Lab 6: *The Skin (Integumentary System)*

Keratinocytes

(Literally keratin cells): The most abundant epidermal cells. They function to produce *keratin*, a tough fibrous protein that gives the epidermis durability and protective capabilities.

Two-Point Threshold

*The measurement of the smallest distance at which two points of contact can be felt.* The density of the touch receptors varies significantly in different areas of the body. In general, areas that have the greatest density of tactile receptors have a high ended ability to "feel". These areas correspond to areas that receive the greatest motor innervation; thus, they are also typically areas of fine motor control.

Activity 4: Determining the Two-Point Threshold

- It would be reasonable to predict the fingertips and lips would have the greatest density of touch receptors.

Cells of the Epidermis

- Keratinocytes - Melanocytes - Dendritic Cells - Tactile (Merkel) Cells

Activity 5: Testing Tactile Localization

- The ability to locate the stimulus should not improve with repeated trials because the receptor density remains unchanged. - Fingertips have the smallest error of localization because they have the highest density of touch receptors per area. Therefore, they are most sensitive.

Activity 2: Visualizing Changes in Skin Colour Due to Continuous External Pressure

- The colour of compressed skin will be pale - The reason for the colour change is obstruction of the capillary flow to the area. - If the pressure is maintained long enough, the skin under pressure will obtain a bluish colour. This condition is called cyanosis and is due to the presence of deoxygenated blood because blood flow has been blocked.

Stratum Granulosum (Granular Layer)

A thin layer named for the abundant granules its cells contain. These cells are (1) *lamellar granules*, which contain a waterproofing glycolipid that is secreted into the extracellular space; and (2) *keratohyaline granules*, which help to form keratin in the more superficial layers. At the upper border of this layer, the cells are beginning to die.

Black Heads

Accumulations of dried sebum and bacteria

Hair Follicle

Formed from both epidermal and dermal cells. Its external and internal epithelial root sheaths are enclosed in a thickened basement membrane, the glassy membrane, and by a peripheral connective tissue (or fibrous) sheath, which is essentially dermal tissue.

Apocrine Sweat Glands

Found chiefly in the axillary and genital areas, secrete a milky protein- and fat-rich substance (also containing water, salts, and urea) that is an excellent source of nutrients for the bacteria typically found on the skin.

Layers of the Epidermis

From deep to superficial, the layers of the epidermis of thick skin are the *stratum basale, stratum spinosum, stratum granulosum, stratum lucidum, and stratum corneum*. This order of the epidermal layers represents the progression that skin cells take as they age, from the young epidermal cells in the stratum basale to the dead cells in the stratum corneum. This represents a journey that takes approximately 25-45 days.

Hair

Hairs are found over the entire body surface, except for the palms of the hands, the soles of the feet, parts of the external genitalia, the nipples, and the lips. A hair, enclosed in a hair follicle, is also an epithelial structure. A hair consists of a central medulla surrounded first by the cortex and then by a protective cuticle.

Arrector Pili Muscle

If you look carefully at the structure of the hair follicle, you will see that it generally is in a slanted position. Small bands of smooth muscle cells- *arrector pili*- connect each hair follicle to the dermis. When these muscles contract (during cold or fright), the hair follicle is pulled upright, dimpling the surface with "goose bumps".

Adaption

In many cases, when a stimulus is applied for a prolonged period, the rate of receptor response slows and conscious awareness of the stimulus declines or is lost until some type of stimulus change occurs. The phenomenon is referred to as *adaption*. The touch receptors adapt particularity rapidly, which is highly desirable.

Tactile (Merkel) Cells

When combined with sensory nerve endings, these cells form sensitive touch receptors called *Tactile* or *Merkel Discs* located at the epidermal-dermal junction.

What are the four protective functions of the skin?

Protection from mechanical damage, chemical damage, thermal damage, and bacterial invasion.

Stratum Spinosum (Spiny Layer)

Several layers of cells that contain thick, weblike bundles of intermediate filaments made of pre-keratin protein. The cells in this layer appear spiky because when the tissue is prepared, the cells shrink, but their desmosomes hold tight to adjacent cells. Cells in this layer and the basal layer are the only ones to receive adequate nourishment from diffusion of nutrients from the dermis.

Plotting the Distribution of Sweat Glands: Which other body areas would, if tested, prove to have a high density of sweat glands?

Soles of feet, underarms, forehead

Melanocytes

Spidery black cells that produce the reddish yellow to brownish black pigments collectively called *melanin*. Melanin provides a protective pigment umbrella that shields the nuclei of deeper epidermal layers from the damaging effects of ultraviolet radiation in sunlight.

Plotting the Distribution of Sweat Glands: With what substance in the bond paper does the iodine painted on the skin react?

