B6.025 Skin Development Flashcards
functions of the skin
part of integumentary system protects inner body from the environment involved in thermoregulation via blood vessels and glands storage center for lipids and water sensation
size of the skin
largest organ in the body
0.5mm-4mm thickness
excretion functions of skin
sweat removes water, small amounts of salt, uric acid, and ammonia
blood reservoir of skin
dermal blood vessels carry 8-10% of total blood flow in resting adult
function of skin in the synthesis of vitamin D
requires UV rays
later converts to calcitriol (in liver and kidney) that aids in absorption of calcium
origin of epidermis
non neural ectoderm
examples of epidermal appendages
hair follicles
apocrine & eccrine sweat glands
mammary glands
nails
neural ectoderm (neural crest cell) derivatives
melanocytes
nerves and sensory receptors
cranial mesenchyme/CT
ectodermal germ layer derivatives in skin
epidermis
epidermal appendages
neural ectoderm derivatives
mesodermal germ layer derivatives in skin
dermis hypodermis blood and lymph vessels Langerhans cells, fibrocytes, adipocytes muscles
type of muscle in mesodermal germ layer of skin
erector pilli in hair follicles
smooth muscle within glands is an ectodermal derivative
how is skin connected to underlying fascia
retinacular system
-retinacular “ligaments”, blood vessels, nerves, and lymphatics
function of retinacular system
connection of skin to fascia
limitation of skin movements
-tether skin to fascia so 2 tissue planes glide on each other
what are skin wrinkles
visible signs of aging of retinacular components
skin formation at 5 weeks
2 germ layers + neural crest cells
- non neural ectoderm
- neural ectoderm
- mesodermal mesenchyme
formation of the epidermis in the 2-4 month of development
ectoderm divides to form an overlying layer of flattened cells (periderm) in week 5
basal layer cells proliferate to form intermediate layer
describe the periderm
flat cells that are cast off in development in amniotic fluid
5 layers of epidermis
from top to bottom:
- stratum corneum
- stratum lucidum (only in thick skin)
- stratum granulosum
- stratum spinosum
- stratum basale
how do the 5 layers of epidermis form?
keratinocytes form and differentiate as they move to higher layers
eventually these die and form stratum corneum
stratum corneum
closely packed dead cells containing keratin
-protects and waterproofs
stratum lucidum
extra layer of dead cells in thick skin such as soles/palms
-provides extra protection
stratum granulosum
cells containing kerato-hyaline granules
-keratin precursors
stratum spinosum
polyhedral cells with tonofibrils (intracellular filaments)
-converge on desmosomes
Langerhans cells prominent
-phagocytose
stratum basale
produces new cells that move into upper layers
forms ridges/hollows reflected in fingerprints
contains melanocytes
function of melanocytes
makes pigment/melanin in melanosomes
transported to keratinocytes and hair bulb
cell types found in the epidermis
keratinocytes
melanocytes
Merkel cells
Langerhans cells
role of keratinocytes in skin
- make up 90% of epidermis (formed in basal layer and push way up to surface)
- function to form barrier against environmental damage via keratinization (keratin protein production and terminal differentiation and shedding)
- protect against pathogens, fungi, parasites, viruses, heat, UV radiation, water loss
- produce pro-inflammatory mediators to attract leukocytes to site of pathogen invasion
what is Harlequin Ichthyosis
excessive keratinization of the skin
keratin layers crack to form fissures
hereditary disorder: either autosomal or X linked recessive
symptoms of Harlequin Ichthyosis
difficult to control water loss, regulate body temp, and fight infections due to disruption of barrier
some infants survive to childhood and adolescence
how does the neural tube give rise to melanocytes
neural tube comprised of neural ectoderms
a subset undergoes epithelial to mesenchymal transformation (EMT)
neural crest cells leave tube and migrate to form new structures (including melanocytes)
function of melanocytes
located in stratum basale
branch to form dendrites
produce melanin that accumulates in melanosomes
also present in hair follices
movement/function of melanin in the skin
dermal melanin transported along dendrites to keratinocytes
accumulates on superficial aspect of keratinocytes
shields keratinocytes from harmful UV light
albinism
globally reduced or absent pigmentation (skin, hair eyes)
vitiligo
