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The epidermis, the outermost layer of skin, serves as a waterproof barrier and creates our skin tone!

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The fingernail is an important structure made of keratin. The fingernail generally serve two purposes. It serves as a protective plate and enhances sensation of the fingertip. The protection function of the fingernail is commonly known, but the sensation function is equally important. The fingertip has many nerve endings in it allowing us to receive volumes of information about objects we touch. The nail acts as a counterforce to the fingertip providing even more sensory input when an object is touched. The structure we know of as the nail is divided into six specific parts: the root, nail bed, nail plate, eponychium (cuticle), perionychium, and hyponychium.

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1. Lanugo Hair is the fine, unpigmented hair that covers the body of a fetus.
2. Vellus Hair is the short, "Peach Fuzz" hair that covers most places on the human body.
3. Terminal Hiar is the thicker, longer, and fully developed hair that can be found on the axillary region,  pubic region, and in the male beard.

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The functions of hair include protection, regulation of body temperature, and facilitation of evaporation of perspiration; hairs also act as sense organs.

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The skin of the integument serves as protection to the immune system by serving as a barrier between the outside world and the internal organs. By helping to synthesize and absorb vitamin D, the integumentary system works with the digestive system to encourage the uptake of calcium from our diet. The integumentary system also works closely with the circulatory system and the surface capillaries through your body.

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The dermis is a fibrous structure composed of collagen, elastic tissue, and other extracellular components that includes vasculature, nerve endings, hair follicles, and glands. The role of the dermis is to support and protect the skin and deeper layers, assist in thermoregulation, and aid in sensation.

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Created 2008.

Learning objectives

Name structural components of the normal epidermis and its appendages

Introduction

The skin of an adult occupies an area of 1.5 to 2 m ². It varies in thickness from 0.3 to several centimetres in thickness. The thinnest sites are the eyelids (a few cells thick) and scrotum. The thickest are the soles and palms (about 30 cells thick). The total weight of skin can reach 20 kg, about 16% of total body weight.

Skin is made up of:

These layers are modified according to the needs of the specific area of the body. For example, the scalp is covered with thick hair, the palms have particularly thick epidermis and the face contains large numbers of sebaceous glands.

Each square centimetre of skin is said to have approximately:

6 million cells

5,000 sense end organs

400 cm nerve fibres

200 pain sensors

100 cm blood vessels

100 sweat glands

15 sebum glands

12 cold receptors

5 hairs

2 heat receptors

Acid mantle
Skin has an average pH value of 5.5, creating the acid mantle. This is the result of acidic substances such as amino acids, lactic acid and fatty acids in perspiration, sebum and the hormones. There are resident protective microflora (bacteria and yeasts) but the acid mantle repels pathogenic micro-organisms and reduces body odour.

Epidermis
The epidermis is a dynamic structure acting as a semi-permeable barrier with a layer of flat anuclear cells at the surface (stratum corneum). The epidermis regenerates in orderly fashion by cell division of keratinocytes in the basal layer, with maturing daughter cells becoming increasingly keratinised as they move to the skin surface. Immune cells within the epidermis recognise and process small molecules penetrating the skin surface. Pigment cells in the basal layer (melanocytes) protect the skin from ultraviolet radiation. The basement membrane zone is the communication channel between epidermis and dermis.

The epidermis has a complex structure designed to protect from the environment. It has an undulating surface with cross-crossing ridges and valleys, with invaginations due to follicles and sweat duct ostia. Epidermis is thickest on palms and soles, and thinnest on eyelid and scrotum.

Friction ridges
Ridges are particularly well developed on the fingers and toes where they are known as friction ridges with characteristic patterns commonly referred to as fingerprints. The science of ridgeology has been well developed for forensic purposes.

Proliferation
Keratinocytes make up 95% of the skin surface and are normally renewed every 15 to 30 days. The speed of renewal is greater if the epidermis is injured and in certain skin diseases (particularly psoriasis). Keratinocytes are created in the basal layer and gradually move towards the surface, flattening out and becoming more differentiated towards the anuclear horny cell of the stratum corneum.

The appearance and structure of normal skin varies according to the site of origin of the tissue and the age, sex and ethnicity of the subject.

Epidermal cells

Basal cell layer

Keratinocytes

Columnar cells derived from ectoderm.

Stain with haematoxylin (i.e. pink on H&E sections).

Produce protein (keratin) and lipids.

Produce inflammatory cytokines including interleukin-1.

Express adhesion molecules.

Attached to surrounding cells by desmosomes.

Dividing cells with a roughly 19-day cycle. Daughter cells move to surface to form stratum corneum (28 to 60-days).

