The integumentary system includes the skin, hair, nails, and glands, and it also houses sensory receptors and a vascular network. The skin, being the body’s largest organ, covers an area of approximately 1.5 to 2 square meters and it weighs more than 6 pounds in the average adult. Located between the skin and deeper tissues is the subcutaneous layer, made up of areolar connective tissue and adipose (fat) tissue.
What role does skin has?
- The skin forms a relatively a waterproof layer which protects the deeper and more delicate structures
- Regulation of body temperature
- Formation of vitamin D from UV rays
- Cutaneous sensation- from sensory receptor nerves
- The skin is a minor excretory organ for some substances including sodium chloride in sweat; excess sweating may lead to low blood sodium levels (hyponatraemia)
Stratum corneum(horny layer) called corneocytes or squames. Dead, dried-out hard cells without nuclei.
Stratum granulosum(granular layer) cells contain basophilic granules. Waxy material is secreted into the intercellular spaces.
Stratum spinulosum(spinous, spiny or prickle cell layer) intercellular bridges called desmosomes link the cells together. The cells become increasingly flattened as they move upward.
Stratum basale(basal layer) columnar (tall) regenerative cells. As the basal cell divides, a daughter cell migrates upwards to replenish the layer above.
Figure 1.1 Cells of the Epidermis (Yousef, Alhajj and Sharma, 2024)
Skin disorders:
- Skin cancer- such as melanoma
- Dermatitis- Skin inflammation
- Burns
- Acne
- Alopecia
You can find more about this subject by visiting the National Institute of Arthritis and Musculoskeletal and Skin Diseases https://www.niams.nih.gov/health-topics/skin-diseases
Figure 1.2 Cross Section, Layers of the Skin (Yousef, Alhajj and Sharma, 2024)
The skin derivatives
Hair (pili)
Humans are relatively hairless compared to other mammals. Specific areas of the body—such as the palms, soles, lips, nipples, penis, and labia minora—are naturally hairless.
Hair serves a protective function and consists of three main parts:
- Shaft – the visible part above the skin.
- Root – located beneath the skin.
- Bulb – the base of the root, responsible for hair growth.
The root and bulb are enclosed within the hair follicle, which has two main layers:
- Inner root sheath
- Outer root sheath
The hair bulb is surrounded by touch receptors and contains actively dividing cells that produce new hair.
Hair Color is genetically determined and depends on the amount of melanin present.
White hair occurs when melanin is replaced by tiny air bubbles.
Arrector Pili Muscle
- Hair typically lies at an angle on the skin.
- Arrector pili muscles cause the hair to stand upright, producing “goosebumps.”
- These muscles are activated by sympathetic nerve fibers in response to fear or cold.
- This is part of an efficient warming mechanism, especially when accompanied by shivering (involuntary skeletal muscle contractions).
Glands
Glands are organs or groups of cells that produce substances such as hormones or enzymes and release them either into the bloodstream or through ducts. In the skin, glands open onto the epidermal layer but are located in the dermis.
Types of Glands
- Exocrine glands: Release substances into ducts (e.g., sweat, sebum).
- Endocrine glands: Release hormones directly into the bloodstream.
- Note: Most skin glands are exocrine.
Examples of Skin Glands
1. Sudoriferous (Sweat) Glands
Found all over the body, but most numerous on the palms, soles, axillae, pubic region, and forehead.
Two Types:
a) Merocrine (Eccrine) Glands
- Most abundant
- Located mainly on the palms, soles, and forehead
- Ducts open directly onto the skin surface
- Active at birth
b) Apocrine Glands
- Found in the axillary and pubic regions
- Ducts open into hair follicles
- Become active during puberty
2. Mammary Glands
- Specialized sweat glands
- Produce milk to nourish newborns
3. Sebaceous Glands
- Made of secretory epithelial cells derived from the same tissue as hair follicles
- Secrete sebum (an oily substance) into hair follicles
- Found throughout the body except on the palms and soles
- In certain areas like the lips, eyelids, nipples, labia minora, and glans penis, sebaceous glands open directly onto the skin surface (independent of hair follicles)
4. Ceruminous Glands
- Located in the ear canal
- Modified apocrine glands
- Produce cerumen (earwax)
- Protect the ear and keep the tympanic membrane (eardrum) from drying out
Functions of Skin Glands
- Temperature regulation (via sweating)
- Protection (sebum lubricates the skin and hair, forming a barrier against pathogens)
- Waste excretion (through sweat)
- Immune defense (sebum contains antimicrobial substances)
Nails
Human nails are structurally similar to the claws, horns, and hooves of animals. They are made of hard keratin plates derived from epidermal cells.
