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Animal Tissues

** Note: it is best to print this lab handout in color **

Learning Objectives

Building on the learning objectives from your lab syllabus, you will be expected to:

1. List the four major types of animal tissues and describe the structural characteristics and function(s) of each. 2. Identify slides of various types of epithelium, connective, muscle and nervous tissues. 3. Provide examples of where each tissue type is located within the body. 4. Relate the structure of individual tissue types to their function.

Introduction

Animals are composed of specific tissues, which are groups of cells similar in structure that perform a common function. Groups of two or more tissue types form organs, and two or more organs form organ systems. For example, nervous tissue is composed of cells called neurons that receive and conduct electrical signals. Nervous tissue, along with other tissue types, forms the brain, spinal cord and nerves (organs), which collectively make up the nervous system (organ system).

In this lab, we will focus on the tissue level of organization. There are four types of tissues found in animals: epithelial tissue, connective tissue, muscle tissue, and nervous tissue. You will learn the major characteristics and locations of each tissue and examine various types of each tissue under the microscope.

There are a few important things to keep in mind during your studies:

As with all levels of organization, the structure of the tissue is related to its function. As you examine each tissue under the microscope, think about how the appearance of the tissue relates to the function it performs.

Most of the slides you will see today contain more than one tissue type. Pay attention to the descriptions and figures provided in the handout to make sure you’re looking at the correct tissue.

Make sure you examine each tissue at all magnifications (4x, 10x, 40x objectives). Certain tissues will be best seen at lower magnifications while others (including specific cell types) are best viewed at higher magnifications. Any magnification may be used on the lab practical.

You are strongly encouraged to draw each tissue, and space is provided in the handout for this purpose.

Activity 1: Epithelial Tissue

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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. Note the cell shapes as well as the position & shape of the nucleus for each cell type in the Figure above. 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.

Epithelial tissues generally function in protection, absorption and secretion, but the function 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 typically be found on the edge of the tissue.

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Simple squamous epithelium

Artery, vein, and nerve slide: Simple squamous epithelium comprises the inner lining of blood vessels (arteries, capillaries and veins), where it provides a smooth surface that reduces friction as blood travels through the vessels. In capillaries, gases and nutrients diffuse between the blood and tissues across this thin barrier. 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 simple squamous epithelium allow for efficient exchange o gases and solutes across the capillaries and lungs? ________________________________________________________________ ____________________________________________________________________________________________________________________________________________________________________________________

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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.

Simple columnar epithelium

Duodenum/Small intestine slide: The intestinal lining is a simple columnar epithelium. The primary function of these cells is absorption of nutrients. As you examine the slide, note the large, oval shaped nuclei that are positioned near the basal edge of the cells. Also note the large, clear goblet cells that are interspersed in the epithelial layer. These are glandular cells that secrete mucus that helps protect the underlying tissues.

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Stratified squamous epithelium

Esophagus/stomach slide: Stratified squamous epithelium consists of multiple layers, with squamous cells at the apical surface. The primary function of this type of epithelium is protection. Areas subject to abrasion, like the mouth, esophagus, and skin, have stratified epithelium. Cells at the apical surface can be scraped away (for instance, by food particles traveling down the esophagus), but the layered nature of the epithelium ensures that the underlying tissues are protected. Note the thick layer of epithelium on the esophagus slide. (*This slide also contains stomach tissue, which has a simple columnar epithelium)

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 slides. How are they similar? How are they different?

__________________________________________________________________________________________ __________________________________________________________________________________________

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Activity 2: 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 (which gives 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.

Connective tissues can be grouped into four categories based on their extracellular matrix: loose, dense, supporting and fluid connective tissues. Using your textbook (6th edition, pg 822), fill in the table below with a short description of the matrix and the overall function for each type.

Type of Connective Tissue Description of Matrix Function

Loose connective tissue

Dense connective tissue

Supporting connective tissue

Fluid connective tissue

**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.

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Loose connective tissue

Areolar tissue slide: Areolar tissue is widely distributed throughout the body. It is located underneath (at the basal surface) of all epithelial tissues, and forms a soft cushioning around organs. Collagen and elastin fibers are visible and widely dispersed within the matrix, with fibroblasts (the cells that secrete the matrix) scattered amid the fibers. In addition to supporting epithelia and cushioning organs, areolar tissue contains immune cells that help fight infection and regulate inflammation.

Adipose tissue slide: Adipose tissue consists of adipocytes, or fat storage cells. It functions in energy storage, insulation, and cushioning. Small pockets of adipose tissue can be found all over the body, but accumulates under the skin (subcutaneous fat) and around certain organs, such as the kidneys. Unlike other connective tissues, it has very little matrix and the cells are closely packed together. Each cell contains a large fat droplet, which pushes the nucleus to the side. Note the clear cytoplasm and the peripherally located nuclei of the fat cells in the slide.

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Dense connective tissue

Palmar Skin (Human skin corpuscle) slide: The layer of skin that lies deep to the epidermis is called the dermis and is composed of dense connective tissue. This tissue contains densely packed bundles of irregularly arranged collagen fibers. It is found in areas of the body that are subject to tension from many different directions. Note the thick layer of dense connective tissue that lies deep to the epithelium on the skin slide. Nuclei of the connective tissue cells are scattered throughout the collagen

fibers.

