7 Integumentary System

The integumentary system, which is comprised of skin, hair, nails, and various exocrine glands, is the largest organ of the human body.

Human skin is divided into two main parts: the dermis and the epidermis.  The dermis is made of connective tissue and is covered on its surface by a thick layer of stratified squamous epithelium that we call the epidermis.  The dermis is a highly vascularized tissue, while the epidermis – – like any other type of epithelial tissue – – is avascular.  Just under the skin, there is a third layer, the hypodermis, that is often incorrectly called a part of the skin.  The hypodermis is also made of connective tissue, most of it being subcutaneous fat.  The hypodermis supplies blood and nerves to the overlying skin and attaches the dermis to the underlying muscles and bones.

The epidermis

The epidermis is a thick, stratified squamous epithelium that covers the dermis.  The epidermis can be divided into four or five distinct layers, depending on its location on the body.  The majority of skin, called thin skin, has four layers of epidermal cells and is covered by tiny hairs.  Thick skin has five layers of cells and has no body hair.  Thick skin is only found on the palms of the hands and the soles of the feet.  Whether it has four or five layers, the epidermis is incredibly thin – – only 0.5 to 1.5 millimeters thick.

The epidermis is comprised of many cells piled up on top of one another.  Common cytologic characteristics of the cells can be used to visually divide the epidermis into distinct zones known as layers, however, cells do not remain fixed within one layer through the entirety of their entire life cycles.  New cells are added to the epidermis via division of cells in the two deepest layers.  As they age, cells change their appearance, moving through the layers towards the surface, where dead cells slough off.

The epidermis is made of cells that are particularly rich in a tough, fibrous cytoskeletal protein called keratin.  Because of this high keratin content, epithelial cells are called keratinocytes and the epithelium is called keratinized stratified squamous epithelium.  When keratinocytes on the surface of the epidermis die, their internal networks of keratin are exposed, forming a layer of what is commonly called dead skin.  This enhances the epidermis’ role in making the skin a waterproof barrier that is resistant to penetration by bacteria and viruses.

The layers of the epidermis, going from the dermis to the surface, are:

  • stratum basale
  • stratum spinosum
  • stratum granulosum
  • stratum lucidum – only in thick skin
  • stratum corneum
This illustration shows a cross section of the epidermis. The cells of the innermost layer, the stratum basale, are large and have a purple nucleus. The stratum basale curls around the dermis, which projects into the epidermis. The stratum basale contains four layers of large, triangle-shaped keratinocytes. Fibers are visible within the spaces between keratinocytes in the stratum basale. A melanocyte is also present in this layer. The melanocyte possesses finger-like projections extending from its main cell body. The projections branch through the extracellular spaces between nearby keratinocytes. Above the stratum basale is the stratum spinosum which consists of 8 layers of oval-shaped keratinocytes. The nucleus is present in these keratinocytes, but has faded to a lighter purple. The stratum granulosum contains five layers of keratinocytes, each containing spots in its cytoplasm, labeled the lamellar granules. The stratum lucidium contains 4 layers of diamond-shaped cells with no nucleus. The stratum corneum contains 9 layers of keratinocytes with no nucleus , nor cytoplasm. A few of the cells in the topmost layer of the stratum corneum are flaking off from the skin.
illustration of the layers of the epidermis

The stratum basale (also called the stratum germinativum) is the deepest epidermal layer.  The stratum basale is a single layer of cuboidal-to-low columnar basal stem cells that are bound to the underlying basal lamina and are constantly dividing.  All of the keratinocytes in outer layers of the epidermis derive from this single layer of basal cells.  As new cells are produced, the existing cells are pushed toward the surface of the skin.

Two other types of cells found in the stratum basale are melanocytes and Merkel cells.  Melanocytes are highly branched cells that produce the pigment melanin.  Located within the cytosol of melanocytes, melanin granules not only give skin its color, but also protect the basal layer of the epidermis from ultraviolet (UV) radiation.  Merkel cells are receptor cells that react to touch by exciting connecting sensory nerves within the dermis to transmit sensory information to the central nervous system.  Merkel cells are especially abundant on the surfaces of the hands and feet.

The stratum spinosum is composed of eight to ten rows of keratinocytes, which are polygonal shaped, and have a spiny appearance when stained with H&E.  It is in the stratum spinosum that keratinocytes first begin to produce keratin.

Interspersed among the keratinocytes are Langerhans cells, which function like macrophages by engulfing bacteria and foreign particles that have penetrated to this layer from the surface.

As new cells are produced in the stratum basale, the keratinocytes of the stratum spinosum are pushed upwards into the next layer, the stratum granulosum, where their cell membranes thicken and they produce large amounts of keratin.  Cells within the stratum granulosum are so separated from the nutrient-rich blood vessels in the dermis – – remember, the epidermis is avascular – – that they begin to die and disintegrate.  Keratohyalin, a protein found within granules in the cytoplasm of stratum granulosum cells, promotes the dehydration of dying cells and binds keratin fibers together, creating a tightly interlocked layer of cells.

