THE INTEGUMENTARY SYSTEM

The integumentary system, from the word integument, derived from the Latin word “integumentum,” meaning “a covering,” is composed mainly of the body surfaces or known as the skin. This is the external covering of the body comprising of the skin, its pigments, and various exocrine glands that produce sweat, tears, and other oils, mucous, waxes, scents, and milk. It also includes all derived structures such as hair, feathers, scales, teeth, nails, claws, horns, and beaks. This system also contains specialized nerve receptors for stimuli.

As an organ system, the integumentary system is the largest. In human beings, it amounts to about 16 percent of the body weight. It is composed of many types of tissue, including epithelial, connective, neural, and muscular. This organ system, composed of many diverse cells and tissues, provides a wall of defense between the body and the environment.

The integumentary system serves multiple functions in order to promote body homeostasis. These functions are the following:

o Protect the body’s internal living tissues and organs o Protect against invasion by infectious organisms o Protect the body from dehydration o Protect the body against abrupt changes in temperature o Help excrete waste materials through perspiration o Act as a receptor for touch, pressure, pain, heat, and coldo Protect the body against sunburns o Generate vitamin D through exposure to ultraviolet light o Store water, fat, glucose, and vitamin D o Participate in temperature regulation

These functions are then simplified into seven chief functions. These are the:

o Regulation of body temperature

o Protection

o Sensation

o Excretion

o Immunity

o Blood reservoir

o Synthesis of vitamin D

How does the integumentary system of human differ from other vertebrates, from invertebrates and plants?

Humans are vertebrates but they have structures that cannot be found in other vertebrates and vertebrates have structures that cannot be found in humans. The integumentary system of vertebrates comprises skin, scales, feathers, hair and glands while that of only humans is made up of the skin which includes glands, hair, and nails. For humans, this system provides protective functions for the body. It prevents excessive water loss, keeps out microorganisms that could cause illness, and protects the underlying tissues from mechanical damage. Pigments in the skin called melanin absorb and reflect the sun's harmful ultraviolet radiation. The skin helps to regulate the body temperature. If heat builds up in the body, sweat glands in the skin produce more sweat which evaporates and cools the skin. In addition, when the body overheats, blood vessels in the skin expand and bring more blood to the surface, which allows body heat to be lost. If the body is too cold, on the other hand, the blood vessels in the skin contract, resulting in less blood is at the body surface, and heat is conserved. In addition to temperature regulation, the skin serves as a minor excretory organ, since sweat removes small amounts of nitrogenous wastes produced by the body. The skin also functions as a sense organ since it contains millions of nerve endings that detect touch, heat, cold, pain, and pressure. Finally, the skin produces vitamin D in the presence of sunlight, and renews and repairs damage to itself.

Snails, slugs, oysters, and clams are known as invertebrates. They are protected by a hard shell made of calcium carbonate secreted by the mantle, a heavy fold of tissue that surrounds the mollusc's internal organs. Spiders, insects, lobsters, and shrimp, have bodies covered by an external skeleton, the exoskeleton, which is strong, impermeable, and allows some arthropods to live on land. The exoskeleton is composed of layers of protein and a tough polysaccharide called chitin, and can be a thick hard armor or a flexible paper-thin covering.

The integumentary system of plants includes the epidermis, cuticle, plant hairs, and glands. The epidermis is the main surface tissue of young plants and the covering material of all leaves. Usually the epidermis is one cell deep; its cells have thick outer and side walls. The epidermis in aerial parts of the plant gives rise to plant hairs, spines, and glands. In leaves, the epidermis develops guard cells that regulate the size of pores or stomata, which allows the exchange of gases with the atmosphere. The epidermis of roots lacks the waxy cuticle found in the parts of the plant above ground, allowing the root epidermis to absorb water from the soil. Root hairs that increase the absorbing surface of the root arise from epidermal tissue. When a plant grows in diameter, the epidermis is replaced by the periderm, in the stem and the roots.

The periderm contains cork cells whose walls after cell death provide a protective waterproof outer covering for plants making up the bark of older trees.

THE INTEGUMENTARY SYSTEM STRUCTURE

The skin is composed of three major parts, these are:

An illustration of the skin

1. Epidermis - The epidermis is composed of stratified squamous epithelium and is separated from the dermis by a thin basement membrane. The epidermis is not as thick as the dermis and varies in thickness from approximately 0.3 mm on the eyelids to 1.5 mm on the palms of the hands and soles of the feet.

