Lesson 4 - Biology of Friction Ridge Skin

Structure, Development & Persistence of FRS

Weeks 8-18

Crease Formation

• Major & minor creases form

  • Most develop concurrently with the volar pads but some develop independently

• Part of the same skin structure as our ridges

8 weeks - Radial Longitudinal

9 weeks - Distal Interphalangeal & Metacarpophalangeal

10 / 18 weeks - Proximal Interphalangeal

11 weeks - Distal Transverse

11.5 weeks - 1st hand movement

13 weeks - Proximal Transverse

Creases vs. White Lines

Creases

• Present on the dermal layer of the skin

• Embryonic in nature - develop ~ 8-18 weeks EGA

• Fixed shape, size & position

• Ridges end at borders of a crease

• Persistent & unique

White Lines

• Superficial folds in the epidermal skin to the Hyalin layer (Stratum Lucidum)

• Develop over time & named for the white lines they leave in inked fingerprint cards

• Change shape, size & position

• Ridges flow thru to the other side of a white line

• NOT persistent or unique

Weeks 7-11

Volar Pad Development

• Volar pads begin to swell

  • Palms ~6.5 weeks

  • Fingers ~7.5 weeks

  • Feet ~8 to 9 weeks

• Volar pad regression = the slowing growth of volar pads & the simultaneous, more rapid, growth of the hand / feet around the pad

• Regression can start ~11 weeks but is not complete until 16 weeks - right before the secondary ridges begin forming

Stratum Lucidum

“Hyalin” layer

• Thin layer of cells

• Clear flattened dead cells - part of the “Cornified Zone”

• Found ONLY in thick skin

Differentiation

Change a cell goes through from birth until death

• Loss of ability to divide

• Increase in cell size & keratin while cell flattens

• Internal changes including synthesis of new proteins / lipids and the addition / degeneration of organelles

• External changes including changes to the plasma membrane properties, surface antigens & receptor sites

• Dehydration & eventually cell death

• Takes about 30-45 days

Hypodermis

Innermost layer of the skin

• Composed of adipose tissue

• No function in regards to friction ridge skin

Function:

• Insulates the body

• Serves as an energy reserve

• Cushions & protects the body

• Allows for skins mobility over underlying structures

Papillae Pegs

Peg-like projections from the dermal papillae that provide the template for the friction ridge arrangement

• Double rows - one on either side of primary ridges

• Helps increase the bond between the epidermis & dermis

• Anastomoses provide additional support by acting like glue bonded to the papillae pegs

Tight Junctions

Attach the keratinocytes to each other - allow them to migrate upward in tandem

Mesenchyme

Loosely organized undifferentiated cells which give rise to numerous structures

Weeks 24-27

Dermal Papillae

• Papillae pegs begin forming after the secondary ridge depth is equal to the primary ridge depth

• Anastomoses appear

• Adherence of the epidermis through anastomoses & the basement membrane zone

• Around 24 weeks the friction ridges are fully developed & set for life

Langerhans Cells

Trigger T-cells → immune response

Phases of Scar Formation

Phase I: Inflammation

• Blood flows to wound immediately

• Platelets send out signals recruiting immune cells to kill bacteria & scavenge damaged cells

• Fibroblasts begin to repair the dermis & endothelial cells begin to repair blood vessels

Phase II: Proliferation & Tissue Formation

• Desmosomes & hemidesmosomes dissolve to allow cell movement

• Pseudopodia & actin filaments help the basal keratinocytes to crawl across the wound

• Dermis contracts & the ridges on the surface pucker

• Mitosis & upward migration of cells in the wound continue until the proper skin thickness is obtained

Phase III: Tissue Remodeling

• Fibroblasts of the dermis continue to reinforce the scar tissue for weeks or months

• Scar replaces the friction ridge skin at the wounded area and remains persistent

Stratum Corneum

“Horny” layer

• Outer layer of cells

• Cells are fully keratinized

• Can be up to 100 cells thick (making it 20 or 30 times thicker than anywhere else)

• Large, flat overlapping dead cells - part of the “Cornified Zone”

• Cells that “slough” off

Stratum Spinosum

“Spinous” layer

• Several cell layers thick

• Cells begin to shrink due to water loss & look spiny due to the abundant desmosomes attaching them together

Supra-basal layer

• Located between the basal & spinous layers

• Found ONLY in primary ridges

• Transitional area found in thick skin

• Contains transient amplifying cells → Arise from stem cells & divide a finite number of times until they become differentiated

Stratum Basale

“Basal” or “Generating” layer

• Innermost layer of the epidermis

• One cell thick → cells are columnar in shape

• “Blueprint” for the friction ridges

• Gives rise to the rest of the cells in the epidermis

• Attached to the basement membrane

Stratum Granulosum

“Granular” layer

• Several cell layers thick

• Last living layer of the epidermis → “Malpighian”