Supporting connective tissue

Hyaline cartilage slide: Hyaline cartilage is the most abundant type of cartilage in the body and is found in the rib cage, the nose, the trachea, and the ends of long bones. It provides structural support (but is more flexible than bone) and has cushioning properties. Hyaline cartilage has a firm matrix with abundant collagen fibers, but the individual fibers cannot be seen under the microscope. When viewed under the microscope the matrix an amorphous quality (no discernable structures). The cells, which are known as chondrocytes, reside in small cavities within the matrix called lacunae.

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Bone tissue slide: Bone tissue forms the skeletal system. It functions in structural support, protection, and mineral (calcium) storage. The extracellular matrix of bone tissue contains abundant collagen fibers as well as a hard, calcified ground substance. Mature bone cells, called osteocytes, reside in cavities within the matrix called lacunae. As bone tissue is formed, channels remain in the hardened matrix that provide passageways for blood vessels and nerves. The larger channels are called central canals (Haversian canals). Bone tissue forms in rings (lamellae) around these canals, creating a structure called an osteon. Examine the bone tissue slide, noting the osteons with their lamellae and bulls-eye like central canals. The lacunae, which contain the bone cells, are visible as small dark patches in the lamellae.

Fluid connective tissue

Blood slide: Blood consists of several cell types suspended in a liquid matrix (plasma). The most numerous cells are the erythrocytes (red blood cells), which transport oxygen. In mammals, these cells lack a nucleus and have a biconcave shape that increases their surface area/volume ratio for more efficient transport. Various types of leukocytes (white blood cells) are also present. These cells are components of the immune system, and function to protect the body against foreign invaders (bacteria, viruses, etc) as well as clean up cell debris. A third type of solid component you may see on these slides are cellular fragments called platelets, which function in blood clotting.

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Activity 3: Muscle Tissue

Muscle tissue is specialized for contraction. The cells are elongated, and are also known as muscle fibers. They contain the contractile proteins actin and myosin, which interact to shorten and elongate the cells. There are three different types of muscle tissue: skeletal, cardiac, and smooth. Examine each type of tissue using the muscle composite slide. (*The skeletal and smooth muscle are shown as part of organs, so they are not the only tissue present).

Skeletal muscle (muscle composite slide)

Skeletal muscles are attached to bones, and contraction of these muscles generates body movements (limb movement, jaw movement, breathing, etc.). They are categorized as voluntary muscle fibers because we can consciously control their contractions. The skeletal muscle fibers are long and cylindrical, with multiple peripherally located nuclei. The cells have striations, alternating light and dark bands that result from the ordered arrangement of actin and myosin within the cell.

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Cardiac muscle (muscle composite slide)

Cardiac muscle is present in the heart, and is categorized as involuntary (not under conscious control). Cells are striated, but the striations are much less obvious than in skeletal muscle tissue. The cells are shorter than skeletal muscle fibers, have a single nucleus and are often branched. Individual cells are connected via gap junctions and desmosomes. These cellular connections are visible under the microscope as dark bands called intercalated disks. These cellular communication junctions are necessary for the coordinated beating of the heart.

Smooth muscle (muscle composite slide & artery/vein/nerve slide)

Smooth muscle tissue is found in the walls of hollow organs, such as the gastrointestinal tract, blood vessels, and the urinary bladder. Contractions of these muscles propel fluid or materials through the organs (i.e. food through the GI tract, blood through blood vessels, urine pushed out of bladder). Like cardiac muscle, it is categorized as involuntary.

Smooth muscle cells are not striated (hence the name “smooth” muscle); they have a single nucleus, and have tapered ends. Examine the smooth muscle on the muscle composite slide as well as the blood vessel slide. In blood vessels there is a layer of smooth muscle deep to the epithelial layer. It is thicker on the artery than on the vein, but can be seen in both.

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Activity 4: Nervous Tissue

Nervous tissue is specialized for communication and composes the brain, spinal cord, and peripheral nerves. The tissue consists of two major cell types: neurons and glial cells. Neurons communicate with each other via electrical and chemical signals. They have nucleated cell bodies and two types of elongated cellular processes: dendrites – which receive signals, and axons – which send signals.

Glial cells are the support cells of nervous tissue. There are several different types with various functions, including maintaining proper ion concentrations in the fluid surrounding neurons, generating myelin (an insulating material that surrounds some axons), and cleaning up debris.

Examine the slide of nervous tissue (giant multipolar neuron slide). Note the large neurons with their elongated cellular processes and the smaller, more numerous glial cells.

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Activity 5: Organs

As explained in the introduction, organs are composed of two or more tissue types. You have examined several organs in the tissue slides you’ve been viewing, including the skin, intestine, and blood vessels. Review each of these slides again, and try to identify the different types of tissues that comprise the whole organ. Label the specific tissue types on the pictures shown below.

Palmer skin

Intestine

Artery

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Post-Lab Activity

Complete the tables below to review today’s lab.

Epithelial Tissues

Appearance Function Location

Simple squamous

Simple cuboidal

Simple columnar

Stratified squamous (nonkeratinized)

Stratified squamous (keratinized)

Muscle Tissues

Appearance Location Conscious control?

Skeletal muscle

Cardiac muscle

Smooth muscle

Connective Tissues

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Type (i.e. loose, dense, etc)

Appearance Function Location

Areolar

Adipose

Dense connective tissue

Hyaline cartilage

Bone

Blood