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Week 5 · Chapter

Chapter 5 — Tissues

Human Anatomy and Physiology I Free sample edition

The module's primary reading · ~40–50 minutes · Objective 3 (identify the four primary tissue types and their subtypes from their structural features, name an epithelium by its layers and cell shape, and describe the layers, accessory structures, and homeostatic functions of the integumentary system, including thermoregulation as a negative-feedback loop), Objective 2 (relate the chemistry of the body to its function — atoms and the three bond types, water's properties, the logarithmic pH scale with the blood's buffered range, and the four classes of organic compounds — and explain the cell's membrane, organelles, and transport (diffusion, osmosis and tonicity, active transport, and bulk transport), the overview of how a cell makes ATP, and how a gene is read into a protein) · Read it first this week; the lecture, the slides, the tutorial, the quiz, and the lab all build on it.

In plain language

Your body is not built straight from cells. Cells first team up into tissues (groups of similar cells that sit together and share one job), and only then do tissues team up into organs. There are four kinds of tissue, and each one has a job you can say with a verb: cover, support, move, or communicate. This week you learn to look at a slice of the body under a microscope and say which kind of tissue you are seeing, and why its build fits its job. That skill is called histology (the study of tissues under the microscope), and you will use it to read every organ for the rest of this course.

Why you already care about this

Think about the last long hike you took. Your heels pressed into your boots for thousands of steps, and the skin there held up. The inside of your mouth handled a crunchy snack at the rest stop without bleeding. Your thigh muscles pulled, your heart kept a steady beat, and your nerves told you where every rock was. Five different jobs, and each one was done by a tissue built for it.

The numbers this week are small, and every one of them is a count you can hold in one hand. There are 4 primary tissue types. The lab's slide set has 7 slides: 2 epithelial, 2 connective, 2 muscle, and 1 nervous. An epithelium's name always has 2 words. Once you can read those four families off a slide, any organ becomes a combination you can take apart.

By the end of this chapter you will be able to:

name the four primary tissue types and give each one its verb;
list the signatures of epithelium and explain why an avascular tissue depends on diffusion;
give any epithelium its two-word name, layers first and cell shape second, and say how that build fits its location;
sort a gland as exocrine or endocrine by where its secretion goes;
explain why connective tissue is cells scattered in a matrix, and place blood, cartilage, tendons, and ligaments in its categories;
tell skeletal, cardiac, and smooth muscle apart by striations, control, nuclei, and intercalated discs;
read a tissue slide in the order type, subtype, location, function, and name the four tissue membranes.

Carry this line with you: Read the slide for its build, not its color — then name it.

Where we are

Already covered: Week 1 set out the ladder of organization and said a tissue is similar cells on one job. Week 2 built the proteins, and Week 3 opened the cell and its membrane, including diffusion. Week 4 showed how cells make ATP and build proteins such as collagen.
This week: the four primary tissue types, how to name an epithelium, how connective tissue is built around its matrix, the three muscle tissues, nervous tissue, and the four tissue membranes.
How they connect: a tissue is cells plus whatever the cells make around themselves, so this week uses the cell from Week 3 and the proteins from Week 4 as its raw material.
Where we're going next: Week 6, The Integumentary System, assembles all four tissues into the skin, the first whole organ you will study.

In Week 1 you climbed the levels of structural organization: chemical, cellular, tissue, organ, organ system, organism. Until now, the tissue rung was a single sentence. In Week 3 you watched particles diffuse from higher to lower concentration, and that idea returns here, because some tissues have no blood vessels and live on diffusion alone. In Week 4 you saw that proteins such as collagen do jobs all over the body; this week you find collagen fibers lying in the matrix of connective tissue. Look for one more thread as you read: the course's spine, structure to function, is how every tissue name earns its meaning.

1. Meet the four families: tissue types and their verbs

Picture a quilt sewn from four fabrics: a smooth one for the surface, a tough one for the backing, a stretchy one that can pull, and a wired one that carries signals. Every organ in your body is a quilt like that. The fabrics are tissues.

Tissue and histology. A tissue is a group of similar cells found together that share an origin and an orderly arrangement and do a common job. Histology is the study of tissues under the microscope: how they look, how they are organized, and what they do.

There are four primary tissue types, and each has its verb:

Epithelial tissue covers and lines.
Connective tissue supports, binds, and protects.
Muscle tissue contracts to move.
Nervous tissue communicates by electrochemical signals.

