Connective tissue cells

Connective tissue cells are classified as either resident or migratory. Resident cells are present in tissues continuously and typically exhibit little movement. They can be regarded as permanent residents of the tissue.

Fibroblasts are the principal cell type of connective tissue. They are responsible for the synthesis of the extracellular matrix, both fibers and the ground substance. In routine histological preparations, fibroblast cytoplasm is often difficult to distinguish from the ground substance.

Adipocyte’s function is to store neutral lipids, primarily triglycerides. They are large, spherical cells that contain a single droplet of lipid that almost completely fills the cell, thus constraining the sparse cytoplasm and flattened nucleus to the periphery of the cell. In routine histologic sections, the lipid is lost through extraction by organic solvents making the cell appear empty.

Macrophages, also called histiocytes, are resident cells derived from monocytes that circulate in the blood. Macrophages are the major phagocytes in tissue. They have an oval-shaped, heterochromatic nucleus which often has a distinct indentation.

Mast cells, derived from hematopoietic stem cells in the bone marrow, travel in the blood stream to reach tissues throughout the body. Their cytoplasm is filled with large, intensely staining granules that contain bioactive substances with roles in the inflammatory response, innate immunity and tissue repair.

Connective tissue cells are classified as either resident or migratory. Migratory or wandering cells are present only transiently and are mobile. Migratory cells, for the most part, are able to enter and leave the tissue in response to specific, local stimuli.

Lymphocytes are the smallest of the migratory cells and are derived from the bone marrow. They possess only a thin rim of cytoplasm that surrounds a deeply staining, heterochromatic nucleus. Lymphocytes are broadly classified as T and B lymphocytes, with B lymphocytes further capable of differentiating into plasma cells. T and B lymphocytes cannot be distinguished in H & E-stained sections.

Plasma cells are antibody-producing cells that develop from B lymphocytes. Plasma cells are ovoid, with a strong cytoplasmic basophilia (due to an extensive rough endoplasmic reticulum) and a prominent pale-staining region (indicating the position of the Golgi apparatus). The nucleus is eccentric and spherical with peripherally-located clumps of heterochromatin.  Plasma cells are considered to have limited migratory ability.

Granular leukocytes consist of neutrophils, eosinophils and basophils, their names reflecting the staining affinities of their cytoplasmic granules. They are derived from the bone marrow.

Neutrophils, also called polymorphonuclear leukocytes or PMNs, have a 3-5 lobed nucleus and weakly staining cytoplasmic granules. They function as phagocytes.

Eosinophils have a bi-lobed nucleus and red-staining cytoplasmic granules. They respond to allergic reactions and parasitic infections.

Basophils have an irregular or bi-lobed nucleus and blue-staining granules. Their function is similar to that of mast cells.