Frontispiece · Ordo Conjunctiva — the in-betweeners

The plates

The twelve specimens

Twelve specimens, and almost all of them are told apart by proportion rather than by any single feature. Every one of these tissues is made of the same three things. What changes is how much of each, and which way the fibres run.

Pl. I

Laxus vagus var. tillandsius

the packing peanut  ·  areolar connective tissue

Open Pl. I →

Pl. II

Laxus adiposus var. mellifer

the vault  ·  adipose tissue

Open Pl. II →

Pl. III

Laxus reticularis var. mycelialis

the invisible net  ·  reticular connective tissue

Open Pl. III →

Pl. IV

Densus rectus var. lineus

captain lengthwise  ·  dense regular connective tissue

Open Pl. IV →

Pl. V

Densus confusus var. textor

the basketweaver  ·  dense irregular connective tissue

Open Pl. V →

Pl. VI

Densus elasticus var. resurgens

the snapback  ·  elastic connective tissue

Open Pl. VI →

Pl. VII

Sustinens hyalinus var. tremellus

the glider  ·  hyaline cartilageThe most abundant cartilage type, found in joints, ribs, and the nose.

Open Pl. VII →

Pl. VIII

Sustinens elasticus var. auritus

the bouncer  ·  elastic cartilageA flexible cartilage type found in the ear and epiglottis.

Open Pl. VIII →

Pl. IX

Sustinens fibrosus var. nuciferus

the shock absorber  ·  fibrocartilageA tough, shock-absorbing cartilage found in intervertebral discs and menisci.

Open Pl. IX →

Pl. X

Sustinens osseus var. corallinus

captain concrete  ·  compact boneDense, strong bone tissue forming the outer layer of bones.

Open Pl. X →

Pl. XI

Sustinens osseus var. trabecularis

the lattice  ·  spongy boneA porous bone tissue found inside bones, providing lightweight support.

Open Pl. XI →

Pl. XII

Sustinens transiens var. bambusinus

the shapeshifter  ·  the epiphyseal plateA growth plate in developing bones made of cartilage.

Open Pl. XII →

The ground plan

Three ingredients, and the proportions decide

Every connective tissue in this volume is cellsThe basic structural and functional units of life., fibres, and ground substance — and that is the whole list. Epithelium is nearly all cells; connective tissue is nearly all matrix, and the cells are scattered through it like currants. So the questions that identify one of these tissues are not really about the cells at all. They are about how much matrix there is, what is in it, and whether it has a direction.

Fig. 1 · cells, fibres, and the space between

Areolar tissue, the plainest case. A few cells, fibres of all three kinds, and a great deal of space that is not empty but full of ground substance.

The words you need

• Ground substance — the gel the rest of it sits in. Mostly waterThe universal solvent essential for life., held by large sugar-protein moleculesGroups of atoms bonded together.. You cannot see it; you see the space it occupies.
• Collagen — thick, strong, pink in H&E, and the commonest protein in your body. Enormously strong in tension and useless in compression.
• Elastic fibres — thinner, branching, and able to stretch and recoil. They need their own stain to show up properly.
• Reticular fibres — fine branching type III collagenA structural protein in the dermis that provides strength and elasticity. that forms nets rather than bundles. Shown by silver, not by H&E.
• Fibroblast — the cell that makes all of the above. In cartilageA flexible connective tissue found in joints, the ear, nose, and rib cage. Cartilage can be of three it is called a chondrocyteMature cartilage cells housed in lacunae., in bone an osteocyteMature bone cells that maintain bone tissue., and it sits in a little hole called a lacunaSmall spaces in bone tissue where osteocytes reside..

Decide what the matrix is made of before you try to name the tissue. Almost every mistake in this volume comes from naming it first.

First question

Loose, dense, or supporting?

This is the first question and it splits the twelve into three families. Do not look at the cells. Look at how much of the field is fibre and how much is space, and whether the matrix is something you could put your finger through.

Fig. 2 · how much of it is fibre

A dense tissue. Compare the amount of open space here with Fig. 1 — that difference is the whole of the loose-versus-dense distinction.

