Connective or supportive tissue is the most abundant tissue in the body. As its name suggests, it connects and binds or supports the various tissues. Connective tissue is unique in that its cells produce the extracellular matrix that supports and holds tissues together. The capsules that surround organs of the body are composed of connective tissue. Bone, adipose tissue, and cartilage are specialized types of connective tissue that function to support the soft tissues of the body and store fat. The proximity of the extracellular matrix to blood vessels allows it to function as an exchange medium through which nutrients and metabolic wastes pass.
Origin and Characteristics
Most connective tissues is derived from the embryonic mesoderm, but some is derived from the neural crest, a derivative of the ectoderm. During embryonic development, mesodermal cells migrate from their site of origin and then surround and penetrate the developing organ. These cells are called mesenchymal cells, and the tissue they form is called mesenchyme. Tissues derived from embryonic mesenchymal cells include bone, cartilage, and adipose (fat) cells. Besides providing the source or origin of most connective tissues, mesenchyme develops into other structures such as blood cells and blood vessels. Connective tissue cells include fibroblasts, chondroblasts, osteoblasts, hematopoietic stem cells, blood cells, macrophages, mast cells, and adipocytes. The matrix of the umbilical cord is composed of a second type of embryonic mesoderm called mucous connective tissue or Wharton jelly.
Types of Connective Tissue
Adult connective tissue can be divided into two types: connective tissue proper, which is the focus of the discussion in this chapter, and specialized connective tissue (cartilage, bone, and blood cells), which is discussed in other chapters. There are four recognized types of connective tissue proper: loose (areolar), adipose, reticular, and dense connective tissue.
Loose Connective Tissue. Loose connective tissue, also known as areolar tissue, is soft and pliable. It fills spaces between muscle sheaths and forms a layer that encases blood and lymphatic vessels (Fig. 4.21). Areolar connective tissue supports the epithelial tissues and provides the means by which these tissues are nourished. In an organ containing functioning epithelial tissue and supporting connective tissue, the term parenchymal tissue is used to describe the functioning epithelium as opposed to the connective tissue framework, or stroma.
Loose connective tissue is characterized by an abundance of ground substance and tissue fluid housing the fixed connective tissue cells: fibroblasts, mast cells, adipose or fat cells, macrophages, and leukocytes. Loose connective tissue cells secrete substances that form the extracellular matrix that supports and connects body cells. Fibroblasts are the most abundant of these cells. They are responsible for the synthesis of the fibrous and gel-like substance that fills the intercellular spaces of the body and for the production of collagen, elastic, and reticular fibers.
The basal lamina is a special type of intercellular matrix that is present where connective tissue contacts the tissue it supports. It is visible only with an electron microscope and is produced by the epithelial cells. In many locations, reticular fibers, produced by the connective tissue cells, are associated with the basal lamina. Together the basal lamina and the reticular layer form the basement membrane seen by light micros-copy. A basement membrane is found along the interface between connective tissue and muscle fibers, on Schwann cells of the PNS, on the basal surface of endothelial cells, and on fat cells. These basement membranes bond cells to the underlying or surrounding connective tissues, serve as selective filters for particles that pass between connective tissue and other cells, and contribute to cell regeneration and repair.
Adipose Tissue. Adipose tissue is a special form of connective tissue in which adipocytes predominate. Adipocytes do not generate an extracellular matrix but maintain a large intra-cellular space. These cells store large quantities of triglycerides and are the largest repository of energy in the body. Adipose tissue helps fill spaces between tissues and helps to keep organs in place. The subcutaneous fat helps to shape the body. Because fat is a poor conductor of heat, adipose tissue serves as thermal insulation for the body. Adipose tissue exists in two forms: unilocular and multilocular. Unilocular (white) adipose tissue is composed of cells in which the fat is contained in a single, large droplet in the cytoplasm. Multilocular (brown) adipose tissue is composed of cells that contain multiple droplets of fat and numerous mitochondria.
Reticular Connective Tissue. Reticular tissue is characterized by a network of fibers interspersed with fibroblasts and macrophages. The fibroblasts synthesize type III collagen fibers. Reticular tissue forms the architecture of liver sinusoids, adipose tissue, bone marrow, and lymphoid tissues such as the spleen.
Dense Connective Tissue. Dense connective tissue exists in two forms: dense irregular and dense regular. Dense irregular connective tissue consists of the same components found in loose connective tissue, but exhibits a predominance of collagen fibers and fewer cells. This type of tissue can be found in the dermis of the skin (i.e., reticular layer), the fibrous capsules of many organs, and the fibrous sheaths of cartilage (i.e., perichondrium) and bone (i.e., periosteum). It also forms the fascia that invests muscles and organs. Dense regular connective tissues are rich in collagen fibers and form the tendons and aponeuroses that join muscles to bone or other muscles and the ligaments that join bone to bone.