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INTRODUCTION

Subcutaneous tissue, or the hypodermis, is one of the largest tissues in the human body. The major components of this layer are adipocytes, fibrous tissue, and blood vessels. It is estimated that this layer represents 9% to 18% of body weight in normal-weight men and 14% to 20% in women of normal weight.1 Fat mass can increase up to four fold in severe obesity, which may represent 60% to 70% of total body weight.2 Although gaining fat in the body is undesirable for many, losing fat in the face has cosmetic implications as well. Adipose tissue gain and loss and volume changes contribute to the aged appearance of the face and body. This chapter will review the importance of the subcutaneous tissue and its various functions.

The subcutaneous tissue is usually not given as much attention as the dermis and epidermis because pathology at superficial layers is easier to detect or diagnose by a shave or small punch biopsy. Subcutaneous tissue usually must have an extensive defect before it is noticed, and in order to biopsy this area, an incision or large punch biopsy (e.g., 6 mm) is required. During histologic tissue processing of biopsy tissue, the triglyceride component, which is the major component of adipocytes, is removed by alcohol and xylol. For this reason, subcutaneous tissue has long been ignored. However, with advances in diagnostic methods and new treatments, much more has been learned about the subcutaneous layer (Box 3-1). It is important for dermatologists and cosmetically oriented physicians to pay close attention to this tissue because it has many roles in cosmetic dermatology and general appearance.

BOX 3-1 Functions of the Subcutaneous Tissue

  • The largest repository of energy in the body.

  • Stores fat-soluble vitamins (A, D, E, K), including their derivatives such as retinoic acids.

  • Helps to shape the surface of the body, and form fat pads that act as shock absorbers.

  • Helps distribute force or stress to mitigate damage to underlying organs.

  • Protects against physical injury from excessive heat, cold, or mechanical factors.

  • Fills up spaces between other tissues and helps to keep organs in place.

  • Involved in thermoregulation by insulating the body from heat loss.

  • Functions as a secretory organ that releases many cytokines.

  • Plays a role in regulating androgen and estrogen levels.3

ADIPOCYTES

In the past, adipocytes in adults were considered stable, nondividing cells, like other mature cells. However, recent data reveal that adipocytes in adults have the potential to increase in number or revert back into stem cells. These stem calls can differentiate to other tissue, such as fibroblasts, collagen, elastic fibers, and hematopoietic stromal cells.4 Fat cells are derived from undifferentiated fibroblast-like mesenchymal cells. Under certain conditions, these mesenchymal cells give rise to adipose cells. Adipose tissue is classified into two morphologic types: white and brown adipose tissue. White adipose tissue normally appears yellow because of ...

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