On haematoxylin and eosin (H&E) routine sections keratinocytes appear pink (taking up eosin dye) with blue nuclei (haematoxylin).

Melanocytes

Dendritic cells with clear cytoplasm and small dark-staining nuclei.

Derived from neural crest.

Ratio to basal keratinocytes is 1:10.

Produce melanin in melanosomes.

Dendritic processes allow transfer of melanin to adjacent keratinocytes by pinocytosis (tips of dendrites pinched off and engulfed).

Similar number of melanocytes in all races but melanogenesis is variable.

White skin: melanin mainly in basal layer. Blacks: melanin throughout epidermis. Tanning: melanin shifts into keratinocytes, production increased.

Merkel cells

Touch (sensory-mechanical) receptors.

Scarce small round cells found only using electron microscopy.

Most numerous in palms and soles.

Probably neural crest derived, possibly from keratinocytes.

Attached to keratinocytes by desmosomes.

Squamous cell layer (stratum spinosum, prickle cell layer)

Keratinocytes

5-12 layers of polygonal cells that become flatter near surface.

Daughter cells produced by basal cells progress outwards.

Communicate with surrounding keratinocytes, melanocytes and Langerhans cells.

Langerhans cells

Dendritic immune cells with clear cytoplasm on haematoxylin-eosin (H&E) stained sections; identified by special stains (such as gold).

Characteristic "tennis racquet" granules on electron microscopy.

Derived from bone marrow.

Antigen-presenting cell: collects contact antigens and presents them to sensitised T lymphocytes .

Circulate via dermal lymphatics to regional lymph nodes.

Surface receptors for C3, Fc portion of IgG; express Ia antigens on surface.

Desmosomes

Attached to cell membrane opposite similar complex on adjacent cell.

Tonofilaments connect the keratinocyte cytoplasm with the desmosome.

In the intercellular space there is a lattice-work transmembrane linker.

Desmosomes make and break as keratinocytes move from basal layer to surface.

Inflammatory cells

Migrating neutrophils , eosinophils, lymphocytes, erythrocytes can be present in epidermis transiently in diseased states.

Granular cell layer (stratum granulosum)

Keratinocytes

3- 5 layers of flattened cells filled with irregular dark keratohyaline granules.

Stratum lucidum

Keratinocytes

Thin homogeneous eosinophilic area (no organelles visible).

Cytoplasm contains keratin filaments.

Most obvious in areas of friction e.g. palms and soles.

Horny cell layer (stratum corneum)

Keratinocytes

Vertical stacks of flat cells move outwards over 14 days.

At base, adherent cell membranes.

At top, cells loosen and fall off.

Readily absorb and lose water.

Epidermal appendages

Structure

Description

Sweat glands produce hypotonic solution of water, sodium chloride, urea, ammonia and uric acid.

Abundant, except vermilion of lips, labia minora, glans penis, prepuce.

Most dense on palms, soles, axillae and forehead.

Sweat on palms and soles enhances grip.

Secretory coil deep in dermis, duct opens directly onto skin surface.

Duct composed of two layers of small cuboidal cells.

Immune function: secrete IgA.

Hypothalamic control via specific nerve fibres results in increased production with heat, emotional stress and spicy foods.

Develop tolerance to high ambient temperatures by increased sweat production and increasing hypotonicity.

Scent glands that become active after puberty.

Mainly found in axillae and perianal regions.

Primary secretion is thick and odourless; smell derives from bacterial colonisation.

Modified as Moll's glands (eyelids), ceruminous glands in ear canal, mammary glands,

Open into pilosebaceous follicle.

Pilosebaceous structures

Terminal hair on scalp; vellus hair on body surface (short, thin, light coloured).

None on palms and soles.

Root sheath buds downward from epithelium at a consistent angle, depending on site.

Growth phase (anagen) with pointed tip lasts several years; short involutional phase (catagen); resting phase for several months, with clubbed or bulbous tip (telogen).

Hair cortex is produced at a rapid metabolic rate from medulla (loose cuboidal cells) within hair bulb. Cortex contains densely packed keratin with extra sulphur and cystine.

Cortex surrounded by cuticle: a single layer of shingle-like cells.

Hair colour depends on amount of melanin in cortex during anagen: dark hair has more eumelanin melanosomes; fair hair fewer with more lamellated phaeomelanin; red hair has erythromelanin; grey/white hair very few melanocytes.

Anatomy and physiology of hair

Sebaceous glands

Most concentrated on scalp and face where circulating androgens induce increased secretion at puberty.

Several lobules lead into common excretory duct, mostly opening into outer portion of hair follicle. Opens directly onto skin surface on labia, prepuce, nipple and areola.

