b. These three concentric layers form complete envelopes around the body,
except for the various openings.
3-3. THE INTEGUMENTARY SYSTEM
An organ system is a group of organs performing a common overall function.
The outermost covering of the body is the integument proper, the largest single organ of
the body. A number of structures are formed or derived from the various layers of the
integument proper. These structures are known as the integumentary derivatives,
sometimes referred to as "appendages." Together, the integument proper and the
integumentary derivatives make up the integumentary system.
Section II. INTEGUMENT PROPER
The integument proper has two major parts--the dermis and the epidermis. The
dermis (or corium) is made up of rather dense FCT, forming a continuous layer around
the body. On top of the dermis is the epidermis. The epidermis and dermis are
interlocked by extensions of the dermis up into the epidermis. These extensions are
known as papillae.
3-5. LAYERS OF EPIDERMIS
The epidermis is a stratified squamous epithelial tissue. This means that it has
several layers of epithelial cells and that its outermost layer is made up of squamous
(flat) epithelial cells.
a. Mitotic Activity. The layer adjacent to the dermis is known as the basal
layer. The basal layer is made up of columnar epithelial cells. Since all of the mitotic
(cell-multiplying) activity of the epidermis occurs in the basal layer, the basal layer is
often called the germinative layer. This mitotic activity involves about 4 percent of the
cells in the basal layer at any given time. It occurs primarily between midnight and 0400
b. Migration of Cells to the Surface. Over a period of weeks, new cells
gradually migrate from the basal layer to the surface. During this migration to the
surface, the cells change in shape from the original columnar to cuboidal and then
finally to squamous. As the cells become squamous in form, they also become
hardened, or cornified, through the development of a special type of protein. As they
approach the surface, they die. Thus, the outermost layers of the epidermis are dead,