CHAPTER 13
Integumentary System
255
Development of the Skin
SYNOPSIS 13-2
Pathological Terms for the Integumentary System
Acanthosis
: Thickening of the stratum spinosum of the epidermis, typically seen in epidermal hyperplasia.
Hypergranulosis
:
Thickening and prominence of the stratum granulosum of the epidermis, often in response to chronic
mechanical irritation of the skin. Hypergranulosis may also be seen in the declivities of papillary lesions such as verruca
vulgaris, or warts.
Hyperkeratosis
: Thickening of the stratum corneum of the epidermis. Orthokeratotic hyperkeratosis refers to
hyperkeratosis
without the presence of nuclei.
Papillomatosis
: Fingerlike projections from the epidermal surface, often with hyperkeratosis, seen in a variety of conditions
including verruca vulgaris, or warts.
Parakeratosis
: A form of hyperkeratosis in which nuclei are retained in the stratum corneum, seen in many conditions
including psoriasis.
Spongiosis
: Intercellular edema of the epidermis frequently seen in various etiologies of dermatitis such as allergic contact
dermatitis or irritant dermatitis.
Ulceration
: The discontinuity of an epithelial surface including the epidermis or mucous membranes.
Figure 13-12A.
Fetal skin (5–9 weeks).
H&E,
3
331
The two layers (
epidermis
and
dermis
) of the skin develop from two
different embryonic tissues. The
epidermis
develops from the
ecto-
derm
, and the
dermis
from the
mesoderm
. About 4 weeks after con-
ception, the human embryo is covered by a single layer of ectodermal
cells, which are loosely arranged on the
basement membrane
and
over the
mesenchymal tissue
. After 5 weeks, the epidermis has two
layers of cells: the super±
cial layer (
periderm
) and
basal layers
. At
2 to 3 months, basal cells are dividing rapidly, and the epidermis
becomes several cell layers thick. At the same time, the mesenchyme
differentiates into a more mature connective tissue with blood
vessels. By 4 months, neural crest cells migrate into the basal layer of
the epidermis and differentiate into melanocytes and Merkel cells.
The connective tissue layer beneath the epithelium develops into
the dermis and a deeper layer, the hypodermis. At about 5 months,
the appendages of the skin (hair follicles and glands) start to form.
This section shows an early stage of skin development in an embryo.
There are only two layers of epithelial cells in the epidermis, and fetal
blood vessels are located within the mesenchyme tissue.
Nucleated
erythrocytes
are shown inside the blood vessels.
Periderm
Periderm
Periderm
Basement
Basement
membrane
membrane
Basement
membrane
Mesenchymal
Mesenchymal
tissue
tissue
Mesenchymal
tissue
Nucleated
Nucleated
erythrocytes
erythrocytes
Nucleated
erythrocytes
Mesenchymal
Mesenchymal
cells
cells
Mesenchymal
cells
Basal layer
Basal layer
Basal layer
A
Figure 13-12B.
Fetal skin (±
fth month).
H&E,
3
438
This section shows a later stage of fetal skin development. The
epidermis has formed multiple cell layers, and four layers of
epi-
dermis
can be distinguished to some extent. The
basal cells
in
the stratum basale layer are highly active and appear as column-
shaped cells. The underlying mesenchymal tissue has differentiated
into connective tissue (
dermis
). There are many active
±
broblasts
in the dermis. An accumulation of basal cells forms a fold called
an
epidermal bud
, which projects into the dermis. These accu-
mulated cells will interact with the dermis and differentiate into
appendages
(
hair follicles
and
glands
).
Epidermis
Epidermis
Epidermis
Dermis
Dermis
Dermis
Accumulated
Accumulated
basal cells
basal cells
(epidermal bud)
(epidermal bud)
Accumulated
basal cells
(epidermal bud)
Erythrocytes in
Erythrocytes in
the blood vessels
the blood vessels
Erythrocytes in
the blood vessels
Basal cells
Basal cells
Basal cells
Fibroblasts
Fibroblasts
Fibroblasts
B
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