Focus: Human Anatomy

Human Anatomy · Integumentary

Focus: Human Anatomy

The Integumentary System

Skin is the body's largest organ. It is built from all four tissue types working together, layered into the epidermis and dermis, with appendages (hair, nails, glands) growing down from the surface. In a combined class this anatomy unit pairs with the skin physiology unit. Dr. Sharilyn Rennie

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Part 1

The Skin and Its Layers

Skin is an organ made of two attached layers, the epidermis and the dermis, resting on the hypodermis. Learn the plan, then the parts.

Part 1 · The big picture

The integument: the body's largest organ

Overview

The cutaneous membrane (skin) is the epidermis (a surface epithelium) firmly attached to the dermis (a connective tissue layer) beneath it. The hypodermis is a fatty layer below that anchors skin to the body.

  • Protection: a physical, chemical, and biological barrier against abrasion, water loss, UV, and microbes.
  • Temperature regulation: blood flow and sweat glands shed or hold heat.
  • Sensation: receptors for touch, pressure, temperature, and pain.
  • Metabolic: begins vitamin D production when sunlight strikes the skin.
  • Excretion and storage: small amounts of waste leave in sweat; the dermis stores water and fat sits in the hypodermis.

Part 1 · The layered model

Two layers, plus the hypodermis

Anatomy
  • Epidermis: keratinized stratified squamous epithelium. No blood vessels of its own.
  • Dermis: connective tissue in two zones, the thin papillary layer and the thick reticular layer. Holds vessels, nerves, and glands.
  • Hypodermis (subcutaneous): areolar and adipose tissue. Not part of the skin itself, but anchors it and cushions.
  • Embedded structures: sebaceous gland, arrector pili muscle, apocrine and eccrine sweat glands, and the lamellated (Pacinian) corpuscle.
Labeled skin model in cross section. From top: epidermis, then the papillary dermis and reticular dermis, then the hypodermis with fat. Pointing into the layers are a sebaceous gland and arrector pili muscle on a hair, an apocrine sweat gland, an eccrine sweat gland, and a deep lamellated (Pacinian) corpuscle.

The full-thickness skin model: layers and embedded structures.
Skin model photographed and labeled by Dr. Sharilyn Rennie.

Part 2

The Epidermis

A tough, renewing sheet of keratinized stratified squamous epithelium. Cells are born deep and die at the surface.

Part 2 · The strata

The epidermis: layers from deep to superficial

Epithelium

Keratinocytes are pushed up from the base, fill with keratin, and die. Read the strata from deep to superficial:

  • Stratum basale: single row of dividing stem cells on the basement membrane. Melanocytes and tactile cells sit here.
  • Stratum spinosum: several rows of keratinocytes linked by desmosomes (a spiny look).
  • Stratum granulosum: cells fill with granules and begin to die.
  • Stratum lucidum: a clear band found only in thick skin (palms, soles).
  • Stratum corneum: many flat, dead, keratin-filled cells. The barrier you see and touch.

Deep to superficial memory line: Basale, Spinosum, Granulosum, Lucidum, Corneum.

Skin model and labels showing the epidermal strata from deep to superficial: stratum basale, spinosum, granulosum, lucidum, and corneum. The dermis below shows a dermal papilla, Meissner corpuscles, capillary loops, and free nerve endings.

The five epidermal strata, with dermal papillae beneath.
Skin model photographed and labeled by Dr. Sharilyn Rennie.

Part 2 · Thick and thin

Thick skin and thin skin

Histology
  • Thin skin covers most of the body. It has four strata (no stratum lucidum) and a thinner corneum, with hair follicles.
  • Thick skin covers the palms and soles. It adds the stratum lucidum, has a very thick corneum, and has no hair follicles.
  • Spot it: on a slide, thick skin shows a tall pink corneum and deep epidermal ridges; thin skin shows a slim corneum and shallower ridges.
Two light micrographs side by side. Thin skin on the left has a slim surface keratin layer and shallow waves at the epidermal-dermal border. Thick skin on the right has a very tall pale keratin layer at the surface and deep finger-like ridges between epidermis and dermis.

