BIOL 319 TAMU Cohn Exam 2

What two ions do the kidneys effect that in turn impacts pH?

·       H⁺ (hydrogen ion)

·       HCO₃⁻ (bicarbonate)

·       The kidneys regulate blood pH by secreting H⁺ and reabsorbing or generating HCO₃⁻.

The blood levels of what molecule is directly affected by the respiratory system that in turn regulates the levels of carbonic acid?

CO₂ (carbon dioxide) - The lungs control blood CO₂ by changing ventilation rate.

And if carbonic acid is affected, what molecules of the blood will also be affected and which one of these directly and solely impacts pH?

Carbonic acid equilibrium:

So changes in carbonic acid affect:

H⁺ (hydrogen ion)

HCO₃⁻ (bicarbonate

H⁺ (hydrogen ion)

pH is mathematically defined by [H⁺], so hydrogen ion concentration is the direct determinant of blood pH.

What is the overall purpose of the digestive system?

Its primary, integrated roles include ingestion, mechanical and chemical breakdown of food, nutrient absorption into the bloodstream, and elimination of indigestible waste. It processes nutrients—proteins, fats, carbohydrates, vitamins, and minerals—for growth, cell repair, and energy.

Define digestion.

the process of breaking down food into nutrients for energy, growth, and repair through mechanical (chewing, churning) and chemical (enzymes, acids) methods

Where does digestion begin

in the mouth (oral cavity) before food is even swallowed. It starts through a combination of mechanical digestion (chewing/mastication) and chemical digestion (saliva containing enzymes like amylase breaking down starches)

You should be able to define/describe the following terms. Esophagus

· muscular tube that moves food from the throat to the stomach by peristalsis.

· Peristalsis: coordinated, wave-like smooth muscle contractions

o Used to push food along digestive tract; ensures that once swallowed, it will move efficient toward stomach for furth digestion (chemical & mechanical)

· Does not significantly digest food

· Main function: transport

You should be able to define/describe the following terms. stomach

·       secretes hydrochloric acid = low pH

·       Lining is protected by lots of mucous

·       Two roles:

1.      stimulates the activity of important enzymes that further processes food chemically

2.     Very potent bacteria killer

·       Churns food into liquefied food mixture (chyme), then goes to small intestine

You should be able to define/describe the following terms. small intestine

· main site of chemical digestion & nutrient absorption, food is broken down further by enzymes from pancreas and bile from liver;

· nutrients absorbed thru intestinal lining:

o Fats enter lymphatic system first

o Amino acids, sugars, vitamins, + other nutrients absorbed straight to bloodstream

· Most important organ for absorption in digestive tract

You should be able to define/describe the following terms. large intestine.

· Receives indigestible material left after digestion in small intestine

· As material moves slowly through colon, body reabsorbs significant amount of water, helps form solid waste

· Hosts gut microbiome

o Large community of symbiotic bacteria

- Mutualistic relationship: benefits both sides

§ Bacteria get warm, protected environment

§ Body gains useful products, like vitamin K

What is the pH of chyme in the stomach and what purpose(s) does this pH serve?

· pH: btw 1.5 & 3.5

what purpose(s) does this pH of chyme serve?

essential for activating digestive enzymes, breaking down food proteins, killing harmful bacteria, facilitating nutrient absorption

How does the stomach protect itself from the potentially damaging effects of such pH?

Its mucous lining

How and where is this pH of chyme neutralized?

o   When acidic chyme enters the duodenum (small intestine), bicarbonate ions (HCO₃⁻) are released.

Bicarbonate buffers the hydrochloric acid, raising the pH toward neutral so intestinal enzymes can function and the intestinal lining is protected'

what organ is responsible for enacting this pH of chyme neutralization?

o   The pancreas is the primary organ responsible.

§  It secretes bicarbonate-rich pancreatic juice into the duodenum via the pancreatic duct.

o   (Secondarily, Brunner’s glands in the duodenum also secrete alkaline mucus, but the pancreas does the major neutralization.)

