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What are the main functions of the skin?

Protection, sensation, thermoregulation, metabolic, physical and sexual identity


What are some of the protective functions of the skin?

Physical barrier to bacteria
Excessive dehydration, UV radiation
Physical, chemical, thermal insults
Penetration of drugs & chemicals


What are the metabolic functions of the skin?

Adipose tissue is a major energy store
Vitamin D synthesised in epidermis


What are the three layers of the skin?

Dermis - dense irregular CT, highly vascular, many sensory receptors
Hypodermis - loose CT contains adipose tissue


What type of epithelial cells make up the epidermis?

Stratified squamous


What are the four major layers of the epidermis?

Stratum basale
Stratum spinosum
Stratum granulosum
Stratum corneum
(Stratum lucidum in v thick skin between SG & SC)


What is keratinocyte?

Epithelial cell that produces keratin
Is abundant in the epidermis and has abundant intercellular junctions (desmosomes and adherens)


Describe keratin

Family of fibrous structural proteins
Intermediate filament made of 4 protofilaments which are pairs of coiled coils of 2 a-helices
Acidic (1) and basic (2) types
Is abundant in stratum corneum in soft form (undergone keratinisation) S-S bonds of cysteine define soft/hardness


Describe the germ layer

Stem cells and transit amplifying cells sitting on basement membrane
SC - unlimited self renewal, TA - limited division before terminal differentiation


What is keratinisation?

Migration of keratinocytes, which become tightly bound by desmosomes, from basal to corneal layer


Describe the stratum spinosum

Very thick layer; at least 3-4 cells thick
Has numerous desmosomes giving cells prickly appearance
Prominent nuclei and cytoplasmic basophilia-active protein synthesis, highly expressed keratin


Describe stratum granulosum

2-3 cells thick
Large, numerous basophilic keratohyalin granules - filaggrin, involucrin
Synthesise glycoprotein granules - intercellular cementing substance
Cell death occurs at the outermost layer


Describe the stratum corneum

Dead, terminally differentiated cells with unique morphology and staining
Fused flattened cells lacking organelles, filled with mature keratin providing protective barrier of skin
Thick, cornified cell envelope beneath PM


Describe the dermis

Complex mix of macromolecules supplied by many blood vessels which provides strength and elasticity to skin
Acts a support for epidermis
Split into Papillary and reticular


Describe papillary dermis

Is loose CT
Loosely packed T3 collagen with elastin fibres on superficial layer
Contains many blood capillaries (vascular papillae)


Describe reticular dermis

Dense CT
Closely packed T1 collagen and elastin
Provides mechanical strength of skin
Hydrophilic gel but flexibility decreases with age
GS - amorphous matrix that embeds collagenous and elastic fibres, skin appendages
GAGs - hyaluronic acid, dermatan sulphates, chondroitin sulphates


What is the role of fibroblasts in the dermis?

Repair of dermis
Synthesis of collagen, elastin, proteoglycans


What is the pilosebaceous unit?

Hair follicle and sebaceous gland


What are the two types of hair follicle?

Vellus - body hair
Terminal - scalp, secondary sexual hair


Describe the structure of hair follicles

Dermal papilla in hair bulb at root, contains fibroblasts which control hair growth by supplying growth factors
Matrix - surrounding papilla, keratinocytes produce hair
Bulge further up contains hair follicle stem cells, also repair skin
Shaft - dead, exposed head of hair
Root - 5 concentric layers of epithelial; inner 3 form hair shaft, outer 2 form epithelial sheath


What are the 3 stages of the hair cycle?

Anagen - active growth phase
Catagen - regressive, shaft cut off from blood supply and cells
Telogen - resting, hair sheds off


Describe the sebaceous gland

Exocrine gland which is androgen (male sex hormone) sensitive
Enlarges during puberty causing acne
Mature sebocytes contain sebum, cell ruptures and sebum released into sebaceous duct and onto skin (lubricates skin and hair)


Describe the eccrine sweat gland

Excretory duct - 2 layers of smaller cuboidal cells
Compact secretory coil - single layer of large cuboidal/columnar cells


What is the composition of sweat and its function?

99% water, aides thermoregulation as evaporating water cools skin


Describe the apocrine gland

Large sweat glands - widely dilated lumen in coiled secretory portion
Present in axilla (underarms) and pubic region
Releases volatile milky, viscous fluid that is odourless BO produced by breaking down of fluid by bacteria
Not functional until puberty


Describe a melanocyte

Dendritic (antigen-presenting immune cell) cell in epidermis on BM
Produce melanin in melanosome - eumelanin (brown/black), pheomelanin (red/brown) - which is injected into keratinocytes
Protects against UV


Describe langerhan cells

Dendritic present in basal and spinous layers
Antigen presenting cell that is 1st line of defence, presents antigen to T lymphocytes


Describe merkel and mast cells

Merkel - in stratum basale, sensory perception (differences in texture)
Mast - in dermis, immune response, produces histamine


What is endochondral ossification?

Formation of bone during fetal development from hyaline cartilage (T2 collagen) model
Model provides rough shape of mature bone
Is typical of long bone formation, allows stresses to be handled during growth


Describe endochondral ossification

Cartilage model forms in embryo
Blood capillaries invade perichondrium converting to periosteum around centre of model
Bone collar (periosteal bone around diaphysis) produced by osteoblasts in periosteum
Chondrocytes at middle of diaphysis mature, hyper trophy and die, cartilage matrix calcified leaving spicules of calcified cartilage
Nutrient artery from periosteum enters diaphysis through bony collar, carries osteoblasts that lay down trabecular bone in place of calcified cartilage
Appositional growth of epiphysis, chondrocytes in centre mature and die. Calcification of this cartilage occurs
Blood vessel enters degenerating cartilage, osteoblast activity replaces calcified cartilage with bone