• Nuclei in various stages of degradation marking the beginning states of cell death

• Cells are still alive but start to break down & flatten out

Desmosomes

• Attachments between cells

• Exist between cells throughout the entire epidermis

• New cells are pushed up in concert from basal layer

• Undergo modifications as cells progress outward from basal layer

• They are broken down to allow cells to slough off as they reach the surface of the skin

Dermis

• Middle layer of the skin

• Composed of collagen fibrils, elastic fibers, blood vessels, lymphatic vessels, & nerves

• Almost 90% of T-cells are located within the dermis

• Has two layers → Dermal Papillae & Reticular Dermis

Function:

• Regulates fluids

• Provides nutrients to epidermis & removes waste

• Protects body from mechanical injury

• Thermal regulation

• Has sensory receptors

Reticular Dermis

• Innermost region of the dermis

• Dermal strength & resilience

• Connects to hypodermis by a fibrous network

Dermal Papillae

• Outermost region of the dermis

• Consists of malleable rounded projections called “papillae pegs”

Weeks 5-7

Precursory Development

• Paddle like hand - thick tissue

• Cartilaginous bone develops from the mesenchyme

• Fingers elongate & separate

• Thumbs rotate

• Nerve innervation occurs

Thin Skin vs Thick Skin

Thin Skin

• Found on most of the body

• Approx. 1.5mm thick

• Contains hair follicles & hair

• Contains eccrine & sebaceous glands

Thick Skin

• Palmer side of hands & soles of feet

• Approx. 4mm thick

• Hairless

• Contains eccrine glands ONLY

• Contains primary & secondary ridges that form FRS

Weeks 14-16

Sweat Gland Anlagen

• Precursor to sweat glands

• Appear ~14 weeks where the primary ridges are forming

Anastomoses

• Sheets of tissue that act like glue bonding to papillae pegs

• Fills in around the pegs

• Continues to fill in gaps as the pegs branch during aging

Hemidesmosmes

• Located in basement membrane zone

• Attachment plaques within basal cells that lock cells to the Basal Lamina

• Tonofilaments secure the plaques to cells & receptor sites accept anchoring filaments from the Lamina Lucida

Merkel cells

• Sensory input → extension of the body’s nervous system

Primary Ridges vs. Secondary Ridges

Primary Ridges

• Under the surface ridges

• Flanked by papillae pegs

• Contains supra-basal layer → transient amplifying cells

• Start to form ~10.5 weeks

Secondary Ridges

• Under the surface furrows

• Appear between primary ridges

• Penetrate as deep as the primary ridges

• Lack sweat glands

• Start to form ~17 weeks

Weeks 10-17

Primary Ridge Formation

• Epidermis remains undifferentiated until 10-11 weeks

• Around 10.5 weeks primary ridges begin to form on bottom of epidermis → “Critical Stage”

• As the skin grows, it separates the existing primary ridges & new primary ridges form

• 15-17 weeks = Downward penetration of sweat glands & upward push of new cell growth

• The entire surface is ridged by 15-17 weeks

Incipient Ridge

• Thin, fragmented ridges that appear in furrows between normal mature friction ridges

• Immature & not fully formed

Homeostasis

• Condition in which the body’s internal environment remains relatively constant within physiology limits

• Achieved in the skin thru physical attachments & regulation of cell production via cell communication

• Keeps everything in equilibrium → The number of cells being produced needs to match the number of cells being sloughed off

Cell Surface Receptors

• Protein sites on cell membrane that accept chemical signals

• Help the cells to “communicate” & signals when to start / stop cellular reproduction

Epidermis

• Outermost layer of the skin

• Thick Skin = 5 layers deep

• Approx. 1.8mm thick

• Mostly comprised of keratinocytes

Volar Pads

• Transient swellings of the mesenchymal tissue that vary in symmetry & size

  • Palms ~6.5 weeks

  • Fingers ~7.5 weeks (starts with thumb and progresses to little finger → distal to proximal on each finger)

  • Soles of feet ~9 weeks

• Regression is slowing growth of volar pad and simultaneous more rapid growth of hand/foot around pad

Gap Junctions

• Connections between cell membranes that permit exchange

Keratinocytes

• Primary cell of the epidermis (90-95%)

• Contains keratin = providing strength

  • Durable protein

  • Organized into bundles (filaments)

  • Structural support

  • Reinforced cells so they do not break when subjected to physical stress

  • K9 (only in thick skin) is present in primary ridges which makes them more durable than secondary ridges

Hyperkeratosis

Overproduction of skin cells that can cause…

• thickened / rough skin

• clogged pores

• dryness / irritation

• dull / patchy / scaly

Conditions:

