The Integument and related Structures: Ch 5

List and describe the three types of hair

Hair has been categorized into three broad groups: primary or guard hairs, secondary or wool-type hairs, and tactile or sinus hairs. Primary hairs are generally straight or arched and are thicker and longer than secondary hairs. They are the dominant hairs in a complex hair follicle. Secondary hairs are softer and shorter than primary hairs. They are generally wavy or bristled in dogs and are the predominant hair type in species with wool-type coats. Tactile hairs are used as probes and feelers. They are well supplied with sensory endings that make them particularly sensitive to the slightest bending or touch. These hairs are commonly known as whiskers and can be found around the mouth and on the muzzle of many species, as well as mixed intermittently throughout the hair coat. The tactile hair is also called the sinus hair because of the presence of a large blood sinus, which is located in the connective tissue portion of the follicle.

Describe the parts of the hair follicle and explain how hair grows

Hair is anchored by the hair follicle, which is an invagination of the epidermis that extends from the skin surface to the dermis or, occasionally, to the hypodermis. The deepest part of the hair follicle expands to form a hair bulb. The wall of the 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. 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.

Describe the process of keratinization

Keratinocytes located along the basement membrane are well nourished by the bloody supply of the underlying dermis, therefore these cells can grow and divide. When new daughter cells are produced, they push older cells away from the life-sustaining nutrients of the dermis toward the outer layers of the epidermis. As older cells travel from the basal to the superficial layers, they undergo profound changes: they fill with keratohyaline granules; lose their nuclei, cytosol, and organelles; and ultimately become lifeless sheets of keratin. Keratinization is important because keratin is a tough, fibrous, waterproof protein that gives skin its resiliency and strength.

List the cell types that comprise the epidermis and describe the function of each cell type

Keratinocytes: produce a tough, fibrous, water-proof protein called keratin that gives skin its resiliency and strength. Melanocytes: Merkel cells: Langerhans cells:

Describe the structure and location of sebaceous glands

Sebaceous glands are generally found all over the body except in certain specialized regions, such as paw pads and the planum nasale. The glands are located in the dermis and may be simple or complex alveolar structures. Although most sebaceous glands have a single duct that empties into a hair follicle, others have ducts that empty directly into the surface of the skin. This latter group of sebaceous glands is found at the mucocutaneous junctions of the lips, labia, vulvae, penis, prepuce, anus, eyelid, and in the ear canal. The sebaceous glands associated with hair follicles are found in the triangle formed by the surface of the skin, the hair follicle, and the arrector pili muscle.

List the five layers of the epidermis

Stratum germinativum, aka stratum basale or basal layer stratum spinosum, aka spiny layer stratum granulosum, aka granular layer stratum lucidum, aka clear layer stratum corneum, aka horny layer

List the structures that comprise the dermis and describe the function of each

The dermis is composed of two layers: the thin, superficial papillary layer and the thick, deep reticular layer. The papillary layer lies just beneath the epithelial layer of the epidermis and is composed of loose connective tissue with loosely woven fibers and ground substance. The deeper, reticular layer, which consists of dense irregular connective tissue, accounts for 80% of the dermis. It is intimately associated with the papillary layer by collagen fibers.

List the structures that comprise the hypodermis

The hypodermis is a thick layer that resides below the dermis. It is a loose layer of areolar tissue that is rich with adipose, blood, and lymphatic vessels, and nerves. The hypodermis is important because it permits the skin to move freely over underlying bone and muscle without putting tension on the skin that would result in tearing.

Describe the unique features of the paw pads and planum nasale

The pad's outer surface is the toughest 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 variances, enabling the animal to walk on rough surfaces, hot roads, and cold snow. In addition to thick adipose layers, the pad is composed of exocrine sweat glands and lamellar corpuscles. The top of the nose in cats, pigs, sheep, and dogs is called the planum nasale. Like paw pads, the planum nasale represents an unusual form of skin. 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 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, particularly in dogs.

Differentiate between eccrine and apocrine sweat glands

Two types of sweat glands are eccrine and apocrine. The excretory portion of the eccrine gland consists of a simple coiled tube located in the dermis or hypodermis and connected to the surface of the skin by a long duct. In dogs, eccrine sweat glands are found only in the deep layers of the fat and connective tissue of footpads. Like eccrine sweat glands, 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 the dog, apocrine glands are located in the external ear canal.