what is fat?

Adipose or fat tissue is a fascinating organ. It plays a crucial role in our body and has several important functions. It also contains stem cells that can be used for regenerative medicine applications.

The combination of various adipose tissue depots in a given organism has been referred to as the “adipose organ”. The panniculus adiposus is the term for the adipose tissue, which consists of adipocytes and a stromal-vascular fraction (SVF) with connective tissue. Adipose tissue is a unique human mesenchymal tissue with specific properties and characteristics.

The Swiss naturalist Conrad Gessner first identified adipose tissue in 1551. The derivation of the terms “adipose” and “lipid” originate, respectively, from the Latin “adeps” and the Greek “lipos” for fat.

No other organ than the adipose organ has been more widely debated on an emotional, cultural, artistic, socio-economical and medical level in the last decades.

Adipose tissue is present in all mammalian species and a variety of non-mammalian species. Subcutaneous adipose tissue covers underlying neurovascular structures, protects tendons and muscles, cushions, harbors lymphatic vessels and nodes, stores energy in the form of fat, preserves thermoregulation, insulating the body, coordinates endocrinological homeostasis and defines someone’s body contour. White adipose tissue is primarily located in three major anatomical areas: (1) subcutaneous, (2) dermal and (3) intraperitoneal.

As an important endocrine organ, its produces hormones such as leptin, resistin and the cytokine TNFα. Obesity or being overweight in humans and most animals does not depend on the body weight but on the amount of body fat – specifically, adipose tissue.

White adipose tissue is the only tissue in the body that can markedly change its mass after adult size is reached. Fat mass can range from 2 to 3 % of body weight in well-conditioned athletes to 60-70% of body weight in massively obese individuals. Normal values for fat mass are 9 – 18% in males and 14-28% in females. The cellular development associated with adipose tissue growth involves both cellular hypertrophy (increase in size) and hyperplasia (increase in number). Hypertrophy is the result of excess triglyceride accumulation in existing adipocytes due to a positive energy balance. Hyperplasia also referred to herein as ‘adipogenesis’, results from the recruitment of new adipocytes from precursor cells in adipose tissue and involves the proliferation and differentiation of preadipocytes. Both adipocyte hypertrophy and hyperplasia occur in association with positive energy balance during normal growth and during the development of obesity, with hypertrophy often preceding hyperplasia in a cyclic manner.

The development of fat tissue

Fat tissue is derived from the mesoderm and a connective tissue.

In the very early stages of life, fat cell development is characterized by the appearance of a number of fat cell clusters, or “primitive fat organ”, which increase in number and size throughout fetal development. Primitive fat organs are vascular structures in presumptive adipose tissue with few or no fat cells. Fetal adipocyte development is spatially and temporally related to capillary development. Arteriolar development clearly precedes adipocyte differentiation in internal fat depots and be used to distinguish adipose tissue depots in the fetus. The degree of overlap between fat cell and vascular morphological development is depot dependent, whereas fat cell size is independent of depot. There is multifaceted evidence of autocrine/paracrine or developmental relationships between capillaries/endothelial cells and preadipocytes.

Human preadipocytes and endothelial cells express the αVβ3integrin and express and secrete PAI-1, which regulates preadipocyte and endothelial cell migration in vitro (Crandall et al., 2000). These findings provide a mechanism for the formation of primitive fat organs because preadipocytes would migrate with developing capillary endothelial cells during angiogenesis (Crandall et al., 2000).