• Psoriasis

• Eczema

• Corns, calluses, & warts

Causes:

• Genetics

• UV / sun exposure

• Autoimmune diseases

• Dermatitis

• Pressure / rubbing of skin

• Allergies

Basement Membrane Zone

Two-part fibrous zone that attaches the epidermis & dermis

• Contains elements from both the epidermis & dermis

• Provides structural support to the skin

• Filters nutrients from the dermal blood vessels to the keratinocytes

Weeks 17-24

Secondary Ridge Formation

• Second proliferation of cells into the dermis - begins ~17 weeks

• All primary ridge growth STOPS

• Secondary ridges appear between the primary ridges & lack eccrine glands

• As the secondary ridges penetrate into the dermis they pull the already tight skin down with them which forms the surface furrows

• Surface ridges spread across the finger & converge at the delta areas

Skin Regeneration

1) Skin cells slough off from the Horny layer

2) Cells communicate to maintain homeostasis

3) Basal cells replicate DNA & start mitosis

4) Additional cells replicate in the Supra-basal layer

5) Cells migrate upward in concert & differentiate

Lamina Lucida

A transparent layer containing anchoring filaments that attach to the plasma membrane of the basal keratinocytes

• Top layer of the Basal Lamina

• The filaments are attached below & perpendicular to the hemidesmosomes

Lamina Densa

A dense layer containing anchoring fibrils that attach to collagen fibrils of the dermis

• Bottom layer of the Basal Lamina

Melanocytes

Pigment cells → Protect us from UV / sun exposure

Dermal-Epidermal Junction

Multiple parts that connect the epidermis and the dermis of the skin together

Basal Lamina

Lower half of the basement membrane zone that contains the Lamina Lucida & Lamina Densa

• Helps attach the epidermis to the dermis

Causes of Variation in the Formation of our Tissues and Body Parts

1. Genetics

2. Environment

3. Developmental Noise

Human variation exists due to…

genotype and phenotype

What do our genetics direct?

• Might direct when and where but NOT how

• Pattern type is influenced by genetics but there are no genes for pattern type

Genotype

The set of chromosomes we inherit from our parents

Phenotype

Particular trait a person displays → the traits that are actually “expressed”

• Ex. Hair color, eye color, etc.

• Different genotypes can cause different phenotypes

Environmental Factors

• Intrauterine stressors

  • Fetal environment

  • Maternal hormone levels

  • Maternal diet

  • Chemical intake, alcohol or drug use

  • Excess fluids

• Disease

• Radiation
  

Developmental Stability

• The ability of an organism to produce a consist phenotype despite environmental and genetic variations

• Growth & development of the embryo & fetus are developmentally stable

  • Ex. # of fingers / toes, length of bones, etc.

Developmentally Stable Growth Factors

• Growth & development of fetus

• Development of hands and feet

• Placement of regular flexion creases

• Placement of volar pads

Developmental Noise

Features that are not hardwired in genetic code and therefore are at the mercy of random formation

• Exact placement of each ridge and crease is left up to chance events that take place during development

• Causes different phenotypes to arise from same genotype in same environment

• Source of randomness

• Provides tremendous variation in the arrangement of ridges

Growth Stress

Impacts the directionality of ridges = guides direction of ridges

• Differential growth establishes growth stress

• Ridges grow perpendicular to growth stress

Ridge Formation

• Link between growth stresses & friction ridge formation is MERKEL CELLS

  • Differentiate and band together in the epidermis

  • Perpendicular to growth stresses

  • Prior to ridge formation

Merkel Cell Activity

• Stimulates basal cells which proliferate into dermis, beginning primary ridge development

• Basal cells divide along bands of Merkle cells

• Cells continue to divide & deepen primary ridges

Bands of Merkel Cells

• Wherever the merkel cells band together the first primary ridges form

• Longest & most continuous ridges form first with few minutiae

• 1st minutiae form in areas where bands of Merkel cells diverge to cover large space or converge to squeeze into a smaller space… this results in ending ridges & bifurcations

• Funnel areas have less unique minutiae

• Hand continues to grow in length & width

• Existing primary ridges are pulled away from each other & new ridges form… they tend to be shorter in length & fill in space which adds more minutiae

• Exact arrangement of ridges not dictated by DNA

• Developmental Noise has its greatest impact during this process (ridge formation)

What three things are driven by growth stresses?

1. Patterns

2. Ridge counts

3. Minutiae densities

3 Main Growth Stresses

1. Growth of hands and feet - increase in length faster than width

2. Regular flexion creases - form prior to ridges & are sites of tension in skin

3. Volar pads - create localized growth stresses that redirect ridge flows in small areas