Transcript Skin Structure & Histology

Alfred Ammoury
Division of Dermatology, St George Hospital UMC
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Skin is a complex organ
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Horizontally stratified into 3 compartments
-epidermis
-dermis
-subcutis (hypodermis)
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Vertically penetrated by appendages
-hair follicles
-sebaceous glands
-eccrine and apocrine sweat glands
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
Bilaminar embryonic disc :
Epiblast and the hypoblast
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Primitive streak – raised groove on the dorsal
surface of the epiblast
Gastrulation – a process of invagination of
epiblast cells
Endoderm – formed from migrating cells that
replace the hypoblast
Mesoderm – formed between epiblast and
endoderm
Ectoderm – formed from epiblast cells that
stay on dorsal surface
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Skin layers are derived from 2 different germ
layers:
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Ectoderm (lateral to the neural plate): epidermis
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Mesoderm: dermis
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Earliest skin study (4 week): single layered epidermis and a
thin mesenchymal dermis.
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Progressive development over the first 6 months: by the end
of the second trimester, the skin is a stratified squamous
epithelium.
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Dermis lags behind epidermis in developemnt.
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Dermal bulk increases post nataly and its maturation
continues
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
Day 20-30: skin of embryo starts to develop
(organogenesis).
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Day 60: most organ systems have formed
(including the skin). Embryo enters fetal period
of growth and differentiation.
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
Gastrulation occurs during the third week after
fertilization
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Complex process of involution and cell
redistribution resulting in the formation of the
three primary embryonal germ layers:
Ectoderm, Mesoderm, endoderm
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Shortly after gastrulation, the ectoderm further
subdivides into neuroectoderm and the
presumptive epidermis that covers the embryo
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At this stage, the ectoderm (presumptive
epidermis) that covers the embryo is made of
basal cells and superficial periderm cells
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Form part of the vernix
caseosa
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Form a pavement epithelium that blanket the
developing epidermis.
Exfoliated peridermal cells and sebum form the
Vernix caseosa , a white greasy substance that
covers the fetal body, protects it from amniotic
fluid contents and facilitates delivery.
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Hematopoiesis has switched from the
extraembryonic yolk sac to the bone marrow.
The classical division between embryonal and
fetal development.
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Epidermal stratification occurs: formation of an
intermediate layer, highly proliferative cells that
eventually evolve into a multi layer structure that
will replace the periderm.
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Basal layer begins to elaborate proteins that will
anchor them to the developing basal lamina.
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Late fetal development reveals the further
differenciation of keratinocytes in the epidermis:
granular layer, stratum corneum that replaces
the periderm at W21.
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Stratum germninativum starts to extend
downward growth (ridges) into the developing
dermis.
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Melanocytes, Merkel cells and Langerhans cells
can be detected by the end of the embryonic
period.
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Neural crest derived
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W8 neural crest cells mgrates
into mesenchymal tissue and
differentiate into melanoblasts.
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Melanoblasts localize to DEJ
and hair bulbs, and
differenciates into melanocytes
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Bone marrow derived
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Detectable by 40 days
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By the third trmester most of
the adult number of these
cells will have been produced
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Slowly adapting touch receptors.
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Controversial origin: neural crest Vs in situ
differentiation of ectodermal cells.
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Present at 11-12 weeks.
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Develops from mesenchyme
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W11: mesenchymal cells produce
collagenous and elastic tissue.
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Part of the dermis project into the
epidermis forming pappillae.
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Capillary loops and nerves
develop in these papillae
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Major vasculature : by the end of
the 1st trimester.
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Hair follicles, sebaceous glands and sweat glands
develop from the epidermis and they grow into
the dermis.
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W12: Hair first appears on eyebrow
& upper lip
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1st hair is lanugo hair (soft, fine,
lightly pigmented and non
mediullated).
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Replaced by coarse terminal hair
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Stratum germinativum
proliferates into the dermis.
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Hair bud becomes club
shaped=Hair bulb containing
germinal matrix cells that
willlatinize to form the hair
shaft.
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Hair bulbs are soon invaginated
by small mesenchymal hair
papilla.
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Arrector pilorum develop from
the surrounding mesenchyme.
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Buds from the sides of
developing epithelial root
sheath of hair follicles
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Inactive until puberty.
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They develop as epidermal downgrowth
Into the underlying dermis
Starts at about W20
Functional at birth
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Develop from
downgrowths of the
stratum germinativum
that gives rise to hair
follicle
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Begin secretion around
7-10 years.
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Fingernails reach the edge at W32
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Toenails reach the edge at W36
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Highly modified and highly specialized type of
sweat glands (development similar to that of SG)
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W6: downgrowths of epidermal ridges in the
pectoral area
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Primary duct forms many 2ndary ducts that in
turn develop to form lactiferous duct (15-20 at
term).
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Epidermis at the site forms a shallow pit
mammary pit where the nipple will develop.
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