The human egg cell and sperm compared

The egg cell and the sperm cell are the two germ cells from which every human life begins. For a conception an egg cell and many sperm are needed. When the cells find each other, they fuse after a while and the chromosomes come together. Then the zygote (= fertilized egg) is formed and can begin to divide. A phenomenological comparison brings out the characteristics of the ovum and sperm. There are no cells in humans that are so different and yet belong so much together.

The ovum and sperm

Size and shape

The egg cell (or ovum, or oocyte) is the largest human cell. It measures 0.15 to 0.2 mm and is just visible to the naked eye. It is also the roundest cell, almost perfectly round (Fig. 4). It therefore has the largest volume in relation to its surface. The cell consists of a large amount of cytoplasm (= cell fluid) in which the nucleus is dissolved (and therefore invisible) until just before conception.

Sperm cells are the smallest human cells. They are no more than a nucleus with a small amount of cytoplasm, some mitochondria (the energy suppliers of the cell) and a long tail. They have hardly any content and are the straightest cells.

Egg cell and sperm are each other's opposite. Large versus small, round versus straight, cytoplasm versus nucleus. The differences are great, at the same time they belong together when the ovum is seen as a sphere and the straight sperm as the corresponding radius.

Mobility

The cytoplasm of a normal body-cell is in movement, the nucleus is not. The two gametes (= germ cells) show different features. The egg cell consists primarily of cytoplasm and is internally mobile. The nucleus is outspread, the chromosomes are unwound (not folded up). The cell is internally active and mobile. The sperm cells have hardly any cytoplasm and are concentrated in their nuclear DNA. They have a crystalline structure. These cells are internally structured and rigid.

In contrast, the ovum is externally not active. After its release, it is passively moved by the fluid-flow in the oviduct (uterine tube), while the sperm cells are active, using their tails to swim against the stream of fluid in the oviduct. They are externally active and mobile.

The ovum is internally mobile and externally passive, this is a polarity. The sperm shows the opposite: internally passive and externally mobile. Egg cell and sperm have a polarity and are opposite to each other: a double polarity.

Metabolism

An egg cell is a metabolically active cell; substances are absorbed and released. E.g. nutrients are absorbed, substances that affect the uterus and substances that attract the sperm are released. An egg cell lives only 12 to 24 hours in its own environment and cannot be preserved. The egg cell can easily be destroyed. It is an active cell and open to the environment.

Sperm cells hardly absorb or release substances. There is almost no interaction with the environment. They live about 3 to 5 days in the female reproductive tract and can be preserved and frozen at temperatures below −60 °C. They are not easy to destroy. They are closed off from the environment and metabolically passive.

The open and vulnerable state of the egg cell is polar to the closed and resistant state of the sperm cells.

Number

For a conception one ovum and millions of sperm are required. The one ovum is worth as much as all those millions of sperm. An ejaculate with fewer than about 20–40 million sperm is regarded as a sign of reduced fertility. Such great numbers are necessary because most sperm do not reach the ovum. Also, for a conception more than one spermatozoon is necessary. See the page on conception.

The ovum is alone and the sperm are with millions. One sperm cell is nothing, one ovum determines everything. One is polar to millions. One comprises everything, it is all there is, whereas the millions of sperm cells are countless and have no importance on their own.

Location

The egg cell develops in one of the two ovaries in the warm abdominal cavity, the sperm develop in the testicles just outside the body in a relatively cold environment.

The ovum develops in warm- and sperm in relative cold conditions.

Development

Egg cells are produced well before birth in a huge number of so called primordial egg cells (primordial oocytes). From the beginning on, there is a continuous process of dying, so that at birth 2 million (!) are left. That process of dying goes on after birth. At the onset of puberty a few hundred thousand remain. Then every four weeks a number of them begin a process of maturation. Of these, only one (sometimes two or three) ovum matures; the rest die. In total about 400 ova mature (13 per year for 30 years). At menopause, no primordial egg cells are left.

In men, a very different process is going on. The first sperm cells are formed only from puberty on, before that they are not produced. Then the production goes on and on and never stops, hundreds per second, millions each day. Sperm cells are constantly being newly formed.

Egg cells are old cells that became mature. Primordial oocytes are in a process of dying. Sperm cells are newly formed and are young. The maturation process of ova is an expiring process, it stops. The formation of the sperm is a vital process, it never stops.