Thin skin (left) and thick skin (right).
Light micrographs of thin and thick skin, used for teaching.

Part 3

Epidermal Cells and Skin Color

Most epidermal cells are keratinocytes, but three other cell types live among them, and three pigments set skin color.

Part 3 · Four cell types

The four cells of the epidermis

Epithelium

Keratinocytes

  • Where: every stratum; they are the bulk of the epidermis.
  • Job: make keratin for the barrier and produce water-resistant cells.
  • Spot it: linked by desmosomes; flatten and die as they rise.

Melanocytes

  • Where: stratum basale.
  • Job: make melanin and hand it to keratinocytes to shield DNA from UV.
  • Spot it: spidery cells with long processes in the deepest layer.

Tactile (Merkel) cells

  • Where: stratum basale, paired with a nerve ending.
  • Job: sense light touch.
  • Spot it: sit at the dermal border with a sensory disc.

Dendritic (Langerhans) cells

  • Where: mainly stratum spinosum.
  • Job: immune cells that patrol for invaders.
  • Spot it: star-shaped, mobile, arise from bone marrow.

Part 3 · Four cell types

The four cells of the epidermis, mapped

Epithelium

Read it top down: the four cells, then each cell's where, job, and what it looks like.

  • Top-down mind map. Level one: the four cells of the epidermis. Level two: the four cell types. Level three under each type: where it sits, its job, and what it looks like.
    • Keratinocytes. Where: every stratum; they are the bulk of the epidermis. Job: make keratin for the barrier and produce water-resistant cells. Looks like: linked by desmosomes; they flatten and die as they rise.
    • Melanocytes. Where: stratum basale. Job: make melanin and hand it to keratinocytes to shield DNA from UV. Looks like: spidery cells with long processes in the deepest layer.
    • Tactile (Merkel). Where: stratum basale, paired with a nerve ending. Job: sense light touch. Looks like: sits at the dermal border with a sensory disc.
    • Dendritic (Langerhans). Where: mainly stratum spinosum. Job: immune cells that patrol for invaders. Looks like: star-shaped, mobile, and arise from bone marrow.

Part 3 · Pigment

What gives skin its color

Anatomy

Three pigments combine to set skin tone:

  • Melanin: made by melanocytes; ranges from brown-black to red-yellow. The main source of differences in skin color and a UV shield.
  • Carotene: a yellow-orange pigment from the diet, most visible in the stratum corneum of the palms and soles.
  • Hemoglobin: the red of oxygenated blood in dermal vessels, seen through lighter epidermis as pink.
Color shifts read as clinical clues: redness (erythema), blue (cyanosis, low oxygen), yellow (jaundice), and pallor (blanching).

Part 3 · Clinical color

Reading skin color: four clinical signs

Clinical

A change in skin color is a clinical clue. Each of these four points to a different cause.

Jaundice

Liver / pancreatic problems; newborns.

A newborn with yellow-tinted skin, showing jaundice.

Pallor

Anemia.

An adult face with notably pale skin, showing pallor.

Erythema

Fever, infection or inflammation, carbon monoxide poisoning, sunburn.

A patch of red, inflamed skin on a limb, showing erythema.

Cyanosis

Hypoxia.

A newborn with dusky bluish-red skin, showing cyanosis.
These signs can be subtle in darker skin tones, so confirm them in the nail beds, palms and soles, the conjunctiva, and the oral mucosa.

Part 4

The Dermis and Hypodermis

Below the epidermis sits the strong, living core of the skin, then the fatty layer that anchors it.

Part 4 · The dermis

The dermis: papillary and reticular layers

Connective

Papillary layer (superficial, thin)

  • Where: just under the epidermis; throws up dermal papillae.
  • Job: feeds the epidermis and grips it with interlocking ridges (these form fingerprints).
  • Spot it: loose areolar tissue with capillary loops, free nerve endings, and tactile (Meissner) corpuscles.