What is the gall bladder

· Small muscular organ that stores bile produced by liver

what does gall bladder contain

· Contains bile,

o acts as an emulsifier

How does bile act as an emulsifier in gall bladder

helps break large fat droplets into smaller droplets so lipids can be efficiently absorbed in small intestine & enter lymphatic system

what prompts the gall bladder to push its contents into the lumen of the small intestine?

·       after fatty meal from duodenum, triggers release of hormone (cholecystokinin) from small intestine, gall bladder contracts and releases bile into lumen of small intestine

   doesn’t chemically dissolve fat but disperses it to make digestion/absorption much more effective

What organ does the gall bladder get its emulsification solution from?

Liver

o Produces bile

o Gall bladder stores & concentrates bile, and when needed gallbladder releases it into small intestine

Define/describe emulsification

· physical process of breaking large fat globules into small droplets using bile salts

o Increases surface area available for digestive enzymes (like pancreatic lipase)

o Mechanical/physical, not chemical digestion

o Makes fat absorption & digestion much more efficient in small intestine

what class of macromolecules are emulsified by the bile from the gall bladder?

lipids (fats)

what is the liver and its function?

· vital accessory organ located in upper right abdomen

o Serves as major metabolic hub

o Serves as detoxifier (processes things like alcohol)

Describe the liver's function to the extent discussed in class (Hint: processing of glucose absorbed from the small intestine).

o After carbs are absorbed in small intestine, glucose goes first to liver via bloodstream

o Liver then:

1. Uses some glucose immediately for own high metabolic needs

2. Releases some glucose into bloodstream to supply body cells after a meal

3. Stores excess glucose by polymerizing into glycogen (storage form of glucose)

what does liver do with glucose regulation between meals

Under endocrine control, liver breaks down glycogen back to glucose and releases it in blood @ controlled rate

This helps maintain stable blood glucose levels during gaps btw meals

What is the gut microbiome

Large community of symbiotic bacteria

where in the GI tract is located?

located in large intestine

Does the gut microbiome have a symbiotic relationship with the humans it inhabits.

Mutualistic relationship: benefits both sides

· Bacteria get warm, protected environment

· Body gains useful products, like vitamin K

In general, what are vitamins?

essential organic molecule that your body can't make in significant amounts so has to be obtained from outside sources

· Not used for energy, but need for normal biochemical + physiological functions

Why is vitamin K important

· important for blood clotting

- You can't produce it, but gut microbiome in large intestine can synthesize it

what ways can one obtain vitamin K?

o Gut bacteria production (major internal source)

o Dietary intake

§ Like leafy greens, vegetable oils, certain fermented foods

Define gonads

Gonads: umbrella term for testes or ovaries

· Produce germ cells

describe the male gonads and the female gonads to the extent discussed in class.

o Produces gametes (germ cells)

- Gametes: haploid cells w/ 23 chromosomes

o Endocrine function

- Gonads act major endocrine glands

- Secrete sex hormones that regulate reproduction, development, and secondary sex characteristics

Describe the centrality of each gonad type in producing the germ cells (what is the name of each respective germ cell, and its chromosome number). male

· Germ cell name: sperm

· Chromosome number:

o Haploid (n=23)

o Contains either X or Y chromosome

· Goal: deliver paternal DNA to egg

Describe the centrality of each gonad type in producing the germ cells (what is the name of each respective germ cell, and its chromosome number). female

· Germ cell: ovum (or egg)

· Chromosome number:

o Haploid (n=23)

o Contains either X chromosome

· Goal: provides maternal DNA and most cytoplasm/organelles

Are the testes and ovaries solely and only considered part of the reproductive system? If not, what other system(s) do each of these organs play a central role?

No; testes & ovaries are apart of both reproductive system and endocrine system.

