Page 156 - Fundamentals of anatomy physiology

Chapter 4 The Tissue Level of Organization 143

Figure 4–2 Cell Junctions. Interlocking 4
junctional
Embedded proteins Tight
proteins junction
(connexons) Tight junction

Adhesion belt
Terminal web

b Gap junctions permit the Gap junctions Adhesion
free diffusion of ions and belt
small molecules between
two cells. Spot desmosome

Hemidesmosome c A tight junction is formed by the
fusion of the outer layers of two
a This is a diagrammatic view of an plasma membranes. Tight junctions
epithelial cell, showing the major prevent the diffusion of fluids and
types of intercellular connections. solutes between the cells. A continu-
ous adhesion belt lies deep to the
tight junction. This belt is tied to the
microfilaments of the terminal web.

Clear Basement Intermediate Cell adhesion
layer membrane filaments molecules
(CAMs)
Dense Dense area
layer

e Hemidesmosomes attach a cell to Proteoglycans
extracellular structures, such as the
protein fibers in the basement d A spot desmosome ties
membrane. adjacent cells together.

A typical desmosome is formed by two cells. Within each Hemidesmosomes resemble half of a spot desmosome.
cell, a complex known as a dense area is connected to the
c­ ytoskeleton (Figure 4–2d). This connection to the cytoskeleton Rather than attaching one cell to another, a hemidesmo-
gives the desmosome—and the epithelium—its strength. For some attaches a cell to extracellular filaments in the base-
example, desmosomes are abundant between cells in the su- ment membrane (Figure 4–2e). This attachment helps
perficial layers of the skin. As a result, damaged skin cells are stabilize the position of the epithelial cell and anchors it to
usually lost in sheets rather than as individual cells. (That is underlying tissues.
why your skin peels rather than sheds off as a powder after
a sunburn.) Attachment to the Basement Membrane

There are two types of desmosomes: Not only do epithelial cells hold onto one another, but they also
remain firmly connected to the rest of the body. The inner sur-
Spot desmosomes are small discs connected to bands of in- face of each epithelium is attached to a two-part basement mem-
brane. The layer closer to the epithelium, the clear layer, (also
termediate filaments (Figure 4–2d). The intermediate fila- called the lamina lucida) contains glycoproteins and a network
ments stabilize the shape of the cell.