Key Features of the Integumentary Organ System

Parts: Epidermis and Dermis, Hair, Nails and associated Muscles and Glands. The Hypodermis (aka subcutaneous fat aka superficial fascia) located immediately deep to the dermis is not considered to be part of the skin.

Major Tissue Types:

Epithelium: Keratinized stratified squamous epithelium, Glandular epithelium
Connective tissue (CT): Loose CT in dermal papillae, Dense irregular CT in the reticular region of the dermis
Muscular: Smooth muscle in the Arrector pili muscle and in blood vessels and
Nervous: Sensory nerve fibers that contact Merkel's cells and other receptors and Motor nerve fibers to smooth muscle

Characteristic Cell Types: Keratinocytes, Melanocytes, Langerhans cells, Merkel's cells

Functional Unit: None described

Layers of the Epidermis

The skin is a fascinating organ! It has so many different functions that that's why no single functional unit is described. I prefer to remember the layers of the epidermis, beginning with the deepest layer and moving superficially. This deep layer is named for its location--the stratum basale; and also for its very important function which is to generate or "germinate" the epidermal cells--the stratum germinativum. Keratinocytes are not the only cell type seen in this layer. Cells known as melanocytes, responsible for making the dark-brown to black pigment called melanin, are seen here. Macrophages called Langerhans cells are the part of the immune system that reside in this layer. Langerhans cells also occur in the stratum spinosum. Their decrease in number as we age explains some of the increased susceptibility to skin infections and cancer many people over 70 experience. Both desmosomes and hemidesmosomes occur in the stratum basale.

Epidermal cells in the stratum spinosum also use desmosomes to attach to one another. Cells in the stratum granulosum make keratohyalin, a substance that looks dark and "granular or grainy". The stratum lucidum or clear layer is only found in thick skin. The transparent stratum corneum contains dead cells whose cell membranes remain intact and surround the keratin that has formed. This superficial layer is very thick in "thick" skin like that found on the sole of the foot; and relatively thin in the "thin"skin on the abdomen. Constant abrasion--like going barefoot outdoors--can increase the amount of keratin in this layer. Some people have really hard feet! ;-)

The Dermis

The skin is a cutaneous membrane. It consists of the superficial epithelium or epidermis described above, and a connective tissue deep to the epithelium called the dermis. Although blood vessels do not penetrate the epithelium, the dermis is highly vascularized. The papillary region of the dermis contains loose connective tissue; the reticular region contains dense, irregular CT.

Receptors in the Skin

I know that you already realize that you can feel touch, pressure, and temperature. We will study this topic in detail in the nervous system. For now, I want to introduce the basic concept of a receptor.

Important Definitions

• Receptors are the "Transducer" in a sensory system.
• Receptors respond to a particular stimulus called a "proper stimulus".
• Their function is to change the form of energy of the proper stimulus into the electrochemical energy of the Nervous system.

Glands Found in the Skin

Let's follow up our discussion of glands from lecture 3. The exocrine glands in skin contain both secretory and ductal epithelium. Use the table below to practice the functional classifications of glands that were introduced in chapter 3 of the text.

Peptalk first:
Look around and notice that people have very different skin types. I asked you to look at the skin slides in lab and to compare the amount of melanin in light versus dark skinned people. Now, as you review the chart I have put together on this topic from the dermatological literature and many health sites, think about the differences you saw in the slides. Also, read reports about the changes in the ozone (protective) layer over the past several years (environmental information is not tested in this course).

For sun-safety, everyone should use sunscreen!

*SPF--Sun Protection Factor; Used to rate sunscreens

Now a word of warning about sunscreens: If you are allergic to sunscreen, use one that is hypoallergenic and free of fragrance. It takes some searching and label-reading, but these sunscreens do exist!

Food for thought: What is the difference between the terms stratified squamous epithelium, cutaneous membrane and the skin!