Organization of tissues into an organ
An organ in an animal is composed of varying amounts of all four basic tissues. As an example, you will examine the largest organ of the body, the skin. As you examine this organ, attempt to classify each of the components into one of the four tissues.
The skin is an organ forming a protective covering over the entire exterior of the body. It is subdivided into three regions: epidermis, dermis and hypodermis (subcutaneous region). The epidermis is the outermost layer and is composed of stratified squamous epithelium. It protects the underlying tissues. Epidermis is very thick and becomes keratinized when covering surfaces of the body that receive excessive abrasion (e.g. soles of the feet, palms of the hands). Keratin is a type of protein. Several layers of cells are present within this type of epithelium, with surface layers being squamous shaped and deepest layers showing a cuboidal shape. In the process of keratinization, new cells are pushed toward the surface as they are gradually transformed into flat, dead cells completely filled with keratin. It is also important to realize that epidermis is non-vascular (i.e. lacks blood vessels) and receives all its nutrition from the dermis. As a result, as the cells are pushed away from the deepest layer, they begin to die from lack of nutrients and oxygen. These cells are constantly abraded from the surface and are replace by cell division of the deeper cells, located close to the basement membrane. The epidermis is attached to the dermis below it by a series of folds, know as dermal papillae (singular, papilla).
Beneath the epidermis is the dermis which is the active area of the skin. The dermis is subdivided into the upper papillary and the lower reticular regions. The papillary region consists of loose (areolar) connective tissue, while the reticular region consists of dense irregular connective tissue (the detailed structure of the dense irregular connective tissue will not be studied in this laboratory). The dermis also contains blood vessels, specialized nerve endings, sweat glands, sebaceous (oil) glands and hair follicles. Beneath the dermis is hypodermis. It consists of both loose (areolar) and adipose connective tissues. It also contains arteries and veins (Figures 19, 20, 21).
Examine Slide #51 of a section of mammalian skin. Distinguish between the epidermis, dermis, and hypodermis. Locate a hair follicle, the infolded pit of the epidermis in which the hair is located. The portion of the hair which is embedded in the dermis is called the root and that which is visible above the surface of the skin is the shaft. Locate the sebaceous glands which are associated with the follicle. These glands are modified epithelial tissue and secrete sebum (an oil) into the follicle to keep the hair soft and pliable. Find the arrector pili muscle associated with the follicle. This is smooth muscle tissue which makes our hair "stand on end" when you are cold or frightened. Locate the coiled tubular sweat gland. This gland releases its secretion (sweat) into a duct which leads to the skin surface opening into a sweat pore. The dermis also contains various sensory receptors of the nervous system. Some are simply nerve endings while others are specialized to perceive specific types of sensory information. For example, Meissner's and Pacinian corpuscles detect pressure (Figure 20). Meissner's corpuscles are located in the dermal papillae and detect light touch. Pacinian corpuscles are located deeper in the dermis and detect stronger pressures. Locate each of these corpuscles on your slide or on the demonstration slides. Also, locate adipose cells in the hypodermis.
Figure 19: Microscopic view of mammalian skin.
Figure 20A: Meissnerís corpuscle. Figure 20B: Pacinian corpuscle.
For study purposes, use the following space to do the sketches of the tissues.
STUDENT'S NAME: ____________________________________________
LAB SECTION: ________________________________________________
1) Match the following terms with corresponding number(s) seen in Figure 21.
______ Epidermis ______ Hair root
______ Dermis ______ Hair shaft
______ Hypodermis ______ Hair follicle
______ Dermal papillae ______ Arrector pili muscle
______ Nerve ending ______ Sweat gland
______ Sebaceous gland ______ Sweat gland duct
______ Adipose cell ______ Pacinian corpuscle
______ Meissnerís corpuscle
Figure 21: Schematic representation of mammalian skin.
2) Fill in the table below with examples of the specific structures in the skin composed of each of the four basic tissues and give the function of each structure.
Structure(s) in the skin
Adipose cells in hypodermis
3) Complete the following table.
Simple columnar epithelium
Simple cuboidal epithelium
Simple squamous epithelium
Stratified squamous epithelium
Pseudostratified ciliated epithelium
Areolar connective tissue
Adipose connective tissue
Dense regular connective tissue
4) Complete the following table.
Shape of cells
Do the cells branch?
Position of the nucleus/nuclei
Number of nuclei per fibre
Location in body
Function in body