What type of molecule is testosterone and estrogen?

steroid hormones

What is the precursor of all steroid hormones?

Cholesterol

Is cholesterol "bad"? If not, is it "good"? Describe the "good" and or "bad" to the extend discussed in class.

Cholesterol itself is not bad, but high cholesterol is (can clause clots and clogged arteries)

Good: stabilizes cell membranes & serves as precursor for all steroid hormones

Describe the organ/organ system that gives rise to the mammary glands in females?

Integumentary system -> mammary glands evolved from modified sweat glands

o Oxytocin stimulates milk ejection during breastfeeding

Despite the origin of mammary glands, what system is the system that the mammary glands are physiologically associated with?

Usually studied with reproductive system

Describe the overall functions of the integumentary system as discussed in class.

To protect

Prevention against water loss

To sense

Thermoregulation

Vitamin D synthesis

Excretion (to small extent)

Integumentary System Function Description: To protect

The skin forms a physical and chemical barrier that:

▪ Prevents entry of microorganisms (bacteria, fungi, pathogens)

▪ Protects against mechanical damage (cuts, abrasions)

▪ Shields from some UV radiation

▪ Prevents opportunistic infections

Integumentary System Function Description: Prevention against water loss

Skin is almost impermeable to water, which helps maintain body hydration.

▪ Prevents excessive evaporation

▪ Maintains ~60% body water composition

▪ Critical for survival

Prevention against water loss: 3rd degree burns

In 3rd degree burns, lose lots of water and hard to keep hydrated

o Severe dehydration risk

o Requires aggressive fluid replacement

Integumentary System Function Description: To sense

The skin has extensive sensory receptors connected to the nervous system.

o Close w/ nervous system

o Has specialized nerve endings

▪ Touch

▪ Pressure

▪ Pain

▪ Vibration

▪ Itch

▪ Temperature

o Two types of receptors:

▪ Warm

▪ cold

Integumentary System Function Description: Thermoregulation

Close w/ circulatory system

o Mechanisms:

▪ Vasodilation

▪ Vasoconstriction

▪ Sweating (Evaporative cooling)

Thermoregulation Mechanisms: Vasodilation

Dermal blood vessels widen

• Skin can hold up to ~20% more blood

• Increases heat loss (radiation)

Thermoregulation Mechanisms: Vasoconstriction

• Reduces blood flow to skin

• Conserves heat

Thermoregulation Mechanisms: Sweating (Evaporative cooling)

• Sweat glands activate quickly

• Evaporation removes heat

o Essential for keeping body near 98.6°F ± ~1°F

Integumentary System Function Description: Vitamin D Synthesis

o Only exception to vitamins → we make it, but can also get it elsewhere

▪ Originally classified as a vitamin, but really a hormone

o Skin helps produce vitamin D when exposed to UV light

▪ Vitamin acts like hormone

▪ Essential to calcium absorption & bone health

Integumentary System Function Description: Excretion (to small extent)

Sweat

▪ excretes some ions/nitrogenous waste out of body

Is the epidermis vascularized?

No epidermis is avascular

What does "thick skin" actually mean and where is it overwhelmingly found?

Thick skin refers to thick epidermis, and found in places like on soles of feet and palms of hands, ventral (palmar) surfaces of fingers

Describe each layer's histological function: Stratum Basale

▪ 1 cell layer thick (often described as one row)

▪ Cells are attached to the basement membrane via desmosomes

▪ Very mitotically active “stem cell zone”, not squamous

Describe each layer's salient physiological function: Stratum Basale

Stem cells divide (mitosis) to continually replace epidermal cells

▪ One daughter cell stays a stem cell

▪ One daughter cell becomes a new keratinocyte and moves

upward ~1 layer (1 cell thick)

Describe each layer's order that the layers occur from deep to superficial and vice versa: Stratum Basale

deepest epidermal layer (right on the basement membrane)

