Chapter 4: Histology

Q: What is another name for the skin?

A: The skin is also called the cutaneous membrane.

Q: What are the two main layers of the skin?

A: The two main layers of the skin are the epidermis and the dermis.

Q: What is the hypodermis?

A: The hypodermis, or subcutaneous tissue, is a layer of supporting adipose tissue found under the dermis.

Q: Is the hypodermis considered a layer of the skin?

A: No, the hypodermis is not technically a layer of the skin, but it plays a key supporting role.

Q: What accessory structures are found in the skin?

A: The skin's accessory structures include hair, nails, and exocrine glands.

Q: What are the six main functions of the integumentary system?

A: Its main functions are protection, sensation, Vitamin D production, immunity, maintenance of body temperature, and excretion.

Q: How does the skin protect the body?

A: The skin protects us from pathogens, ultraviolet (UV) radiation, chemical hazards, mechanical damage, and dehydration.

Q: What can the sensory receptors in the skin detect?

A: They can detect pain, pressure, vibrations, and temperature changes.

Q: How does the skin produce Vitamin D?

A: Vitamin D synthesis begins in the skin in response to UV radiation. This vitamin is important for calcium absorption.

Q: Besides acting as a physical barrier, how does the skin provide immunity?

A: The skin contains antimicrobial proteins and immune system cells that ward off microorganisms.

Q: How does the integumentary system regulate body temperature?

A: It regulates temperature through changes in blood flow to the skin and by secreting sweat.

Q: What does the skin excrete through sweat?

A: The skin helps regulate fluid, electrolytes, and waste products in the blood through sweat production.

Q: What is the epidermis?

A: The epidermis is the uppermost layer of the skin.

Q: How is the epidermis classified as a tissue?

A: It is classified as a stratified squamous keratinized epithelial tissue.

Q: What is the main cell type of the epidermis?

A: The main cell type is the keratinocyte.

Q: What do keratinocytes produce?

A: They produce keratin intermediate filaments, which provide structure to the skin, hair, and nails.

Q: What are the functions of keratin?

A: Keratin is a water-proofing protein that provides most of the protective function of the skin.

Q: What is the main difference between keratinized and non-keratinized epithelial tissues?

A: Keratinized tissues are dry (like the surface of the skin), while non-keratinized tissues must be kept moist (like the inside of the lips and mouth).

Q: What are melanocytes?

A: They are a type of cell in the epidermis that produces the pigment melanin.

Q: What are the two main functions of melanin?

A: Melanin gives skin and hair its color and protects the skin from harmful UV radiation.

Q: What are the two types of melanin?

A: The two types of melanin are eumelanin and pheomelanin. Differences in these pigments create the variations we see in skin and hair color.

Q: How many layers does the epidermis have?

A: The epidermis has four layers in thin skin (most of the body) and five layers in thick skin (palms of the hand and soles of the feet). These layers are also called "strata."

Q: What is the deepest layer of the epidermis?

A: The deepest layer is the stratum basale (or stratum germinativum), located at the junction between the epidermis and the dermis.

Q: What is the stratum basale made of?

A: It consists of a single layer of newly produced, cube-shaped keratinocytes attached to a basement membrane. Melanocytes are also found in this layer.

Q: Why does cell division only happen in the stratum basale?

A: The epidermis is avascular (lacks blood vessels), and this layer is closest to the capillaries of the dermis, which supply the necessary oxygen and nutrients for cells to divide.

Q: What are epidermal ridges and what is their function?

A: Epidermal ridges are extensions from the stratum basale into the dermis. On our fingers and toes, they form fingerprints, which increase grip and friction.

Q: What is the second layer of the epidermis called?

A: The second layer is the stratum spinosum, also known as the prickly layer. It is one of the thicker layers, with eight to ten rows of keratinocytes.

Q: Why does the stratum spinosum have a "prickly" appearance?

A: When the tissue is prepared for viewing under a microscope, the cells shrink, but their connecting junctions (desmosomes) remain, giving the layer a spiny look.

