In this note, we are going to discuss the anatomy and function of the skin, as well as its appendages – hairs, nails, sweat and sebaceous glands.

Skin

The skin is the sensory organ of the integumentary system, the largest organ of the human body.

It performs a broad variety of functions:

• It serves as a protective barrier against various forms of damage, including mechanical, chemical, thermal, and pathogenic factors.
• It participates in biochemical synthetic processes, such as vitamin D production
• It regulates body temperature by adjusting blood flow and sweating.
• It is also a major sensory organ, containing numerous receptors for touch, temperature, pain, and other stimuli

Skin layers

The skin has several layers:

• Epidermis

• Dermis

• Hypodermis

Let’s discuss them in greater detail.

Эпидермис

The epidermis is the outermost layer of the skin. It is represented by the stratified squamous epithelium, composed of five sub-layers:

• Basal layer (stratum basale/germinativum) – the deepest one. Acts as the stem cell region, regenerating other layers

• Prickle cell layer (stratum spinosum) – contains post-mitotic cells from the basal layer as well as keratin fibrils

• Granular layer (stratum granulosum) – contains pycnotic cells rich in keratohyaline granules

• Clear layer (stratum lucidum) – is present only in palms and soles (the so-called glabrous skin, that is hairless and lacks of sebaceous glands). Contains translucent cells filled with keratin filaments.

• Cornified layer (stratum corneum) – the most superficial layer, composed of dead cells known as squames, undergoes constant desquamation, making way for new cells that originate in the basal layer below

Dermis

The dermis is situated underneath the epidermis and consists of two layers:

• Papillary layer, which is the superficial one.

It is characterized by irregular projections, termed dermal papillae, that interlace with epidermal ridges of the epidermis. This forms the individual pattern visible on the skin’s surface that we refer to as a fingerprint. The papillary layer is primarily composed of collagen fibers and capillary loops, providing mechanical support and nutrition to the epidermis.

• Reticular layer, which is located deep to the papillary layer.

It mostly consists of the extensive clusters of collagen fibers, forming an interconnected network. In addition to collagen fibers, there is a network of elastic fibers and a limited presence of reticular fibers.

Langer’s lines are topographical landmarks used in surgery for determining optimal incision points and are aligned with the orientation of dermal collagen fibers. These topographical landmarks aid surgeons in minimizing skin tension and facilitating wound closure.

Hypodermis

The deepest skin layer – the hypodermis – contains fascial fibers and adipose tissue.

Adipose tissue plays a vital role in thermoregulation.

This layer contains small bundles of smooth muscles known as arrector pili muscles.

The contraction of these muscles, triggered by cold stimulus, causes the erection of hair and puckering of the skin, commonly known as goose bumps.

The hypodermis is also filled with subcutaneous nerves, vessels, and lymphatics.

Skin appendages

Skin appendages include:

• Hairs.

Hairs are elongated, cornified structures extending from the skin, covering a large portion of the body surface. They detect various stimuli (sensory function), regulate body temperature (thermoregulatory function), and provide protection from external factors (protective function). The hair grows out of the hair follicle,

and the lowest expanded part of the follicle is called the hair bulb.

• Nails

Nails are homologous to the stratum corneum of the epidermis and contain a variety of minerals, such as calcium. A nail consists of: the nail plate,

nail folds,

nail matrix,

nail bed and hyponychium.

• Sweat glands.

Sweat glands are exocrine glands, producing sweat and secreting it via ducts on the epithelial surface. This secretion plays a crucial role in maintaining appropriate body temperature by regulating heat loss or gain. There are two types of sweat glands – eccrine and apocrine. Apocrine glands secrete a substance rich in proteins, which gives it a specific smell. These glands are particularly well-developed in the genital and axillary regions.

• Sebaceous glands.

Sebaceous glands are located within the dermis, they secrete the substance called sebum, which is an oily and fatty secretion. Sebum plays a role in the epidermal barrier and the immune system of the skin.

Sensory function of the skin

The skin contains numerous receptors that provide information about the location, duration, and intensity of various stimuli.

In terms of anatomy, these receptors are the dendrites of sensitive neurons. Signals carried by these fibers travel to the central nervous system via either the dorsal column-medial lemniscus pathway or the spinothalamic pathway. More information is provided in the corresponding videos.

There are several types of receptors. Let’s discuss them in greater detail.

Free nerve endings

The most numerous receptors are free nerve endings, mostly located within the papillary layer of the dermis. They are unspecialized and can function as nociceptors (responding to pain), mechanoreceptors (detecting touch and compression of the skin), or thermoreceptors (responding to temperature).

Merkel nerve ending (Merkel’s discs)

The Merkel nerve ending consists of a free nerve ending combined with a so-called Merkel cell. Merkel cells are located within the basal layer of the epidermis in highly sensitive areas, such as the fingertips. Their receptors are sensitive to fine touch and pressure.

Ruffini endings / corpuscles

Ruffini endings, also known as bulbous corpuscles, are located in the dermis. They consist of an encapsulated sensory fiber, surrounded by collagen fibers and are sensitive to pressure and distortion of the skin.

Hair follicle receptors

Some free nerve endings surround hair follicles, forming a root hair plexus. These receptors detect distortions and movements across the body surface.

Pacinian corpuscles

Pacinian corpuscles also known as lamellar corpuscles are large encapsulated receptors, located within the dermis, and sensitive to mechanical pressure and vibrations. The distribution of these receptors varies across the body, with a higher concentration in certain areas such as fingers, breasts, and external genitalia. Additionally, they can be located in joints, the periosteum, connective tissue, and internal organs, indicating their widespread distribution throughout the body.

Meissner’s corpuscles

Meissner’s corpuscles, also known as tactile corpuscles, are found in the dermal papillae of hairless skin in highly sensitive areas, such as eyelids, lips, fingertips, nipples, and external genitalia. These are encapsulated nerve endings sensitive to light pressure and low-frequency vibrations

Golosary

Skin and skin appendages

skin

cutis

epidermis

epidermis

dermis

dermis

hypodermis

hypodermis

basal layer

stratum basale/germinativum

spinous layer

stratum spinosum

granular layer

stratum granulosum

lucid layer

stratum lucidum

corneal layer

stratum corneum

papillary layer

stratum papillare

reticular layer

stratum reticulare

arrector pili muscle

musculus arrector pili

hair follicle

folliculus pili

hair bulb

bulbus pili

nails

ungues

nail plate

lamina unguis

nail folds

vallum unguis

nail bed

matrix unguis

hyponychium

hyponychium

sweat glands

glandulae sudoriferae

sebaceous glands

glandulae sebaceae