An organ is a structure built from more than one tissue type working together. In fact, an organ is never one tissue. The skin, which you will study next week, is built from all four.

Worked example — sorting by verb. Read each job and name the family that does it:

A sheet that lines the inside of the mouth: covering and lining, so epithelial.
A tough cord that holds a joint together under pull: supporting and binding, so connective.
A wall of cells that shortens to squeeze: contracting, so muscle.
Cells that send a message from the foot to the spinal cord: communicating, so nervous.

In words: the verb picks the family before you ever look at your first slide.

Misconception check. Students often think an organ is made of one tissue, so "the heart is muscle" and "the skin is epithelium." That is tempting because each organ has one tissue that stands out. But an organ is never one tissue. The skin, for example, carries all four tissue types, even though its surface sheet is the part you see.

Memory hook: cover, support, move, communicate.

2. Spot the signatures of epithelium

Run your tongue along the inside of your cheek. That smooth surface is a living sheet of cells, packed side by side like tiles on a floor. A tile floor has almost no grout between the tiles, one side faces the room, and the other side sits on a base.

Epithelium's signatures. Epithelium has three signatures:

It is packed with cells and has little material between them.
It has an apical surface, the free surface, and a basal surface that rests on a basement membrane, the thin layer that separates the epithelium from the tissue beneath it.
It is avascular: no blood vessel crosses the basement membrane, so its nutrients arrive by diffusion.

That last signature ties back to Week 3. Diffusion moves particles from higher to lower concentration. The tissue under the basement membrane has the blood vessels, so nutrients reach the epithelium by spreading up from below.

Worked example — reading the signatures on a slide. A slide shows a single row of flat cells, edge to edge, on a thin line. Read it:

The cells are packed with almost nothing between them, so the tissue is highly cellular.
One side faces an open space, so that is the apical surface.
The other side rests on the thin line, so that line is the basement membrane and that side is the basal surface.
No vessel crosses that line, so the cells are fed by diffusion from below.

In words: packed cells, one free surface, a basement membrane, and no vessels add up to epithelium.

Misconception check. Students often think epithelium is full of blood vessels, because the surfaces it lines can bleed. Epithelium is avascular. A scrape that does not bleed has reached only the avascular epithelium of the skin. Bleeding begins when an injury reaches the tissue below, where the vessels are.

Memory hook: tiles on a floor, no pipes in the tiles.

3. Name an epithelium: layers first, shape second

A coffee shop calls a drink by two words, like "iced latte": first how it is served, then what it is. An epithelium is named the same way, by two words in a fixed order.

The two-word name. An epithelium's name gives its layers first and its cell shape second.

The layer words are simple and stratified. A simple epithelium is one layer of cells. A stratified epithelium is stacked layers. You will also see the name pseudostratified.
The shape words are squamous (flat), cuboidal (cube-shaped), and columnar (column-shaped).

The name then predicts the job. Read each one as structure to function:

Simple squamous epithelium lines the alveoli and the capillaries, because one flat layer is the shortest path for diffusion.
Stratified squamous epithelium forms the skin and lines the mouth, because outer layers can be lost to wear.
Simple columnar epithelium lines most of the digestive tract.

Worked example — Dataset E, the tissue-identification key: slides 1 and 2. The lab's slide set is illustrative and engineered by the course pack. Read its two epithelial slides in the order type, subtype, location, function:

Slide 1 shows one layer of flat cells on a basement membrane, so epithelial — simple squamous — the alveoli and the capillary linings — the thinnest barrier.
Slide 2 shows many stacked layers, flat at the surface, so epithelial — stratified squamous — the epidermis and the mouth — wear and protection.

In words: one flat layer means a short path across; many layers mean surface cells can wear away while deeper ones remain.

Worked example — predict an epithelium from a job. Two places, two jobs:

An air sac in the lung must let gases cross fast. One flat layer gives the shortest path, so predict simple squamous epithelium.
The skin of your heel takes rubbing on every step. Layers that can be lost to wear protect what lies beneath, so predict stratified squamous epithelium.

Misconception check. Students often read squamous, cuboidal, and columnar as numbers of layers, so "columnar" sounds like "many rows." Those words are shapes, never counts. The layer word comes first and answers "how many layers?"; the shape word comes second and answers "what shape are the cells?" A second slip is calling stratified "simple." Simple is single; stratified is stacked.