Q: What is the third layer of the epidermis?

A: The third layer is the stratum granulosum, or granular layer. It consists of three to five rows of keratinocytes.

Q: Why is it called the "granular layer"?

A: The keratinocytes in this layer produce large amounts of keratohyalin proteins, which look like granules. As cells move through this layer, they flatten and die.

Q: What is the fourth layer of the epidermis and where is it found?

A: The fourth layer is the stratum lucidum (clear layer), and it is found only in thick skin.

Q: What gives the stratum lucidum its clear appearance?

A: It appears clear under a microscope because of a protein called eleidin. This extra layer helps reduce the effects of friction.

Q: What is the most superficial layer of the epidermis?

A: The most superficial (topmost) layer is the stratum corneum, also known as the horny layer.

Q: What is the stratum corneum made of?

A: It consists of 15-30 layers of dead keratinocytes that are full of keratin and surrounded by a lipid coating that makes the skin water-resistant.

Q: What is desquamation?

A: Desquamation is the process where dead keratinocytes are gradually sloughed off from the surface of the stratum corneum. (These cells make up about 50% of household dust!)

Q: How does the epidermis protect against foreign invaders?

A: The epidermis is our first line of defense. Its dry surface is a poor environment for the growth of microorganisms.

Q: What are Langerhans cells?

A: They are specialized immune cells found in all layers of the epidermis (except the stratum corneum) that act as a first line of defense against foreign antigens, including those on skin cancer cells.

Q: What is the most prominent contributor to skin pigmentation?

A: The most prominent contributor is melanin.

Q: Where do melanocytes originate?

A: Melanocytes are derived from neural crest cells and migrate to the epidermis during development.

Q: What is an epidermal-melanin unit?

A: This is the relationship formed when a single melanocyte connects with several neighboring keratinocytes.

Q: Where is melanin produced?

A: Melanin is produced by melanocytes inside an organelle called a melanosome.

Q: How does melanin from a melanocyte get to a keratinocyte?

A: When stimulated by a keratinocyte, the melanocyte transports the melanosome (containing the melanin) to the keratinocyte, which contributes to skin pigmentation.

Q: Is the number of melanocytes different between people with dark and fair skin?

A: No, the number of melanocytes is the same. The difference in skin color comes from the quantity and quality of the melanosomes they produce.

Q: What are some factors that determine a person's skin pigmentation?

A: These include the type of melanin produced, the number and size of melanosomes, and how long the melanosomes last in the keratinocytes before being destroyed.

Q: What is the primary function of melanin besides providing color?

A: Melanin's other key function is to protect us from ultraviolet (UV) radiation from the sun.

Q: How does melanin protect the skin's cells?

A: When stimulated by sun exposure, melanocytes produce more melanin. The melanosomes then form a protective cap around the nucleus of the keratinocytes to shield them from UV light.

Q: What are the negative long-term effects of sun exposure?

A: Cumulative sun exposure can damage the dermis, leading to wrinkles, and can cause various forms of skin cancer, including melanoma.

Q: What structures in the epidermis are responsible for sensing light touch?

A: Tactile discs, which are formed from Merkel cells and their associated nerve endings in the stratum basale, are responsible for sensing light touch.

Q: What is the function of tactile discs?

A: They function as mechanoreceptors that sense and respond to fine touch and pressure, helping to determine an object's shape and texture. They are found in sensitive areas like the fingertips.

Q: What are the free nerve endings in the epidermis responsible for?

A: They are responsible for the sensation of pain.

Q: What other sensations are associated with free nerve endings?

A: Free nerve endings are also associated with the sensations of tickling and itching. Itch can be stimulated by chemical, mechanical, or thermal factors.

Q: Why is some UV exposure from the sun important?

A: UV exposure is necessary for the synthesis of vitamin D3.

Q: What is the first step of vitamin D synthesis in the skin?

A: When exposed to UV rays, cells in the epidermis convert a compound called 7-dehydrocholesterol into cholecalciferol, which is vitamin D3.

