the hair matrix.

    The hair bulb surrounds the hair papilla, which is made of connective tissue and contains blood capillaries and nerve endings from the dermis.

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    Figure 4.4 Hair Hair follicles originate in the epidermis and have many different parts.

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    Figure 4.5 Hair Follicle The slide shows a cross-section of a hair follicle. Basal cells of the hair matrix in the center differentiate into cells of the inner root sheath. Basal cells at the base of the hair root form the outer root sheath. LM × 4. (credit: modification of work by “kilbad”/Wikimedia Commons)

    Nails

    The nail bed is a specialized structure of the epidermis that is found at the tips of our fingers and toes. The nail body is formed on the nail bed, and protects the tips of our fingers and toes as they are the farthest extremities and the parts of the body that experience the maximum mechanical stress. In addition, the nail body forms a back-support for picking up small objects with the fingers. The nail body is composed of densely packed dead keratinocytes. The epidermis in this part of the body has evolved a specialized structure upon which nails can form. The nail body forms at the nail root, which has a matrix of proliferating cells from the stratum basale that enables the nail to grow continuously. The lateral nail fold overlaps the nail on the sides, helping to anchor the nail body. The nail fold that meets the proximal end of the nail body forms the nail cuticle, also called the eponychium. The nail bed is rich in blood vessels, making it appear pink, except at the base, where a thick layer of epithelium over the nail matrix forms a crescent-shaped region called the lunula (the “little moon”). The area beneath the free edge of the nail, furthest from the cuticle, is called the hyponychium. It consists of a thickened layer of stratum corneum.

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    Figure 4.6 Nails The nail is an accessory structure of the integumentary system.

    Skin Models

    View the laboratory models of thin and thick skin and be able to identify each of the following.

     Thin skin

     Thick skin

    Layers of the skin:

    • Epidermis

       Stratum corneum

       Stratum lucidum

       Stratum granulosum

       Stratum spinosum

       Stratum basale

    • Dermis

       Papillary layer

       Reticular layer

      Layers beneath the skin (subcutaneous)

    • Hypodermis

      Skin structures:

    • Dermal papillae

    • Hair follicle

       Root

       Shaft

       Hair follicle

       Papilla

       Arrector pilli (or piloerector m.)

       Bulb

    • Blood vessels

    • Nerve endings:

       Lamellar corpuscle

       Tactile corpuscle

    • Glands:

       Eccrine (merocrine) sweat gland

       Pore

       Duct

       Apocrine sweat gland

       Sebaceous glands

    • Nail

       Eponychium

       Hyponychium

       Free edge

       Lateral nail fold

       Proximal nail fold

       Lunula

       Nail bed

       Nail plate (body)

       Nail matrix

      Skin cells:

    • Keratinocytes

    • Melanocytes

    • Fibroblasts

      Tissues types:

    • Stratified squamous epithelium

    • Areolar CT

    • Dense irregular CT

    • CT – adipose

    • Glandular epithelium (simple or stratified cuboidal)

    EPIDERMAL LAYERS

    & Physiology Lab Homework by Laird C. Sheldahl, under a Creative Commons Attribution-ShareAlike License 4.0

     

    LAB 4 EXERCISE 4-1

    Integument Layers

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    Label the following: Hair follicle * Sebaceous gland * Epidermis * Dermis (papillary layer) *

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    Dermis (reticular layer) * Hypodermis * Arrector pili muscle * Sweat gland.

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    LAB 4 EXERCISE 4-2

    EPIDERMAL LAYERS

    & Physiology Lab Homework by Laird C. Sheldahl, under a Creative Commons Attribution-ShareAlike License 4.0

     

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    Label the following: Stratum corneum * Stratum lucidum * Stratum granulosum * Stratum spinosum * Stratum basale * Dermal papilla * Sweat duct * Sweat pore * Stratified squamous epithelium * Dendritic cell * Melanocyte

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    LABELING:

     

    LAB 4 EXERCISE 4-3

    Skin Accessories

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    Label the parts of a hair follicle: Sebaceous gland * Hair shaft * Hair bulb * Hair root * Hair papilla

    Label the parts of a nail: Eponychium * Free edge * Hyponychium * Lunula * Nail matrix * Nail root * Lateral nail fold * Nail bed * Nail plate.

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    LAB 4 EXERCISE 4-4

    Histology: Hair * Hair follicle * Dermal papilla * Sebaceous gland * Eccrine sweat gland * Stratified squamous epithelium * Areolar CT * Dense irregular CT.

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