The conception is the moment at which two very different cells, the egg cell and the sperm cell, meet and become one new cell. This page describes that meeting as embryology knows it and then looks at what the process shows when it is observed as a whole. The two cells themselves are described on the page about the human egg cell and sperm.
The way to the egg
After ovulation the egg cell is caught by the fimbriae, the finger-like fringe at the open end of the oviduct. The egg cell (or ovum or oocyte), with the nutritive cells that surround it, is now in the oviduct (or fallopian tube). It is transported towards the uterus by the flow of liquid in the oviduct and by the beating of the cilia that line it. Both the egg and the mucous membrane of the fallopian tube produce substances that attract sperm cells. The sperm cells swim against the stream from the vagina through the uterus and oviduct to the egg. Many sperm cells (more than 90%) get stuck behind barriers such as cilia or are damaged by antibodies. Only some dozens to a few hundred sperm cells reach the egg and surround it with their heads directed towards the oocyte (Fig. 7).
On the way the sperm cells change. In the female genital tract their membranes are altered so that they become able to react to the egg, a process called capacitation. Freshly released sperm cells cannot fertilise an egg; they need this time of preparation. The egg, on its side, can be fertilised only for a limited time after ovulation, roughly a day. The meeting therefore depends on timing as much as on movement.
The pre-conception attraction complex
Microscopic observation of the meeting shows that for a few hours a so-called pre-conception attraction complex exists: a state in which the egg is surrounded by several dozens of sperm cells, lying with their heads against the envelope of the egg cell and their tails pointing away from her. It has been observed, too, that this complex starts to rotate. The rotation is caused by the movement of the tails of the sperm cells. After a while they begin to move more or less rhythmically in the same way. The embryologist Jaap van der Wal calls this a "mating dance".
The conception
During this pre-conception attraction complex the egg secretes substances that alter the sperm. Their cap, the acrosome, opens and releases enzymes, which enables them to connect themselves to the egg and to fertilise it. The sperm cells in turn ensure that the zona pellucida changes, so that it lets one, and only one, sperm cell pass through. Substances from both sides are required to prepare the fusion of the gametes. Both cells have to be prepared and to change to become receptive to the other. The cells interact and change each other. Whether the fusion succeeds is uncertain; that depends on the interaction of the two cells.
Then the cell membrane of the egg merges with that of the sperm cell, and the contents of the sperm fuse with the contents of the egg. The large egg receives the small sperm. Both cell membranes open up and connect with each other. The cell membrane is continuous: there is at no time an opening or hole in it through which the sperm creeps. The membranes of the egg cell and the sperm cell always form one continuous entity. The question remains how it is determined which sperm cell will fuse with the egg.
After the fusion of the egg with a sperm cell has taken place, the zona pellucida changes again and no other sperm cell can fuse with the egg any more. In textbooks this is called the cortical reaction: small granules just under the egg membrane release their contents into the space around the egg, and the zona pellucida hardens.
The pre-conception attraction complex is a process that takes time. It is not a race in which one sperm cell arrives first and wins ("survival of the fittest"), and it is not true either that the sperm enters or penetrates the egg, since the cell membranes fuse. There is no race and no aggression. Both cells are equal, need each other and prepare each other for the fusion and the conception. The conception is a gradual process.
The first hours of the new cell
After the fusion the egg completes its last maturation division, which it had interrupted at ovulation. The nucleus of the egg and the nucleus of the sperm, now called pronuclei, move towards each other in the centre of the cell. Each carries 23 chromosomes. When their chromosomes come together on the first spindle, the new cell has 46 chromosomes, half from the mother and half from the father. The sperm cell brings either an X or a Y chromosome, and with that the genetic sex of the child is determined at conception.
Besides its chromosomes the sperm cell contributes very little material. Almost all of the cytoplasm, the nutrient reserves and the mitochondria of the new cell come from the egg. The zygote that arises is a single, large, round cell, and from here the story continues on the page about the first week.
Conception in other animals
In many animals the meeting of egg and sperm takes place outside the body. Sea urchins release their eggs and sperm into the sea water, and the same happens in most fish and in frogs, whose eggs are fertilised as they are laid. In such animals the egg is exposed to countless sperm cells at once, and it protects itself against the entry of more than one by a rapid change of its membrane and envelope, much as the human egg does. In mammals, birds and reptiles fertilisation is internal. In the mammals it takes place in the oviduct, and the young embryo then remains in the protected space of the mother's body, where it will settle in the wall of the uterus. The page on plant and animal shows the sea urchin's further development.
