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Soft tissue

Soft tissue is a broad term often used for mesenchymal tissue that support and surround more well-defined organs and specific tissues. The cells and structures of soft tissue are present throughout the human body. The major cell types of soft tissues are non-epithelial and of mesodermal origin, denoted as “mesenchymal cells”. In the tissue dictionary soft tissue is exemplified by cartilage (from the bronchus), fibroblasts (from the gallbladder), adipocytes (from gallbladder and rectum), ganglion (from the rectum) and peripheral nerve (from the breast). Other main types of soft tissue include muscle tissue (smooth and striated muscle), which is described separately.

Chondrocytes are the main cell type in cartilage. Cartilage, similar to bone, is a specialized form of connective tissue. Cartilage is composed of a non-vascularized extracellular matrix of collagen fibers embedded in a gel-like proteoglycan matrix. Mature chondrocytes synthesize and secrete extracellular matrix, which separates the cells from each other and result in the appearance of isolated chondrocytes surrounded by a lacuna.

Fibroblasts are the main and prototypic mesenchymal cell type that produces collagen fibers. Collagen, which exists in various subtypes, is the main component of connective tissue. Fibroblasts are slender, elongated cells with indistinct cytoplasmic borders and bipolar oval nuclei. Fibroblasts are sparsely distributed in between collagen fibers that support various epithelial tissues and other organs. With the exception of brain tissue, fibroblastic cell types can be found throughout the human body.

Adipocytes are the main cell type in adipose tissue (fat). Adipose tissue is typically homogenous and finely divided by faint septa. Adipose tissue is spread throughout the body and surrounds most organs and tissues in the human body. In the skin, underlying adipose tissue forms the subcutis as an integral component of the skin. Microscopically adipose tissue is mainly composed of ill-defined lobules of adipocytes surrounded by thin bands of collagen and small blood vessels. The cytoplasm of the adipocyte is compressed at the perimeter of the cell as it is displaced by a single lipid vacuole and only a thin rim of cell membrane is evident in the microscopic image. Adipocytes contain a small, thin and oval nucleus located peripheral to the dominating lipid vacuole, whereas nuclei of capillary endothelial cells are present at intersections between multiple adipocytes.

Ganglion cells are peripheral neurons (nerve cells) that are distributed as groups of cells denoted as a ganglion. A ganglion provides a relay and connection point for nerves in the peripheral nervous system. Ganglia are present at various locations throughout the human body. In the gastrointestinal tract they are involved in transmitting signals coordinating bowel movement. Ganglia can often be visualized in between the two layers of smooth muscle surrounding the intestinal mucosa.

Peripheral nerves are branching extensions of the peripheral nervous system. Large nerves originate from the spinal chord and through smaller and smaller branches end up at distal locations in various tissues throughout the body. Peripheral nerves appear as white fascicles bound together by connective tissue. Microscopically, transverse sections of a peripheral nerve show how endoneurial compartments containing axons and Schwann cells are surrounded by perineurium to form individual fasicles, which are embedded in epineural fibrous tissue. Most peripheral nerves contain a mixture of myelin and unmyelinated nerve fibers. Myelin is produced by Schwann cells, which account for 90% of nuclei found in peripheral nerves.