Abstract
This chapter introduces an in vitro model as a means of studying human hormonal aging. For this purpose, human sebaceous gland cells were maintained under a hormone-substituted environment. This environment consisted of growth factors and sex steroids in concentrations corresponding to those circulating in young and postmenopausal women. The authors suggest that hormone decline, occurring with age, may play a significant role not only in the maintenance of skin homeostasis but also in the initiation of aging. Furthermore, skin, the largest organ of the body, offers an alternative approach to understanding the molecular mechanisms underlining the aging process.
TopThe Sebaceous Gland
Sebaceous glands or holocrine glands are skin appendages and are found over the entire surface of the body except the palms, soles and dorsum of the feet. They are largest and most concentrated in the face and scalp where they are the sites prone for acne. The normal function of sebaceous glands is to produce and secrete sebum, a group of complex oils including triglycerides and fatty acid breakdown products, wax esters, squalene, cholesterol esters and cholesterol (Downing et al., 1987; Nikkari, Schreibman, & Ahrens, 1974; Ramasastry, Downing, Pochi, & Strauss, 1970; Thody & Shuster, 1989). The most accepted function of sebum is skin lubrication in order to protect it against friction and to make it more impervious to moisture.
Furthermore, sebum lipids transport antioxidants in and on the skin and exhibit a natural light protective activity. They exhibit an innate antibacterial activity and have a pro- and anti-inflammatory function. The sebaceous gland can regulate the activity of xenobiotics and is actively involved in the wound healing process (Zouboulis, 2004). It possesses all enzymes required for the intracellular androgen metabolism and confers upon the skin an independent endocrine function (Fritsch, Orfanos, & Zouboulis, 2001).
Key Terms in this Chapter
Exogenous Skin Aging: Exogenous aging, otherwise called extrinsic aging, takes place in exposed areas of the body (e.g. head, neck) which are constantly influenced by various environmental factors including ionizing and non-ionizing irradiation, air pollution, natural deleterious gases (e.g. ozone and high concentrations of oxygen), smoking, invasion of pathogenic bacteria, viruses, xenobiotics and mechanical stress. Among them UV-irradiation is the most fundamental one, as it can damage skin to such an extent, that makes it seem prematurely aged (photoaging). This premature aging process is cumulative with sun exposure and affects more individuals of skin phototypes I and II.
Endogenous Skin Aging: Endogenous aging, otherwise called intrinsic or chronological aging, is influenced by genetics, hormonal changes and metabolic processes, which appear at advanced age. Endogenous skin aging can be viewed on non UV-exposed areas of the body and can be considered as model of the aging process taking place in internal organs.
Sebum: (Latin, meaning fat or tallow ) Is produced by sebaceous glands and its main function is to protect and waterproof hair and skin, and keep them from becoming dry, brittle, and cracked. It can also inhibit the growth of microorganisms on skin. In the sebaceous glands, sebum is produced within specialized cells and is released as these cells burst; sebaceous glands are thus classified as holocrine glands. In humans, the composition of sebum is as follows: 25% wax monoesters, 41% triglycerides, 16% free fatty acids and 12% squalene.
Sebaceous Glands: Can usually be found in hair-covered areas where they are connected to hair follicles to deposit sebum on the hairs, and bring it to the skin surface along the hair shaft. The structure consisting of hair, hair follicle and sebaceous gland is known as pilosebaceous unit. They are largest and most concentrated in the face and scalp where they are the sites prone for acne.
Skin: The largest organ of the body, exhibits multiple functions, among them it serves as a protective barrier between internal organs and the environment and is a complex organ with multiple cell types and structures. It is divided into three major compartments: epidermis, dermis and subcutaneous tissue.
SZ95 Sebocytes: Are sebaceous gland cells derived from facial skin and transfected with the SV-40 large T antigen. They functionally behave in a manner concomitant to nontransfected human sebaceous gland cells.
Hormone: (from Greek ??µ? - “to set in motion”) A chemical messenger from one cell (or group of cells) to another. The function of hormones is to serve as a signal to the target cells. Endocrine hormone molecules are secreted (released) directly into the bloodstream, while exocrine hormones (or ectohormones) are secreted directly into a duct, and from the duct they either flow into the bloodstream or they flow from cell to cell by diffusion in a process known as paracrine signaling.
Aging: A complex process that defines the changes observed throughout the organism’s lifespan and cannot be defined by a single pathway or a single cause. Aging is controlled by both environmental factors and the genetic constitution of the individual and has been described as a progressive decline of the ability to withdraw stress, damage and disease. Furthermore, it is characterized by an increase of degenerative and neoplastic disorders.