• More space than fibre → loose, FAM. LAXIDAE. Areolar, adipose, reticular.
• More fibre than space → dense, FAM. DENSIDAE. Dense regular, dense irregular, elastic.
• A solid matrix you could not push a finger through → supporting, FAM. SUSTINENTIDAE. The cartilages, and bone.

Loose and dense are both connective tissue proper, and both have cells you would recognise as fibroblasts. Supporting tissues put their cells in lacunae and change the name.

Second question

Which supporting tissue?

If you have decided the matrix is solid, there are only four answers left, and the first question is whether it is mineralised. Bone is hard because of calcium; cartilage is firm because of bound water.

Fig. 3 · matrix, and what is in it

Compact bone. Rings around a central canalA hollow canal in an osteon containing blood vessels and nerves., with cells trapped in lacunae between them — the structure you only need when nothing can diffuse through the matrix.

• Mineralised, with concentric rings around a canal → compact bone.
• Mineralised, as an open lattice of struts → spongy bone.
• Not mineralised, matrix featureless → hyaline cartilage.
• Not mineralised, matrix full of a dark fibre mesh → elastic cartilage.
• Not mineralised, thick visible bundles with cells in rows → fibrocartilage.

And one case that is none of these: a band with cartilage at one end and bone at the other, in five zones. That is the epiphyseal plate, and it is a place rather than a tissue.

The naming key

How to read a name

Put the two answers together — how much matrix first, then what the matrix is doing — and the name writes itself. The nicknames in this volume are the ones you already know from the In-Betweeners; the binomials are new.

Worked both ways

• Sustinens hyalinus → supporting, glassy matrix → hyaline cartilage.
• Densus confusus → dense, no grain → dense irregular, which is your dermisThe thick inner layer of the skin that contains blood vessels, nerves, and connective tissue..
• You saw empty circles packed edge to edge with flattened nucleiClusters of neurons in the CNS responsible for processing information. → Laxus adiposus → adipose.

Two that trip people up

The look-alikes

Neither of these breaks the key. They catch people out because the pairs look identical until you ask the one question that separates them — so ask it first, before you have talked yourself into an answer.

Fig. 4 · the pair that settles it

Dense regular. Every bundle in this field shares one direction — which is exactly what dense irregular never does.

Keying out a specimen

Four questions, in this order

This is the procedure. It works on a slide you have never seen before, and it works best if you refuse to guess the answer before you have asked all four.

1. Is the matrix solid? No → connective tissue proper, go to 2. Yes → supporting, go to 3.
2. More space than fibre, or more fibre than space? Space → loose: empty circles mean adipose, a silver net means reticular, otherwise areolar. Fibre → dense: go to 4.
3. Is it mineralised? Yes → bone: rings mean compact, struts mean spongy. No → cartilage: featureless matrix means hyaline, a dark mesh means elastic, thick bundles with cells in rows means fibrocartilage.
4. Do the bundles agree about direction? All one way → dense regular. Every way at once → dense irregular. Wavy, dark and branching → elastic connective tissue.

One override: if what you have is a band of cartilage in stacked columns running down into new bone, you are not looking at a tissue at all. You are looking at the epiphyseal plate, and it is Pl. XII.

YOur missions

Field Notes for Future Missions

Before you go to the plates, try the key on a specimen nobody has named for you.

Field notes

Why twelve organisms

Connective tissue is proportion, and proportion is hard to teach from a picture because every micrograph is pink. The twelve organisms in this volume are each an argument about proportion made at a scale you can see without a microscope.

Spanish moss is mostly air. A honeypot ant is mostly cargo. A weaver nest has no plan and cannot be pulled apart; a hank of flax has nothing but a plan and frays if you cut across it. A coconut husk exists to be dropped. A reef is strong because it is mostly holes. Hold the organismA living individual made up of cells. in your headRounded proximal end that fits into the acetabulum of the hip bone. and the micrograph stops being pink and starts being an answer.