Modified in Montgomery's tubercles of areolae secreting into lactiferous duct.

Produce sebum: triglycerides, phosopholipids, esterified cholesterol.

Nail matrix produces cornified cells of nail plate; outer cells formed by proximal matrix, innermost by distal matrix (lunula).

Fingernails grow 0.1mm per day; toenails 0.03mm per day.

Cuticle protects matrix from environment.

Nails protect and scratch.

Normal nail

Activity

Download and label the components shown on the Diagram: Normal Epidermis Empty - PDF file

Check the answers: Normal Epidermis - PDF file
Take the Structure of the epidermis quiz

Sponsored content: melanomas are notoriously difficult to discover and diagnose.

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Created 2008. Name structural components of the normal epidermis and its appendages The skin of an adult occupies an area of 1.5 to 2 m 2. It varies in thickness from 0.3 to several centimetres in thickness. The thinnest sites are the eyelids (a few cells thick) and scrotum.

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The Four Types of Glands in the Integument System Are: 

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The Four Types of Glands in the Integument System Are: 

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- sudoriferous (sweat) glands
- sebaceous glands
- ceruminous glands
- and mammary glands

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Sensory receptors in the skin primarily serve as the creators of the sensation of touch.

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The main function of these glands is to secrete materials outside the cells and body.

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Recall that the epidermis is the outermost layer of skin in mammals. It is relatively thin, is composed of keratin-filled cells, and has no blood supply. The epidermis serves as a barrier to water and to invasion by pathogens. Below this, the much thicker dermis contains blood vessels, sweat glands, hair follicles, lymph vessels, and lipid-secreting sebaceous glands (Figure 1). Below the epidermis and dermis is the subcutaneous tissue, or hypodermis, the fatty layer that contains blood vessels, connective tissue, and the axons of sensory neurons. The hypodermis, which holds about 50 percent of the body's fat, attaches the dermis to the bone and muscle, and supplies nerves and blood vessels to the dermis.

Sensory receptors are classified into five categories: mechanoreceptors, thermoreceptors, proprioceptors, pain receptors, and chemoreceptors. These categories are based on the nature of stimuli each receptor class transduces. What is commonly referred to as "touch" involves more than one kind of stimulus and more than one kind of receptor. Mechanoreceptors in the skin are described as encapsulated (that is, surrounded by a capsule) or unencapsulated (a group that includes free nerve endings). A free nerve ending, as its name implies, is an unencapsulated dendrite of a sensory neuron. Free nerve endings are the most common nerve endings in skin, and they extend into the middle of the epidermis. Free nerve endings are sensitive to painful stimuli, to hot and cold, and to light touch. They are slow to adjust to a stimulus and so are less sensitive to abrupt changes in stimulation.

There are three classes of mechanoreceptors: tactile, proprioceptors, and baroreceptors. Mechanoreceptors sense stimuli due to physical deformation of their plasma membranes. They contain mechanically gated ion channels whose gates open or close in response to pressure, touch, stretching, and sound." There are four primary tactile mechanoreceptors in human skin: Merkel's disks, Meissner's corpuscles, Ruffini endings, and Pacinian corpuscle; two are located toward the surface of the skin and two are located deeper. A fifth type of mechanoreceptor, Krause end bulbs, are found only in specialized regions. Merkel's disks (shown in Figure 2) are found in the upper layers of skin near the base of the epidermis, both in skin that has hair and on glabrous skin, that is, the hairless skin found on the palms and fingers, the soles of the feet, and the lips of humans and other primates. Merkel's disks are densely distributed in the fingertips and lips. They are slow-adapting, encapsulated nerve endings, and they respond to light touch. Light touch, also known as discriminative touch, is a light pressure that allows the location of a stimulus to be pinpointed. The receptive fields of Merkel's disks are small with well-defined borders. That makes them finely sensitive to edges and they come into use in tasks such as typing on a keyboard.

Practice Question

Which of the following statements about mechanoreceptors is false?

1. Pacini corpuscles are found in both glabrous and hairy skin.

2. Merkel's disks are abundant on the fingertips and lips.

3. Ruffini endings are encapsulated mechanoreceptors.

4. Meissner's corpuscles extend into the lower dermis.

Meissner's corpuscles, (shown in Figure 3) also known as tactile corpuscles, are found in the upper dermis, but they project into the epidermis. They, too, are found primarily in the glabrous skin on the fingertips and eyelids. They respond to fine touch and pressure, but they also respond to low-frequency vibration or flutter. They are rapidly adapting, fluid-filled, encapsulated neurons with small, well-defined borders and are responsive to fine details. Like Merkel's disks, Meissner's corpuscles are not as plentiful in the palms as they are in the fingertips.

