ADI POS E TI SS U E 266 Adipose tissue is widely distributed throughout the body and in varying amounts in different individuals. It is a specialized connective tissue consisting of fat-storing cells, adipocytes, and has a rich blood supply. Two types of adipose tissue are recognized. The more common and abundant is referred to as white adipose tissue. Its adipocytes are very large cells whose cytoplasm contains a single large vacuole in which the fat is stored in the form of triglycerides. When observed in a typical H&E section, white adi-pose tissue appears as a mesh-like structure (see orientation micrograph). The second type is brown adipose tissue. It consists of smaller cells. Their cytoplasm is characterized by numerous vesicles that occupy much of the cells’ volume. It also is very richly vascu-larized. Brown adipose tissue is found in human newborns where it assists in maintaining body temperature. ORIENTATION MICROGRAPH: Shown here is white adipose tissue from the hypo-dermis of skin. It consists of numerous adipocytes closely packed in lobules. Dense con-nective tissue (DICT) surrounds the adipose tissue. The loss of the fat within the cell gives the adipose tissue a mesh-like appearance. Note the small blood vessels (BV) ob-served at the periphery of the tissue. They provide a rich capillary network within the adi-pose tissue. Several sweat gland ducts (SGD) are also present in the dense connective tissue. DICTBVSGDDICT BV SGD
White adipose tissue, human, H&E, ×363; inset ×700.
This is a higher magnification micrograph of white adipose tissue from the specimen shown in the orientation micro-graph. It reveals portions of several lobules of adipose cells. Dense connective tissue (DICT) separates the lobules from surrounding structures. In well preserved specimens, the adipocytes (A) have a spherical profile in which they exhibit a very thin rim of cytoplasm surrounding a single, large fat-containing vacuole. Because the fat is lost during tissue preparation, one only sees the rim of cytoplasm and an almost clear space. Between the cells, there is an extremely thin, delicate connective tissue stroma holding the adipocytes together and within this stroma are small blood vessels (BV), mostly capillaries and venules. The majority of nuclei that are observed within the adipose tissue belong to fibroblasts, adipocytes, or cells of small blood vessels. However, distinguishing between fibroblast nuclei and adipocyte nuclei is often difficult. The inset shows an adipocyte whose nucleus (N) is relatively easy to identify. It appears to reside within the rim of cytoplasm (Cy), giving the adipocyte the classic “signet ring” appearance. A second nucleus (N’), partially out of the plane of section, appears to reside between the cytoplasmic rim of two adjacent cells. This is probably the nucleus of a fibroblast. Because of the relatively large size of the adipocyte, it is very infrequent that the nucleus of the cell is included in the plane of section of a given cell. Other cells that may be seen within the delicate connective tissue stroma are mast cells (MC).
Brown adipose tissue, human, H&E, ×450; inset ×1100.