Adipose tissue

Abstract

Adipose tissue is a type of connective tissue that is composed mainly of adipocytes. It is found throughout the body and fulfills a number of important functions: it provides structural support and protective padding for major organs (e.g., kidneys), it serves as an insulating layer that prevents cutaneous heat loss, and it stores energy for longer periods of fasting. While the insulating and energy storage functions of adipose tissue provide decisive evolutionary advantages, their roles have diminished in modern times. Today, humans also rely on clothing for insulation and, in industrialized societies where food is readily available and affordable, its positive role as an energy reservoir has reversed to become a major health concern (obesity). However, in infants, adipose tissue continues to play a central role in the maintenance of body heat, and in individuals with severe illness, energy stores within the body make an essential contribution to sustaining the body.

There are two types of adipose tissue. Brown adipose tissue is abundant in neonates but recedes in adulthood. Its primary function is to prevent hypothermia through thermogenesis. White adipose tissue remains present throughout a lifetime and serves as the body's most important energy reservoir and also produces hormones.

White adipose tissue

Structure

Function

  • Energy storage: energy reservoir (triglycerides)
  • Regulation
    • An anabolic state of metabolism results in increased lipogenesis through both hypertrophy (lipid vacuole expands) and hyperplasia (via stem cell reserves) of adipocytes.
    • A catabolic state of metabolism results in lipolysis.
  • Insulation: subcutaneous tissue provides thermal insulation
  • Structural support
    • Padding in areas exposed to mechanical stress (foot sole)
    • Holds organs in place (e.g., keeping eyeball in orbit, renal pelvis in renal sinus)
  • Hormone secretion
    • Leptin
    • Estrogens: adipocytes express cytochrome P-450 aromatase, which catalyzes the conversion of steroids to estrogens (i.e., it converts androstenedione to estrone).
  • Reservoir for lipophilic biomolecules (e.g., fat-soluble vitamins)

Individuals with congenital leptin deficiency (e.g., due to leptin gene mutations) present with constant hunger, hyperphagia, and severe obesity beginning in the first few months of life.

Blocking aromatase activity is the mechanism of treatment of estrogen-dependent diseases such as breast cancer, endometriosis, and endometrial cancer. Obese women are more prone to develop these conditions due to higher circulating levels of estrogens synthesized by their adipose tissue.

Occurrence

Development

Mesoderm germ layer → mesenchymal stem cells → preadipocytes (mitotically active) → adipocytes (ripe, mitotically inactive)

A persistently high-calorie intake that exceeds daily needs can lead to a pathological increase of white adipose tissue, causing obesity.

Brown adipose tissue

Structure

  • Macroscopic
    • Color: brown due to high iron-containing mitochondrial content
    • Same structure as white adipose tissue
  • Microscopic
    • Rich in mitochondria and capillaries (see “Function” below)
    • Smaller than adipocytes of white adipose tissue
    • Contain multiple vacuoles (instead of only one as seen in white adipose tissue)

Function

Nonshivering thermogenesis (heat production)

Occurrence

  • Infants: especially in the neck and thorax
  • Adults: only surrounding the vertebra, vessels (along large arteries), mediastinum, and clavicle

Development

  • Embryonic: mesoderm germ layer → mesenchymal stem cellsbrown adipose tissue
  • In adult organism: regression over the course of a lifetime

Clinical significance

last updated 08/15/2018
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