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Connective and supportive tissues (textus connectivus) 63

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3.2 Differentiation of cells from the mesoderm (schematic). Via fusion of mononuclear precursor cells of
a
granulocyte-monocyte line.

ing further into mature, less active fibrocytes, chondrocytes and differentiate into macrophages (Figure 3.1). They are
and osteocytes. relatively numerous in loose connective tissue where, in
Fibroblasts have irregularly developed cell processes their resting form, they may be difficult to distinguish from
and nuclei rich in euchromatin. Their cytoplasm con- fibroblasts. The primary function of histiocytes is localised
tains numerous organelles that are dedicated primarily to phagocytosis. Histiocytes can move freely in an amoeboid
protein biosynthesis (rough endoplasmic reticulum, ribo- fashion through the tissue. Their nucleus is round to oval
somes, Golgi apparatus). Fibroblasts synthesise the fibres and the cell surface appears irregular due to the presence
and ground substance of the extracellular matrix. of numerous processes (pseudopodia). The cytoplasm
Fibrocytes are small, spindle-shaped cells that usually contains granules (lysosomes). In addition to their endo-
lie between bundles of fibres. Under the light microscope and phagocytic function, histiocytes (macrophages) have
their nuclei appear dense and elongated. The cytoplasm a role in antigen presentation and the immune response.
is weakly acidophilic with few metabolically active They synthesise proteins (e.g. interferon, complement,
organelles. cytokines) that act as immune mediators (see Chapter 8,
Histiocytes, or connective tissue macrophages, are ‘Immune system and lymphatic organs’). The lifespan of
derived from circulating monocytes that enter the tissue active macrophages is approximately 2–3 months.

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