Describe each layer's approximate thickness of the layer: Stratum Basale

One daughter cell becomes a new keratinocyte and moves upward ~1 layer (1 cell thick)

Describe the origin of the name of the layer: Stratum Basale

“Basale” = base/basement layer (basal layer attached to basement membrane)

Describe each layer's histological function: Stratum spinosum

~6–10 cells thick (your prof said “about ~8,” not splitting hairs)

• Increased desmosomes between cells (stronger cell-to-cell attachment)

• Keratinocytes begin making keratin (bundled intermediate filaments)

• Presence of lamellar bodies containing glycolipids (waterproofing material)

• Often contains immune “dendritic” cells (Langerhans-type) scattered here (prof started this at end)

Describe each layer's salient physiological function: Stratum spinosum

• Provides strength via increased desmosomes

• Begins keratinization → more toughness + water resistance

• Starts producing/packaging glycolipids (lamellar bodies) that will help waterproof the spaces

between cells later

Describe each layer's order that the layers occur from deep to superficial and vice versa: Stratum spinosum

just above basale

Describe each layer's approximate thickness of the layer: Stratum Spinosum

~6–10 layers (≈ 8)

Describe the origin of the name of the layer: Stratum Spinosum

• “Spinosum” = spiny layer

• “Spiny” appearance is largely an artifact of tissue prep:

- Cells shrink in hypertonic prep and pull apart

- Desmosomes keep “spike-like” connections → looks spiny under microscope

Describe each layer's histological physiological function: Stratum granulosum

• Only a few cells thick (≈ 2–3 layers)

• Cells are on their “deathbed” (nucleus/organelles breaking down)

• Lots of keratohyalin granules (“grainy” look)

• Lamellar bodies release glycolipids → forms waterproof seal between cells

Describe each layer's salient physiological function: Stratum granulosum

• Major waterproofing + final keratinization push

o Keratohyalin granules drive heavy keratin buildup

o Glycolipids dumped between cells → blocks water movement between cells

o Combined effect: water can’t pass well through cells (keratin) or between cells (lipids +

tight junctions)

• Cells undergo apoptosis (programmed “good” cell death for organism benefit)

o Prof contrasted: apoptosis = good/normal, necrosis = bad (trauma/disease)

Describe the order that the layers occur from deep to superficial: Stratum granulosum

above spinosum (the "death zone")

Describe each layer's approximate thickness of the layer: Stratum granulosum

~2–3 layers

Describe the origin of the name of the layer: Stratum granulosum

“Granulosum” = grainy layer (due to visible keratohyalin granules)

Describe each layer’s histological physiological function: Stratum lucidum (ONLY in thick skin)

• Few layers thick

• Cells are dead and look clear/translucent (no granules, no nuclei)

Describe each layer’s salient physiological function: Stratum lucidum (ONLY in thick skin)

• Adds extra protective layer in high-friction regions

Describe the order that the layers occur from deep to superficial: Stratum lucidum (ONLY in thick skin)

between granulosum and corneum, only in thick skin

Describe each layer’s approximate thickness of the layer: Stratum lucidum (ONLY in thick skin)

A few layers (thin, but present)

Describe the origin of the name of the layer: Stratum lucidum (ONLY in thick skin)

“Lucidum” = clear layer (“lucid” = clear)

Describe where present: Stratum lucidum

• Thick skin = thick epidermis (not thick dermis!)

