Animal cells and their shapes

Cells are the building blocks of life – all living organisms are made up of them. Textbooks often show a single ‘typical’ example of a plant cell or an animal cell, but in reality, the shapes of cells can vary widely. Animal cells in particular come in all kinds of shapes and sizes. Plant cell shapes tend to be quite similar to each other because of their rigid cell wall.

We can learn a lot about what a cell does by looking at its shape and size, and microscopes are the ideal tool for this.

Shaped for the task

Cells have different shapes because they do different things. Each cell type has its own role to play in helping our bodies to work properly, and their shapes help them carry out these roles effectively. The following cell types all have unusual shapes that are important for their function.

Neurons are cells in the brain and nervous system. Their job is to carry electrical messages all the way from the brain to the rest of the body and back (almost like electrical wire), so they are very long, thin cells. They also need to connect with other neurons to form communication networks, so they have many long branches. You can learn more about neurons elsewhere in this context and in the context Uniquely Me.

Photoreceptor cells (rods and cones) are cells in the eye that detect light. They’re actually a very specialised form of neuron. Photoreceptors need to collect light as efficiently as possible, so they have a specialised protrusion from the cell (called the outer segment) that is full of the molecules that absorb light. Rods, which are especially good at detecting light, have a bigger protrusion. The outer segment is now known to be a highly modified kind of primary cilium, a recently discovered organelle. You can read more about the primary cilium elsewhere in this context.

Immune cells are cells that respond when the body is infected (by a bacterium, for instance). To do their job, they need to be able to change shape. For instance, lymphocytes may need to move through body tissue to get to the site of infection, so they change their shape to squeeze past tightly packed tissue cells. Some immune cells (such as neutrophils) engulf bacteria and viruses, so they need to change their shape to ‘swallow’ them. You can learn more about the different kinds of immune cells in the context Fighting Infection.

Microscopes on cells

Virtually all our understanding of cell shape comes from years of accumulated microscope experiments. There is no other tool that lets us look at cell shape directly. Using light microscopy, scientists have been able to view living cells to see how their shapes change over time. They have also been able to view cellular processes that involve shape changes such as mitosis.

Because of the design of electron microscopes, living cells can’t survive in the harsh conditions inside the microscope and therefore can’t be viewed directly. However, electron microscopes can give high-resolution information about the shape of individual cells that have been prepared for viewing, including small areas of the cell that have specific shapes, such as the primary cilium and microvilli.

Cells in 3D

Cells are three-dimensional objects with complex shapes, but the images generated from most microscopes are two-dimensional. This has made it difficult to understand the overall shape of cells and how they interact with one another. Now, though, several microscopic techniques make it possible to make three-dimensional models of cells or parts of cells. This is done by collecting multiple two-dimensional images digitally, then combining them using computer programs.

Dr Rebecca Campbell and Associate Professor Tony Poole are two scientists at the University of Otago who use microscope images to make three-dimensional models of the cells they study. Rebecca uses multiple images from the confocal laser scanning microscope to build up 3D views of whole neurons, and Tony has been building a 3D model of the primary cilium. You can learn more about Rebecca and Tony’s work elsewhere in this context.

Nature of Science

Scientific experiments often reveal unexpected information that can lead to new hypotheses and theories. Scientists in the 1600s were surprised to see tiny ‘building blocks’ when they looked at tissue under optical microscopes. Their observations eventually led to the development of cell theory – the idea that the cell is the basic unit of life.

Useful links

Watch this short video to see an immune cell (a neutrophil) changing its shape as it follows a bacterium through the blood and eventually engulfs it.
www.biochemweb.org/neutrophil.shtml external link

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