Cells are often described as the smallest unit of life. That applies to both uni- and multicellular organisms. This is a characteristic they maintain as long as they survive. As such, they possess an ‘individuality’.
To the benefit of the organism as a whole, some cells divide while others, also to the benefit of the organism, do not. Still others may usefully disintegrate - as in the case of megakaryocytes which give rise to platelets important in blood clotting.
Obvious - yet rarely stated - is the fact that cells do not typically merge with other cells. Here, the fusion of sperm and ova is seen as a special case. Each contains only 23 chromosomes and so only half of the entire genetic material. Thus, sperm and ova are - in a sense - only half cells. (Indeed, whether sperm qualify for the title ‘cell’ is a moot point.)
With very few exceptions, cells remain separate from each other throughout their lifespan. They do not blend or merge to form mega-cells. Those cells that do fuse do not fuse with cells of another type. They fuse only with cells of the same type. In so doing they become multinucleated. The cytoplasm is combined but not the nuclei. In this, a hint of the ‘individuality’ I mentioned above, persists.
What does having two nuclei within the same cytoplasm imply? In practical terms, there is an increase in the efficiency of the cell. (If by cell we mean that bounded by the cell membrane.) It implies double input/output and information processing capacity.
Those cells in our bodies that do undergo fusion do so in specialized circumstances, such as during development and tissue repair. For example:
- Muscle Cells: Muscle cells, known as myocytes, can fuse to form multinucleated muscle fibres. This process contributes to the growth and repair of skeletal muscle tissue.
- Osteoclasts: Osteoclasts, which are responsible for bone resorption, are formed by the fusion of monocyte/macrophage precursor cells.
- Syncytiotrophoblasts: In the placenta, syncytiotrophoblasts are formed by the fusion of cytotrophoblasts. These multinucleated cells play a crucial role in facilitating nutrient and gas exchange between the maternal and fetal circulations.
Cell fusion is not a random process. It is tightly regulated and involves complex molecular mechanisms. This implies that cell fusion is a process that has evolved. As such, it offers the organism a biological - that is, a survival and/or reproductive - benefit. To use a word rarely used biologically, it is ‘deliberate’.
Having introduced this word, I will stop here and ponder what, in a biological context, ‘deliberate’ means and how it might be used.
