Integumentary System

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Layers of the Skin

Skin Tissue
This colored (SEM) of human skin tissue reveals pseudo-stratified epithelium (below surface) and the underlying dermis (connective tissue, green). The folded surface of the skin (upper frame) is covered by mature keratinocytes. STEVE GSCHMEISSNER/Getty Images

The integumentary system consists of the largest organ in the body, which is the skin. This extraordinary organ system protects the internal structures of the body from damage, prevents dehydration, stores fat, and produces vitamins and hormones. It also helps to maintain homeostasis within the body by assisting in the regulation of body temperature and water balance. The integumentary system is the body's first line of defense against bacteria, viruses, and other pathogens. It also helps to provide protection from harmful ultraviolet radiation. The skin is a sensory organ in that it has receptors for detecting heat and cold, touch, pressure, and pain. Components of the skin include hair, nails, sweat glands, oil glands, blood vessels, lymph vessels, nerves, and muscles. Concerning integumentary system anatomy, the skin is composed of a layer of epithelial tissue (epidermis) that is supported by a layer of connective tissue (dermis) and an underlying subcutaneous layer (hypodermis or subcutis).

Skin Layers Overview

  • Epidermis - outermost layer of the skin composed of squamous cells. This layer is characterized into two distinct types: thick skin and thin skin.
  • Dermis - thickest layer of skin that lies beneath and supports the epidermis.
  • Hypodermis (Subcutis) - innermost layer of the skin that helps to insulate the body and cushion internal organs.

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Epidermis Skin Layer

Epidermis Skin Layers
Drawing of skin layers and cell types. Don Bliss/National Cancer Institute

The outermost layer of the skin is composed of epithelial tissue and is known as the epidermis. It contains squamous cells or keratinocytes, which synthesize a tough protein called keratin. Keratin is a major component of skin, hair, and nails. Keratinocytes on the surface of the epidermis are dead and are continually shed and replaced by cells from beneath. This layer also contains specialized cells called Langerhans cells that signal the immune system of an infection by presenting antigenic information to lymphocytes in lymph nodes. This aids in the development of antigen immunity.

The innermost layer of the epidermis contains keratinocytes called basal cells. These cells constantly divide to produce new cells that are pushed upward to the layers above. Basal cells become new keratinocytes, which replace the older ones that die and are shed. Within the basal layer are melanin producing cells known as melanocytes. Melanin is a pigment that helps to protect the skin from harmful ultraviolet solar radiation by giving it a brown hue. Also found in the basal layer of the skin are touch receptor cells called Merkel cells. The epidermis is composed of five sublayers.

Epidermal Sublayers

  • stratum corneum - top layer of dead, extremely flat cells. Cell nuclei are not visible.
  • stratum lucidum - thin, flattened layer of dead cells. Not visible in thin skin.
  • stratum granulosum - rectangular-shaped cells that become increasingly flattened as they move to the surface of the epidermis.
  • stratum spinosum - polyhedral-shaped cells that flatten as they get closer to the stratum granulosum.
  • stratum basale - innermost layer of elongated columnar (column-shaped) cells. Consists of basal cells that produce new skin cells.

Thick and Thin Skin

The epidermis is characterized into two distinct types: thick skin and thin skin. Thick skin is about 1.5 mm thick and is found only on the palms of the hands and soles of the feet. The rest of the body is covered by thin skin, the thinnest of which covers the eyelids.

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Dermis Skin Layer

Epidermis Stain
This is a hematoxylin and eosin stained slide at 10x of normal epidermis. Kilbad/Wikimedia Commons/Public Domain

The layer beneath the epidermis is the dermis. This is the thickest layer of skin composing almost 90 percent of its thickness. Fibroblasts are the main cell type found in the dermis. These cells generate connective tissue as well as the extracellular matrix that exists between the epidemis and dermis. The dermis also contains specialized cells that help regulate temperature, fight infection, store water, and supply blood and nutrients to the skin. Other specialized cells of the dermis help in the detection of sensations and give strength and flexibility to the skin. Components of the dermis include:

  • Blood vessels - transport oxygen and nutrients to the skin and remove waste products. These vessels also transport vitamin D from the skin to the body.
  • Lymph vessels - supply lymph (milky fluid containing white blood cells of the immune system) to skin tissue to fight microbes.
  • Sweat glands - regulate body temperature by transporting water to the skin's surface where it can evaporate to cool down the skin.
  • Sebaceous (oil) glands - secret oil that helps to waterproof the skin and protect against microbe build-up. They are attached to hair follicles.
  • Hair follicles - tube-shaped cavities that enclose the hair root and provide nourishment to the hair.
  • Sensory receptors - nerve endings that transmit sensations such as touch, pain, and heat intensity to the brain.
  • Collagen - generated from dermal fibroblasts, this tough structural protein holds muscles and organs in place and gives strength and form to body tissues.
  • Elastin - generated from dermal fibroblasts, this rubbery protein provides elasticity and helps to make the skin stretchable. It is also found in ligaments, organs, muscles and artery walls.

 

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Hypodermis Skin Layers

Structure of the Skin
This image illustrates the structure and layers of the skin. OpenStax, Anatomy & Physiology/Wikimedia Commons/CC BY Attribution 3.0

The innermost layer of the skin is the hypodermis or subcutis. Composed of fat and loose connective tissue, this layer of the skin insulates the body and cushions and protects internal organs and bones from injury. The hypodermis also connects skin to underlying tissues through collagen, elastin, and reticular fibers that extend from the dermis.

A major component of the hypodermis is a type of specialized connective tissue called adipose tissue that stores excess energy as fat. Adipose tissue consists primarily of cells called adipocytes that are capable of storing fat droplets. Adipocytes swell when fat is being stored and shrink when fat is being used. The storage of fat helps to insulate the body and the burning of fat helps to generate heat. Areas of the body in which the hypodermis is most thick include the buttocks, palms, and soles of the feet.

Other components of the hypodermis include blood vessels, lymph vessels, nerves, hair follicles, and white blood cells known as mast cells. Mast cells help to protect the body against pathogens, heal wounds, and aid in blood vessel formation.

Source:
SEER Training Modules, Module Skin Cancer: Melanoma. U. S. National Institutes of Health, National Cancer Institute. 3, March 2010 .