Q: What happens to vitamin D3 after it's made in the skin?

A: The liver converts it into calcifediol, and then the kidneys convert that into calcitriol, the active form of vitamin D.

Q: What does calcitriol do in the body?

A: Calcitriol is a hormone that helps regulate calcium and phosphorus levels in the blood. It significantly increases the absorption of calcium from the small intestine.

Q: What is the dermis?

A: The dermis is the second tissue layer of the skin, located just below the epidermis.

Q: What are the two layers of the dermis?

A: The superficial (top) layer is the papillary layer, and the deeper layer is the reticular layer.

Q: What is the papillary layer made of?

A: It consists of loose connective tissue made of fine collagen and elastic fibers.

Q: How do the epidermis and dermis connect?

A: They connect through the interaction of the epidermal ridges (from the epidermis) and the dermal papillae (from the dermis).

Q: Why is the connection between the epidermis and dermis stronger in thick skin?

A: Thick skin has more numerous and deeper ridges and papillae, which increases the surface area for a stronger connection that can resist pressure and friction.

Q: What are the main functions of the papillary layer?

A: Its main functions are to support the epidermis and to regulate body temperature by controlling blood flow through its vessels.

Q: How does the papillary layer help regulate body temperature?

A: When the air is cool, its blood vessels constrict to reduce blood flow and conserve heat. When the air is warm, the vessels dilate to increase blood flow and dissipate excess heat, like a radiator.

Q: What is the reticular layer and what is it made of?

A: The reticular layer is the toughest layer of the skin, composed of densely packed collagen fibers with some elastic fibers mixed in. Processed animal leather comes from this layer.

Q: What are cleavage lines?

A: They are lines formed by the natural orientation of the thick collagen bundles within the reticular layer.

Q: Why are cleavage lines important in surgery?

A: Incisions made parallel to cleavage lines tend to heal with minimal scarring. Incisions made perpendicular to the lines can gape open and are more likely to form a noticeable scar.

Q: How does the dermis repair itself after being wounded?

A: The dermis doesn't constantly regenerate. When a wound occurs, cells called fibroblasts produce large amounts of connective tissue fibers to repair the damage. Scar tissue is made of fibers that are not replaced by normal tissue during the healing process.

Q: What causes stretch marks?

A: Stretch marks are caused when the skin is stretched beyond the limits of its elastic fibers, causing them to tear. This is common during pregnancy or periods of extreme weight gain.

Q: What accessory structures are found in the reticular layer?

A: This layer contains hair follicles, nails, and glands.

Q: How do the vessels in the dermis support the epidermis?

A: Blood vessels provide oxygen and nutrients, while lymphatic vessels maintain fluid balance and provide a path for immune cells.

Q: What are the three main sensory receptors in the dermis?

A: They are Meissner's corpuscles, Pacinian corpuscles, and Ruffini corpuscles.

Q: What do Meissner's (tactile) corpuscles detect?

A: Located in the dermal papillae, they are receptive to light touch, pressure, and vibration. They are very important for learning to read Braille.

Q: What is the function of Pacinian and Ruffini corpuscles?

A: Pacinian corpuscles detect deep pressure and vibration. Ruffini corpuscles detect pressure and distortion of the skin. Both are found in the reticular layer.

Q: Is the hypodermis part of the skin (integumentary system)?

A: No, the hypodermis is not technically part of the skin. It is also known as the subcutaneous layer and it lies beneath the dermis.

Q: What are the main functions of the hypodermis?

A: It connects the skin to the underlying tissues and serves as an important energy reserve.

Q: What are the main cell types of the hypodermis?

A: The main cell types are fibroblasts (which produce collagen and elastic fibers) and adipocytes (which store fat).

Q: What is subcutaneous fat and what does it do?

A: It is the adipose tissue in the hypodermis made of adipocytes. It serves as padding to protect muscle and bone, provides an energy reserve, and helps regulate body temperature.

