Learn about animal tissues and their types
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Animal Tissues
There are four types of tissues found in animals: epithelial tissue, connective tissue, muscle tissue, and nervous tissue. In this lab you will learn the major characteristics of each tissue and examine various types of each tissue under the microscope.
I. Epithelial Tissue
Epithelial tissues line body surfaces and cavities, as well as form glands. The cells of the tissue are
closely connected to each other via cellular junctions and because epithelium is found on the edges of
organs, it has two distinct surfaces. The apical surface is exposed to the body cavity or exterior, while
the basal surface is adjacent to the underlying tissue (see the figure below). Epithelia contain no blood
vessels (they are non-vascular) and are dependent upon the underlying connective tissue for nutrients.
Epithelial cells can be one of three shapes: squamous (flattened), cuboidal, or columnar. If the
epithelium consists of one cell layer it is called simple epithelium. Stratified epithelium has more than
one layer. The combination of cell shape and the number of cell layers is used to classify epithelial
tissue. (i.e. simple columnar epithelium would consist of a single layer of column shaped cells). Cell
shape in stratified epithelium can vary, but the shape of the cells at the apical surface determines the
name.
The function of epithelial tissue varies depending on its location. As you explore the types of epithelial
tissues below, think about how the structure of each tissue fits its function.
**Hint: Many of the slides you will examine contain other types of tissue in additional to epithelium.
Keep in mind that epithelial tissues line either the outer or inner surfaces of organs and will thus be
found on the edge of the tissue.Simple squamous epithelium
Blood vessels (artery, vein, and nerve slide): Simple squamous epithelium comprises the inner lining of
blood vessels, where it provides a smooth surface that reduces friction as blood travels through the
vessels. The blood vessel slide shows a cross section of an artery and a vein. The wavy lining of the
vessel lumen (interior) is simple squamous epithelium.
Lung slide: The walls of lung air sacs (alveoli) are also composed of simple squamous epithelium. Air
sacs are the location of gas exchange between the air and blood. How does the structure of this
epithelial type allow for efficient gas exchange? (hint: the gases have to travel through the epithelial
cells to move between the air and the blood)
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Simple cuboidal epithelium
Simple cuboidal epithelium (Kidney) slide: The tubules of the kidney are composed of a single layer of
cuboidal cells. The kidney slide shows cross sections of many tubules, all of which are lined with simple
cuboidal epithelium. These cells are active in absorption and secretion of various substances from or
into the kidney filtrate (which ultimately becomes urine). Note the shape of the epithelial cells and the
centrally located nuclei.Keratinized stratified squamous epithelium
Palmar Skin (Human skin corpuscle) slide: The epidermis (most superficial layer) of the skin is composed
of stratified squamous epithelial cells that contain large quantities of the protein keratin. Keratin is a
tough fibrous protein that offers protection from abrasion and water loss. New cells are produced at
the basal surface of the epithelium and are gradually pushed towards the apical surface. As they move
upwards, they become filled with keratin and eventually die, forming a layer of dead, keratin filled cells
on the apical surface of the epidermis. Examine the palmer skin slide, noting the entire epidermis and
the layer of dead cells at the apical surface. The dermis, which lies deep to the epidermis, is composed
of connective tissue.
Compare the skin and esophagus slide. How are they similar? How are they different?
II. Connective Tissue
Connective tissues vary widely in their form and function, but they are all characterized by the presence
of extracellular matrix. The extracellular matrix is nonliving material composed of protein fibers and
ground substance. The protein fibers are composed of collagen (which gives strength) or elastin (whichgives flexibility). The number and type of fibers differs between the various types of connective tissue.
The ground substance fills the spaces between the cells and the fibers. It contains interstitial fluid
(tissue fluid) and large polysaccharide molecules. The consistency of the ground substance can vary
from liquid to gel-like to a solid.
**Hint: Because connective tissue consists largely of extracellular matrix, the cells that are present will
be scattered among the matrix components. For most of these slides (adipose tissue is an exception),
you will not see cells directly adjacent to other cells as they are in epithelial tissue.