Memory hook: layers first, shape second.

4. Sort the glands: exocrine versus endocrine

Some cells in an epithelium do more than cover; they make and release a product. A group of such cells is a gland, and you sort glands by one question: where does the product go?

Exocrine and endocrine. An exocrine gland releases its secretion through a duct onto a surface or into a cavity. An endocrine gland has no duct and releases its secretion into the surrounding tissue fluid and the blood.

Worked example — follow the product. Sort two described glands:

A gland empties its product through a small tube onto the inner surface of a passage. A tube means a duct, and a surface is the destination, so it is exocrine.
A gland has no tube at all and its product enters the tissue fluid and then the blood. No duct and a blood destination mean it is endocrine.

In words: when you see a duct to a surface, you say exocrine; when you see no duct and a blood destination, you say endocrine.

Misconception check. Students often decide by the size of the gland or by how important its product sounds. Size and importance do not sort a gland. The duct does: a duct onto a surface or into a cavity is exocrine, and no duct, with the secretion going into the tissue fluid and the blood, is endocrine.

Memory hook: exo exits through a duct; endo enters the blood.

5. Build around the matrix: connective tissue

Picture a bowl of fruit gelatin with a few grapes scattered through it and a handful of threads stirred in. The grapes are far apart, and most of the bowl is gelatin and threads. Connective tissue is built like that bowl, which is the opposite of epithelium's packed tiles.

Connective tissue and its matrix. Connective tissue is cells scattered in a matrix. The matrix is ground substance plus protein fibers, collagen, elastic, and reticular, and the cells make it themselves. The collagen in that matrix is one of the proteins Week 4 said cells build.

Conventions for grouping connective tissue differ; this course uses three categories:

Connective tissue proper is loose or dense. Tendons and ligaments are dense regular connective tissue: their fibers run in one line and resist pull along that line. Adipose tissue, the fat-storing tissue you will meet under the skin next week, is also counted among the connective tissues.
Supportive connective tissue is cartilage and bone. Cartilage has no blood vessels, so its nutrients diffuse in through the matrix and it is slow to heal. A torn piece of cartilage illustrates that pairing in clinical terms: no vessels, slow repair. Bone gets its own week later.
Fluid connective tissue is blood and lymph. Blood is a connective tissue whose cells travel in a liquid matrix, the plasma.

Worked example — Dataset E, slides 3 and 4. Read the two connective slides in the order type, subtype, location, function:

Slide 3 shows scattered cells in an abundant fibrous matrix, so connective — connective tissue proper — loose under epithelia, dense in tendons and ligaments — packing and pull.
Slide 4 shows formed cells in a fluid matrix, the plasma, so connective — blood — the vessels — transport.

In words: when the space between cells is larger than the cells, read connective tissue, whether the matrix is a fiber mesh or a liquid.

Worked example — sort by the amount of matrix. At a clinic, a blood sample is drawn into a tube and a second tube holds a scraping from the inside of the cheek. Sort each:

The blood: cells float apart in a liquid, so the matrix is abundant. Connective, fluid.
The cheek scraping: flat cells come off in packed sheets with little between them. Epithelial.

Misconception check. Students often call blood a muscle, because the heart pumps it, or "only a fluid," because it pours. Blood is a connective tissue. Its cells are scattered in a liquid matrix, the plasma, which is the same build as every connective tissue: cells apart, matrix between.

Memory hook: epithelium is packed; connective tissue is scattered.

6. Tell the three muscle tissues apart

At the gym, you decide to curl a dumbbell, and your arm muscle obeys. Meanwhile, your heart speeds up on its own, and the walls of your stomach keep churning without asking you. Three kinds of movement, three kinds of muscle tissue.

The three muscle tissues. Read each one by its striations, its control, its nuclei, and its location:

Skeletal muscle is long, cylindrical, and striated, with many nuclei at the edge of each fiber, and it is voluntary.
Cardiac muscle is short, branched, and striated, usually with one central nucleus. Its cells are joined by intercalated discs, and it is found only in the heart wall. It is involuntary.
Smooth muscle is spindle-shaped, with one nucleus and no striations, the visible stripes across the other two. It is involuntary, and it moves the walls of hollow organs.

