directly between cells. Apical specializations are prominent in epithelia and
includemicrovilli that increase surface area; stereocilia,which are non-
motile modified microvilli; and cilia and flagella,which are motile struc-
tures. Cilia and flagella have the classic “9+2”microtubular arrangement
emanating from basal bodies.The basal surface may be modified with
infoldings that house numerous mitochondria as found in proximal and
distal tubule cells of the kidney and striated duct cells of the salivary
glands. Those cells are involved in extensive ion transport.
CONNECTIVE TISSUEConnective tissue consists of cells and a matrix (fibers and ground sub-
stance). The cells include fibroblasts(the source of collagen and other
fibers),plasma cells(the source of antibodies), macrophages(the cells
responsible for phagocytosis), mast cells(the source of heparin and hista-
mine), and a variety of transient blood cells: lymphocytes (B and T),
eosinophils, basophils,andneutrophils (PMNs).B cells are involved in
humoral immunity and T cells in cell-mediated immunityas well as
humoral immunity(helper T cells). Neutrophils phagocytose bacteria; the
dead neutrophils are a major component of pus. Basophils, like mast cells,
release histamine although they originate from a different bone marrow
stem cell. Eosinophils are involved in response to parasitic infection.
Eosinophilic granules contain a crystalline core of major basic protein,
which is toxic for parasites and histaminase, which breaks down histamine
and limits the allergic response. Type I collagenand elastin make up the
predominant fibers found in connective tissue. Ground substance includes
proteoglycans and glycoproteins that organize and stabilize the fibrillar
network.Type II collagenis associated with hyaline cartilage; type III col-
lagenforms the collagenous component of reticular connective tissue found
in highly cellular organs, such as the liver and lymphoid organs. Type IV
collagenforms a sheet-like meshworkorinsoluble scaffoldingof the
basal lamina. Other types of collagen exist and include the fibril-associated
collagenswith interrupted triple helices (FACIT).Collagen fibrils are con-
nected to other extracellular matrix molecules by the FACIT collagens.
SPECIALIZED CONNECTIVE TISSUES:
BONE AND CARTILAGEBone contains three major cell types: osteoblaststhat secrete type I colla-
genand noncollagenous proteins; osteocytes,which maintain mature
High-Yield Facts 19