loss of melanocytes in patches of skin, hair, and oral mucosa due to an autoimmune disorder
piebaldism
patchy absence of hair pigment
waardenburg syndrome
patches of white skin and hair
irises of different colors
deafness
description of merkel cells
originate from epidermis
found in stratum basale
attached to keratinocytes via desmosomes
function of merkel cells
make contact with sensory nerve fibers (Merkel nerve endings)
mechanoreceptors
react to low vibrations and pressures over long periods
where are merkel cells present in the body
high density on fingertips and lips/face
description of Langerhans cells
star shaped dendritic cells arising from BM (mesodermal) that migrate to epidermis
found throughout epidermis, but prominent in stratum spinosum
function of Langerhans cells in skin
antigen presenting immune cells
phagocytic
easily damaged by UV light
dermis formation
formed from corium during 3rd and 4th months of development
what is the origin of the dermis
mesenchyme of 3 sources
lateral plate mesoderm (parietal)
paraxial mesoderm
neural crest cells (neural ectoderm)
lateral plate mesoderm dermis derivatives
dermis of limbs and body wall
paraxial mesoderm dermis derivatives
dermis of back
neural crest cell dermis derivatives
dermis of face and neck
components of dermis
connective tissue fibroblasts blood vessels sensory nerve endings oil and sweat glands hair follicles
what is the papillary layer of dermis
forms irregular dermal papillae which project upward into the epidermis
retinacular layer of dermis
inner dermis
function of papillary layer
supports overlying epidermis
ridges increase surface area for exchange of oxygen, nutrients, and waste
increased surface area also prevents separation of dermal and epidermal layers
dermal papillae play role in hair formation, growth, and cycling
composition of reticular layer
composed of dense irregular connective tissue
densely packed collagen, elastic, and reticular fibers
orientation of collagen fibers in reticular layer
creates lines of tension, Langer’s lines
relevant to wound healing/surgical procedures: incisions made parallel may heal better and produce less scarring
properties of reticular layer
strength, extensibility, elasticity
contains roots of hair and hair bulbs (bases of hair follicles)
describe the skin at birth
Vernix Caseosa
whitish paste formed by secretions from sebaceous glands, degenerated epidermal cells, and degenerated lanugo hair
function of vernix caseosa
covers skin
protects against macerating action of amniotic fluid
what are epidermal appendages
derivatives of epidermis
develop as downgrowths of epidermis into dermis
dermis provides signals that lead to differentiated structures
examples of epidermal appendages
hair follicles sweat glands sebaceous glands mammary glands nails
functions of hair follicles
involved in thermoregulation
provide protection from sun/elements
initial development of hair follicles
first in cranial region (9th week)
by 5th month, most are formed (5 mil)
hair follicle development
- solid epidermal proliferations from stratum basale project and penetrate underlying dermis
- terminal end (hair bud) invaginates to form hair papilla
- hair papilla fills with mesoderm in which vessels and nerve endings develop
- cells in hair bud center become spindle shaped and keratinized to form the hair shaft
- cells in periphery become cuboidal and form the epithelial hair sheath
- surrounding mesenchyme forms the dermal root sheath and a small smooth muscle attachment (erector pili)
how does hair grow
via continuous proliferation of epithelial cells at the follicle base/ hair bulb that pushes hair up the shaft
what is the erector pili
small smooth muscle (mesoderm derivative) attached to the dermal root sheath
function of erector pili
contracts in response to cold, emotional state (adrenaline)
creates shallow depression on the skin surface; thus protrusion of surrounding area (goose bump)
contraction causes hair to stand up
role in insulation/warmth
first hair on a fetus
lanugo
appears on eyebrow and upper lip
shed at time of birth
hair development in newborn
coarser hair arises from new hair follicles after birth
melanin is transferred to hair forming cells several weeks before birth to give hair color
process of sebum production
- small bud grows from epithelial wall of hair follicle and penetrates mesoderm
- sebaceous glands are formed from cells in bud
- cells in center of gland degenerate, forming sebum that is secreted into hair follicle and reaches skin
hypertrichosis
excessive hairiness
overabundance of hair follicles
