Download 5. Tissue Organization

5. Tissue Organization
Cells in the human body are organized into tissues in order that they may better carry out their
functions. Histology is that field of biology that focuses on the study of tissues. There are four
basic types of tissue in the human body: epithelial tissue, connective tissue, muscle tissue, and
nervous tissue (Table 5.1). We will discuss some basics of epithelial and connective tissues at
this time, and we will see more of them as we discuss specific organ systems. Muscle tissue and
nervous tissue will be considered in detail later this semester.
I. Epithelial Tissue: Surfaces, Linings, and Secretory Functions
Epithelial tissues cover body surfaces, line body cavities, and form various glands. All
substances passing into or out of the body pass through an epithelium. General functions of
epithelial tissues include protection, absorption, filtration, excretion, and secretion.
Special characteristics of epithelia (Fig. 5.1)
A. Cellularity. Whereas other tissues often include a considerable amount of extracellular
material, epithelial tissues are composed almost completely of tightly packed cells. There is
very little extracellular material.
B. Polarity. An epithelium has an apical surface, exposed to an internal or external space, and a
basal surface, attached to underlying tissue. Adjacent cells are bound tightly together, often
with specialized connections such as tight junctions and desmosomes. Why is this particularly
important for epithelia?
C. Attachment to a basement membrane. Sheets of epithelial tissue are supported by underlying
layers of connective tissue. A structure called the basement membrane forms the interface
between the epithelial tissue and connective tissue.
D. Avascularity. Epithelia do not contain blood vessels and must obtain nutrients, oxygen, etc.
from the exposed surface or the underlying tissue.
E. Extensive innervation. Epithelia are richly supplied with nerve endings to monitor the
condition of that particular location in the body.
F. Regeneration. Since they exist on an exposed surface, epithelial cells are exposed to damage.
Thus, they are continually replaced.
Classification of epithelia
There are two general schemes for classifying epithelia (Fig. 5.2). One classification scheme is
by shape of the cells; three shapes are identified: squamous, cuboidal, and columnar. The
other scheme is by the degree of layering: simple or stratified.
Simple epithelia consist of just a single layer of epithelial cells. They are fragile layers of tissue
and are found inside the body. Simple epithelia typically line surfaces used for exchange, such
as the linings of the respiratory surface and intestine. Simple epithelia also line the inside of the
blood vessels and heart chambers. Stratified epithelia contain several layers of cells, which give
additional protection. (Since the cells in different layers may be of different shapes, stratified
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epithelia are named for the cells in the apical layer.) Such tissues form the surface of the skin
and the lining of the mouth.
Six types of epithelia can be described by combining the three types of cells (squamous,
cuboidal, and columnar) with the degree of layering (simple or stratified). In the laboratory, we
will study four of these tissues. Two additional epithelia are also described: pseudostratified and
transitional. In the lab, we will study pseudostratified. Here is a summary of the five types of
epithelia that we will study in the lab. You will also be questioned about these tissues on the
lecture exam, based on the information given below:
Simple squamous epithelia are thin and delicate (Table 5.2a). The cells are shaped like fried
eggs. This type of tissue forms the alveoli of the lungs and lines the blood vessels.
Simple cuboidal epithelia provide limited protection and are often found on surfaces for
absorption or secretion (Table 5.2b).
Simple columnar epithelia also occur on surfaces used for absorption or secretion, such as the
stomach and intestine (Table 5.2c and d). The apical surfaces of the cells are typically lined with
microvilli to aid in absorption, and the tissue may contain goblet cells, which secrete mucus.
Pseudostratified columnar epithelia typically possess cilia. They are found lining the nasal
cavity, trachea, and bronchi. The tissue is only one cell layer thick, but the cells are of different
lengths, giving the false appearance of many layers (Table 5.2e and f).
Stratified squamous epithelia are quite resistant to mechanical stress (Table 5.3a and b). This
type of tissue forms the skin and the linings of the mouth and esophagus. In exposed surfaces
(i.e., the skin), cells of the apical layers contain fibers of the protein keratin, which strengthen
the tissue and make it resistant to water loss.
Glandular epithelia
A gland is a group of cells or a single cell that makes and secretes some watery product, which
is called a secretion.
Glands are classified into two major types: endocrine glands and exocrine glands. Endocrine
glands are ductless, and their products are released into the bloodstream or lymph. Exocrine
glands (Figs. 5.5, 5.6, and 5.7) release their products onto the surface of the body or into body
cavities. Multicellular exocrine glands have ducts.
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II. Connective Tissue
Connective tissues include bone, fat, and blood. Although the functions of these tissues are very
diverse, they share two properties in common:
A. Common origin. All connective tissues arise during embryonic development from tissue
called mesenchyme.
B. Extracellular matrix. Whereas epithelial tissue is composed mostly of cells, connective tissue
is composed mostly of extracellular material called the extracellular matrix. When observing
different tissues, this is the key to distinguishing a connective tissue.
The extracellular matrix of a connective tissue has two major components: ground substance
and fibers. The ground substance is unstructured material that fills the space between cells and
fibers. Fibers provide support and strength for the connective tissue. Three types of fibers may
be found in the connective tissue matrix:
A. Collagen fibers are long, flexible, and tough. They are particularly important in resisting
pulling forces (tension) on the tissue.
B. Elastic fibers are able to stretch and return to their original lengths.
C. Reticular fibers are similar in structure to collagen, and they are also strong and flexible.
These fibers are most abundant where connective tissue joins another type of tissue, such as in a
basement membrane. They are also abundant in the lymph nodes, spleen, and liver.
Two particular types of cells are often found wandering through the extracellular matrix of
connective tissues. Briefly describe the functions of these two cells:
Mast cell
Macrophage
Types of connective tissue
A. Connective tissue proper includes a variety of connective tissues that can be viewed as the
“proper” connective tissues. It can be divided into two subclasses:
Loose connective tissues (Table 5.5) include areolar tissue (found throughout the body acting
as a packing material between other tissues), adipose tissue (stores fat), and reticular tissue
(found in the lymphatic system where it supports cells of the immune system).
Dense connective tissues (Table 5.6) include dense regular connective tissue (which includes
tendons and ligaments) and dense irregular connective tissue (which is found in the skin, joint
capsules, and surrounding various organs).
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B. Supporting connective tissue includes cartilage and bone.
Cartilage has properties intermediate between dense connective tissue and bone. The primary
cells of cartilage are the chondrocytes. There are three basic varieties of cartilage (Table 5.7):
Hyaline cartilage is the most common type, and we will study it in the laboratory. The matrix
contains closely packed collagen fibers for strength and flexibility. This type of cartilage is
found connecting the ribs to the sternum, supporting the tubes of the respiratory tract, and on the
articular surfaces of many joints.
Elastic cartilage contains many elastic fibers. This type of cartilage is found in the outer ear,
the epiglottis, and the larynx.
Fibrocartilage is densely packed with collagen and is extremely tough. Pads of fibrocartilage
are found between the vertebrae (intervertebral discs) and between the pubic bones of the pelvis
(pubic symphysis).
Bone tissue (Table 5.8) will be studied in greater detail in Chapter 7.
C. Fluid connective tissue includes blood and lymph (Table 5.9). You will learn much more
about these tissues when you study the circulatory and lymphatic systems in BIO 2060.
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