Anatomia skóry i jej przydatków | Skin and Its Appendages Anatomy

Introduction to Integumentary System Anatomy

The skin is not only the largest organ of the human body but also one of the most complex. As a dynamic structure, it serves as a critical barrier between the external environment and the internal organs, while providing structural integrity, thermoregulation, and sensory feedback. The skin is composed of three primary layers: the epidermis, dermis, and subcutaneous tissue.

Epidermis

The epidermis is the outermost layer of the skin, serving as the body’s first line of defense against environmental hazards. This layer is thin, yet it performs essential roles in shielding deeper tissues from mechanical injury, pathogens, and dehydration. It is composed primarily of keratinocytes – cells that produce keratin, a tough protein that provides durability and strength. The epidermis is an avascular layer, meaning it lacks blood vessels and relies on nutrient exchange with the underlying dermis. This is why minor superficial scratches on the epidermis do not bleed. The epidermis is divided into several distinct sublayers.

Sublayers of the Epidermis

Layer	Description
Stratum Corneum	The outermost layer of the epidermis is composed of dead, flattened keratinocytes (also known as corneocytes), which are filled with keratin and arranged in overlapping layers to form a strong, protective barrier. Cells are continuously shed and replaced.
Stratum Lucidum	A thin, transparent layer present only in thicker areas of the skin, such as the palms of the hands and soles of the feet. It consists of flattened, dead keratinocytes that add an extra layer of protection.
Stratum Granulosum	This layer contains keratohyalin granules (rich in filaggrin, which binds keratin filaments together) and lamellar bodies, which release the lipids that make the skin waterproof. Keratinocytes in this layer flatten and lose their nuclei as they move upwards, and at the top of the layer they become the dead corneocytes of the stratum corneum.
Stratum Spinosum	Known as the “spiny layer” due to the appearance of cells under a microscope, this layer contains desmosomes, which join the keratinocytes to one another and give the layer its strength (the autoantibodies in pemphigus vulgaris attack them). Langerhans cells, involved in immune response, are also found in this layer.
Stratum Basale	The deepest layer, where new keratinocytes are generated through mitosis. This layer also contains melanocytes, which produce melanin for skin color and UV protection, and Merkel cells, which contribute to the sense of touch by detecting pressure changes. The basal cells are attached to the basement membrane by hemidesmosomes, which are the target of the autoantibodies in bullous pemphigoid.

Cell Types in the Epidermis

• Keratinocytes: The most abundant cells in the epidermis, responsible for producing keratin, which forms the waterproof barrier of the skin.
• Melanocytes: Cells that produce melanin, providing pigmentation to the skin and protecting it from UV radiation. Skin tone is also influenced by hemoglobin in blood vessels (giving a reddish/pink hue) and carotene from the diet (giving a yellowish hue).
• Langerhans Cells: Immune cells that patrol the epidermis, identifying and capturing harmful pathogens.
• Merkel Cells: Sensory cells located in the stratum basale, responsible for detecting fine touch and pressure.

The constant renewal of the epidermis ensures that the skin can maintain its protective functions, with old cells being shed and replaced by new cells generated in the deeper layers. Epidermal turnover, which is the time a cell needs to move from the stratum basale to the surface and be shed, takes roughly 4 to 8 weeks in healthy adults (figures differ between sources). It is faster in children and slower in older adults, which is one reason why wounds heal more slowly with age.

Dermis

Beneath the epidermis lies the dermis, a much thicker and more structurally complex layer. The dermis provides the skin with its strength and elasticity, owing to its rich network of collagen and elastin fibers. Unlike the epidermis, the dermis is well-vascularized and contains a variety of structures, including blood vessels, nerve endings, hair follicles, and glands. The dermis is composed of two main sublayers: the papillary layer, closer to the epidermis, and the reticular layer – deeper within.

Sublayers of the Dermis

Layer	Description
Papillary Layer	This upper layer of the dermis is composed of loose connective tissue and forms dermal papillae, which are small, finger-like projections extending into the epidermis. These projections increase the surface area for nutrient exchange and help anchor the dermis to the epidermis. The papillary layer is rich in capillaries, supplying nutrients to the avascular epidermis, and contains Meissner’s corpuscles, specialized nerve endings that detect light touch.
Reticular Layer	The deeper and thicker layer of the dermis is composed of dense, irregular connective tissue rich in collagen and elastin fibers. These fibers provide the skin with tensile strength and elasticity, allowing it to stretch and recoil without tearing. The reticular layer also contains various structures such as hair follicles, sebaceous glands (oil glands), sweat glands, and larger blood vessels. Pacinian corpuscles, which detect deep pressure and vibration, lie at the border between the reticular dermis and the hypodermis.