The three muscle tissues compared by striations, nuclei, control, and location
Muscle tissue	Striations	Nuclei	Control	Location
Skeletal	yes	many, at the edge of each fiber	voluntary	attached to bone
Cardiac	yes	usually one, central	involuntary	the heart wall only
Smooth	no	one	involuntary	the walls of hollow organs

Worked example — Dataset E, slides 5 and 6. Read the two muscle slides:

Slide 5 shows long striated fibers with many nuclei, so muscle — skeletal — attached to bone — voluntary movement.
Slide 6 shows striated branching cells joined by intercalated discs, so muscle — cardiac — the heart wall — pumping.

In words: both slides are striped, so the stripes alone cannot decide; the branching and the discs mark cardiac muscle.

Misconception check. Students often confuse cardiac and skeletal muscle, because both are striated. Stop at the stripes and look one step further. Cardiac cells are short and branched and are joined by intercalated discs; skeletal fibers are long, cylindrical, and carry many nuclei at the edge. Cardiac muscle is involuntary, and skeletal muscle is voluntary.

Memory hook: stripes plus discs means heart.

7. Name nervous tissue at the tissue level

At night, you reach for a light switch in the dark and find it on the first try. Signals from your fingertips reported where the switch was, and signals back told your arm muscles to move. The tissue that carried them is nervous tissue.

Nervous tissue. Nervous tissue is large neurons that carry signals among smaller neuroglia that support them. This week names it at the tissue level only; how a neuron fires is a later week's story.

Worked example — Dataset E, slide 7, and the full count. Read the last slide, then count the families:

Slide 7 shows large neurons among smaller neuroglia, so nervous — nervous tissue — the brain, the cord, and the nerves — signaling.
Count the families: 2 epithelial + 2 connective + 2 muscle + 1 nervous = 7 slides.
Check the families: 4 tissue types appear, and every slide belongs to exactly one of them.

Dataset E, the illustrative tissue-identification key: seven slides named by type, subtype, location, and function
Slide	Identifying feature	Type	Subtype	Location	Function
1	one layer of flat cells on a basement membrane	epithelial	simple squamous	the alveoli and the capillary linings	the thinnest barrier
2	many stacked layers, flat at the surface	epithelial	stratified squamous	the epidermis and the mouth	wear and protection
3	scattered cells in an abundant fibrous matrix	connective	connective tissue proper	loose under epithelia, dense in tendons and ligaments	packing and pull
4	formed cells in a fluid matrix, the plasma	connective	blood	the vessels	transport
5	long striated fibers with many nuclei	muscle	skeletal	attached to bone	voluntary movement
6	striated branching cells joined by intercalated discs	muscle	cardiac	the heart wall	pumping
7	large neurons among smaller neuroglia	nervous	nervous tissue	the brain, the cord, and the nerves	signaling

In words: you read seven slides, you find four families, and you read every name in the same order.

Misconception check. Students often try to name a nervous-tissue slide by its smaller cells, because there are more of them. The smaller cells are the neuroglia, the support. The large cells are the neurons, the signal carriers, and they are what make the tissue nervous.

Caveat: the slide set is an engineered teaching set. Real slides vary in their stain and their cut, which is why you read the build, never the color.

Memory hook: big cells signal, small cells support.

8. Line the body: the four tissue membranes

Think of the sheets inside a house: a liner in a shower that faces the outside water, a liner inside a sealed box, the outer wall itself, and the grease-lined sleeve of a door hinge. The body has four tissue membranes that match those four places.

The four tissue membranes. You can sort every one of them by the place it lines:

A mucous membrane lines a cavity open to the outside.
A serous membrane lines a closed cavity and covers its organs.
The cutaneous membrane is the skin.
A synovial membrane lines the cavity of a freely movable joint.

Worked example — match the membrane to the place. Read each place and name its membrane:

The lining of the mouth opens to the outside world, so it is a mucous membrane.
The lining around the lungs sits in a closed cavity and covers the organs in it, so it is a serous membrane.
The outer covering of the body is the skin, so it is the cutaneous membrane.
The lining inside the elbow joint, a freely movable joint, is a synovial membrane.

In words: you ask where the lining sits. Open to the outside is mucous, closed is serous, the skin is cutaneous, and a movable joint is synovial.