The stratum granulosum is three to five layers deep.  Visible nuclei can still be found in the deeper layers of stratum granulosum, though the nuclei are flattened, and these cells have very little cytoplasm.  When stained, keratin and keratohyalin give the stratum granulosum its grainy appearance.

The stratum lucidum is a clear layer of the epidermis found only in the thick skin of the palms of the hands, the soles of the feet, and the lateral surfaces of the fingers and toes.  It appears clear because of the presence of eleidin, an intermediate form of keratin that is the product of the transformation of keratohyalin.

The stratum corneum is the most superficial layer of the epidermis and is the layer exposed to the outside environment.  The stratum corneum is comprised of 15 to 30 layers of dead corneocytes, which themselves are primarily made of keratin fibers.  The thickness of the stratum corneum depends on localization, as it is thicker in thick skin due to additional amounts of keratin produced from the eleidin found in the stratum lucidum.  The cornified cells in this layer slowly shed from the surface and are replaced by cells pushed up from the deeper layers of the epidermis.  The stratum corneum provides protection against abrasive forces acting on the skin, forms a barrier against invading microbes, and prevents water loss from the underlying layers.

 

the five layers of thick skin epidermis

The epidermis lies on the finger-like projections of the upper layer of dermis known as the dermal papillae.  These dermal papillae cause the overlying epidermis to fold into ridges on the fingers that are called fingerprints, which are unique to each individual.  Because fingerprints do not change with the growth or aging processes, they can be used for lifelong identification of the individual.  The entire epidermis is replaced each month.  Cosmetic procedures, such as microdermabrasion, remove the upper layers of the epidermis to create a younger appearance.

layers of the epidermis
cells within the epidermis

The histological features of each layer of the epidermis are summarized in the table below.

skin layer

Skin features

Stratum corneum

A layer of dead corneocytes.  No nuclei or organelles are present, just dried membranes and keratin fibers.  Is thicker in thick skin than in thin skin.

Stratum lucidum

A thin, transparent layer of lightly stained, flattened keratinocytes.  Keratinocytes in this layer have lost their organelles.  Keratohyalin has been transformed into translucent eleidin.  Remember, stratum lucidum is not present in thin skin.

Stratum granulosum

A thin layer containing keratinocytes that are filled with darkly staining keratohyalin granules.  Keratinocytes are semi-dry and flat, with elongated nuclei.

Stratum spinosum

Thickest of the epidermal layers.  A few rows of irregular, polygonal-shaped keratinocytes that change from almost cuboidal to flat, as they move away from the stratum basale.  The nuclei are round, and cells have visible rings of pink-stained cytoplasm.

Stratum basale

The first layer of cells on the basal membrane.  A single layer of cuboidal cells, interspersed with melanocytes that contain brown-colored granules of melanin.

The dermis

The dermis is the core of the skin and is composed of connective tissue.  It is located internally, under the epidermis.  The dermis is a highly vascularized tissue, containing blood and lymph vessels, as well as nerves, hair follicles, and sweat glands.  The dermis can be divided into two layers: a superficial layer called the papillary layer, and a deeper layer called the reticular layer.  The papillary and reticular layers are both made of connective tissue, but differ in the type of tissue present.

The papillary layer is made of areolar connective tissue, a type of loose connective tissue.  The papillary layer has a low number of scattered cells, including fibroblasts, mast cells, and macrophages, and an abundance of extracellular matrix.  The extracellular matrix includes a loose network of fine collagen and elastic fibers, and ground substance.  The main ingredients of ground substance are hyaluronic acid and various proteoglycans.  Blood vessels in the papillary layer are smaller, and there are some sensory receptors present, such as Meissner’s corpuscles, which are sensitive to touch.

The papillary layer contains dermal papillae, finger-like projections covered by the overlying epidermal ridges.  These papillae are important in binding the epidermis to the dermis and are prominent in hairless thick skin, such as the fingertips, but not in the thin skin that has hair.  Dermal papillae are responsible for the pattern of fingerprints.  If the overlying epidermis is destroyed, it regrows with the same pattern as before, based on the layout of the dermal papillae below.

The reticular layer is made of dense irregular connective tissue and is the deeper, thicker layer of the dermis.  The name “reticular layer” comes from the way the meshwork fibers appear under a microscope as reticulated, or net-like.  The reticular layer has fewer fibroblasts and immune cells, more collagen, and less ground substance than the papillary layer.  The reticular layer also has elastin fibers that are invisible after typical H&E staining.  Collagen fibers are larger and often arranged in bundles, making the extracellular matrix of the reticular layer much coarser when compared to the almost invisible, evenly distributed collagen fibers seen in the loose connective tissue of the papillary layer.  The reticular layer has plentiful blood vessels and a rich nerve supply.

