The Integument and Related Structures

Can you list all five layers of the epidermis? What is happening in each layer?

The deepest layer of the epidermis is the basal layer, called the stratum basale. This layer consists of a single layer of columnar to cuboidal cells. These cells are actively dividing, giving birth to the cells that are pushed upward to form the upper layers of the epidermis

How is the skin of hairy animals different from that of humans?

This process is called keratinization, and it enables millions of dead cells to rub off or exfoliate daily at no expense to the health of the animal. Remarkably, an entirely new epidermis forms in humans every 7 to 8 weeks.

How does hair form and grow?

2. The part of hair visible above the skin is called the shaft, and the portion buried within the skin is called the root. A web of sensory nerve endings called the root hair plexus envelops the root, making it an important touch receptor when the hair is bent. Hair is anchored by the hair follicle. The follicle is an invagination of the epidermis that extends from the skin surface to the dermis or, occasionally, the hypodermis. The deepest part of the hair follicle expands to form a hair bulb. At the base of the bulb is a mound of dermal cells called the papilla. The papilla is covered with rapidly dividing epithelial cells called the matrix. These cells are nourished by blood flow from vessels in the underlying papilla, which stimulates much cell division and growth. As cells divide, older cells are pushed upward into the follicle tunnel away from the papilla. These cells become keratinized, and as they lose contact with the nutrition provided by the papilla, they die and become part of the developing hair. In this way, hair is constructed from dead epithelial cells. The wall of a hair follicle is composed of three layers: an internal epithelial root sheath, an outer epithelial root sheath, and a dermal or connective tissue root sheath. Animals with fur often have compound follicles in which multiple hair strands emerge from a single epidermal orifice, or pore, although each strand has its own follicle and bulb. As many as 15 hairs may be associated with one pore. Usually in compound follicles a single long primary hair (also known as a guard hair or cover hair) is surrounded by shorter secondary hairs called satellite hairs. In dogs, usually three compound follicles are grouped together to emerge from the same epidermal fold. Hair is formed in three concentric layers. The innermost layer (and central core) is called the medulla. It is composed of two to three layers of loosely arranged cells that are separated by spaces filled with liquid or air. The cells themselves contain flexible, soft keratin similar to that found in the stratum corneum of the epidermis. Surrounding the medulla is the cortex. Unlike the flexible medulla, the cortex is stiff and rigid because it is composed of hard keratin and is the thickest of the three layers. A single layer of cells arising from the edge of the papilla form the hair surface, the cuticle, which is also composed of hard keratin. The cells of the cuticle are layered like shingles on a roof, which prevents the hairs from sticking together and forming mats. In some animals such as sheep, however, the edges of the cells in the cuticle are raised, enabling them to grab onto the cuticle cells from other hair strands.

What are the three cycles of hair growth?

Anlagen, catagen, and telogen (A.C.T)

What is keratinization and why is it an important process?

Once alive, in histologically deeper layers and in earlier stages of development, these cells gave up vital organelles and nuclei to make room for the tough, protective substance called keratin. It is during this process, called keratinization, that the cells expire and in doing so form the vital protective barrier that helps enable an animal's survival.

Why does hair turn gray and then white as animals age?

As animals age, melanin production decreases, and the hair begins to turn gray. White hair is formed when the cortex loses its pigment entirely and the medulla becomes completely filled with air.

Name two types of sweat gland. How are they different from one another?

Eccrine gland consists of a simple coiled tube located in the dermis or hypodermis. It is connected to the surface of the skin by a long duct. In dogs, eccrine sweat glands are found only in the deep layers of fat and in the connective tissue of footpads. apocrine sweat glands have a coiled, excretory portion buried in the dermis or hypodermis with a single excretory duct. However, unlike eccrine sweat glands, apocrine glands empty into hair follicles, rather than onto the surface of the skin. In dogs, apocrine glands are located in the external ear canal. Interestingly, dogs with long hair have more sebaceous and apocrine glands in their external ear canals than do dogs with short hair. Dogs with more hair in their ear canals have an increased incidence of otitis externa, which is an infection of the ear canal.

Why is skin important? Can you think of six important functions of skin?

It protects against excessively humid or dry conditions. It protects against environmental chemicals. It protects against infectious organisms, such as bacteria, viruses, fungi, and parasites. It helps regulate body temperature.