• Found on palms, palmar fingers, soles

Describe each layer's histological physiological function:

• Thickest layer by far

• Dead, flattened “shingle-like” cells packed with keratin

• Strong “brick-and-mortar” barrier:

o “Bricks” = dead keratin-filled cells

o “Mortar” = glycolipids between cells + tight junction effects + remaining cell connections

Describe each layer's salient physiological function:

• Main protective barrier:

o Mechanical protection (friction, abrasion)

o Water resistance (prevents dehydration)

o Helps protect from environmental insult (including radiation exposure via pigment

content)

• Constant turnover: cells move from basale to surface and flake off

Describe the order that the layers occur from deep to superficial:

most superficial (outermost barrier you shed)

Describe each layer's approximate thickness of the layer:

• Typical skin: ~25–35 cell layers

• Thick skin (palms/soles): can be ~50+ layers

Describe the origin of the name of the layer: Stratum corneum

“Corneum” = horn/cornified layer (tough outer layer)

For describing the above, you should incorporate the structure and function of hemidesmosomes, desmosomes, stem cells, apoptosis, keratin (soft or hard?), keratohyalin granules, lamellar bodies, glycolipid, keratinocytes and keratin: Stratum basale (germinativum)

Histology ID:

• Sits directly on the basement membrane

• Highly mitotic “birthplace” of epidermal cells

Key structures & what they do here:

• Stem cells are the “exception/nuance”: these basal cells aren’t fully mature squamous yet — they

are epidermal stem cells.

• Hemidesmosomes: anchor basal cells to the basement membrane

o This is how the epidermis stays “glued” to the dermis.

• Desmosomes: also present between neighboring basal cells to help hold them together.

Salient physiological function:

• Mitosis produces new cells:

o One daughter cell remains a stem cell

o The other becomes a keratinocyte and moves into the next layer (spinosum)

For describing the above, you should incorporate the structure and function of hemidesmosomes, desmosomes, stem cells, apoptosis, keratin (soft or hard?), keratohyalin granules, lamellar bodies, glycolipid, keratinocytes and keratin: Stratum spinosum ("spiny layer")

Histology ID:

• Cells look “spiny” in prepared slides due to artifact

• Increased cell-to-cell connections

Key structures & what they do here:

• Desmosomes increase (high density)

o Function: resist pulling forces; strengthen the tissue against friction/shear.

• Keratinocytes dominateo These are the cells actively starting to become the protective barrier.

• Keratin production begins

o Keratin is made from intermediate filaments bundled into tough, water-resistant

material.

o In epidermis: soft keratin (flexible protective keratin).

▪ (Hard keratin is in hair and nails.)

• Lamellar bodies form

o Keratinocytes package glycolipids into lamellar bodies (little packets).

Salient physiological function:

• Mechanical strength (desmosomes = “spot welds”)

• Start building the water barrier:

o Keratin resists water moving through cells

o Lamellar bodies prepare glycolipids to block water between cells later

For describing the above, you should incorporate the structure and function of hemidesmosomes, desmosomes, stem cells, apoptosis, keratin (soft or hard?), keratohyalin granules, lamellar bodies, glycolipid, keratinocytes and keratin: Stratum granulosum ("grainy layer")

Histology ID:

• Cells look grainy due to granules

• Cells are degenerating (organelles breaking down)

Key structures & what they do here:

• Keratohyalin granules

o The visible “grains” that mark this layer

o Serve as centers for massive keratin buildup/organization

• Lamellar bodies release their contents

o Dump glycolipids into the spaces between cells

o Glycolipids are viscous/waxy → strong water repellent

• Apoptosis begins in earnest

o Cells are on the “deathbed”: nucleus and organelles break down

o This is programmed cell death (good/intentional), not necrosis

Salient physiological function:

This is where the epidermis becomes a true barrier:

• Blocks water through cells: tons of keratin (water resistant)• Blocks water between cells: glycolipids fill intercellular space (“mortar”)

• Tightens overall sealing (often includes tight junction influence as your lecture noted)

For describing the above, you should incorporate the structure and function of hemidesmosomes, desmosomes, stem cells, apoptosis, keratin (soft or hard?), keratohyalin granules, lamellar bodies, glycolipid, keratinocytes and keratin: Stratum lucidum (“clear layer”) (only in thick skin)

Histology ID:

• Clear/translucent layer

• Cells are already dead

• No nuclei, no keratohyalin granules → clear appearance

Key structures & what they do here:

