Chapter 5, Problem 1LO

Summary Introduction

To describe: The main structural features of the epidermis and to explain the functional significance of each feature.

Introduction: The skin is the chief organ of the integumentary system. It makes up about 16 percent of the total body weight. It is also the place or region where the body communicates with the external environment. The skin, along with the integumentary system, forms the first line of defense against the hostile environment.

Explanation of Solution

The epidermis is the outermost layer made up of five layers listed as follows:

• Stratum corneum: It is the exposed surface of both thick and thin skin and contains 15 to 30 layers of the keratinized cells. The dead cells present in this layer remain interconnected by the desmosomes. This layer is water-resistant but it is not waterproof.
• Stratum lucidum: It is also known as clear skin and it separates the stratum corneum from the deeper tissues. The cells present in this layer are flat and densely packed without any granules. They are also filled with protein called keratohyalin. They are synthesized in the underlying stratum granulosum and are dead when they reach the stratum lucidum.
• Stratum granulosum: It is made up of three to four layers of keratinocytes. When the cells reach this layer, most of the cells stop dividing and produce large amounts of keratin and keratohyalin. These cells become thinner but their membranes become thicken and less permeable as the protein fibers start developing.
• Stratum spinosum: This spiny layer is made up of 8 to 10 keratinocyte layers that are held together by the desmosomes. As the name indicates, the cells are like miniature pincushions in the standard histological sections. It also contains Langerhans cells or dendritic cells, stimulating the defense system thereby participating to promote immune response.
• Stratum basale: This is the basal layer of the epidermis and hemidesmosomes help in anchoring the cells to the basement membrane. This membrane separates the epidermis from the areolar tissue present in the neighboring papillary layer of the dermis. This layer is dominated by the basal cells. These are stem cells that help to replace the lost or shed superficial keratinocytes.