Deeper in the epidermis, near the base, are Ruffini endings, which are also known as bulbous corpuscles. They are found in both glabrous and hairy skin. These are slow-adapting, encapsulated mechanoreceptors that detect skin stretch and deformations within joints, so they provide valuable feedback for gripping objects and controlling finger position and movement. Thus, they also contribute to proprioception and kinesthesia. Ruffini endings also detect warmth. Note that these warmth detectors are situated deeper in the skin than are the cold detectors. It is not surprising, then, that humans detect cold stimuli before they detect warm stimuli.

Pacinian corpuscles (seen in Figure 4) are located deep in the dermis of both glabrous and hairy skin and are structurally similar to Meissner's corpuscles; they are found in the bone periosteum, joint capsules, pancreas and other viscera, breast, and genitals. They are rapidly adapting mechanoreceptors that sense deep transient (but not prolonged) pressure and high-frequency vibration. Pacinian receptors detect pressure and vibration by being compressed, stimulating their internal dendrites. There are fewer Pacinian corpuscles and Ruffini endings in skin than there are Merkel's disks and Meissner's corpuscles.

In proprioception, proprioceptive and kinesthetic signals travel through myelinated afferent neurons running from the spinal cord to the medulla. Neurons are not physically connected, but communicate via neurotransmitters secreted into synapses or "gaps" between communicating neurons. Once in the medulla, the neurons continue carrying the signals to the thalamus.

Muscle spindles are stretch receptors that detect the amount of stretch, or lengthening of muscles. Related to these are Golgi tendon organs, which are tension receptors that detect the force of muscle contraction. Proprioceptive and kinesthetic signals come from limbs. Unconscious proprioceptive signals run from the spinal cord to the cerebellum, the brain region that coordinates muscle contraction, rather than to the thalamus, like most other sensory information.

Barorecptors detect pressure changes in an organ. They are found in the walls of the carotid artery and the aorta where they monitor blood pressure, and in the lungs where they detect the degree of lung expansion. Stretch receptors are found at various sites in the digestive and urinary systems.

In addition to these two types of deeper receptors, there are also rapidly adapting hair receptors, which are found on nerve endings that wrap around the base of hair follicles. There are a few types of hair receptors that detect slow and rapid hair movement, and they differ in their sensitivity to movement. Some hair receptors also detect skin deflection, and certain rapidly adapting hair receptors allow detection of stimuli that have not yet touched the skin.

Integration of Signals from Mechanoreceptors

The configuration of the different types of receptors working in concert in human skin results in a very refined sense of touch. The nociceptive receptors-those that detect pain-are located near the surface. Small, finely calibrated mechanoreceptors-Merkel's disks and Meissner's corpuscles-are located in the upper layers and can precisely localize even gentle touch. The large mechanoreceptors-Pacinian corpuscles and Ruffini endings-are located in the lower layers and respond to deeper touch. (Consider that the deep pressure that reaches those deeper receptors would not need to be finely localized.) Both the upper and lower layers of the skin hold rapidly and slowly adapting receptors. Both primary somatosensory cortex and secondary cortical areas are responsible for processing the complex picture of stimuli transmitted from the interplay of mechanoreceptors.

Density of Mechanoreceptors

The distribution of touch receptors in human skin is not consistent over the body. In humans, touch receptors are less dense in skin covered with any type of hair, such as the arms, legs, torso, and face. Touch receptors are denser in glabrous skin (the type found on human fingertips and lips, for example), which is typically more sensitive and is thicker than hairy skin (4 to 5 mm versus 2 to 3 mm).

How is receptor density estimated in a human subject? The relative density of pressure receptors in different locations on the body can be demonstrated experimentally using a two-point discrimination test. In this demonstration, two sharp points, such as two thumbtacks, are brought into contact with the subject's skin (though not hard enough to cause pain or break the skin). The subject reports if he or she feels one point or two points. If the two points are felt as one point, it can be inferred that the two points are both in the receptive field of a single sensory receptor. If two points are felt as two separate points, each is in the receptive field of two separate sensory receptors. The points could then be moved closer and re-tested until the subject reports feeling only one point, and the size of the receptive field of a single receptor could be estimated from that distance.

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Recall that the epidermis is the outermost layer of skin in mammals. It is relatively thin, is composed of keratin-filled cells, and has no blood supply. The epidermis serves as a barrier to water and to invasion by pathogens. Below this, the much thicker dermis contains blood vessels, sweat glands, hair follicles, lymph vessels, and lipid-secreting sebaceous glands (Figure 1).