Hair Follicles and Associated Structures

The dermis is home to hair follicles, which are responsible for the growth of hair. Each follicle is anchored deep in the dermis, and its root extends down into the subcutaneous tissue. Hair follicles are also associated with sebaceous glands and arrector pili muscles — both are covered in detail in the “Associated Skin Structures” section below.

Subcutaneous Tissue

The subcutaneous tissue, also known as the hypodermis, is the deepest layer of the skin, located beneath the dermis. This layer serves as a critical interface between the skin and the underlying muscles and bones. It is primarily composed of adipose tissue (fat), which acts as a cushion to absorb mechanical shocks and insulate the body from extreme temperatures.

Components of the Subcutaneous Tissue

• Adipose Tissue: The primary component of the hypodermis, adipose tissue stores fat, which serves as an energy reserve and provides insulation. The thickness of this layer varies across different parts of the body and among individuals.
• Connective Tissue: In addition to adipose cells, the subcutaneous layer contains fibrous connective tissue that binds the skin to underlying structures, such as muscles and bones.
• Blood Vessels and Nerves: Larger blood vessels and nerves pass through the subcutaneous tissue to supply the dermis and epidermis. This layer also contains lymphatic vessels, which help drain excess fluid from the tissues and return it to the bloodstream.

Basement Membrane Zone

The epidermis and the dermis are joined by the basement membrane zone (the dermo-epidermal junction). The basal keratinocytes attach to it with hemidesmosomes, and type VII collagen in the anchoring fibrils fixes it to the dermis. This is clinically important, because the level at which the skin splits decides the type of blister.

Level of the split	Cause	Example
Inside the epidermis (intraepidermal)	Autoantibodies against desmosomes, so the keratinocytes separate from each other (acantholysis)	Pemphigus vulgaris: flaccid blisters that rupture easily, Nikolsky sign positive
At the dermo-epidermal junction (subepidermal)	Autoantibodies against hemidesmosomes	Bullous pemphigoid: tense blisters, mainly in older adults
At the dermo-epidermal junction (genetic)	Mutations in keratins (EB simplex, splitting inside the basal layer) or in the anchoring proteins (junctional and dystrophic EB)	Epidermolysis bullosa

Blood Supply, Lymphatics and Innervation

• Blood supply: two horizontal vascular plexuses are joined by vertical vessels. The deep plexus lies between the dermis and the hypodermis, and the superficial plexus lies under the papillary dermis and sends capillary loops into each dermal papilla. At rest the skin receives only about 5% of the cardiac output, but this can rise several times during heat stress, because the skin is the main organ that releases heat.
• Lymphatics: a superficial network drains to the regional lymph nodes. This is why a skin infection or a melanoma can first show up in the nodes of the same region (the sentinel lymph node).
• Sensory nerves: each spinal nerve root supplies a strip of skin called a dermatome. Shingles (herpes zoster) follows one dermatome and does not cross the midline.
• Autonomic nerves: sympathetic fibers supply the arrector pili muscles, the skin blood vessels and the eccrine sweat glands (which, unusually, use acetylcholine as the transmitter).

Regional Variation and Skin Phototypes

The skin covers about 1.5 to 2 m² and makes up around 15% of body weight. The epidermis is only about 0.05 mm thick on the eyelids but up to 1.5 mm on the palms and soles. This thick skin has a stratum lucidum and many eccrine glands but no hair follicles or sebaceous glands, while thin skin covers the rest of the body.

The Fitzpatrick scale classifies the skin by its reaction to sun and is used to estimate the risk of sunburn and skin cancer.

Phototype	Typical reaction to the sun
I	Always burns, never tans (very fair skin, often red or blond hair)
II	Burns easily, tans minimally
III	Sometimes burns mildly, tans gradually
IV	Burns minimally, tans well (olive skin)
V	Rarely burns, tans darkly (brown skin)
VI	Never burns (deeply pigmented skin)

Phototype also matters at the bedside: erythema, cyanosis, jaundice and pallor look different on dark skin, so look at the mucous membranes, the nail beds, the palms and the sclerae as well.

