CONNECTIVE TISSUE

Connective tissues support and bind. Unlike epithelial tissues,
connective tissues are distributed throughout an extracellular matrix.
This matrix frequently contains fibers that are embedded in a
ground substance with a consistency anywhere from liquid to solid.
To a large extent, the nature of this extracellular material determines
the functional properties of the various connective tissues.
Connective tissues are of two general types, depending on
whether the fibers are loosely or densely packed. In loose connective
tissue strong, flexible fibers of the protein collagen are
interwoven with fine, elastic, and reticular fibers, giving loose
connective tissue its elastic consistency and making it an excellent
binding tissue (e.g., binding the skin to underlying muscle
tissue). In fibrous connective tissue, the collagen
fibers are densely packed and may lie parallel to one another, creating
very strong cords, such as tendons (which connect muscles
to bones or to other muscles) and ligaments (which connect bones
to bones).
Adipose tissue is a type of loose connective tissue that consists
of large cells that store lipid. Most often, the cells
accumulate in large numbers to form what is commonly called fat.
Cartilage is a hard yet flexible tissue that supports such
structures as the outer ear and forms the entire skeleton of such
animals as sharks and rays. Cells called chondrocytes
lie within spaces called lacunae that are surrounded by a rubbery
matrix that chondroblasts secrete. This matrix, along with
the collagen fibers, gives cartilage its strength and elasticity.
Bone cells (osteocytes) also lie within lacunae, but the
matrix around them is heavily impregnated with calcium phosphate,
making this kind of tissue hard and ideally suited for its
functions of support and protection.
Blood is a connective tissue in which a fluid called plasma
suspends specialized red and white blood cells plus platelets.
Blood transports various substances throughout the bodies
of animals.