(00:36) Hey wait up for me. OW...Are you Ok? Yeah, I think I just fell over a root... Oh my knee...

(00:55) Your body is an incredible living system made up of billions of cells. We're following blood cells as they're propelled through a blood vessel toward the boy's injured knee. If you are hurt, your cells work together to repair the damage. They communicate using their own language of chemical signals.

(01:20) Now we've arrived at the wound site. Blood cells are flowing out of the broken blood vessel ahead. Moments after an injury, these blood cells and cell fragments start to form a mesh-like clot. Many different types of cells are involved in tissue repair. The flat, light-colored cells are fibroblasts. Early in the healing process, fibroblasts multiply and produce proteins that help to repair the damage. The smaller dark cell fragments between the fibroblasts are platelets. When activated, platelets release a stream of protein messengers, called growth factors, to stimulate cell growth and tissue repair. To see how a growth factor from a platelet signals a nearby fibroblast cell, we need to swoop in close to the rippling fibroblast surface.

(02:45) Fibroblasts, like all your cells, have a fluid, outer membrane that regulates the flow of molecules in and out. The gray structures sticking out of the cell membrane are receptors for incoming signals. When the growth factor from the platelet (shown in purple and blue) encounters a matching receptor, it binds to it. A second receptor protein joins in, making the growth factor fit like a key in a lock. The binding of the growth factor causes the receptor to change shape. This change in the protein conducts the signal through the membrane and into the cell's interior – the cytoplasm. You'll see this better from inside the cell . . .

(03:46) Beneath the cell membrane, you see the grey receptor ends surrounded by pink fibers – these structures help to give the cell its shape – and a range of messenger proteins that will carry the signal through the cytoplasm. While active – as shown by the yellow flashes of light – the ends of the receptor interact with the messenger proteins.

(04:25) Now we'll watch the action again from our position in the cell's cytoplasm. The growth factor binds the receptor proteins outside the cell, drawing the receptor ends together. The signal is transmitted through the cell membrane, and each new protein is activated in turn. If you look closely, you can see the proteins change shape as they become activated with the signal. Each step of a pathway is under tight control to ensure that the correct message is relayed. For example, as this white protein accepts the signal, the blue protein comes in to deactivate the red one. Now let's follow the white protein on its journey through the cytoplasm, toward the center of the cell.

(03:56) As we follow the signal to the nucleus, you'll see it passed from messenger to messenger. The first exchange is with this brown protein, the second will be with a purple protein in a few seconds time. Although we're only following one path, a single cell has many different ways to transmit signals through the cytoplasm. The fibers (shown in green) are part of the cytoskeleton. Like the pink fibers you saw before, these give the cell shape and help to organize its contents. Crawling along the fibers are motor proteins that reshape the cytoskeleton and help this fibroblast cell to move. On our way, we will encounter other structures in the cytoplasm, known as organelles. On the right, you see glowing organelles, called mitochondria, which generate energy for the cell. The activated protein passes by a network of membranes (here in light brown) known as the endoplasmic reticulum.

(06:57) The protein is transported into the nucleus through a pore in the nuclear membrane. The nucleus contains tightly wound coils of DNA (shown in green). The protein messenger passes the signal to two other molecules that team up to locate a specific gene along the DNA. In this case, the gene carries the information to make a growth factor. Other molecules then unwind a small section of the DNA molecule and allow an enzyme called RNA polymerase (shown in brown) to make an RNA copy of the gene. The "copy," called messenger RNA (here in light green), is packaged with a set of carrier proteins and leaves the nucleus. The cell will use this copy to make the growth factor. Now we'll follow the messenger RNA copy back out of the nucleus to see how a new protein is made.

(08:18) On the left is the endoplasmic reticulum (which we've seen before) and on the right is a new structure called the Golgi apparatus (which we'll visit again later). Straight ahead are more of the glowing mitochondria. In the cytoplasm, the messenger RNA is released from its carrier proteins and binds to a complex called a ribosome. Here, the ribosome, a huge molecule, is shown as a multicolored structure. The ribosome reads the information encoded in the RNA and assembles a protein from amino acids found in the cell. Many ribosomes can operate at the same time to make multiple copies of the protein. The ribosomes anchor on the outer membrane of the endoplasmic reticulum. If you look carefully, you can see the ghostly shapes of the newly made proteins accumulating on the inner side of the membrane. Once the job is done the ribosomes and RNA part company.

(10:00) The newly made proteins leave the endoplasmic reticulum wrapped in a layer of membrane called a vesicle. They travel toward the Golgi apparatus (on the right) where the proteins are modified and sorted for transport. The loops of the Golgi are busy with protein traffic moving in and out. The vesicle fuses with the membrane at one end of the Golgi and a new vesicle containing the modified proteins is pinched off the other side. The new vesicle transports the proteins through the cytoplasm – delivering the proteins to where they are needed. Some proteins are used inside the cell. Others, like these growth factors, must be exported. Here, the vesicle fuses with the cell membrane, dumping the proteins outside the cell.

(11:48) The released growth factors will communicate with other cells to continue the healing process. These growth factors will attract more fibroblasts to the wound site and remodel the clot for better healing. Other proteins produced by this signaling pathway will tell the fibroblast cell to grow and divide, making many new cells to heal the wound. With the cooperation of many different cells, damage to the injured knee can be quickly repaired. Every day, your cells communicate and cooperate to keep you healthy. They act and interact; they grow, divide and die; all through the amazing language of cell signals.