The epidermis has several layers that contain four cell types. These layers (from the deepest to the most superficial) are the:

o Stratum basale - is the deepest layer of the epidermis (closest to the dermis). It consists of a single layer of columnar or cuboidal cells which rest on the basement membrane. Basal cells are the stem cells of the epidermis. Their mitotic activity replenishes the cells in more superficial layers as these are eventually shed from the epidermis.

o Stratum spinosum – consists of spiny prickle cells that interlock to support the skin.

o Stratum granulosum – the thin middle layer which initiates keratinization (production of keratin).

o Stratum lucidum - consists of several layers of flattened dead cells. This thick layer appears only in frequently used areas such as the palms of the hands and soles of the feet.

o Stratum corneum – this outermost layer is thick with rows of dead cells. These cells contain soft keratin, which keeps the skin elastic and protects underlying cells from drying out.

An illustration of the different layers of the epidermis

The cell types are the following:

Keratinocytes- Comprise approximately 90% of all epidermal cells. These cells produce a protein mixture known as keratin which helps waterproof and protect the skin.

Melanocytes- Comprise approximately 8% of all epidermal cells. These cells produce a group of pigments known as melanin which are responsible for skin, hair and eye color.

Langerhans cells- These cells arise from the bone marrow and migrate to the epidermis. These cells play an important role in the immune response. This helps the immune system by processing antigens.

Merkel cells- These cells are located in the deepest regions of the epidermis and are associated with sensory neurons and are thought to function in the sensation of touch. This is involved with touch reception.

An illustration of the different cell types found in the epidermis

2. Dermis – this is known as the “true skin,” this is the layer beneath the epidermis. Its major parts are collagen (a protein that adds strength), reticular fibers (thin protein that add support), and elastic fibers (a protein that adds flexibility).

The dermis can be divided into two layers, these are the:

o Reticular layer – this contains criss-crossing collagen fibers that form a strong elastic network. This network forms a pattern called cleavage lines or the Langer’s lines. The reticular layer also contains Pacinian corpuscles, sensory receptors for deep pressure. This layer contains sweat glands, lymph vessels, smooth muscle, and hair follicles.

o Papillary layer - this layer lies directly beneath the epidermis and connects to it through papillae (finger-like projections). Some papillae contain capillaries that nourish the epidermis; others contain Meissner’s corpuscles, sensory touch receptors.

The two layers of the dermis

Corpuscles associated with the two layers of the dermis

3. Hypodermis - The hypodermis which is also known as the subcutaneous tissue attaches the skin to underlying bones and muscles and also supplies it with blood vessels and nerves. The hypodermis consists mostly of connective tissue and adipose cells. As much as one half of the body's stored fat is located in the hypodermis

The human integumentary system also includes hairs, nails, and glands.

HAIR

The hair develops from the embryonic epidermis and its primary function is protection.

Illustration of the hair structure

A hair is divided into the shaft which is the aspect of the hair that protrudes above the surface of the skin and the root which is the aspect of the hair that is beneath the surface of the skin. The base of the root is expanded and is known as the hair bulb. A hair is composed of numerous columns of dead yet keratinized cells held tightly together in three concentric layers known as the medulla, cortex and cuticle.

NAILS

Nails are plates of hard, tightly packed keratinized cells of epidermis. Nails allow us to grasp and manipulate small objects. This is composed of three principal parts:

o Nail body- The visible portion of the nail.

o Free edge- The aspect of the nail that may extend past the distal end of the digit.

o Nail root- The aspect of the nail that is buried underneath a fold of skin.

Illustration of the nail structure

Other structures associated with the nail include:

o Lunula- The whitish semilunar area of the proximal end of the nail body.

o Eponychium- Also known as the cuticle. This is a narrow band of epidermis which extends from the lateral border of the nail wall.

o Nail matrix- Epithelial tissue deep to the nail root where actual nail growth occurs.

GLANDS

The two major glands of the integumentary system are the sebaceous and sweat glands.

o Sebaceous glands - Sebaceous glands are located in the dermis and are usually connected to hair follicles. These glands produce an oily, white substance known

as sebum which oils the hair and skin and thus prevents drying and also provides protection against some bacteria.

The sebaceous gland structure

o Sweat glands – are divided into two types, the eccrine and apocrine sweat glands. Eccrine sweat glands also known as merocrine sweat glands, are the most common type of sweat glands. These sweat glands are composed of simple coiled tubular glands that open directly onto the surface of the skin through sweat pores. These glands are most numerous on the palms of the hands & the soles of the feet. The apocrine sweat glands are composed of compound coiled tubular glands that usually open into hair follicles superficial to the opening of sebaceous glands.

The sweat gland structure

There are several types of cutaneous glands including:

Sweat glands (also known as sudoriferous glands) - excrete sweat to regulate temperature Sebaceous glands - oil-producing glands that keep skin and hair moist and soft Ceruminous glands - glands of the ear canal that produce earwax

Mammary glands - milk-producing glands located in the breasts. Uropygial or preening glands of birds repel water from feathers Poison glands of frogs Mucous glands of fish and amphibians, which help prevent infection Lacrimal glands produce tears to moisten the eyes Scent glands in the anal or genital area may attract mates or repel attackers