Associated Skin Structures: Hair, Nails, and Glands

The skin not only serves as a protective barrier but also houses several specialized structures that play critical roles in maintaining its overall function. These structures include hair follicles, nails, and glands, all of which contribute to sensory perception, protection, and homeostasis.

Types of Sweat and Sebaceous Glands

Gland Type	Description
Eccrine Sweat Glands	Found throughout the body, especially on the palms, soles, and forehead, eccrine glands secrete a watery sweat directly onto the skin surface. This sweat is primarily composed of water, salts, and waste products, and its evaporation helps cool the body.
Apocrine Sweat Glands	Located mainly in the armpits, groin, and around the nipples, apocrine glands secrete a thicker, milky fluid into hair follicles. This sweat is initially odorless, but when broken down by skin bacteria, it produces body odor. These glands become active during puberty and are linked to emotional and hormonal changes.
Sebaceous Glands	Found throughout the skin except on the palms and soles, sebaceous glands are closely associated with hair follicles. They secrete an oily substance called sebum, which lubricates the hair and skin, preventing them from becoming dry and brittle. Sebum also plays a role in maintaining the skin’s barrier function, protecting against environmental damage and the growth of harmful microorganisms. Unlike sweat glands, sebaceous glands do not play a direct role in thermoregulation but are essential for skin health and hydration.

Nails grow continuously throughout life, with the growth rate influenced by factors such as age, nutrition, and overall health. On average, fingernails grow about 3 mm per month, while toenails grow roughly two to three times more slowly (a fingernail grows back completely in about 6 months, a toenail in 12 to 18 months).

Common Congenital Anomalies

Congenital anomalies of the integumentary system, which includes the skin, hair, nails, and associated glands, are often visible at birth and can range from benign to conditions with significant health implications. These anomalies can affect appearance, skin function, and, in some cases, increase the risk of complications like infections or malignancies. Below are some frequently observed congenital anomalies in the integumentary system:

Congenital Anomaly	Description
Congenital Melanocytic Nevus (CMN)	CMNs are pigmented birthmarks that vary in size and can appear anywhere on the body. Small CMNs are generally benign, but large and giant nevi (a projected adult diameter above 20 cm) carry a higher risk of melanoma and, when they lie over the spine or are multiple, of neurocutaneous melanosis. Regular monitoring and, in some cases, surgical removal may be recommended for larger or atypical lesions.
Epidermal Nevus	These are skin lesions formed by an overgrowth of epidermal cells, often appearing as raised, pigmented patches. Epidermal nevi are generally benign but may become more pigmented or change in appearance. Some cases are associated with systemic conditions, requiring further medical evaluation and monitoring.
Port-Wine Stain (PWS)	A type of capillary malformation, PWS presents as a flat, reddish-purple birthmark typically on the face or neck. Although usually asymptomatic, PWS can darken and thicken over time. Laser therapy is often used to reduce visibility, particularly if located in prominent areas. In rare cases, PWS can be associated with Sturge-Weber syndrome.
Albinism	Albinism is a genetic condition characterized by reduced or absent melanin production, affecting skin, hair, and eye pigmentation. Individuals with albinism are more sensitive to sunlight and have an increased risk of skin cancer. Management includes protective sun measures and regular dermatological evaluations.
Ichthyosis	Ichthyosis encompasses a group of disorders that cause dry, scaly skin due to abnormal keratinization (cornification) of the epidermis. Symptoms range from mild to severe, with certain types, like lamellar ichthyosis, appearing at birth. Treatment focuses on regular moisturization and specialized skincare to prevent cracking and infection.
Epidermolysis Bullosa (EB)	EB is a group of genetic conditions causing fragile skin that easily blisters from minor friction. Symptoms can range from mild to life-threatening, depending on the type. Management includes wound care, infection prevention, and, in severe cases, supportive therapies to address complications.
Congenital Hypertrichosis	A rare genetic condition, often called “werewolf syndrome”, that causes excessive hair growth from birth, typically on the face, arms, and other parts of the body. The condition is caused by genetic mutations and, depending on the subtype, can be inherited in an X-linked dominant pattern or an autosomal dominant pattern. While it has cosmetic effects, there are no specific treatments beyond hair removal methods.
Ectodermal Dysplasia	A group of genetic disorders affecting the development of two or more ectodermal structures — skin, hair, nails, teeth, and sweat glands — at the same time. It often causes sparse hair, missing or malformed teeth, and reduced sweating due to underdeveloped sweat glands, which can lead to heat intolerance.