Biophotonics_Concepts_to_Applications

1.6.3 Biological Tissues and Organs.

Abiological tissue(or simply atissue) is an ensemble of similar cells that col-
lectively carry out a specific function. The four basic categories are epithelial,
connective, muscle, and nervous tissues. The characteristics of these tissues are
described in the following paragraphs and are summarized in Table1.8.
Epithelial tissuescover the outside surface of the skin and line the inner surfaces
of organs and cavities within the body. The cells of an epithelium are connected by
tight junctions and thus serve as a barrier to protect against mechanical injury,
invading microorganisms, andfluid loss. In addition to protecting the organs that
they cover or line, some epithelial tissues are specialized for absorbing or secreting
chemical solutions. One example is the mucous membrane, which secretes a slip-
pery substance called the mucous that lubricates and moistens a specific tissue
surface.
The functions ofconnective tissueare to bind together other tissues, to give
shape to organs, and to hold them in place. Examples of connective tissue are loose
connective tissue, adipose tissue,fibrous connective tissue, cartilage, bone, and
blood.Loose connective tissueconsists of a loose weave of proteinfibers, which are
categorized as collagenousfibers (a rope-like coil of three collagen molecules that
resist stretching), elasticfibers (rubbery stretchablefibers made of elastin that
quickly restore skin shape if it is stretched),fibroblasts (cells that maintain the
structural integrity of connective tissue), and microphages (travelling cells that
swallow bacteria and the debris of dead cells and eliminate them from the body).
Adipose tissueis located beneath the skin, around internal organs, and in bone
marrow. Its functions include providing insulation from heat and cold, providing a
protective padding around organs, and storing lipids that can be used to meet the
energy needs of the body.Fibrous connective tissueare dense collagen-enriched

Table 1.8 Characteristics and functions of basic biological tissues

Biological

tissue

Characteristic Functions

Epithelial

tissue

Cover outside skin surfaces; line inner

surfaces of organs and cavities in the

body

• Protect against invading
  microorganisms,fluid loss, and
  mechanical injury


• Absorb or secret special chemical
  solutions
  Connective
  tissue

Examples: loose andfibrous

connective tissue, adipose tissue,

cartilage, bone, blood

Bind together various tissues, give

shape to organs, and hold organs in

place

Muscle

tissue

Long, excitable cells capable of

considerable contraction

The three main types are skeletal,

cardiac, and visceral muscles

Nervous

tissue

Cells that make up the central and

peripheral nervous systems

• Central nervous system: brain and
  spinal cord


• Peripheral nervous system: cranial
  and spinal nerves

1.6 Structures of Biological Cells and Tissues 19