Chapter 4. White Adipose Tissue

A. Introduction

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Image: University of Delaware Histology

Adipose tissue is a highly specialized loose connective tissue designed to store large quantities of triacylglycerols (triglycerides) and fat-soluble substances. It is classified as a connective tissues due, in part, to the fact that its is derived from embryonic mesenchyme. Each adipose cell, or adipocyte, stores lipid in cytoplasmic inclusions called lipid droplets. Two types of adipose tissue are found in animals: white adipose tissue and brown adipose tissue.

Brown adipose tissue is present in significant amounts in human infants. Its distriburion is very limited in the adult, where it is found in the mediastinum, the subcutaneous tissue between the scapula, the area around the kidney and the area along the aorta. While shares many physiological characteristics with white adipose tissue, it displays some distinctive histologic and physiologic characteristics:

  • Brown adipocytes are small cells that contain a centrally-placed nucleus and large numbers of mitochondria.
  • Brown adipocytes are multilocular cells, i.e., each cell contains multiple small lipid droplets.
  • Brown adipose tissue has a high metabolic rate capable of generating relatively high amounts of heat, a process that is physiologically important to infants prior to the maturation of their thermoregulatory mechanisms.

White adipose tissue is the principal type of adipose tissue of the adult human. The tissue, which is scattered about the body, occupying positions in skin and around and within a variety of other organs, is itself a large, physiologically important organ.  It is the largest storage reservoir of metabolic fuel in the body, it serves as a thermal insulator, in the case of the skin, and a protective cushion, in the case of the adipose that surrounds organs. Its size and distribution vary according to a variety of factors, including age and sex.

The total size and distribution of what adipose tissue vary according to a variety of factors, including age and sex. In the adult, white adipose tissue is primarily distributed subcutaneously and viscerally. Subcutaneously, white adipose tissue forms a distinct tissue layer beneath the dermis of the skin called the panniculus adiposus or hypodermis. Viscerally, white adipose tissue is generally found in significant amounts in the omentum and mesentery and around the kidneys and heart. It may also be found in small amounts in other organs.

Its important histologic characteristics are:

  • The adipocytes are relatively large, compared to brown adipocytes, and are dominated by the lipid droplet.
  • White adipocytes are unilocular cells, i.e., each cell contains a single large lipid droplet, a cytoplasmic inclusion that contains triacylglycerols and fat-soluble substances.
  • The large lipid droplet appears empty because the lipd is lost during routine histolkogical preparation, and it displaces the rest of the cell toward the cells' periphery, flattening the nucleus along the edge of the cell, giving the cell a signet ring appearance.
  • The tissue is well vascularized, with microvascular vessels found in the sliver of loose connective tissue found between adjacent cells.

You may find small sections of adipose tissue is a variety of tissue sections. We have chosen the hypodermis as our the specific study for this manual, however, you may find well-preserved adipose tissue in other slides as well.