BIOPRINTING FOR SKIN - 3D Bioprinting and Nanotechnology in Tissue Engineering and Regenerative Medicine - page 276

13.2.1 BASEMENT MEMBRANE
The basement membrane is essential for the attachment of the epidermis to the dermis and consequent-
ly the mechanical strength of skin. Moreover, it stabilizes the connection between the epithelial cells of
the basal layer and prevents them from sliding apart. Furthermore, it promotes survival, proliferation,
and differentiation of the cells. The basement membrane consists of special extracellular matrix (ECM)
molecules, and is partly secreted by the epidermal cells and partly by the dermal cells. It contains colla-
gen type IV and VII, glycoproteins like laminin and fibronectin, nidogen, proteoglycans, and integrins,
among others. Collagen type IV forms the basis of the membrane, developed as a felt-like network.
It gives the basement membrane its high tensile strength. The other components are mostly integrated
in this network. To attach the membrane to the underlying dermis, collagen type VII forms anchoring
fibrils ( Michael, 2013c ).
The basement membrane acts as a barrier between the epidermis and the dermis, keeping the ke-
ratinocytes and fibroblasts in their spatial surroundings. In contrast, macrophages, lymphocytes, and
nerve cells can cross the membrane by secreting specialized enzymes, which are able to degrade com-
ponents of the membrane. Of course, nutrients, oxygen, and metabolites must be able to pass the barrier
( Michael, 2013c ).
13.2.2 DERMIS
The dermis is located underneath the basement membrane. Its thickness varies between 0.6 mm at the
eyelids and more than 3 mm at the back and the sole of the feet. The dermis is divided into two parts,
with the upper papillary region containing most of the cells and the lower reticular region comprising
hair follicles and glands.
The main characteristic of the dermis is the abundant presence of ECM. It is secreted by the resident
cells, the fibroblasts. The dermis has several different tasks. Via the proteoglycans and glycoproteins
of the ECM, liquid in the form of hydrated water is bound. This results in a gel-like substance that
can well resist compressive forces, thereby protecting the underlying tissue. Diffusion of nutrients,
metabolites, and hormones becomes possible, ensuring the supply of the skin. The tensile strength and
mechanical stability of the dermis and therefore the skin is due to collagen type I, which forms long
fibers. Equally essential is elasticity, which is enabled by elastic fibers. Altogether, the dermis can
absorb mechanical compression or shear forces, while remaining very pliable ( Michael, 2013c ).
In contrast to the epidermis, the dermis contains many blood vessels. They are organized in two
plexuses, one located directly above the hypodermis, the other directly underneath the basement mem-
brane. The latter is responsible for the supply of the epidermis via diffusion, and reaches into the papil-
lae. The blood vessel system of the dermis also takes part in temperature regulation of the whole body.
Furthermore, the dermis contains sebaceous and perspiratory glands, hair follicles, and nerves.
The nerves are responsible for the perception of pain, touch, itch, and temperature. Finally, cells of
the innate and the adapted immune system can also be found in the dermis: mast cells, macrophages,
leucocytes, and plasma cells ( Michael, 2013c ).
13.2.3 HYPODERMIS
The hypodermis consists of connective tissue and fat, and has several functions. First, it ensures
heat insulation, energy storage, and protection of internal organs against external mechanical forces.