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Monocyte/macrophage:
There are two types of macrophage - free and fixed.
Free macrophages are derived from circulating monocytes
that enter tissues in response to stimuli and once in the tissue differentiate
into macrophages.
Fixed or resident macrophages are derived from the haematopoietic
stem cell but migrate into the tissues in which they are fixed early in
development.
It is thought that both free and fixed macrophages are
capable of cell division at least to some degree.
Macrophages are larger than neutrophils (15 to 50 microns),
have a single large, usually centrally located, nucleus that many be folded
or kidney shaped (reniform). Macrophages have abundant cytoplasm that
usually stains gray-blue with routine stains. Macrophages also contain
numerous lysosomes in their cytoplasm that contain many of the same products
as are present in neutrophil granules.
Macrophages are the most versatile of the leukocytes. They
possess some of the same basic functions as neutrophils, such as phagocytosis
and killing of microbes. They arrive in numbers at inflammatory sites
after the initial wave of neutrophils and are then the key controlling
element of the inflammatory response from that point on. They have a longer
life span the neutrophils (weeks)
Macrophages are capable of synthesizing a huge array of
inflammatory mediators, oxygen radicals and nitric oxide, and lipid mediators
of inflammation (arachidonic acid metabolites).
Macrophage modifications:
There are several modified forms of macrophages that you
may recognize in tissue section:
-
Epithelioid Macrophages - these are larger than normal macrophages
with abundant cytoplasm. They are often arranged in a cobblestone
or "epithelioid" fashion (i.e. they resemble the normal appearance
of epithelium).They often form when macrophages are chronically stimulated
by hard to kill microbes (e.g. Johne's disease - Mycobacterium paratuberculosis)
-
Foreign Body Giant Cells - these form by fusion of macrophages into
large syncitia that are then capable of ingesting large particles.
Nuclei are randomly scattered throughout the cytoplasm and the cytoplasm
often conforms to the shape of the phagocytosed material. They can
be up to 500 micron in diameter. Foreign body giant cells arise to
engulf endogenous material such as bone, keratin, hair, cholesterol
crystals or exogenous material such as sutures, metal splinters, plant
awns.
-
Langhan's Giant Cells - these are usually seen with epithelioid
cells. They are common in fungal and mycobacterial infections. They
have a distinctive appearance of up to 20 nuclei placed peripherally
in the cytoplasm, often in a horse-shoe arrangement. The abundant
cytoplasm tends to be palely eosinophilic unless many mycobacteria
are present, when the cytoplasm becomes greyish.
-
Touton Giant Cells - these are distinctive cells often associated
with fat breakdown. Cells have a ring of peripherally placed nuclei
with foamy pale eosinophilic cytoplasm.
 
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