Page 169 - Atlas of Histology with Functional Correlations

migrate from the blood vessels and take up residence in the connective tissue of

different regions of the body.

FUNCTIONAL CORRELATIONS 5.1 ■ Cells in

Connective Tissue

Fibroblasts are the dominant cells in the connective tissue. These highly
active cells with irregularly branched cytoplasm synthesize collagen,

reticular, and elastic fibers as well as carbohydrates, such as
glycosaminoglycans, proteoglycans, and adhesive glycoproteins of the
extracellular matrix. The spindle-shaped fibrocytes are smaller than the
fibroblasts and are the mature and less active cells of the fibroblast line.

Macrophages or histiocytes are phagocytes that are attracted to the sites

of inflammation. They ingest bacteria, dead cells, cell debris, and other
foreign matter that enters the connective tissue. Macrophages are part of the
mononuclear phagocyte system, derived from circulating blood monocytes

that are formed in the bone marrow, take up residence in the connective
tissue, and differentiate into macrophages. These cells also enhance
immunologic activities of the lymphocytes. Macrophages are antigen-
processing cells to lymphocytes that are then stimulated to perform specific
immune responses. Although present throughout connective tissue of the

body, the macrophages have specific names in different organs. Dusts cells
are found in the alveoli of the lungs, Kupffer cells line the sinusoids in the
liver, Langerhans cells are in the epidermis of the skin, microglia in the

tissues of the brain, monocytes in the circulating blood, and the osteoclasts
in the bone.

Lymphocytes are the most numerous cells in the loose connective tissue
of the respiratory and gastrointestinal tracts. They do not have any function
in the bloodstream, but leave the circulatory system and enter the connective

tissue through the capillaries. They mediate immune responses to antigens
that enter these organs and, once activated, produce antibodies that kill cells
by inducing cell death (apoptosis). There are three functional types of
lymphocytes: T lymphocytes, B lymphocytes, and NK (natural killer) cells.

These lymphocytes are primarily identified by the specific marker proteins
on the cell membrane.

Plasma cells are derived from B lymphocytes that enter the connective
tissue from blood vessels and then differentiate into plasma cells when they

168