• Essentially an extra layer of dead keratin-filled cells adding toughness in high-friction areas

Salient physiological function:

• Extra protection in thick epidermis regions

For describing the above, you should incorporate the structure and function of hemidesmosomes, desmosomes, stem cells, apoptosis, keratin (soft or hard?), keratohyalin granules, lamellar bodies, glycolipid, keratinocytes and keratin: Stratum corneum ("horn/cornified layer")

Histology ID:

• Thick sheet of dead, flattened, shingle-like cells

• Essentially keratin “sacs”

Key structures & what they do here:

• Cells (keratinocytes) have completed their differentiation → now are cornified dead cells packed

with soft keratin

• Intercellular spaces are filled with glycolipids (from lamellar bodies)

→ waxy seal between cells

• Desmosomal remnants and tight junction effects help cohesion until shedding

Salient physiological function:

• Main physical + waterproof barrier

o Prevents dehydration (water loss)

o Mechanical protection against friction and tearing

o Helps protect from environmental damage

• Constant shedding at surface (turnover pipeline from basale)

What is the role of apoptosis in maturing the skin cells into the most superficial

and by far the thickest layer, the stratum corneum?

Role of apoptosis (programmed cell death)

Converts fragile living cells → tough, inert barrier cells

• Eliminates structures that would require oxygen/nutrients (important because epidermis has no

blood supply)

• Prevents inflammation (unlike necrosis)

Functional outcome: produces the dead protective cells that make up the stratum corneum.

What is the role of keratin in maturing the skin cells into the most superficial

and by far the thickest layer, the stratum corneum?

 Role of keratin (soft keratin in epidermis)

 Makes cells mechanically resistant to friction and tearing

• Makes cells resistant to water passage THROUGH the cells

Functional outcome: creates tough, water-resistant “bricks” of the stratum corneum.

What is the role of glycolipid in maturing the skin cells into the most superficial

and by far the thickest layer, the stratum corneum?

Seal the spaces between corneum cells

• Prevent water movement between cells

• Greatly reduce dehydration risk

Functional outcome: creates the waterproof intercellular seal.

For cells in each layer you should be able to highlight the major physiological changes that occur. How do these physiological changes add up to the final product which is a thick layer of dead, melanized keratinocytes that serve as mechanical protectors of invasion by microbes, prevent indiscriminate water loss and protect underlying cells from the damaging effects of ultraviolet light?: stratum basale

“birth and pigmentation loading”

Major physiological events

• Rapid mitosis of stem cells

• Production of new keratinocytes

• Cells anchored by hemidesmosomes

• Melanin transferred into keratinocytes from melanocytes

Why this matters

• Maintains constant cell supply

• Begins UV protection early by loading melanin

• Keeps epidermis attached to dermis

Contribution to final product: continuous supply of melanized precursor cells.

For cells in each layer you should be able to highlight the major physiological changes that occur. How do these physiological changes add up to the final product which is a thick layer of dead, melanized keratinocytes that serve as mechanical protectors of invasion by microbes, prevent indiscriminate water loss and protect underlying cells from the damaging effects of ultraviolet light?: stratum spinosum

“strengthening and early waterproofing”

Major physiological events

• Keratinocytes enlarge and mature

• Keratin production begins (soft keratin)

• Desmosomes increase → strong cell-cell adhesion

• Lamellar bodies form (contain glycolipids)

• Cells still alive and metabolically active

Why this matters

• Tissue becomes mechanically tough

• Prepares materials for future waterproofing

• Creates strong cohesive sheet of cells

Contribution: builds mechanical strength and prepares lipid barrier.