Maturation

From a primordial oocyte only one mature egg cell develops. During meiosis the rest of the mass of the nucleus is excreted as polar bodies. The cell grows during maturation, the amount of cytoplasm increases. During ripening the ovum moves from the centre of the ovary to the edge (Fig. 5).

From one primary sperm cell four sperm cells develop. The cytoplasm is eliminated, the cell is getting smaller. When some cytoplasm stays behind, the sperm cell cannot swim well and cannot reach the egg cell. Sperm cells are produced at the edge of the testis and stored inside.

At egg cell maturation the focus is on one cell, that expands in volume. Sperm cells show concentration of material and expansion of the number. Egg cells move from the inside to the outside, sperm cells from the outside to the inside.

Conclusion

In appearance and processes egg cell and sperm are mutually antagonistic, each other's opposite. Large versus small, internally - versus externally active, old versus young, concentration versus expansion, etc. During maturation of these gametes two cells are formed which differ maximally and seem to go to extremes in their individuality. The development and maturation show increasing divergence, a process of polarization.

When they are mature, ovum and sperm can come together and resolve the polarity in a conception, so that a new human being can develop, with all the cell shapes that lie between the two extremes. If they do not meet, they are not viable and die. What happens in the days after conception is described on the page on the first week.

The development of the ovum

Ova are created as primordial oocytes in million copies as early as in the embryonic stage and their number is gradually reduced. They lie separated from each other in follicles and are surrounded by a layer of nutritive, so called follicular cells. In the primordial follicles (= initial vesicles) they lead a passive existence. In Fig. 5 the development of the egg cell can be seen clockwise from the left (primordial follicles).

1. The development begins with the thickening of the surrounding layer of nutritive cells, this is called the primary follicle (= first vesicle).
2. This stage leads to the secondary follicle, because in the layer of nutritive cells an antrum (= cave) arises. The ovum grows and gets larger. The follicle produces oestrogen, a hormone that stimulates the wall of the uterus to thicken.
3. The antrum grows larger. Around the ovum a layer is formed, called the zona pellucida (= translucent layer). Around it are the nutritive cells in the corona radiata (= radiating wreath). The growth of the ovum continues. The wall of the uterus continues to thicken.
4. Then the ovum is shot away into the abdominal cavity. There is a moment when the ovum floats freely in the abdominal cavity. It is then collected by the fimbriae of the oviduct. The interception is an active process, the oviduct moves to the ovum. The remaining cavity in the ovary is called the corpus luteum (= yellow small body) that makes progesterone, which also plays a role in the thickening of the uterine wall, so that the fertilized ovum can implant. When a fertilization does not occur, then the thickened wall comes loose and menstruation occurs.

The development of sperm

From a germ, four equal sperm cells are formed by division. Around the nucleus a hard cap is formed, the acrosome (acros = top, soma = body). Then the cytoplasm is ejected and the cell gets smaller. Mitochondria move to the beginning of the tail, that becomes thicker and longer. The cells are stored for some time, after which those that are not released are resorbed. If a small cloud of cytoplasm remains with the nucleus, the sperm is badly damaged and will have trouble moving forward.

Egg cell and sperm in the wider course of development

The comparison of the two germ cells is the first step in the embryology series. The introduction to embryology explains why the early embryo is studied here as a sequence of images, and the page on plant and animal shows the same kind of polarity in simpler organisms before the human embryo is considered.

In textbook terms, both germ cells carry half the usual number of chromosomes: 23 instead of 46. This reduction takes place during meiosis, the special cell division that only germ cells go through. In the egg cell meiosis is not completed until a sperm cell has entered it. The sperm cell, by contrast, has already finished both meiotic divisions before it leaves the testis.

The genetic contribution of the two cells is therefore equal, but almost everything else is not. Nearly all the cytoplasm of the future embryo, including its mitochondria, comes from the egg cell. The sperm cell contributes its nucleus and the structures needed for the first cell division, and little more. The first days of development run largely on what the egg cell has stored.

Seen as a whole, the page describes two cells that develop in opposite directions from the very start. One grows, slows down and waits; the other shrinks, multiplies and moves. That contrast is the starting point for the phenomenological reading of conception.

Further reading

Bie, G van der, 2001. Embryology. Louis Bolk Institute, Driebergen.