The adipocyte is a connective tissue cell specialized to store neutral fat and produce a variety of hormones.
Adipocytes differentiate from mesenchymal stem cells and gradually accumulate fat in their cytoplasm. They are located throughout loose connective tissue as individual cells and groups of cells. When they accumulate in large numbers, they are called adipose tissue. Adipocytes are also involved in the synthesis of a variety of hormones, inflammatory mediators, and growth factors. This specialized connective tissue is discussed in Chapter 9.
Niches of adult stem cells are located in various tissues and organs.
Many tissues in mature individuals contain reservoirs of steam cells called adult stem cells. Compared with embryonic stem cells, adult stem cells cannot differentiate into multiple lineages. They usually are capable of differentiating only into lineage-specific cells. Adult stem cells are found in many tissues and organs, residing in specific sites referred to as niches. Niches of stem cells residing in tissues and organs (excluding bone marrow) are called tissue stem cells. They been identified in the gastrointestinal tract—for instance, in the stomach (isthmus of the gastric gland), small and large intestines (base of the intestinal gland), and many other areas. Bone marrow represents a unique reservoir of stem cells. In addition to containing HSCs (see Chapter 10), bone marrow also contains at least two other populations of stem cells: A heterogenous population of multipotent adult progenitor cells (MAPCs) that appear to have broad developmental capabilities and
FOLDER 6.5 Clinical Correlation: The Role of Mast Cells and Basophils in Allergic Reactions