Reticular layer (deep, thick)

  • Where: the bulk of the dermis.
  • Job: gives skin its strength, stretch, and recoil.
  • Spot it: dense irregular tissue; collagen and elastic fibers in all directions, with lamellated (Pacinian) corpuscles for deep pressure.

Collagen bundles run in parallel lines of tension called cleavage (Langer) lines, which guide how incisions heal.

Labeled full-thickness skin model in cross section. Between the surface epidermis and the deep fatty hypodermis lies the dermis, marked as papillary dermis above reticular dermis. Embedded structures include a sebaceous gland, an arrector pili muscle, apocrine and eccrine sweat glands, and a deep lamellated (Pacinian) corpuscle.

Find the papillary and reticular dermis between the epidermis and the hypodermis.
Skin model photographed and labeled by Dr. Sharilyn Rennie.

Part 4 · The hypodermis

The hypodermis (subcutaneous layer)

Connective
  • Where: deep to the dermis; not part of the skin proper.
  • Job: anchors skin to underlying muscle and bone while letting it slide, insulates, and absorbs shock.
  • Spot it: areolar and adipose tissue; thickens where the body stores fat.
  • Because it is well supplied with blood and fat, it is a common site for injections that absorb slowly (subcutaneous route).
Labeled full-thickness skin model in cross section. The hypodermis is the deep layer of yellow fat (adipose) beneath the reticular dermis, anchoring the skin to the body, with the dermis and epidermis above it.

The hypodermis is the deep fatty layer below the reticular dermis.
Skin model photographed and labeled by Dr. Sharilyn Rennie.

Part 5

Hair and Nails

Appendages of the skin are made of hard keratin and grow from epidermis that dips deep into the dermis.

Part 5 · Hair

Hair and the hair follicle

Anatomy
  • Shaft is the part above the skin; the root is the part in the follicle.
  • Follicle: an epidermal tube that dips into the dermis and ends in a widened bulb.
  • Matrix at the bulb is the growth zone; the hair papilla feeds it with a capillary.
  • Arrector pili: a smooth muscle that pulls the hair upright (goosebumps) and squeezes the sebaceous gland.
  • Job: warmth, light-touch sensing, and protection (scalp from UV, nose and ears from particles).
Skin model showing hair. Hairs project from the surface as the hair root, and within the dermis each follicle widens at its base into the hair bulb seated in fatty tissue, with a sebaceous gland and sweat gland nearby.

Hair roots and bulbs set in the dermis and hypodermis.
Skin model photographed and labeled by Dr. Sharilyn Rennie.

Part 5 · Nails

Nails

Anatomy

A nail is a plate of hard keratin over the tips of fingers and toes.

  • Nail plate (nail body) is the visible nail; its free edge grows past the fingertip.
  • Nail bed is the epidermis under the plate; the nail matrix at the root is the growth zone.
  • Lunula is the pale crescent over the thick matrix; the eponychium (cuticle) seals the proximal edge and the nail folds border the sides.
  • The hyponychium is the skin beneath the free edge. Job: protect the fingertip; nail color can hint at oxygen levels.
Photograph of a fingertip with nail parts labeled. At the base are the nail fold and the eponychium (cuticle); the pale crescent lunula sits just past them. The nail body (with the nail bed underneath) covers the center, the free edge is the growing edge at the tip, and the hyponychium is the skin beneath the free edge.

Surface anatomy of the nail.
Labeled by Dr. Sharilyn Rennie.

Part 6

Glands of the Skin

Two main gland families open onto the skin: oil glands and sweat glands, plus a few modified versions.

Part 6 · Skin glands

Sebaceous and sweat glands

Anatomy

Sebaceous (oil) glands

  • Where: almost everywhere except palms and soles; usually empty into a hair follicle.
  • Job: secrete sebum (oil) that softens skin and hair and slows bacteria.
  • Spot it: holocrine glands; whole cells burst to release the product.

