Skin Development & Microanatomy Flashcards
what germ layers is skin derived from
ectoderm
mesoderm
what structures are derived from ectoderm
epidermis
hair follicles
sweat glands
sebaceous glands
modified glands
what structures are derived from mesoderm
dermis + subcutis
- dermal matrix (collagen, elastin, proteoglycans)
- vessels/lymphatics
- nerves
- arrector pili muscle
- subcutaneous fat
what does skin thickness depend on
thickness of the DERMIS
what are 3 general causes of increased skin thickness
- increased cellularity from inflammation
- neoplastic infiltrate
- matrix deposition/fluid in the dermis
what cell types are present in the epidermis
- keratinocytes
- melanocytes
- langerhans cells
- merkel cells
function of keratinocytes
create a barrier to the outside
structure of keratinocytes
stratified (4-5 cell layers)
- stratum basale
- stratum spinosum
- stratum granulosum
- stratum corneum
basal layers (stratum basale)
innermost layer
contains germinal cells that undergo mitosis to form undifferentiated keratinocytes
spinous layer (stratum spinosum)
middle 1-2 layers of cells that differentiate into keratinocytes
begin to flatten
granular layer (stratum granulosum)
cells that begin producing components of the lipid envelope for the cornified layer
cornified layer (stratum corneum)
outermost layer of dead keratinocytes
acts as a barrier to and communicates with the environment
types of cornified layers
compact
basket weave
compact cornified layer
seen in glabrous (hairless) skin and regions under constant pressure/friction
basket-weave cornified layer
seen in normal haired skin
high lipid content causes loose arrangement of keratin lamellae
lipid envelope
connects dead keratinocytes together in the cornified layer
what connective structures are used to connect keratinocytes
desmosomes
tight junctions
E-cadherins
desmosomes
connect keratinocytes to each other
- should NOT be visible in health
spongiosis
intercellular edema causing keratinocytes to separate
desmosomes become visible
tight junctions
prevent passage of molecules through intercellular spaces to require them to pass within the cells via diffusion/active transport
- maintains health of keratinocytes without needing blood supply
E-cadherins
connects keratinocytes to each other, to melanocytes, and to langerhans cells
what determines the thickness of the epidermis
the thickness of the SPINOUS layer
epidermal thickness vs hair coat
glabrous (hairless) = thick epidermis
haired = thin epidermis
is epidermal atrophy common
no, rare
caused by hyperadrenocorticism
what is epidermal turnover
the time it takes for basal cells to reach the cornified layer
takes 3 weeks
immunologic properties of keratinocytes
- normal flora/microbiome
- antimicrobial peptides
- lipids
- cytokines/chemokines
- express MHC II to act as APCs to T cells in the epidermis
epidermal hyperplasia
acanthosis
increased thickness of the epidermis caused by chronic irritation (to the cells or the envelope)
steps of acanthosis
- injury/irritation causes rapid basal cell division
- spinous layer thickens
- increased turnover rate
- decreased transit time between layers
- decreased differentiation
- changes in granular and keratin layer
subacute acanthosis lesions
- hypogranulosis: lack of granular layer (dec. differentiation)
- parakeratosis: retention of nuclei in keratin layer
chronic acanthosis lesions
- hypergranulosis: adaptation to high turnover
- compact hyperkeratosis: compaction of the cornified layer despite haired skin
melanocytes
provide pigmentation to the skin and hair coat
location of melanocytes
between basal keratinocytes
extend dendritic processes that carry melanosomes to surrounding keratinocytes
melanosomes
synthesize and store melanin pigments
hyperpigmentation
excess melanin stimulating hormone (MSH) caused by chronic irritation or solar exposure
depigmentation
loss of skin or hair pigment caused by injury to the dermis-epidermis junction and basal cells OR matricidal cells of the hair bulbs leading to melanocyte damage
leukoderma
depigmentation of the epidermis
leukotrichia
depigmentation of the hair coat
merkel cells
oval receptor cells located in the epidermis
synapse with somatosensory afferent nerve fibers to sense light touch, shape, and textures
also have a neuroendocrine function
what are other sensory cells in the skin
- merkel cells
- tylotrich pads
- vibrissae
langerhans cells
epidermal dendritic APCs
used for immune surveillance
where are langerhans cells located
in the epidermis and hair follicle epithelium
function of langerhans cells
sample antigens from the environment on the skin surface –> detach from the adjacent keratinocyte –> migrate to draining lymph node –> present to lymphocytes to initiate an immune response
what type of immune response do langerhans cells initiate
adaptive immune response
what is the function of collagen
provide tensile strength
