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CHAPTER
30
Skin in
Osteogenesis Imp
erfecta
Anna L. Chien, Euphemia W. Mu and Sewon Kang
Johns Hopkins School of Medicine, Baltimore, MD, USA
INTRODUCTION
differentiated keratinocytes, or corneocytes, lack nuclei
and cytoplasmic organelles, reside in the stratum cor-
neum and are eventually shed from the skin's surface.
Other cells in the epidermis include melanocytes that
produce melanin pigment, Langerhans cells that pres-
ent foreign antigens and Merkel cells that function as
mechanoreceptors.
The basement membrane is a multilayer structure
between the dermis and epidermis that measures 0.5
to 1 μm in thickness.
7
The dermal-epidermal junction
allows selective transfer of nutrients, water and cell sig-
naling molecules. Below the basement membrane lies
the dermis, a mesoderm-derived layer of supporting
connective tissue ranging in thickness from 0.6 mm in
eyelids to over 5 mm over the back.
6
Broadly, the dermis
has two layers: a thin papillary layer and a thick reticular
layer. The papillary layer is predominantly composed of
loosely arranged type III collagen fibers. In contrast, the
reticular layer is largely formed by thick bundles of type
I collagen running parallel to the skin surface.
8
The principal cells in the dermis are fibroblasts,
which generate collagen, elastin and proteoglycans.
Notably, collagen makes up approximately 90% of
the protein and 70% of the dry weight in the dermis.
6,9
Collagen is a family of over 20 fibrous proteins that
imparts structural stability to the skin. Each collagen
fiber is composed of three chains that form a triple helix
with variable repeats of glycine, proline and hydroxy-
lysine or hydroxyproline. Also contributing to the der-
mis, elastin and proteoglycans provide elasticity and
hydration, respectively. In addition, the dermis incorpo-
rates sweat glands, hair roots, nerves, blood vessels and
lymph vessels.
Just below the dermis lies the
subcutaneous fat
layer,
which contains adipose, large blood vessels and nerves.
The subcutis provides heat insulation, energy stores and
protective cushioning. All together, the epidermis, dermis
Osteogenesis imperfecta (OI) is a connective tissue
disorder due to defects in type I collagen, a major com-
ponent of bone and skin.
1,2
While numerous clinical
investigations have examined the skeletal abnormalities
in OI, relatively little is known about the cutaneous mani-
festations of this disorder. This chapter will review the
normal physiology of skin and describe our emerging
understanding of the dermatologic characteristics of OI.
NORMAL STRUCTURE AND FUNCTION
OF SKIN
Skin, our largest organ, serves as the interface between
our outer and inner environments. Externally, skin pro-
tects against potential sources of injury, including micro-
organisms, ultraviolet (UV) radiation and direct trauma.
Internally, skin maintains homeostasis through tempera-
ture regulation, immunosurveillance, water exchange,
vitamin D synthesis and other biologic processes.
3
Moreover, our cutaneous covering determines our physi-
cal appearance and sensory perception, thereby facilitat-
ing everyday social interactions.
4
Skin consists of three layers: the epidermis, dermis
and subcutaneous fat (
Figure 30.1
). The outermost layer
of skin is the epidermis, which varies in thickness from
0.05 mm in eyelids to 1.5 mm in palms and soles.
5
The
epidermis can be subdivided into four layers based on
cell morphology and intercellular proteins. From inside
out, these layers are the stratum basale, stratum spino-
sum, stratum granulosum and stratum corneum.
Keratinocytes are the predominant cells in the epi-
dermis. Initially attached in the stratum basale, kerati-
nocytes differentiate and migrate through the epidermis
to the stratum corneum in approximately 28 days.
6
Fully