For cells in each layer you should be able to highlight the major physiological changes that occur. How do these physiological changes add up to the final product which is a thick layer of dead, melanized keratinocytes that serve as mechanical protectors of invasion by microbes, prevent indiscriminate water loss and protect underlying cells from the damaging effects of ultraviolet light?: stratum granulosum

“barrier assembly and programmed death”

Major physiological events• Massive keratin accumulation

• Appearance of keratohyalin granules

• Lamellar bodies release glycolipids between cells

• Apoptosis begins

o nucleus and organelles break down

• Tight waterproof seal starts forming

Why this matters

This is the critical transition layer.

It creates BOTH waterproof strategies:

Through cells blocked by:

• heavy keratin (hydrophobic)

Between cells blocked by:

• glycolipid “mortar”

Meanwhile apoptosis:

• removes metabolically active machinery

• creates inert barrier cells

• prevents inflammation

Contribution: converts living cells into waterproofing barrier units.

For cells in each layer you should be able to highlight the major physiological changes that occur. How do these physiological changes add up to the final product which is a thick layer of dead, melanized keratinocytes that serve as mechanical protectors of invasion by microbes, prevent indiscriminate water loss and protect underlying cells from the damaging effects of ultraviolet light?: Stratum lucidum (thick skin only)

Major physiological events

• Cells now fully dead

• Packed with keratin

• Clear appearance

Why this matters

• Adds extra protection in high-friction areas

• Thickens the barrier

Contribution: additional mechanical reinforcement where needed.

For cells in each layer you should be able to highlight the major physiological changes that occur. How do these physiological changes add up to the final product which is a thick layer of dead, melanized keratinocytes that serve as mechanical protectors of invasion by microbes, prevent indiscriminate water loss and protect underlying cells from the damaging effects of ultraviolet light?: Stratum corneum

“final protective shield”

Major physiological state

Cells are now:

• Dead

• Flattened

• Packed with soft keratin

• Surrounded by glycolipid seal

• Strongly linked by residual junctions

• Loaded with melanin

Functional outcomes

Mechanical protection

Microbial barrier

Prevention of water loss

Two complementary mechanisms:

1. Through cells blocked by

• keratin (hydrophobic)

2. Between cells blocked by

• glycolipids (waxy mortar)

→ prevents dehydration (critical in burn patients)

UV protection

Comes from:

• Melanin loaded into keratinocytes early (basale)

• Melanin caps absorb and scatter UV

• Thick corneum adds additional shielding

→ protects DNA of living basal cells

How does the lack of vascularization of the epidermis play into a key developmental event as the skin cells continue to rise from deeper layers to more superficial layers?

Because the epidermis is avascular, keratinocytes rely on diffusion from dermal capillaries. As they move upward through the epidermal layers, they become too far from the blood supply, triggering apoptosis. This programmed cell death transforms them into flattened, keratin- and lipid-filled corneocytes that form the protective, water-resistant stratum corneum barrier.

Describe Merkel cells (location)

• Found in the stratum basale (deepest layer of the epidermis)

• Interspersed among basal keratinocyte stem cells

• Each Merkel cell is closely associated with a sensory nerve ending

Describe Merkel cells (function)

light-touch receptors that help the body detect gentle mechanical stimuli on the skin.

Do these Merkel cells function as mechanoreceptors?

yes

How do these Merkel cells function as mechanoreceptors?

• A mechanoreceptor responds to physical deformation.

• When the skin surface is lightly pressed or compressed:

1. The Merkel cell is mechanically deformed (squished).

2. This deformation changes the Merkel cell’s membrane potential.

3. The associated sensory nerve ending is stimulated.

4. A nerve impulse travels to the spinal cord and brain.

5. The brain interprets this as fine/light touch.

Describe the location of dendritic cells.

• Primarily located in the stratum spinosum of the epidermis

• Scattered (“smattered”) among keratinocytes

• Migrated into the epidermis from the immune system

Describe the histology of dendritic cells

• Irregular cell body with a visible nucleus

• Multiple long, branching cytoplasmic processes (dendrite-like extensions)

• Processes extend between surrounding keratinocytes

• Under the microscope they resemble neurons, but they are not neurons