what is the function of elastin
provide elasticity
what is the function of proteoglycans
provide pliability
dermal dendritic APCs
provide immunosurveillance in the dermis
sample antigens from dermis –> migrate to draining lymph node –> initiate response
what type of immune response to dermal dendritic APCs initiate
adaptive immune response
basement membrane zone
complex structure of proteins and glycoproteins that connects the epidermis and dermis
NOT visible on H&E
dermo-epidermal separation
defects in the basement membrane zone that lead to separation of the dermis from the epidermis
panniculus vs subcutis
panniculus - around the panniculus muscle
subcutis - underneath the panniculus muscle
panniculus muscle
thin muscle layer attached to the skin underneath the dermis
panniculus fat
fat located directly below the dermis (above the panniculus muscle)
subcutaneous fat
fat located directly below the panniculus muscle (in the subcutis)
develops independently of panniculus fat
panniculitis
inflammation in the fatty tissue under the dermis (panniculus or subcutis)
cellulitis
inflammation of the subcutaneous fat due to infection
hair follicles
extensions of the epidermis
develops from germinal cells in the epidermis that grow downwards along with keratinocytes
infundibulum
superficial portion of the hair follicle that opens into the orifice of the follicle
what is the epithelium of the infundibulum
epidermis
where are the majority of the immune cells in hair follicles located
infundibulum (CD4 and CD8 T cells, langerhans cells)
isthmus
middle portion of the hair follicle that includes the bulge
bulge
location of the follicular stem cells and melanocyte stem cells
present throughout life
forms new anlagen phase and pigmentation of hair shaft
supra-bulbar and bulb regions
deepest portions of the hair follicle
compound hair follicles
one primary hair follicle + several secondary hair follicles grouped together
all hair exits through one follicular opening
dogs, cats, rabbits, sheep, goats
primary follicles
produce guard hairs
secondary follicles
produce undercoat/wool
NO sweat glands or arrector pili muscle
more secondary follicles = higher quality wool
simple hair follicles
one primary hair shaft exits from the follicular opening
horse, cattle, pigs, sheep, goats
what does hair growth cyclicity depend on
breed: continuously growing vs prolonged telogen
seasonal synchronized shedding: daylight and temp
body location: mane and tail vs body
what controls cyclic growth
growth factors
hormones (T4, cortisol)
effect of thyroid hormone on hair growth
maintains hair growth and anagen phase
hypothyroid –> shuts down hair follicles
effect of cortisol on hair growth
initiates anagen phase
anagen phase
growing phase
- follicles extend down to subcutis
- fully developed dermal papilla
- hair shaft fully formed/growing
do anagen follicles have immune activity
NO - follicular keratinocytes release anti-inflammatory cytokines to counteract IFN-y by preventing follicles from expressing MHC I
catagen phase
transition phase
- involution of epithelium to initiate detachment of hair bulb
telogen phase
resting phase
- hair detaches from bulb
- shaft is only connected to epithelium by keratin plug (club)
exogen phase
old hair is pushed out when new anagen phase begins
what controls sweat glands
- hypothalamus
- sympathetic NS
do rodents have sweat glands
no
epitrichial glands
apocrine glands
ducts open into hair follicles
- barrier
- pheromone release
- lipid excretion
- thermoregulation
generalized: dogs, cats, horses, ruminants
axillary/inguinal: humans
atrichial glands
eccrine glands
ducts open directly onto the skin surface
- temperature control
- “seal” epidermis
- secrete electrolytes
paw pads/lip/nose: dogs, cats, ruminants
generalized: humans
modified sweat glands
naslabial glands
ceruminous glands (otitis externa)
mammary glands
anal sac glands
mammalian scent glands
sebaceous glands
holocrine gland - balls of cells without a lumen; cells in the center undergo cell death
located in the mid-dermis and open into the isthmus area of the hair follicle
- secrete protective lipids for hair coat/skin
- innate immune function
- removal of follicular keratin
- guide hair shaft through canal
- pheromones
sebum
contains lipids and dead sebocytes
what stimulates sebaceous gland hyperplasia
androgens
what stimulates sebaceous gland involution
estrogens
glucocorticoids
dermal vasculature
3 main plexi:
1. superficial plexus
2. mid-dermal plexus
3. deep plexus
superficial plexus
located directly under the epidermis
mid-dermal plexus
surrounds the hair follicles at the level of the sebaceous glands
deep plexus
located in deep dermis/subcutis
vasculitis
acute vascular wall damage leading to infarction/necrosis of the skin
chronic vasculitis
chronic, low grade damage leading to ischemia and atrophy of the follicles causing ischemic collagen change
