6 Small beginnings – the first two weeks
6.1 Pre-implantation and assymetric division
Let us now return to the Fallopian tube, where a fertilized egg is assembling its chromosomes prior to commencing a series of mitotic divisions which will eventually give rise to the millions of cells that make up the human body. Obviously these millions of cells do not just exist as an amorphous mass: they are differentiated into many different types of cell, and they are organized into recognizable, discrete structures: tissues and organs. This is accomplished by a coordinated sequence of complicated events, yet the fascinating choreography of development is based, like ballet, on a few relatively straightforward steps. In the very first days of embryonic development the stage is set for the full repertoire of movements, so understanding the early days allows us insights into the later processes, not yet fully understood, that lead to the development of a new-born baby.
Figure 17 shows the stages of development of the human conceptus (early embryo) during the first week or so. These stages are called pre-implantation, because during this time the embryo lies free within the mother's reproductive tract. From the time of fertilization until the early blastocyst stage, roughly 5 days, the conceptus is in the Fallopian tube. A blastocyst is the first structure that looks different from the blob of cells that is the conceptus. A blastocyst consists of two distinct types of cell, and is swollen by fuid which is produced by one of these types. We shall discuss this further in a moment. By the time the blastocyst arrives in the uterus, the hormonal cycle has progressed, so that the uterus will be able to accommodate the embryo, and allow it to attach to the thickened uterine wall. Prior to this, implantation is impossible.
Figure 17, The stages of early human development. (Note: from the fertilized egg onwards each cell contains two sets of chromosomes in its nucleus – for simplicity, the cell nuclei are not shown beyond stage (c).)
Q Which hormone is involved in preventing implantation?
A Progestogen (Section 3.1).
The divisions resulting in the stages shown in Figure 17 are called cleavage divisions. This means that the divisions take place without any net cell growth, resulting in the cells roughly halving in size at each division. In this way, the large amount of cytoplasm in the egg is reduced to more manageable amounts. You will recall that the cytoplasm in the egg provides the conceptus with many of the nutrients it needs to get it started; as the conceptus becomes metabolically active, synthesizing its own nucleic acids and proteins, it becomes less dependent on components in the cytoplasm, and more reliant upon external supplies of nutrients from the surrounding fuid.
Three waves of cell division result in an 8-cell conceptus whose cells are all in contact with the ‘outside’, i.e. the uterine fuid, through the zona pellucida (Figure 17d–g). But the fourth division is different, and, as you will see, vital for further development. The division from 8 to 16 cells is asymmetric, that is, the division does not occur across the centre of the cell, but is closer to one end than the other.
Q What is the consequence of this asymmetry?
A The resulting cells will be of two different sizes (Figure 17h).
At the 16-cell stage, the first differentiative event has occurred: there are two populations of cells: eight large cells on the outside, and eight smaller cells on the inside. Size is not the only difference between them; although cells of both populations have a nucleus, cytoplasm and a cell membrane, the constituents of the cytoplasm and the cell membranes are different. In particular, one of the differences in the membranes means that the smaller, inner cells are more adhesive than the larger, outer cells, and form a tight ball of cells lying within, though still connected to, the outer shell. You would not expect the constituents of the nucleus to be different, as the same DNA is present in all cells of this individual, but different proteins are made in the two populations (in particular, different membrane proteins, giving rise to the differential adhesiveness).
Q What does the synthesis of different sets of protein imply is happening at the level of the DNA?
A Different genes are being transcribed.