Page 168 - Atlas of Histology with Functional Correlations

connective tissue types, fibroblasts are the most abundant cells; these are

scattered between the dense collagen bundles.

CELLS OF THE CONNECTIVE TISSUE

The two most common cell types in the connective tissue are the active
fibroblasts and the inactive or resting fibroblasts, the fibrocytes. Fusiform
fibroblasts synthesize all the connective tissue fibers (collagen, elastic, and
reticular) and the extracellular ground substance, including proteoglycans,

glycosaminoglycans, and adhesive glycoproteins.

Adipose (fat) cells store fat and may occur singly or in groups in the
connective tissue. There are two types of adipose cells. Cells with a large, single,
or unilocular lipid droplet in the cytoplasm are white adipose tissue, whereas
cells with numerous or multilocular lipid droplets are brown adipose tissue.

White adipose tissue is more abundant than brown adipose tissue, and when
adipose cells predominate, the connective tissue is called adipose tissue.

Macrophages or histiocytes are phagocytic cells that ingest foreign material
or dead cells and are most numerous in loose connective tissue, after fibroblasts.

They are difficult to distinguish from fibroblasts, unless they are performing
phagocytic activity and contain ingested material in their cytoplasm. The
macrophages, however, are called by different names in different tissues/organs.

Mast cells are normal elements of the connective tissue, usually closely
associated with blood vessels. They are widely distributed in the connective

tissue of the skin, the digestive, and respiratory organs. Mast cells are ovoid cells
filled with fine, regular, dark-staining, basophilic granules. However, mast cells
can exhibit variable sizes and granular content.

Plasma cells arise from the lymphocytes that migrate into the connective

tissue and have a wide distribution in the body. They are especially abundant in
the loose connective tissue and lymphatic tissue of the respiratory and digestive
tracts, respectively.

Leukocytes (white blood cells), neutrophils, and eosinophils migrate from
the blood vessels and capillaries to reside in the connective tissue. Their main

function is to defend the organism against bacterial invasion or foreign matter.

Fibroblasts and adipose cells are permanent or resident connective tissue
cells. Neutrophils, eosinophils, plasma cells, mast cells, and macrophages

167