- Nail Plate: The visible, hardened portion
- Nail Bed: The germinative zone beneath the plate
- Nail Root: Embedded in the skin, covered by the cuticle
- Lunula: The pale, crescent-shaped area at the base of the nail
Fingernails grow faster than toenails, and growth rates increase with warmer temperatures.
Sensory Receptors in the Skin
The skin is a sensory organ that helps detect external stimuli.
Types of Sensory Receptors
- Mechanoreceptors: Detect touch and pressure
- Thermoreceptors: Sense temperature changes
- Pain receptors: Detect harmful stimuli
Sensory Nerves:
- Receptors are located in the dermis
- They send nerve impulses to the spinal cord and brain
- These signals are interpreted in the sensory cortex of the cerebrum
Relevant Medical Terminology:
- Sebaceous gland: Produces sebum (oil)
- Hair follicle: Structure where hair is produced
- Vasodilation: Widening of blood vessels to release heat
- Vasoconstriction: Narrowing of blood vessels to conserve heat
- Derma: Refers to the skin
Epidermis
Figure 1.3 Cells of the Epidermis. The image shows stratum corneum, stratum lucidum, stratum granulosum, stratum spinosum, stratum basale, and dermis (Brown and Krishnamurthy, 2022)
It’s the thinnest and outermost layer of the skin composed of keratinised epithelia cells. It is thickest on the palm of the hand and sole of the feet.
Composed of specialised cells.
The epidermis is subdivided into four basic layers (from deep to superficial): basal cell layer, spinous cell layer, granular layer and cornified (horny) layer. Cells forming these layers are bound together by complex membrane-associated plaques called desmosomes. Desmosomes anchor and unite cytoskeletal components of adjacent epidermal cells. Important molecules critical to keratinocyte–keratinocyte bonding (adhesion) includes cadherins, desmogleins, and desmocollins.
- the epithelial cells are called Keratinocytes – produce keratin
- Melanocytes – produce melanin
- Langerhans cells – arise from bone marrow and help to activate the immune system
- Merkel cells – associated with sensory neurons to aid in our sense of touch
The epidermis is the most superficial layer of the skin and is composed of stratified keratinised squamous epithelium , which varies in thickness in different parts of the body.
There are no blood vessels or nerve endings in the epidermis, but its deeper layers are bathed in interstitial fluid from the dermis, which provides oxygen and nutrients, and drains away as lymph.
The cells on the surface are flat, thin, non-nucleated, dead cells, or squames, in which the cytoplasm has been replaced by the fibrous protein keratin.
The maintenance of healthy epidermis depends upon three processes being synchronised: desquamation of the keratinised cells from the surface effective keratinisation of the cells approaching the surface continual cell division in the deeper layers with newly formed cells being pushed to the surface.
Melanin, a dark pigment derived from the amino acid tyrosine and secreted by melanocytes in the deep germinative layer, is absorbed by surrounding epithelial cells.
Dermis
Figure 1.4 Anatomy of the skin, showing the epidermis, dermis, and subcutaneous tissue (National Cancer Institute, 2019)
It is formed from connective tissue and the matrix contains collagen fibres interlaced with elastic fibres. Rupture of elastic fibres occurs when the skin is overstretched, resulting in permanent striae, or stretch marks, that may be found in pregnancy and obesity. Collagen fibres bind water and give the skin its tensile strength, but as this ability declines with age, wrinkles develop.
The dermis is a complex, dynamic microenvironment that harbours a collection of specialized cells within an intricate matrix (formed by soluble and non-soluble molecules.) It is subdivided into the papillary dermis (superficial) and the reticular dermis (deep.) The dermis is responsible for a wide variety of functions, including maintaining the elastic and mechanical integrity of the skin, cutaneous nutrition, immunosurveillance, sensory perception and temperature regulation.
In a normal physiological state, the dermis contains an ensemble of cells that includes (1) fibroblasts, (2) endothelial cells, (3) monocytes/macrophages, (4) dendritic cells (also called dermal dendrocytes), (5) mast cells, (6) lymphocytes, (7) Schwann cells, axons and specialized nerve endings, (8) pericytes and glomus cells and (9) mesenchymal elements with presumed differentiation plasticity (mesenchymal stem cells.) These cells are contained within an acellular matrix, composed mainly of collagen and glycosaminoglycan.
Hypodermis
Figure 1.5 Anatomy of the Skin (National Cancer Institute, 2019)
The hypodermis, also known as the subcutaneous fascia, the subcutisis, located beneath the dermis. This layer is the deepest skin layer and contains adipose lobules, sensory neurons, blood vessels, and scanty skin appendages, such as hair follicles.
- Serves as an insulator that protects the body
- Stores energy in the form of fat
- Helps to regulate body temperature
- Helps to connect the skin to muscles and bones
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Integumentary System, Bozeman Science
The Integumentary System, Part 1 – Skin Deep: Crash Course Anatomy & Physiology #6, CrashCourse