6 Small beginnings – the first two weeks
6.2 Compaction and adhesion
Around the time of the 8- to 16-cell division, the conceptus undergoes a morphological (shape) change, called compaction, in which the cells fatten on each other, and the outlines of individual cells become hard to distinguish. This stage, sometimes referred to as a morula, from the Greek word for mulberry, is shown in Figure 17i. At this stage it is hard to see individual cells; in fact, unless the cells are separated by various laboratory treatments, it is not possible to see the two different cell types present in an intact embryo at the 16-cell stage. (Figure 17h is included for clarity even though it is not a stage normally seen.) Once compaction has occurred, it is impossible to see subsequent cell divisions, although they do occur at frequent intervals. By the time the fuid-filled cavity of the early blastocyst appears (Figure 17j), there are about 64 cells, and a mature, expanded blastocyst can contain more than 250 cells (Figure 17k).
As mentioned above, the blastocyst consists of two cell types which are derived from the asymmetric division at the 8-cell stage. The larger cells resulting from that division form the trophoblast cells, which surround the fuid-filled cavity, and indeed are responsible for making the fuid. These cells will form the embryo's contribution to the placenta, the organ which will supply nutrients to and remove waste products from the embryo throughout the rest of gestation. The smaller cells from the asymmetric division form the inner cell mass. These are the cells that will give rise to the embryo proper, and to some of the membranes surrounding it. The trophoblast cells do not undergo much further differentiation, but the inner cell mass cells have a long developmental road ahead of them.
How interchangeable are the two cell populations? Is differentiation into two different types of cell really a permanent step, or can it be reversed? The answer seems fairly clear-cut: once a cell has differentiated, there is no turning back. If a trophoblast cell is put into the middle of a group of inner cell mass cells it will not contribute to the inner cell mass, but will be pushed to the outside of the clump, and discarded. This happens because trophoblast cells have different protein molecules on their surfaces from those found on the surfaces of inner cell mass cells.
Q Can you remember a property of some of the surface molecules which might account for this?
A Adhesiveness. Adhesion between cells is mediated by surface proteins whose shapes allow them to interact with each other.
In fact, many surface proteins – the glycoproteins – carry short chains of carbohydrate on them and it is the carbohydrate part that is responsible for the stickiness. Adhesion of one cell to another can be regarded as an early form of communication of the ‘Is there something there – Yes or No’ variety. But it seems that cells are able to recognize similarities and differences between themselves purely on the basis of their adhesive properties. This differential adhesion is what pushes the trophoblast cell out of the clump of inner cell mass cells. The inner cell mass cells adhere more tightly to each other than they do to the trophoblast cell, with the result that it is gradually excluded from the group. The concept of differential adhesion is an important one that you are likely to meet in further study.
It seems clear that by the time the inner cell mass and the trophoblast are visibly distinct, they have differentiated to such an extent that they cannot substitute for each other. But what about earlier stages? It has been known for many years that in some animals the fate of early embryonic cells has not been determined: cells from 2- and 4-cell embryos can substitute for each other and can even, if separated from their fellows, give rise to complete new embryos. It is ethically difficult to do this kind of experiment with human embryos, but what little experimental evidence there is seems to accord with what has been found in other mammals: until the 8-cell stage, embryonic cells are developmentally ‘plastic’, that is, their fate is not yet sealed. By the 8-cell stage, however, preparations are clearly under way for the asymmetric division that marks the first differentiation, and the cells cannot safely be tampered with.