Eccrine (merocrine) sweat glands

  • Where: over the whole body, dense on palms, soles, and forehead; open onto the surface.
  • Job: make watery sweat for cooling (thermoregulation).
  • Spot it: coiled tubes in the dermis; the most common sweat gland.

Apocrine sweat glands

  • Where: axillary and genital regions; open into hair follicles.
  • Job: release a richer sweat that body bacteria turn into body odor; active from puberty.
  • Spot it: larger glands deep in the dermis. Modified versions: ceruminous (earwax) and mammary (milk).
Labeled full-thickness skin model showing the glands and associated structures: a sebaceous gland opening onto a hair follicle, the arrector pili muscle on the hair, an apocrine sweat gland and an eccrine sweat gland coiled in the dermis, and a deep lamellated (Pacinian) corpuscle, set among the epidermis, dermis, and fatty hypodermis.

Glands and associated structures in the full-thickness skin model.
Skin model photographed and labeled by Dr. Sharilyn Rennie.

Part 6 · Synthesis

Form follows function in the skin

Synthesis
  • A dead, keratinized corneum makes the barrier work; living layers below replace it for life.
  • Melanin sits over basal nuclei exactly where UV would do the most genetic damage.
  • Dense irregular dermis takes pulling forces from every direction, so skin resists tearing.
  • Appendages are epidermis that grew downward, so hair, nails, and glands are all keratin or epithelial tubes.
  • Vessels and sweat glands sit in the dermis because temperature control needs blood and water at depth.
Every skin structure is built for its job: barrier, sensor, radiator, and factory in one organ.

Part 7

Check Your Understanding

Three questions from recall to reasoning, a lab identification, and the key takeaways.

Part 7 · Recall (DOK 1)

Name the layer

Practice

Depth of Knowledge 1 · Recall

Which epidermal stratum is the deepest and contains the dividing stem cells?

Part 7 · Skill / concept (DOK 2)

Match structure to type

Practice

Depth of Knowledge 2 · Skill / concept

The reticular layer of the dermis resists pulling forces from all directions. Which tissue gives it that property?

Part 7 · Strategic reasoning (DOK 3)

Reason it through

Practice

Depth of Knowledge 3 · Strategic reasoning

A surgeon plans an incision on the forearm to heal with the least scarring. Why follow cleavage (Langer) lines?

Part 7 · Lab check

Identify it

Practice

Cover the labels and name every structure before lab. State its layer and one function for each.

Full-thickness skin model for self-quiz: name the three layers and each embedded structure.

Name the layers, the glands, the muscle, and the deep corpuscle.
Skin model photographed and labeled by Dr. Sharilyn Rennie.

Epidermal strata model for self-quiz: name each stratum from deep to superficial.

Name each epidermal stratum and the dermal structures below.
Skin model photographed and labeled by Dr. Sharilyn Rennie.

Wrap-up

Key takeaways

Summary
  • Skin is two attached layers: epidermis (keratinized stratified squamous epithelium) over dermis (connective tissue), with a fatty hypodermis beneath.
  • Epidermal strata, deep to superficial: basale, spinosum, granulosum, lucidum (thick skin only), corneum.
  • Four epidermal cells: keratinocytes, melanocytes, tactile (Merkel), dendritic (Langerhans).
  • Dermis = thin papillary (areolar, sensation) over thick reticular (dense irregular, strength).
  • Skin color = melanin + carotene + hemoglobin.
  • Appendages: hair and nails (hard keratin), sebaceous (holocrine oil), and sweat glands (eccrine cooling, apocrine scent).

Dr. Sharilyn Rennie · Focus: Human Anatomy · The Integumentary System: Anatomy & Histology. Skin model plates photographed and labeled by Dr. Rennie; skin micrographs used for teaching. Need a PDF? Use Print / Save PDF above, or download the accessible PDF from Canvas.

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