Normal Skin Histology
The integumentary system is composed of multiple subunits that work in unison.
The skin and its appendageal structures make up the integumentary system. There
are three main layers to the skin: epidermis, dermis, and subcutaneous tissue.
Within the epidermis, the principal skin cell is the keratinocyte. Other cells
found in the epidermis include melanocytes, Merkel cells, and Langerhans cells.
The main cell type found within the dermis is the fibroblast. Fibroblasts make
collagen, which forms the mechanical support for the skin. The dermis is a
region of high vascularity. The subcutaneous fat tissue is found directly
beneath the dermis and is composed primarily of adipocytes.
The normal human epidermis varies extensively in thickness in different
regions of the body. It is thickest on the back and thinnest on the eyelids and
on the scrotal skin. The epidermis can be subdivided into five components:
stratum basale, stratum spinosum, stratum granulosum, stratum lucidum, and
stratum corneum. The stratum lucidum is found only on the skin of the palms and
soles. Each layer of the epidermis has important anatomical and physiological
functions.
The stratum basale is the deepest layer. It consists of cuboidal
epithelium sitting atop a basement membrane zone. The stratum basale contains
the proliferating keratinocytes, which are constantly undergoing replication to
replace the overlying epidermis. It takes approximately 28 days for a basal
keratinocyte to progress to the outermost layer of the stratum corneum. Melanocytes
and Merkel cells can also be found within the stratum basale. Melanocytes are
pigment-forming cells; they transfer their pigment to neighboring keratinocytes.
Merkel cells are modified nerve endings and have been found to be important as
mechanoreceptors.
The stratum spinosum is many cell layers thick and is recognized by the
intercellular connections among adjacent keratinocytes, which are seen on light
micros- copy as tiny spines. From the lower to the upper layers of the stratum
spinosum, the keratinocytes progressively become flatter in appearance.
The stratum granulosum is recognized by the large number of basophilic
keratohyalin granules within its keratinocytes. This stratum is typically 2 to
4 cell layers thick. The keratohyalin granules are composed primarily of the
protein profilaggrin; they vary from 1 to 4 µm in diameter. Profilaggrin is the
precursor to filaggrin, an essential protein that is required for the integrity
of the overlying epidermis.
The stratum lucidum occurs only in the skin of the palms and soles. It is
composed of a translucent eosinophilic layer. The stratum lucidum is made up of
tightly packed squamous keratinocytes.
The stratum corneum, the outermost layer of skin, is made up of
anucleate, cornified keratinocytes. Kerati- nization (cornification) is a
complex process that results in the appearance of the stratum corneum. As cells
progress up the stratum corneum, they are shed in the process known as
desquamation.
The dermis is primarily composed of collagen, which is produced by
fibroblasts. This portion of the skin contains a highly vascular network that
is responsible for the nutrition of the skin and for thermoregulation. This
network includes a deep dermal plexus and a superficial plexus. The superficial
plexus is responsible for thermoregulation. It undergoes vasoconstriction
during exposure to cold temperatures and vasodilation in times of warm temperature.
The dermis can be split into two
regions, called the papillary and the reticular portions. The papillary dermis
is juxtaposed to the overlying epidermis and interdigitates with it. The papillary
dermis and the epidermis are connected by the basement membrane zone. This zone
contains many unique proteins. These proteins are the targets for the various
autoantibodies that can be found in patients with autoimmune blistering diseases.
The subcutaneous tissue is composed of adipocytes. This tissue’s main
functions are storage of energy, insulation, and cushioning. The adipocytes are
closely packed in a connective tissue septum with associated blood vessels and
nerve endings.
There are many types of skin appendages, including hair follicles,
sebaceous glands, eccrine glands, apocrine glands,
and various nerve endings.