Starch

Epidermis

Structurally, the avascular epidermis is *keratinized stratified squamous epithelium* consisting of four distinct cell types and four or five distinct layers.

Tactile Localization

The ability to determine which portion of the skin has been touched. The tactile receptor field of the body periphery has a corresponding "touch" field in the brain. Some body areas are well represented with touch receptors, and tactile stimuli can be localized with great accuracy, but density of touch receptors in other body areas allows only crude discrimination.

Accessory Organs of the Skin

The accessory organs of the skin: *hair, nails, and cutaneous glands*- all derive from the epidermis, but they reside in the dermis. They originate in the stratum basal and grow downward into the deeper skin regions.

Hair Bulb

The base of the hair follicle where hair is formed by mitosis of the *germinal epithelial cells*. As the daughter cells are pushed away from the growing region, they become more keratinized and die; thus the bulk of the hair shaft, like the bulk of the epidermis, is dead material.

Reticular Layer

The deepest skin layer. It contains blood vessels, sweat and sebaceous glands, and pressure receptors (lamellar corpuscles).

Dermis

The dense irregular connective tissue making up the bulk of the dermis has two principal regions: the *papillary* and *reticular* areas. *Collagenic* and *elastic fibres* are found throughout the dermis. The collagenic fibres make the dermis tough and attract and hold water, thus keeping the skin hydrated (moist). The elastic fibres give skin its exceptional elasticity in youth. *Fibroblasts, adipose cells*, various types of *phagocytes* (which are important in the body's defense), and other cell types are found throughout the dermis.

Dermal Cutaneous Receptors

The dermis also has a rich nerve supply. Many of the nerve endings bear specialized receptor organs that respond to pain, pressure, or temperature extremes, and transmit messages to the central nervous system for interpretation. These receptors include free nerve endings (pain and temperature receptors), lamellar corpuscles, and hair follicle receptors.

Some injections hurt more than others. On the basis of what you have learned about skin structure, can you determine why this is so?

The dermis has a very rich nerve supply, some with nerve endings that respond to pain. If these bare nerve endings are stimulated by injection, a pain message will be transmitted to the central nervous system for interpretation.

Nails

The hornlike derivatives of the epidermis, are transparent and nearly colourless, but they appear pink because of the blood supply in the underlying dermis. The exception to this is the proximal region of the thickened nail matrix, which appears as a white present called the *lunule*. When someone is *cyanotic* because of a lack of oxygen in the blood, the nail beds take on a blue cast. Nails consist of a *free edge*, a *body* (visible attached portion), and a *root* (embedded in the skin and adhering to an epithelial *nail bed*). The borders of the nail are overlapped by skin folds called *nail folds*.

Stratum Corneum (Horny Layer)

The outermost layer consisting of 20-30 layers of dead, scalelike keratinocytes. They are constantly being replaced by the division of the deeper cells.

Activity 7: Plotting the Distribution of Sweat Glands

The palm has a greater density of sweat glands than the forearm.

Hair Root

The part of the hair enclosed within the follicle

Hair Shaft

The portion of the hair projecting from the skin

Activity 6: Demonstrating Adaption of Touch Receptors

The pressure sensations returns when coins are added to the stack. The same receptors are probably being used. Generator potentials are graded, and stronger stimuli produce larger potentials and thus increased frequency of nerve impulses.

Hyponychium

The region beneath the free edge of the nail

What did the two-point discrimination test demonstrate?

The relative density of touch receptors in various body areas (lips, fingertips, ect.)

Activity 8: Examining a Skin Slide

- The stratified squamous epithelium of the skin is composed of several recognizable layers, the outermost of which are keratinized. - Both types of epithelia are protective, but their skin epithelium also protects against water loss to the external environment, UV damage, and chemical damage in addition to protecting against mechanical damage and bacterial invasion.

How does the skin help to regulate body temperature? (Describe two different mechanisms)

1. Capillaries in the papillary layer of the dermis allow heat to radiate to the skin surface to cool off the body and will constrict blood flow to the dermis temporarily when body heat needs to be conserved. 2. Sweat glands secrete perspiration that evaporates and carries large amounts of body heat with it.

Hair Matrix

A layer of actively dividing epithelial cells located on top of the hair papilla.

Stratum Basale (Basal Layer)

A single row of cells immediately above the dermis. Its cells are constantly undergoing mitosis to form new cells, hence its alternate name, *stratum germinativum*. Some 10-25% of the cells in this layer are melanocytes, which thread their processes through this and adjacent layers of keratinocytes. Occasional tactile cells are also present in this layer.

Hair Papilla

A small nipple of dermal tissue that protrudes into the hair bulb and provides nutrition to the growing hair.

Dendritic Cells

Also called *Langerhgans cells*, these cells play a role in immunity by performing phagocytosis.

