Multicellular Organisms

There are many different cell types in the human body. All of those cells are descended from a single fertilised cell (the zygote). Image from Accessed on 11 March 2013.

Multicellular organisms are composed of large numbers of differentiated cells. These cells are largely terminally differentiated and perform a number of distinct roles in the multicellular organism. If a truly multicellular organism is dissociated, its cells cannot reconstitute the original organism. However, other apparently multicellular organisms are aggregates of cells, even though they may show signs of cellular differentiation. For example, when sponges are disaggregated, their cells can reconstitute the original organism. Some multicellular animals adopt a colonial mode of life. Such organisms (e.g. bryozoans and colonial tunicates) usually share common vascular systems.

The structural basis of multicellularity lies in the ability of cells to stick to together. This property of cells is referred to as adhesion and cellular adhesion is thought to be a major milestone in the evolution of multicellularity.

Intercellular adhesion is largely mediated by a group of proteins that are generally referred to as Cell Adhesion Molecules (CAMs). Thus, multicellular organisms are composed of numerous cell types that adhere to each other. All of the cells in a multicelluar organism are derived from a single cell - the zygote. In other words, even though there may be many differentiated cell types in a multicellular organism, they all have the same genome. This feature then forms the basis of self/non-self discrimination that the immune systems of metazoans carry out. All cells of the multicellular host animal are genotypically identical, while foreign cells are genotypically different to the host. This means that the proteins on the surface of cells act as 'markers of identity’. Indeed, immune cells and immune receptors recognise the proteins, carbohydrates and lipids on the surfaces of cells. Taken together, these surface molecular structures are called‘molecular patterns’. Those patterns on pathgen cell surfaces are called Pathogen Associated Molecular Patterns (PAMPs).

Sham Nair 2014