Q: Besides the main layers of skin, what are the accessory structures of the integumentary system?

A: The accessory structures include hair, nails, and various glands.

Q: What are the two main functions of hair?

A: Hair has important protective and sensory functions.

Q: How does hair protect the body?

A: The hair of the eyebrows, eyelashes, nose, and ears protects against foreign substances. The hair on your head protects you from UV radiation and helps insulate your head.

Q: How does hair act as a sensory organ?

A: You can feel even the slightest movement of hair, such as when an insect is crawling on your arm or when the wind blows.

Q: Where does hair grow from?

A: Hair is a "dead" structure filled with keratin that grows from a hair follicle.

Q: What is the hair papilla?

A: Located at the base of the hair follicle, the hair papilla is a small area of connective tissue with blood vessels and nerves from which a new hair is formed.

Q: What part of the hair follicle is actually responsible for producing the hair?

A: A layer within the hair papilla called the hair matrix is responsible for producing hair through cell division. The cells it produces make up the hair shaft and are filled with a "hard" keratin protein.

Q: What are the three layers of the hair shaft?

A: The hair shaft consists of three layers: an inner medulla, a middle cortex (which makes up most of the hair), and an outer cuticle.

Q: What gives hair its strength and flexibility?

A: The medulla contains soft keratin, while the cortex and cuticle contain hard keratin. This mixture of different keratin proteins gives hair its strength and flexibility.

Q: What determines a person's hair color?

A: Melanocytes in the hair papilla produce melanin, which is then transferred to the cells of the hair matrix. The type and amount of melanin create the hair's color.

Q: Why does hair turn gray with age?

A: As we age, the melanocytes in the hair papilla stop producing as much melanin. Gray hair results from this lack of pigment.

Q: How do hair dyes work?

A: Chemicals in hair dyes disrupt the hair's outer cuticle to deposit pigment molecules into the inner medulla and cortex. This process damages the hair, often leaving it dry and brittle.

Q: What are the three phases of the hair growth cycle?

A: The three phases are:

1. Anagen: The active phase of hair growth.

2. Catagen: The end of the active phase.

3. Telogen: A resting period for the follicle.

Q: How is hair naturally shed?

A: During the telogen (resting) phase, the hair is known as a club hair. When the follicle becomes active again, the new hair pushes the old club hair out, causing it to be shed. It's normal to lose up to 100 hairs per day.

Q: What can cause excessive hair loss?

A: Factors like stress, hormonal changes, medications, and dietary changes can all affect the activity of hair follicles and cause excessive hair loss.

Q: What is the arrector pili muscle?

A: It is a small piece of smooth muscle attached to each hair follicle.

Q: What happens when the arrector pili muscle contracts?

A: It pulls the hair follicle upright, causing the formation of "goose bumps" on the surface of the skin. goosebumps are often a response to cold temperatures.

Q: What is the function of the arrector pili muscle in humans?

A: Although it's involved in the process of getting goose bumps, its role in regulating body temperature is not as important in humans as it is for other mammals with fur. It is largely considered a vestigial organ but may help maintain the structure of the hair follicle.

Q: What are nails made of and what is their function?

A: A nail (also called unguis) is composed of packed cells containing large amounts of hard keratin. Their main function is to protect the tips of our fingers and toes. 💅

Q: What are the three main parts of a nail?

A: The three parts are the nail plate, the nail matrix, and the nail bed.

Q: What part of the nail is responsible for producing the nail plate?

A: The nail matrix, which is a fold of the epidermis located at the base of the nail, is responsible for producing the nail.

Q: What is the lunula?

A: The lunula is the small, white, half-moon shaped structure at the base of the nail. It is the only visible part of the nail matrix.

Q: What gives the nail plate its pink color?

A: The nail bed, which lies beneath the nail plate, contains a network of capillaries that gives the nail its pink appearance.

Q: Can over-the-counter remedies speed up nail growth?

A: No, special vitamins, creams, or polishes cannot increase the speed at which nails grow, although diet can determine the quality of the nail.