Integration
On the page about the egg cell and the sperm cells it is shown that egg and sperm are polar to each other. Both cells are so far specialised that they are not able to develop further on their own. Only when they meet can the joining of both extremely specialised cells become the starting point for a new development.
That both cells attract each other is not surprising. What the egg is, the sperm is not, and vice versa. When they meet, the one-sidedness of each is resolved by that of the other. What was separated is united.
Reversal
During the hours of the pre-conception attraction complex an ovum with its mobile cytoplasm is surrounded by tens or hundreds of structured sperm nuclei. The usual picture of a cell, in which the nucleus is at the centre and the mobile cytoplasm around it, is reversed. The nuclei are now at the periphery. At the same time the cytoplasm of the ovum is at rest in the centre, around which the movement of the sperm cells' tails induces the rotation. That is a second reversal, since normally the periphery is at rest and the cytoplasm is in motion. And thirdly, a cell is normally in interaction with its environment at the periphery. Now the periphery is occupied by the secluded, non-interactive quality of the sperm cells. The complex is directed inwards. The pre-conception attraction complex is the reversal of the normal conditions and relationships of a cell.
Incarnation
In the anthroposophical view, this dance of ovum and sperm cells, and the reversal of the normal relationships during the hours of the attraction complex, may be what opens the physical for the spiritual. From that perspective an encounter with the soul and spirit of a human being is being prepared, and the pre-conception attraction complex may be the moment at which the spirit germ of the new human being is linked with its physical germ: the moment of incarnation. Within that tradition it is also said that some parents sense this moment, not only as the beginning of a child but as the arrival of a particular person.
The tradition adds that conception and incarnation are never simply made. In its words, a woman and a man do not make a child, but receive it.
No reproduction
Compared with many other mammals, human conception is often described as inefficient: the chance that a sperm cell and an egg actually meet in a given cycle is small. From the anthroposophical perspective, which takes into account that an incarnation has to take place, this is not surprising. The conception of a human being is then not only a matter of reproduction but something of a different order: the arrival of an individual. Every human being is a unique individuality. The parents do not repeat themselves in their offspring; they create the conditions for another human being to come into existence. The same thought returns in the pages that follow, where the embryo is described as a whole that shapes itself, from the second week onwards.
Questions
Where does fertilisation take place in humans?
Fertilisation normally takes place in the oviduct (fallopian tube), in its wide outer part near the ovary. The fertilised egg is then carried towards the uterus in the following days.
What is the pre-conception attraction complex?
It is the phase of a few hours in which the egg is surrounded by dozens of sperm cells lying with their heads against it. Their tails move in the same direction, so that the whole complex slowly rotates; the embryologist Jaap van der Wal called it a "mating dance".
What is the acrosome reaction?
The acrosome is a cap on the head of the sperm cell that contains enzymes. When it opens, these enzymes help the sperm pass through the zona pellucida, the protective layer around the egg.
Why does only one sperm cell fuse with the egg?
As soon as one sperm cell has fused with the egg, the egg releases substances that change the zona pellucida. From then on no other sperm cell can bind to it or pass through, so the egg receives one set of chromosomes from the father.
Does the sperm penetrate the egg?
Not in the sense of making a hole. The cell membranes of egg and sperm fuse, so that there is at no moment an opening in the membrane; the contents of the sperm cell become part of the egg.
Is conception a race between sperm cells?
The image of a race won by the fastest sperm does not fit the observations well. Egg and sperm prepare each other over hours, and which sperm cell fuses depends on the interaction of the cells rather than on speed alone.
What does the sperm cell contribute to the zygote?
It brings 23 chromosomes, including either an X or a Y chromosome, which determines the genetic sex of the child. The egg contributes the other 23 chromosomes and almost all of the cytoplasm.
What is meant by a reversal during conception?
In an ordinary cell the nucleus lies in the centre and the mobile cytoplasm around it. In the attraction complex the sperm nuclei lie at the periphery and the egg's cytoplasm rests in the centre, so the usual relationships are turned around.
How does the anthroposophical tradition view conception?
In that tradition conception is seen as the moment where the spirit of a new human being connects with its physical germ. It is a view from that tradition, described here as such, not a biological finding.