Mod 13 (Section I): Lesson 1

trich/o and pil/o

Two combining forms trich/o and pil/o mean hair.

cutane/o, derm/o, and dermat/o

combining form for skin

arrector pili

A tiny muscle called the arrector pili muscle attaches to the base of the hair follicle at one end and to dermal tissue on the other end. In order to generate heat when the body is cold, the arrector pili muscles contract all at once, causing the hair to stand up on the skin. Contraction of the arrector pili muscles is commonly called gooseflesh, or "goose bumps."

follicle

Each fiber of hair consists of a root and a shaft; the root is embedded within a saclike structure known as the hair follicle. In a manner that parallels the development of new skin cells, hair is formed from cells at the base of the follicle that push upward to create the visible portion of the hair, the tightly woven network of horny cells that is the hair shaft.

Hair

Hair is an outgrowth from the skin of flexible, keratinized fibers. It covers most of the body except the palms of the hands, soles of the feet, and parts of the external reproductive organs. Formed at the junction of the epidermis and dermis, hair provides protection to delicate body structures.

Nails

Nails are hard, flat plates of keratin that cover the tips of the fingers and toes, protecting them from injury. At the base of each nail, hidden by a fold of skin called the cuticle, lies the nail root. The white crescent-shaped area nearest the root is called the lunula; it marks the site of nail growth. Two combining forms for nail are onych/o and ungu/o. Combine the first one with the prefix para-, which means near or beside, and you get paronychium, the soft tissue that surrounds the nail border. Using the second combining form, the term ungula, meaning pertaining to the nail, is formed. For example, ungual tuft is the term for the end portion of the distal phalanx that lies under the nail.

Sebaceous Glands

Sebaceous glands and sweat glands form the third category of the skin's appendages. Sebaceous glands secrete sebum, an oily substance that lubricates the skin and hair. Sebum also prevents excessive water loss from the skin and, because of its acidic nature, helps to protect against infection. Found in the dermal layer of the skin, in close association with hair follicles, sebaceous glands are most numerous on the scalp, face, back, and upper chest.

Dermis

The dermis, also known as the corium, accounts for most of the skin's mass. It consists of two layers of fibrous tissue that enable the skin to stretch and rebound. The dermis contains blood vessels, nerves, lymphatic vessels, muscle fibers, and hair follicles. Do not confuse corium, which refers to the dermis, with corneum, the outer layer of the epidermis. The dermis is also the location of the sebaceous glands and the sweat glands. Sebaceous glands secrete the oily substance called sebum that lubricates the hair and inhibits bacterial growth on the skin. The receptor endings of nerves within the dermis allow us to sense pain, pressure, touch, and temperature. The blood vessels in the dermis play a critical role in the regulation of body temperature.

Epidermis

The outer skin layer, the epidermis, is composed of several layers, or strata. The most superficial layer consists of thin, flat, scalelike cells that are continually shed and replaced by new cells generated at the deepest level of the epidermis, the stratum germinativum. This outermost layer is the stratum corneum. New skin cells form at the lowest level of the epidermis and migrate toward the outermost layer, where they shrink, flatten, and die; their cytoplasm is replaced by a hard, water-repellent scleroprotein called keratin. You should recognize scler/o as the combining form that means hard. The combining form kerat/o, when used in reference to the skin, means horny tissue or hard. The process of cell change that forms the stratum corneum is called keratinization or keratogenesis. This membrane of cells at the skin's surface becomes thick and chemically resistant, preventing excessive loss of water, salts, and heat from the body. As a result, the skin is better able to protect us against a range of environmental threats, including harmful toxins, invading microorganisms, and physical trauma. For these reasons, the stratum corneum is sometimes called the barrier layer of the skin. The keratinized cells, also known as horny cells, are eventually sloughed off of the skin's surface and replaced by the migration of new cells.

Layers of the Skin

The skin is a multilayered organ. It consists of a thin outer, avascular layer—the epidermis—and a deeper, dense layer of connective tissue called the dermis. Beneath the dermis is the subcutaneous tissue, a spongy, porous layer with a rich blood supply and a wealth of fat-producing cells.

Primary Germ Layers of the Skin

The skin is one of the earliest organs to develop, forming during the third week of gestation. After undergoing repeated division, the fertilized egg organizes into three primary layers of cells—the ectoderm, the mesoderm, and the endoderm.

Subcutaneous Layer

The subcutaneous tissue anchors the dermis and the epidermis to the underlying muscle and bone. The nerves and blood vessels that supply the skin run through the subcutaneous layer, which is composed of loose connective tissue filled with fat cells. These deposits of fat within the subcutaneous tissue are the reason we also refer to this stratum as the subcutaneous adipose layer. The term adipose incorporates the combining form adip/o, which means fat, and the suffix -ose, which in this context means full of or pertaining to. The production and storage of fat in the subcutaneous layer creates a cushion for the skin and insulates the body against excessive heat loss.

Sudoriferous Glands

The sweat glands are also called sudoriferous glands. Perspiration produced by these glands helps regulate body temperature through the cooling process of evaporation. Perspiration also rids the body of waste products and moisturizes the surface cells of the skin. The combining form for sweat is hidr/o. Thus, hidrosis is the term for perspiration. This term may also describe a condition of abnormal or excessive amounts of sweat. The suffix -osis means abnormal condition. There are two types of sudoriferous glands: eccrine glands and apocrine glands. Eccrine glands are small, simple glands that excrete sweat through pores, tiny openings on the surface of the skin. They are responsible for most sweat production. Apocrine glands are large sweat glands associated with hair follicles. They secrete both sweat and a fluid containing apocrine cells. In the presence of sex hormones, the apocrine secretions produce a highly individual sexual scent. The apocrine glands become active with the onset of puberty and are most prevalent in the axillary area and the genital area. The term axillary refers to the axilla, or armpit. There are nonsudoriferous apocrine glands in the breasts and the ears. Those in the breasts secrete fat droplets into breast milk, and those in the ears help form cerumen.