Pores

Epithelial openings, called pores, are the outlets for the *sweat (sudoriferous) glands*.

Why is it advantageous to have pain receptors that are sensitive to all vigorous stimuli, whether heat, cold, or pressure?

Because all of these stimuli, if excessive, cause tissue damage

Papillary Layer

Composed of *areolar connective tissue*. It is very uneven and has fingerlike projections from its superior surface, called *dermal papillae*, which attach it to the epidermis above. In the palms of the hands and the soles of the feet, the papillae produce the fingerprints, unique patterns of loops and ridges in the epidermis. Capillaries in the papillary layer furnish nutrients for the epidermis and allow heat to radiate to the skin surface. The pain and touch receptors (tactile corpuscles) are also found here.

Cutaneous Glands

Cutaneous glands fall primarily into two categories: the sebaceous glands and the sweat glands.

Define Adaptation of Sensory Receptors

Decline in receptor sensitivity and stimulation with prolonged unchanging stimuli

Eccrine Sweat Glands

Distributed all over the body, produce clear perspiration, consisting primarily of water, salts (mostly NaCl), and urea.

Acne

Due to active infection of the sebaceous glands

The Integumentary System

Includes the skin and its accessory organs. The skin does much more than just cover the body exterior. Architecturally, the skin in a wonder. It is tough yet pliable, a characteristic that enables it to withstand constant insult from outside agents.

Imagine yourself without any cutaneous sense organs. Why might this be very dangerous?

Many external stimuli (heat, cold, pressure) that can threaten homeostasis might go undetected, and proper protective measures might not be taken.

Was organ system controls the activity of the eccrine sweat glands?

Nervous system

Pain receptors do not adapt. Why is this important?

Pain is a warning of actual or potential tissue damage

Plotting the Distribution of Sweat Glands: Which skin area (the forearm or the palm) has more sweat glands?

Palm of hand

Stratum Lucidium (Clear Layer)

Present only in thick skin. A very thin transparent band of flattened, dead keratinocytes with indistinct boundaries.

Dermal Blood Supply

The rich dermal blood supply allows the skin to play a role in regulating body temperature. When body temperature is high, the capillary network of the dermis becomes engorged with the heated blood, and body heat is allowed to radiate from the skin surface. If the environment is cool and body heat must be conserved, the dermal blood vessels constrict so that the blood bypasses the dermis temporarily.

The Skin Functions

The skin has several functions, most (but not all) concerned with protection. It insulates and cushions the underlying body tissues and protects the entire body from mechanical damage (bumps and cuts), chemical damage, thermal damage, and bacterial invasion. The hardened uppermost layer of the skin prevents water loss from the body surface. The skin has other functions, as well. The skin: - Acts as a mini excretory system; urea, salts, and water are lost when we sweat. - Performs important metabolic duties, such as producing proteins important to our immunity. - It is the site where vitamin D is synthesized for the body. Vitamin D plays a role in calcium absorption in the digestive system. - Contains the cutaneous sense organs (part of the nervous system that is in the dermis) that allows us to sense and enjoy the external environment. - Plays an important role in regulating heat loss from the body surface (a function of its abundant capillary network under the control of the nervous system).

Basic Structure of the Skin

The skin has two distinct regions: the superficial *epidermis* composed of epithelium and the underlying connective tissue *dermis*. These layers are firmly "cemented" together along a wavy basement membrane. But friction, such as the rubbing of a poorly fitted shoe, may cause them to separate, resulting in a blister. Immediately deep to the dermis is the *subcutaneous tissue,* or *hypodermis* (primarily adipose tissue), which is not considered part of the skin.

What substance is manufactured in the skin and plays a role in calcium absorption elsewhere in the body?

The skin is the site of vitamin D synthesis for the body.

Eponychium (Cuticle)

The thickened proximal nail fold.

Nail Matrix

The thickened proximal part of the nail bed containing germinal cells , are responsible for nail growth. As new cells are produced by the matrix, they become heavily keratinized and die.

Sudoriferous (Sweat) Glands

They are *exocrine glands* that are widely distributed in the skin. There are two types of sweat glands: *eccrine* and *apocrine* sweat glands. The sweat glands are controlled by the nervous system and are an important part of the body's heat-regulating apparatus. They secrete perspiration when the external temperature or body temperature is high. When this perspiration evaporates, it carries large amounts or body heat with it.

The Sebaceous (Oil) Glands

They are found nearly all over the skin, except for the palms of the hands and the soles of the feet. Their ducts usually empty into a hair follicle, but some open directly into the skin surface. The product of sebaceous glands, called *sebum*, is a mixture of oily substances and fragmented cells that acts as a natural lubricant that keeps the skin soft and moist.

Which two body areas were least sensitive to touch?

back of calf, back of neck

Which two body areas tested were most sensitive to touch?

lips and fingertips