In the late stage of differentiation, the cells increase in size and become more spherical. Small lipid droplets coalesce to form a single large lipid droplet that occupies the central portion of the cytoplasm. Smooth-surfaced endoplasmic reticulum (sER) is abundant, whereas rER is less prominent. These cells are designated late lipoblasts. Eventually, the lipid mass compresses the nucleus to an eccentric position, producing a signet-ring appearance in hematoxylin-and-eosin (H&E) preparations. Because these cells have a single lipid droplet, they are designated unilocular [Lat. unus, single; loculus, a little place] adipocytes or mature lipocytes.
Structure of Adipocytes and Adipose Tissue
Unilocular adipocytes are large cells, sometimes 100 m or more in diameter.
When isolated, white adipocytes are spherical, but they may appear polyhedral or oval when crowded together in adipose are surrounded by reticular fibers (type III collagen), which are mesechymal stem cell secreted by the adipocytes. Special stains also reveal the pres ence of unmyelinated nerve fibers and numerous mast cells. A summary of white adipose tissue features is listed in Table 9.2.
FIGURE 9.1 • Development of adipose tissue cells. Like all connective tissue cells, adipocytes are derived from undifferentiated mesenchymal stem cells. By expressing PPAR-/ RXR transcription factors, they become committed to becoming early lipoblasts (preadipocytes) committed to white adipocyte lineage development. By expressing PRDM16/PGC-1 transcription factors, these cells will differentiate into early lipoblasts committed to brown adipocyte lineage development. Lipoblasts develop an external (basal) lamina and begin to accumulate numerous lipid droplets in their cytoplasm. In white adipose tissue, these droplets fuse to form a single large lipid droplet that ultimately fills the mature cell, compressing the nucleus, cytoplasm, and cytoplasmic organelles into a thin rim around the droplet. In brown adipose tissue, the individual lipid droplets remain separate.
tissue. Their large size is due to the accumulated lipid in the cell. The nucleus is flattened and displaced to one side of the lipid mass; the cytoplasm forms a thin rim around the lipid. In routine histologic sections, the lipid is lost through extraction by organic solvents such as xylene; consequently, adipose tissue appears as a delicate meshwork of polygonal profiles (Fig. 9.2). The thin strand of meshwork that separates adjacent adipocytes represents the cytoplasm of both cells and the extracellular matrix. The strand is usually so thin, however, that it is not possible to resolve its component parts in the light microscope.
Adipose tissue is richly supplied with blood vessels, and capillaries are found at the angles of the meshwork where adjacent adipocytes meet. Silver stains show that adipocytes
The lipid mass in the adipocyte is not membrane bounded.
The TEM reveals that the interface between the contained lipid and surrounding cytoplasm of the adipocyte is composed of a 5-nm-thick condensed layer of lipid reinforced by parallel vimentin flaments measuring 5 to 10 nm in diameter. This layer separates the hydrophobic contents of the lipid droplet from the hydrophilic cytoplasmic matrix.
The perinuclear cytoplasm of the adipocyte contains a small Golgi apparatus, free ribosomes, short profiles of rER, microfilaments, and intermediate filaments. Filamentous mitochondria and multiple profiles of sER are also found in the thin rim of cytoplasm surrounding the lipid droplet (Fig. 9.3).
Regulation of Adipose Tissue