Misconception check. Students often swap mucous and serous membranes, because both line cavities. Ask one question: is the cavity open to the outside? Open means mucous; closed, with organs inside, means serous.

Limitation: this chapter stops at the tissue level. How tissues repair themselves and how they age are beyond this edition of the course.

Memory hook: open is mucous, closed is serous.

Key terms

Histology — The study of tissues under the microscope: how they look, how they are organized, and what they do. (first used: 1. Meet the four families: tissue types and their verbs)
Epithelial tissue — The primary tissue type that covers body surfaces and lines internal cavities and passages, built as sheets of tightly packed cells. (first used: 1. Meet the four families: tissue types and their verbs)
Connective tissue — The primary tissue type that supports, binds, and protects other tissues, built from cells scattered through a matrix they make themselves. (first used: 1. Meet the four families: tissue types and their verbs)
Muscle tissue — The primary tissue type that contracts to produce movement; it comes in three kinds, skeletal, cardiac, and smooth. (first used: 1. Meet the four families: tissue types and their verbs)
Nervous tissue — The primary tissue type that communicates by electrochemical signals, made of large signaling neurons among smaller supporting neuroglia. (first used: 1. Meet the four families: tissue types and their verbs)
Organ — A body structure built from more than one tissue type working together; the skin, for example, contains all four primary tissue types. (first used: 1. Meet the four families: tissue types and their verbs)
Apical surface — The free surface of an epithelium, the side that does not rest on the basement membrane. (first used: 2. Spot the signatures of epithelium)
Basal surface — The surface of an epithelium opposite the free surface, the side that rests on the basement membrane. (first used: 2. Spot the signatures of epithelium)
Basement membrane — The thin layer on which an epithelium's basal surface rests, separating the epithelium from the tissue beneath it. (first used: 2. Spot the signatures of epithelium)
Avascular — Having no blood vessels of its own; an avascular tissue such as epithelium or cartilage receives its nutrients by diffusion. (first used: 2. Spot the signatures of epithelium)
Simple epithelium — An epithelium that is one cell layer thick; the word simple is the first word of its two-word name. (first used: 3. Name an epithelium: layers first, shape second)
Stratified epithelium — An epithelium built of two or more stacked layers of cells; the word stratified is the first word of its two-word name. (first used: 3. Name an epithelium: layers first, shape second)
Squamous — Describes flat epithelial cells; the shape word in a name such as simple squamous epithelium. (first used: 3. Name an epithelium: layers first, shape second)
Cuboidal — Describes cube-shaped epithelial cells; a shape word, never a count of layers. (first used: 3. Name an epithelium: layers first, shape second)
Columnar — Describes column-shaped epithelial cells; the shape word in a name such as simple columnar epithelium. (first used: 3. Name an epithelium: layers first, shape second)
Exocrine gland — A gland that releases its secretion through a duct onto a surface or into a cavity. (first used: 4. Sort the glands: exocrine versus endocrine)
Endocrine gland — A gland with no duct that releases its secretion into the surrounding tissue fluid and the blood. (first used: 4. Sort the glands: exocrine versus endocrine)
Matrix — The material between the cells of a connective tissue, made of ground substance and protein fibers that the cells themselves secrete. (first used: 5. Build around the matrix: connective tissue)
Ground substance — The part of a connective-tissue matrix that is not fiber; together with the protein fibers it makes up the matrix the cells secrete. (first used: 5. Build around the matrix: connective tissue)
Protein fibers — The collagen, elastic, and reticular fibers that, together with ground substance, make up a connective-tissue matrix. (first used: 5. Build around the matrix: connective tissue)
Connective tissue proper — The category of connective tissue that is either loose or dense; tendons and ligaments are its dense regular form. (first used: 5. Build around the matrix: connective tissue)
Dense regular connective tissue — Connective tissue proper packed with fibers running in one line, the tissue of tendons and ligaments, which resists pull along that line. (first used: 5. Build around the matrix: connective tissue)
Cartilage — A supportive connective tissue with no blood vessels of its own, so its nutrients diffuse in through the matrix and it is slow to heal. (first used: 5. Build around the matrix: connective tissue)
Blood — A fluid connective tissue whose formed cells travel in a liquid matrix, the plasma. (first used: 5. Build around the matrix: connective tissue)
Plasma — The liquid matrix of blood, in which the blood's cells travel. (first used: 5. Build around the matrix: connective tissue)
Skeletal muscle — Muscle tissue made of long, cylindrical, striated fibers with many nuclei at the edge of each fiber, attached to bone and under voluntary control. (first used: 6. Tell the three muscle tissues apart)
Cardiac muscle — Muscle tissue made of short, branched, striated cells, usually with one central nucleus, joined end to end by intercalated discs and found only in the heart wall. (first used: 6. Tell the three muscle tissues apart)
Smooth muscle — Muscle tissue made of spindle-shaped cells with one nucleus and no visible striations that moves the walls of hollow organs and passageways involuntarily. (first used: 6. Tell the three muscle tissues apart)
Striations — The visible stripes across skeletal and cardiac muscle cells under the microscope; smooth muscle lacks them. (first used: 6. Tell the three muscle tissues apart)
Intercalated discs — The junctions that join cardiac muscle cells end to end, the feature that tells cardiac muscle from skeletal muscle. (first used: 6. Tell the three muscle tissues apart)
Mucous membrane — A tissue membrane that lines a cavity or passage open to the outside of the body. (first used: 8. Line the body: the four tissue membranes)
Cutaneous membrane — The tissue membrane that is the skin. (first used: 8. Line the body: the four tissue membranes)
Synovial membrane — A tissue membrane that lines the cavity of a freely movable joint. (first used: 8. Line the body: the four tissue membranes)