What causes pigmentation of skin?

Pigmentation is caused by the presence or absence of melanin granules in the armlike extensions of the melanocytes. Microscopically, no pigmentation is apparent if the granules are concentrated around the nucleus in the cell body of the melanocyte. As the granules move into the cellular arms and into the surrounding tissue, however, pigmentation becomes apparent. The more granules that exist in the arms of the melanocyte and surrounding tissue, the darker the pigmentation. The dispersion of granules is controlled by the release of melanocyte-stimulating hormone, which, in turn, is controlled by the intermediate lobe of the hypophysis. The melanosomes are transported to the tips of the cellular projections, where they are released into the intracellular space and ultimately absorbed by keratinocytes, which arrange the melanin on the side of the cell with the greatest amount of sun exposure. In this way, pigment acts to protect cells from the damaging effects of ultraviolet rays.

Draw and label the parts of a hair follicle.

The labeled drawing should show hair bulb, papilla, and matrix.

How are paw pads and the planum nasale different from other regions of skin?

The pad's outer surface is the toughest and thickest skin on the body. It is often pigmented and is composed of all five epidermal layers. Of these five layers, the outermost epidermal layer, the stratum corneum, is thicker than all of the others combined. The insulating fat and tough outer skin form a protective barrier against abrasion and thermal variation, enabling the animal to walk on rough surfaces, hot roads, and cold snow. The surface of the pad feels rough, and an uneven surface is visible with the naked eye. Although usually pigmented and appearing as a tough, thick region of integument, the planum nasale in dogs is composed of only three epidermal layers; the stratum lucidum and stratum granulosum are not present. The outermost layer, the stratum corneum, is composed of only four to eight cell layers, which is surprisingly thin considering the exposed location of the nose and its heavy use.

How is the dermis different from the epidermis?

Unlike the epidermis, which is primarily cellular, the dermis is highly fibrous. It is composed of dense, irregular connective tissue that contains collagen, elastic, and reticular fibers. Hair follicles, nerve endings, glands, smooth muscle, blood vessels, and lymphatics are all found in the dermis as well, creating a rich and interesting tissue community. Fibroblasts, adipocytes, and macrophages also are present and represent the most commonly found cellular elements. The dermis is a tough layer that binds the superficial epidermis to the underlying tissues. It represents the "hide" of the animal and is used to make leather.

What factors stimulate contraction of the arrector pili muscle? Why is this muscle important?

When frightened or cold, animals can make their hair stand up beyond the normal implantation angle. This is due to the presence of a small, smooth muscle called the arrector pili muscle, which is attached to each hair follicle and is innervated by the sympathetic nervous system. When the muscle contracts, it pulls the hair to an erect position. Perhaps you have seen a frightened cat "puff up." This reaction is a defense mechanism designed to make the animal appear bigger and therefore less vulnerable to potential predators. In addition, hair that stands erect can better trap insulating layers of air than nonerect hair. So animals with erect 162hair coats stay warmer than animals with flat coats. In humans, contraction of the arrector pili muscles causes goose bumps. The arrector pili muscle also is responsible for forcing sebum from the sebaceous gland, which helps keep the integument moist and supple.

Where are anal sacs found and what is their importance to animals?

nal sacs and other related musk glands are famous for their powerful, foul-smelling secretions. Although skunks are shy and not often seen in the wild, it is not uncommon to catch the noxious odor of a skunk's spray from our car as we drive down a suburban or rural road. The odor can linger in the region for days. Cats and dogs have anal sacs similar to musk glands that are located at the 5 and 7 o'clock positions relative to the anus. They are connected to the lateral margin of the anus by a small, single duct. The anal sac is lined with sebaceous and apocrine glands and acts as a reservoir for the secretions that are produced from these glands. When the animal defecates or becomes frightened, some or all of the anal sac contents are expressed, feces become coated with the secretions stored in the anal sac, and the unique smell of the animal is transferred to the environment. Thus defecation serves the purposes of elimination, marking territory, and attracting a mate. Sometimes the small duct of the anal sac clogs and can become infected if left untreated. Animals with irritated or impacted anal sacs often drag their rumps along the ground to help alleviate the discomfort.