All accessory glands and structures of the integument, including sweat glands, sebaceous glands, and hair follicles, are located in the reticular layer.

 

layers of the dermis

 

The histological features of the papillary and reticular layer are summarized in the table.

Papillary layer

Reticular layer

loose connective tissue with relatively little collagen fiber that is uniformly distributed dense irregular connective tissue with many disordered bundles of collagen
purple, elongated nuclei of the fibroblasts are scattered in extracellular matrix very few fibroblasts scattered between collagen bundles
other cells include mast cells and macrophages other cells include macrophages, leukocytes, and limited adipocytes
smaller blood and lymphatic vessels, and nerves very vascular, with larger blood vessels and nerves
contain dermal papillae that form fingerprints contain glands and hair follicles (if in thin skin)

Hair

Hair is a filament made of dead, keratinized cells that emerges from a hair follicle located in the dermis.  The hair can be divided into two regions: the hair root, located in the skin, and the hair shaft, exposed at the skin’s surface.  The hair root ends deep in the dermis at the hair bulb, and includes a layer of dividing 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.

 

illustration of the structure of human hair

 

Hair growth begins at the deepest portion of the follicle, the bulb.  Growth is influenced by hormones, nutrition, temperature, and various growth factors.  Similar to the skin, hair gets its color from the pigment melanin, produced by melanocytes in the hair papilla.  Different hair colors result from differences in the type and amount of melanin, which are genetically determined.  As a person ages, melanin production decreases, and hair tends to lose its color and become gray or white.

 

cross section of a hair follicle

 

The arrector pili muscle is a smooth muscle that attaches to the hair follicle, on one side, and the dermis, on the other.  Contraction of the arrector pili muscle causes the hair follicle to stand on end, creating goose bumps.  Arrector pili muscles are activated by the sympathetic nervous system in response to stressors including cold temperatures, fear, and sexual arousal.  While the effect is vestigial in humans, in other mammals, the erect hairs trap more air, creating insulation against cold temperatures, and visibly increase the size of the animal, hopefully warding off predators intimidated by size.

Sweat glands

Sweat glands, or sudoriferous glands, produce sweat that is used to regulate the temperature of the body via evaporative cooling.  Sweat is produced via exocytosis and it is excreted through a duct directly onto the skin’s surface.  There are two types of sweat glands: eccrine and apocrine.

Eccrine sweat glands are found all over the skin’s surface and are especially numerous on the palms of the hand, the soles of the feet, and the forehead.  Eccrine sweat glands are involved primarily in thermoregulation.  The walls of these coiled glands are made of double-layered stratified cuboidal epithelium.  In histological sections, eccrine sweat glands look like circles located in the reticular layer of dermis, with tube-like channels penetrating toward the surface and ending at a pore.

black arrows indicate eccrine sweat glands; the circle marked with a blue arrow is a hair follicle
the black arrow indicates an eccrine sweat gland duct leading to the skin’s surface

Apocrine sweat glands can be found in hairy areas, such as the armpits and genital regions.  Apocrine sweat glands secrete thicker sweat whose smell is unique to each individual.  The release of this sweat is controlled by the nervous and endocrine systems, and plays a role in human pheromone responses.  Apocrine sweat glands are larger than eccrine sweat glands, are lined by cuboidal epithelial cells that surround a central lumen, and empty their product into the hair follicle.  Ceruminous glands, which produce earwax, and mammary glands, which produce milk, are both modified apocrine sweat glands.

Vocabulary list

apocrine sweat gland

areolar connective tissue

arrector pili muscle

basal cell

dense irregular connective tissue

dermal papillae

dermis

eccrine sweat gland

eleidin

epidermis

fingerprint

hair

hair bulb

hair matrix

hair papilla

hair root

hair shaft

hypodermis

integumentary system

keratin

keratinocyte

keratohyalin

Langerhans cell

Meissner’s corpuscle

melanin

melanocyte

Merkel cell

papillary layer

reticular layer

stratum basale (stratum germinativum)

stratum corneum

stratum granulosum

stratum lucidum

stratum spinosum

sweat gland (sudoriferous gland)

thick skin

thin skin

Study prompts

What areas of the body are covered by thick skin?  Why only these three areas?  (Consider the three main differences between thin and thick skin.  What advantages do these differences convey to these particular areas of the body?)

What are three functions of the epidermis?

What is microdermabrasion?  Would you recommend it to a patient?  Why or why not?

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Histology Copyright © by Malgosia Wilk-Blaszczak. All Rights Reserved.

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