Summary

A tissue is similar cells with a shared origin, an orderly arrangement, and a common job.
Four primary tissue types: epithelial covers, connective supports, muscle moves, nervous communicates.
An organ is never one tissue; the skin carries all four.
Epithelium is packed, has an apical and a basal surface on a basement membrane, and is avascular.
An epithelium's name is layers first, shape second: simple or stratified, then squamous, cuboidal, or columnar.
Simple squamous lines the alveoli and capillaries; stratified squamous forms the skin and lines the mouth.
Exocrine glands use a duct; endocrine glands are ductless and release into the blood.
Connective tissue is cells scattered in a matrix of ground substance and protein fibers.
Blood is a connective tissue with a liquid matrix, the plasma; cartilage is avascular and slow to heal.
Skeletal muscle is striated and voluntary; cardiac is striated, involuntary, with intercalated discs; smooth is unstriated and involuntary.
Nervous tissue is neurons plus neuroglia.
The four tissue membranes are mucous, serous, cutaneous, and synovial.

Check your understanding

Work these before looking at the answers — they are ungraded, and the answers (with reasoning) are at the bottom of the page.

A study card made up for this question describes a slide: one layer of tall, column-shaped cells lining a passage of the digestive tract. Give the tissue its full name in the order type, subtype, location, function.
A classmate's flashcard reads: "Stratified columnar = three columns of cells." What is wrong with the reading, and how should the two words be read?
A described sample from an illustrative lab notebook shows spindle-shaped cells with one nucleus each and no stripes, taken from the wall of a hollow organ. Name the tissue and say whether it is voluntary.
A fitness blog says: "The lining of your wrist joint is a mucous membrane, and your cartilage heals fast because it is packed with blood vessels." Correct both halves.

Answers (worked reasoning — read after attempting)

One layer means simple, and tall, column-shaped cells mean columnar, so the subtype is simple columnar. It covers and lines, so the type is epithelial. Read in order: epithelial — simple columnar — lining the digestive tract — the lining of the passage that food moves through. The layer word came first and the shape word second.
The flashcard reads the shape word as a count. Stratified is the layer word, and it means stacked layers; columnar is the shape word, and it means column-shaped cells. So the name says "stacked layers of cells, with column-shaped cells," and it says nothing about three. Shape words are never counts.
Spindle-shaped cells with one nucleus and no striations are smooth muscle. Smooth muscle moves the walls of hollow organs, which matches where the sample came from. It is involuntary: you do not decide to contract it the way you decide to curl a dumbbell.
The lining inside the wrist joint, a freely movable joint, is a synovial membrane, not a mucous membrane; mucous membranes line cavities open to the outside. And cartilage is avascular: it has no blood vessels of its own, so its nutrients diffuse in through the matrix, and that is why it is slow to heal.

Next up in this module: the lecture, the slides, the Readings & Resources page, the tutorial, the practice set, and the quiz; next week the four tissues are assembled into the skin, the first whole organ and its layered model.

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