Animal tissues: cover, connect, move, control

Animal cells also team up into tissues, each suited to its job. Try four quick things right now, then tap each card to see which tissue made it possible.

Every tissue in your body belongs to one of four families: epithelial tissue covers and lines, connective tissue joins and supports, muscular tissue moves, and nervous tissue senses and controls. In each, the way the cells are built matches the work they do.

Epithelium is on the outside of you, as your skin, and on the inside too: it lines your mouth, your lungs, your blood vessels and your gut. Its cells sit close together with hardly any gap between them. That tight fit keeps germs out and water in, and it lets the layer take in, release and move substances in a controlled way.

The same idea is built five different ways, each suited to a different task, as in Table 3.2 and Fig. 3.11 of your book. Pick a job.

Notice the pattern: where things must pass through quickly (gases in the lungs, liquids across blood vessels) the layer is one cell thick and thin. Where the job is to take a beating (skin, mouth, food pipe) there are many layers. Where cells must absorb food, they are tall pillars with hair-like structures, and where they make things, they are cube or pillar shaped factories.

Blood runs to every corner of your body carrying nutrients, gases and hormones, so it links all the parts together. Bones link and hold you up from head to toe. Tissues whose job is to join other tissues and hold them up are called connective tissues. Blood and bone both belong here.

Then why is one runny and the other rock hard? Because of the matrix, the material the cells sit in. In blood it is a runny, jelly-like liquid; in bone it is solid, stiff and hard.

Activity 3.2 starts from things that have happened to you. Pick one and look inside the blood vessel.

Each of those everyday moments is blood at work:

Red colour: red blood cells (RBCs) contain haemoglobin, an iron-rich protein. An RBC lasts about 4 months and is then replaced. A cut: platelets gather at the wound and help the blood to clot. An infection: white blood cells (WBCs) collect at the spot, which brings redness, swelling and sometimes pus. Running: working muscles need more oxygen, so you breathe faster and more blood flows, which is why your face turns red.

You can find four more connective tissues on your own body without any equipment. Do each action, then tap it to see which tissue you were feeling (Table 3.4 and Fig. 3.12 of your book).

Bone has a rigid matrix with compounds of calcium and phosphorus, which give it strength and rigidity. Cartilage sits in a matrix that is soft and jelly-like by comparison: it gives under pressure and springs back, so it makes a good cushion where bones meet. A tendon ties a muscle to a bone. A ligament ties bone to bone and keeps a joint from bending further than it should.

Running, writing and lifting a bag happen when you decide: they are voluntary movements. Your heartbeat and the churning of food in your intestine carry on whatever you decide: they are involuntary movements. Three types of muscle share this work. Press the button and see which ones obey you.

Skeletal muscle is attached to the skeleton and does voluntary movement. Look closely and it is built from bundles of muscle fibres: long cylinders that never branch, each with many nuclei (multinucleate) and crossed by light and dark bands (striated).

Smooth muscle lines organs such as the stomach and intestine. Each cell tapers at both ends like a spindle, holds just one nucleus and shows no bands. It drives slow, steady movements like those of digestion.

You will find cardiac muscle in one place alone, the heart. Its cylinder shaped fibres branch and join up; each cell has one nucleus, and the bands are only faint. It contracts in a steady rhythm all through life without tiring.

You snatch your hand off a hot pan before you even think about it. You can sing a song you learned years ago. Both rely on nervous tissue, the control and coordination network of the body. Its headquarters is the brain, which coordinates what the body does, stores memories and decides on responses. Muscles, voluntary or not, cannot act by themselves: their instructions come from nervous tissue. When you exercise, it is a signal from the brain that speeds up your heart, so that more oxygen reaches your muscles.

The cells of nervous tissue are neurons (nerve cells), built to receive, process and pass on messages. Fire one.

Look at the three main parts. The cell body holds the nucleus and runs the cell. Dendrites pick up signals from other neurons. The axon is a long fibre that carries the message away from the cell body and ends in axon terminals, which hand the message on to the next cell.