can be focal or over entire body
atrichia
congenital absence of hair
can be associated with other defects in ectodermal derivatives (teeth, nails)
2 types of sweat glands
eccrine (do not lose cytoplasm in their secretions)
apocrine (release some of their cytoplasm into their secretions)
eccrine gland description
form in skin over most of body, high density in palms and soles
form from buds in stratum basale of epidermis that grow down into dermis
end portion coils to form secretory part
epidermal buds also give rise to smooth muscle cells associated with glands
function of eccrine glands
function by merocrine mechanisms (exocytosis) function in temperature control soon after birth
apocrine gland description
develop from down growths of stratum basale
form where body hair is found (axilla, pubic region)
form from same epidermal buds that give rise to hair follicles
form during puberty
open onto hair follicle (not skin)
function of apocrine glands
produce sweat
odor due to bacteria that break down products in sweat
portion of secretory cells is shed ins secretion
components of sweat
lipids
proteins
pheremones
sebaceous gland development
develop as buds from hair follicle epidermal wall (week 5)
function of sebaceous glands
secrete vernix/sebum (oily) function to lubricate skin and hair
what are mammary glands
modified apocrine sweat glands
development of mammary glands
first appear as epidermal mammary lines/ridges in BOTH sexes
at 7 weeks: lines extend bilaterally from base of forelimb to hindlimb region
16-24 epithelial buds of mammary ridge penetrates underlying mesenchyme and form secondary buds
near term: sprouts canalize to form lactiferous ducts
mammary line formation
most disappears shortly after it forms
small portion persists in thoracic region, penetrating underlying mesenchyme
development of lactiferous ducts
prenatally: ducts open into epithelial pit
after birth: pit is transformed into a nipple by proliferation of underlying mesenchyme and epithelial pit everts
at birth duct lacks alveoli and secretory structures
at puberty: increased hormones in females stimulate gland enlargement, ductal development, and breast enlargement
polythelia
accessory nipples due to persistence of mammary line
polymastia
remnant of mammary line develops into complete breast
inverted nipples
lactiferous ducts open into epithelial pit that failed to evert
gynecomastia
excessive development of male mammary gland
can occur transiently in newborns due to maternal sex hormones OR when there is decreased ratio of testosterone to estradiol (Klinefelter XXY)
nail development
- epidermal thickenings appear at tips of digits and form nail fields at 10 weeks
- fields migrate to dorsal side of each digit
- fields grow proximally, forming a nail root
- proliferation of tissue surrounding each field creates a depression for each nail
- from nail root, epidermis differentiates into finger and toe nails
- nails extend to tips of digits in 9th month of development
what is the hypodermis
superficial fascia layer under skin
function of hypodermis
attaches skin to underlying bones, muscles, and deep fascia
attaches to overlying dermis via collagen and elastin fibers
provides blood and nerve supply to underlying bones, muscles
what is the hypodermis made of
loose CT fibroblasts adipose cells macrophages derived from mesoderm but NOT dermatome
cutis aplasia
congenital absence of the skin, generally isolated lesions on the scalp
cutis laxia
skin is inelastic and hangs loosely in folds due to decrease or absence of elastin
may be caused by genetic mutation in elastin of fibulins
Ehlers Danlos
fragile, bruisable skin that heals with peculiar scars
caused by mutation in collagen and altered collagen synthesis
epidermolysis bullosa simplex
autosomal dominant disease of keratin (filaments aggregate)
skin is fragile, thus non scarring blisters result from little or no trauma
wound healing characteristics of embryonic skin
rapid
efficient
decreased platelet aggregation/degranulation events
decreased immune response
altered ECM (more type 3 collagen and hyaluronic acid)
scarless
discuss the rapid nature of embryonic wound healing
gap in embryonic epidermis closed by contraction of a rapidly assembles actin “purse string”
discuss the efficient nature of embryonic wound healing
standard fibroblasts exert tractional forces to bring wound margins close together
adult wound healing
slower and less efficient
epidermal cells migrate over exposed substratum to close a gap
specialized myofibroblasts aid in CT contraction
