New ways of making babies are on the horizon

The human genome can create cells with a remarkable range of capabilities and shapes. Looked at under the microscope, the enterocytes which line the gut bring to mind the ghosts from a Pac-Man video game. Neurons look like medieval morningstars, but with long hair. But none is more special than the big, round egg. It is not just the largest of the cells. It is also, once fertilised, the cell from which all other sorts of cell are descended.

The size of the egg allows it to carry the nutrients and chemical building blocks needed for the first days of an embryo’s development. As those days tick by, the single large cell of the fertilised egg splits into two, then four, eight and onwards until they are countless. In the earliest stages, the divisions produce “stem cells”, from which all sorts of others can be derived. As time passes, the cells specialise, expressing some genes more and some genes less, eventually producing all the intricacies of tissue and organ the newborn needs, right down to its toenails.