The first cell divisions
Once the egg and the sperm cell have fused (see the conception), the nuclei of both come together and the chromosomes form pairs. The zona pellucida closes, so that no more sperm cells can connect with the egg. The zygote (= fertilised egg) has come into being. For a few days there is hardly any exchange of nutrients and other substances with the environment.
The unicellular zygote exists for about one day. Then the cell divides into two equally big cells. They divide again every 16 to 20 hours or so. First there are four, then eight, then sixteen cells. Around the fifth day there are about 32 cells and then about 64, and the divisions are no longer synchronised. The stage of 16 to 64 cells is called a morula (= mulberry; the embryo resembles a mulberry or a blackberry).
Around the eight-cell stage the loosely touching cells press closer together and the ball becomes smooth on the outside. Embryologists call this compaction. From then on the cells on the outside and the cells on the inside begin to behave differently, the first differentiation within the embryo.
On the sixth day most cells lie at the periphery, forming the trophoblast (= nutritive tissue), and a clump of cells stays inside, forming the embryoblast (= embryonic tissue). The trophoblast cells move closer to one another, which makes the tissue firmer. The divisions continue. On the sixth day there are approximately 120 cells, of which some 10 form the inner embryonic cell mass and about 110 the outer trophoblast; the ratio is about 1 : 10. The trophoblast will later form most of the placenta and the membranes, while the embryoblast forms the embryo itself.
All divisions occur within the shell of the zona pellucida. The embryo does not grow. The cells become smaller after each division and contain relatively less cytoplasm and more nuclear material.
Meanwhile the embryo floats passively on the stream of fluid in the oviduct as it is transported to the uterus (Fig. 10). It is carried along by the fluid and by the beating of the cilia in the wall of the tube, and around the fourth day it reaches the cavity of the uterus.
Hatching
On the sixth day, when the embryo is in the uterus, the zona pellucida breaks. This happens because the trophoblast, on the side of the blastocoel, forms enzymes that make the zona pellucida softer. Simultaneously the embryo expands to 2 times its volume, then contracts to 50% and expands again to 3 times its volume. Then the zona pellucida breaks open and the embryo bulges out of its shell. This "first birth" is called hatching and can be compared to the hatching of a chick (Fig. 9). The embryo is now out of its rigid zona pellucida and can start to grow.
The release of the embryo from its shell is an active process of the embryo itself. The softening of the zona pellucida, together with the expansion, causes a break which enables the embryo to bulge out.
Length of time
Remarkably, the development of the zygote up to the blastula stage takes a similar, short time in all mammals, however long the rest of the development lasts. In humans, with an embryonic and foetal development of about nine months, it takes about a week. In mice, with a development time of about three weeks until birth, and in the elephant, whose gestation lasts around 21 months, this first phase is equally a matter of days. For the rest of the development of mice only about two weeks remain.
Roe deer have an extended gestation time. The rut takes place in July and August. The zygote develops into a blastula and then rests for approximately 150 days, a so-called embryopause. Only in December does the development go on, with implantation. After a further development of 144 days the fawns, often twins, are born in May or June. The roe deer is the only hoofed animal known to show this, but a delayed development is also seen in mustelids (including the badger), bears and kangaroos. The embryopause always takes place at the end of the first week, in the blastula stage of the embryo, before implantation.
The shell of the zona pellucida
The zona pellucida is a clear envelope of proteins and sugars that is formed around the egg cell while it matures in the ovary. After conception it has several tasks at once. It keeps the loosely connected cells of the young embryo together as one ball. It prevents more sperm cells from reaching the egg. And it prevents the embryo from attaching itself too early, while it is still in the oviduct; only in the uterus, after hatching, can the trophoblast touch the mother's tissue. The shell is therefore both protection and boundary. Inside it the embryo lives in a closed world of its own for almost a week.
The animal way of beginning
The first week shows the features that the page on plant and animal describes for the sea urchin. There is no growth at first, only division into ever smaller cells. There is at first no differentiation: the early cells are alike, and each of the first few cells could still form a complete embryo, which is why identical twins can arise from one zygote. Then an inner space appears, the blastocoel. The human embryo therefore begins in the animal way, not the plant way: it forms an interior before it grows outward into its surroundings.
