Table 1. Ultrastructural features of adipocytes.
| Ultrastructural Feature |
White adipocyte | Beige adipocyte | Bone marrow adipocyte |
|---|---|---|---|
| Cytoplasm | Thin rim around lipid | Evenly distributed around lipid and organelles | Thin rim around lipid |
| Lipid | Large central unilocular droplet, smaller lipid droplets in cytoplasm | Small lipid droplets dispersed throughout cytoplasm | Large central lipid globule with smaller fat globules noted in the peripehral cytoplasm |
| Surrounding matrix | Collagen-rich; adipocytes are "embraced" by a network of collagen fibers | Vastly reduced collagenous matrix relative to WAT | Lack of association with collagen, basal lamina positive for PAS and reticulin stains |
| Nucleus | Displaced toward periphery, crescent or triangular shape. Peripheral chromatin condensation along the nuclear membrane and in association with nucleoli. | Centrally located. Peripheral chromatin condensation with a central euchromatic area and nucleolus. | Displaced toward periphery, crescent or triangular shape. Peripheral chromatin condensation along the nuclear membrane and in association with nucleoli. |
| Mitochondria | Filamentous and spherical with rare, short, or gently wavy cristae | Numeous, large mitochondria with closely packed, straight cristae. Increased mitochondrial volume (3.7-fold vs WAT). | Dense, usually spherical mitochondria |
| Endoplasmic Reticulum | Mostly smooth ER | Cisternae of rough ER | Some profiles of rough ER |
| Innervation | Unmyelinated nerve fiber with Schwann cells and 'naked' nerve fibers can be found adjacent to the membrane of some cells (~5% of adipocytes are innervated) | Every adipose cell was in close contact with one or more nonmyelinated nerve fibers (often 2, 3, or more contacts per cell) (100% of adipocytes are innervated) | Unknown |
| Vasculature | Adjacent to capillaries (~0.4 apilllaries/cell) | Adjacent to capillaries (~0.8 capillaries/cell) | Adjacent to sinusoidal endothelium |
| Contacts between adipocytes | Very limited (Loncar 1988 found only one direct contact between WAT adipocytes out of 2000 total cells) | Appearance of gap junction connections between adipocytes which connect nearly all cells to one another | Unknown |