144
UNIT ONE
A NEW VIEW OF CELL SPECIALIZATION—PROTEOMICS
extracted from a cell or tissue sample, converted to DNA “probes,” and labeled
with a fluorescent dye. The grid positions where the probes bind fluoresce,
which a laser scanner detects and converts to an image. The intensity of the
F
uorescence reveals the abundance of the mRNAs present. Probes representing
two cell sources can be linked to di±
erent F
uorescent tags so that their gene
expression patterns can be directly compared—such as a healthy and cancer-
ous version of the same cell type. A microarray can scan for activity in all genes
or be customized to paint molecular portraits of speci²
c functions.
Researchers are compiling DNA microarray patterns for the 260
+
types of
normal di±
erentiated cells in a human body. A statistical analysis called hierarchy
clustering groups cells by similarities in gene expression. The results generally
agree with what is known of histology (the study of tissues) from microscopy,
but sometimes reveal new proteins in speci²
c cell types. Although DNA microar-
rays can ²
ll in molecular details that cannot be seen under a microscope, a pair
of discerning human eyes will always be necessary to see the bigger picture of
how cells assemble into tissues.
A
tissue atlas displays groups of cells stained to reveal their
specializations and viewed with the aid of a microscope. It’s
easy to tell skeletal muscle from adipose tissue from blood.
A new way to look at tissues is to pro²
le the proteins that
their cells manufacture. These proteins are responsible for
cell specializations and arise from the expression of subsets of the genome.
Such an approach is called
proteomics.
A skeletal muscle cell, for example,
transcribes messenger RNA molecules from genes that encode contractile
proteins, whereas an adipose cell yields mRNAs whose protein products
enable the cell to store massive amounts of fat. All cells also transcribe many
mRNAs whose encoded proteins make life at the cellular level possible.
In the mid 1990s, technology was developed to display the genes
expressed in particular cell types. The tool is a DNA microarray (also known
as a gene chip). It is a square of glass or plastic smaller than a postage stamp
to which thousands of small pieces of DNA of known sequence are bound, in
a grid pattern, so that the position of each entrant is known. Then mRNAs are
5.1
INTRODUCTION
In all complex organisms, cells are organized into
tissues
(tish
uz), which are layers or groups of similar cells with a
common function. Some cells, such as blood cells, are sepa-
rated from each other in fluid-filled spaces or intercellular
(in
ter-sell
u-lar) spaces. Many other cell types, however, are
tightly packed, with structures called
intercellular junctions
that connect their cell membranes.
In one type of intercellular junction, called a
tight junc-
tion,
the membranes of adjacent cells converge and fuse. The
area of fusion surrounds the cell like a belt, and the junction
closes the space between the cells. Tight junctions typically
join cells that form sheetlike layers, such as those that line
the inside of the digestive tract. The linings of tiny blood ves-
sels in the brain consist of cells held tightly together (From
Science to Technology 5.1).
Another type of intercellular junction, called a
desmo-
some,
rivets or “spot welds” skin cells, enabling them to
form a reinforced structural unit. The cell membranes of cer-
tain other cells, such as those in heart muscle and muscle
of the digestive tract, are interconnected by tubular chan-
nels called
gap junctions.
These channels link the cytoplasm
of adjacent cells and allow ions, nutrients (such as sugars,
amino acids, and nucleotides), and other small molecules to
move between them
(f
g. 5.1)
.
Table 5.1
summarizes inter-
cellular junctions.
Tissues can be distinguished from each other by varia-
tions in cell size, shape, organization, and function. The
study of tissues,
histology,
will assist understanding in later
discussions of the physiology of organs and organ systems.
The tissues of the human body include four major types:
epithelial, connective, muscle,
and
nervous.
These tissues
associate, assemble, and interact to form organs that have
specialized functions.
Table 5.2
compares the four major tis-
sue types.
This chapter examines in detail epithelial and connec-
tive tissues and introduces muscle and nervous tissues.
Throughout this chapter, simpli±
ed line drawings (for exam-
ple, ± g. 5.2
a
) are included with each micrograph (for exam-
ple, ±
g. 5.2
b
) to emphasize the distinguishing characteristics
of the speci± c tissue, as well as a locator icon (an example
of where in the body that particular tissue may be found).
Chapter 9 discusses muscle tissue in more detail, and chap-
ters 10, 11, and 12 detail nervous tissue.
PRACTICE
1
What is a tissue?
2
What are the di±
erent types of intercellular junctions?
3
List the four major types of tissue.
5.2
EPITHELIAL TISSUES
General Characteristics
Epithelial
(ep
ı˘-the
le-al)
tissues
are found throughout the
body. Epithelium covers the body surface and organs, forms
the inner lining of body cavities, and lines hollow organs. It
always has a
free (apical) surface
exposed to the outside or
internally to an open space. A thin, nonliving layer called the
basement membrane
anchors epithelium to underlying con-
nective tissue.
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