Characteristics
On the first day after conception a human being is a large round cell. That is all there is, but no less either. It seems a perfect life: the one-cell human organism is an entity and the one cell includes everything. The zygote is round and floats. Here it is easy to see that the entity comes first and the parts come later. A human being is not built from cells or organs; a human being divides into cells and into organs.
After the first day there are divisions, and after 6 days the blastula is formed: a sphere with an interior space. The embryo has no direction: left and right, up and down are not yet concepts that describe it. Directions come later, in the third week.
Immediately after conception the zona pellucida closes and there is no interaction with the environment. After 6 days of cell divisions the cells have become very small. There is now more nuclear material and less cytoplasm than on the first day, which decreases the inner activity of the cells. The embryo has become more structured compared to the zygote. It now has more of the structured quality of the sperm, while the inner mobile quality of the egg of the first day has diminished.
This process of dividing while the cells are shrinking cannot continue, because the cells would become too small and the embryo would become rigid. There must be a reversal. This reversal is brought about by the blastula expanding and contracting and finally bulging out of the zona pellucida. Then it can absorb nutrients and start to grow, and the cells can grow too. The embryo reaches the uterine wall and its development continues in the second week.
In mammals the development of the embryo until implantation takes about a week or less. In the rest of the embryonic development the differences are big: from about two weeks in mice to around 21 months in elephants. The development of all mammals up to the blastula has its own time and hardly depends on the species. It is not yet part of the life of the animal; it has a life of its own. It is as if the time of the species has not yet arrived.
The floating existence
Throughout the first week the embryo is not attached to anything. It drifts in a thin film of fluid, turned neither up nor down, touched only by the moving wall of the oviduct. It receives almost nothing from outside, apart from oxygen and a little fluid, and lives on what the egg brought with it. Its whole existence in these days is a matter of dividing, rearranging and waiting, while it is carried from the place of conception to the place where it will settle. Only with implantation does it exchange this free, floating existence for a fixed place and a connection with the mother's body.
Summary of the first week
- The embryo is a sphere; there are no directions.
- The development takes place within the seclusion of the zona pellucida.
- The entity or unity comes first, multiplicity comes later.
- The duration of the development in the first week is short and similar in all mammals.
The mineral human being
Characteristics of the mineral are that it falls apart into the same elements, that there is no communication with the environment and that there is no time (Fig. 11). In this phenomenological reading the first week of the embryo can therefore be called the "mineral human being": a form that divides into equal parts, closed off from its surroundings and not yet bound to the time of its species.
Questions
What happens in the first week after conception?
The zygote divides into two, four, eight and more cells while it is carried through the oviduct towards the uterus. It becomes a morula and then a blastula with an inner cavity, and at the end of the week it hatches from its shell.
What is a morula?
A morula is the stage of about 16 to 64 cells, when the embryo looks like a tiny mulberry or blackberry. The name comes from the Latin word for mulberry.
What is the difference between the trophoblast and the embryoblast?
The trophoblast is the outer layer of cells of the blastula; it later forms most of the placenta and membranes. The embryoblast is the small group of cells inside, from which the embryo itself develops.
Why does the embryo not grow in the first week?
All divisions take place inside the zona pellucida, a firm envelope, and the embryo takes up hardly any nutrients from outside. Each division halves the cells, so their number increases while the whole stays the same size.
What is hatching?
Hatching is the moment at the end of the first week when the blastula breaks out of the zona pellucida. The shell is softened by enzymes from the trophoblast, and the embryo expands and contracts until the shell tears and it bulges out.
Where is the embryo during the first week?
It floats freely in the fluid of the oviduct and later of the uterus. It is moved along passively by the stream of fluid and the beating cilia, without any attachment to the mother's tissue.
What is an embryopause?
An embryopause, or delayed implantation, is a resting phase in which the blastula stops developing for weeks or months before it implants. The roe deer, badger and other mustelids, bears and kangaroos show it.
Why is the first week called the mineral human being?
In this phenomenological reading, the embryo in the first week shares features with the mineral: it divides into equal parts, has no exchange with its surroundings and no time of its own. The expression is an image, not a biological term.
Is the length of the first week the same in all mammals?
The phase up to the blastula is short in all mammals, a matter of days, while the rest of gestation varies enormously between species. In humans it takes about a week.