3.4.2 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
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3.4.1 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 sequen
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3.3 Summary of Section 5

  1. After ejaculation some sperm penetrate the cervical mucus, and on arriving in the uterus become capacitated.

  2. A few sperm swim up the Fallopian tube containing the recently ovulated egg.

  3. In the tube the sperm become activated. This involves changes to the membranes and a change in the swimming pattern.

  4. Enzymes from the acrosome allow the sperm to get next to the egg, by removing follicle cells and digesting
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3.1 How fertilization happens

Now that we have dealt with the basic biology, we can resume and give more detail to our story, and return to where we left it: fully mature, strongly swimming sperm have been deposited in the vagina, and will begin their race to the newly ovulated egg.


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4.4 Hormonal control of sperm production

The most important hormone involved in controlling sperm production is a steroid called testosterone. This is produced in the testis itself, by the Leydig cells (see Figure 12a). The testosterone is released from the Leydig cells between the tubules, and taken up by the neighbouring Sertoli cells. The Leydig cells are stimulated to make testosterone by two other hormones, luteinizing hormone (LH) and follicle-stimulating hormone (FSH), which are both produced by the pituitary gland and
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4.3 Gamete production in men

A sexually mature man is producing sperm all the time at a rate of around 300–600 per gram of testis per second. This provides the 500 million or so which are released at each ejaculation. But the formation of an individual sperm takes about nine weeks (64 days). Sperm are produced in the testes, and production is most efficient at a temperature several degrees lower than the normal body temperature of 371°C. For this reason the testes (plural of testis) are suspended outside the body cavi
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4.2 The reduction of chromosome number: meiosis

If you look at the chromosomes shown in Figure 8 you will see that they have been lined up in pairs. The members of each pair are of similar shape and size, and unlike the members of other pairs. At a molecular level these distinctions are maintained: the order of the bases in the DNA is very similar in both members of a pair, but is quite different from that found in other pairs. By ‘very similar’ we mean that the order of the particular genes on each chromosome of the pair is the same,
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4.1 Why are cells different?

Now let us go on with our story and assume that we have decided the time is right to have a baby. The primary requirement for conception is that healthy gametes should be produced. We shall therefore look first at how gametes are made, and then examine some of the factors affecting their quality. But we must start with an explanation of what gametes are, and what sets them apart from other kinds of cell. In other words, what makes gametes special? Gametes are the cells that fuse to form a new
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3.6 Summary of Section 3

  1. Many people wish to limit the number of their offspring, and so resort to contraceptive measures.

  2. Chemical contraceptives interrupt the production of gametes, or prevent implantation.

  3. Mechanical or barrier contraceptives prevent egg and sperm from meeting and, in the case of IUDs, prevent implantation.

  4. Surgical methods of contraception involve physical alteration of the reproductive tract so as to prevent e
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1.3.5 Natural methods of contraception

Many people with particular religious beliefs are fundamentally opposed to the use of artificial methods of contraception. In the developing world, where, as you saw above, the population is frequently increasing at an unsustainable rate, this is a particular problem. For Muslims and Roman Catholics (and others), who may nonetheless wish to limit their families, the preferred option is to use natural family planning methods. The most commonplace method, which involves estimating the ‘fertil
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1.3.4 Surgical methods of contraception

Surgical methods are by and large the most drastic and irreversible ones, ranging from castration to relatively untraumatic tube-tying. Because of the psychological and physiological side-effects, surgical removal of the testes or ovaries is not generally carried out for contraceptive reasons alone, although these operations may be carried out for other reasons, such as the presence of malignant tumours. Any kind of surgical sterilization can be physiologically traumatic for a woman, as it in
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2 How has the human population grown?

For most of human history there have been relatively few people in the world. Figure 2 shows that only over the last 50 years have numbers really shot up, and that, at the turn of the century, the population reached over six billion.

Figure 2

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1 What is development?

In this unit we begin to look at the human being in the context of an individual life cycle, examining some of the processes that contribute to the formation of a new person. This is the first time that many of you will have encountered this level of biological detail; we would ask that you take the time to understand it fully at this stage. We hope to show you that, far from being a dry academic subject, the study of biology allows us to glimpse a dimension of dynamic sophistication and eleg
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Learning outcomes

After completing this unit you should be able to:

  • Define and use, or recognize definitions and applications of, each of the terms printed in bold in the text.

  • Explain the scientific basis for the main methods of contraception. (Question 1)

  • List the factors affecting fertilization. (Questions 2 and 5)

  • Describe with the help of diagrams the early stages of embryonic development. (Question 3)

  • <
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References

Sheldon, P. (2005) Earth’s Physical Resources: An Introduction (Book 1 of S278 Earth’s Physical Resources: Origin, Use and Environmental Impact), The Open University, Milton Keynes
Roy, A. (1999) The Cost of Living, Random House Inc, New York

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4 Summary

  1. Many rivers are fed by springs, which occur at points where groundwater reaches the surface. Springs can occur in different geological settings, forming valley springs, stratum springs or solution channel springs.

  2. The water in a river originates from overland flow, from interflow and from baseflow. Baseflow forms a higher proportion of river water in summer than in winter, and in rivers flowing over good aquifers.

  3. River discharg
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3.5 Big dams in the future?

Throughout the 20th century, reservoir construction to improve water resources was considered a key component of development. It was undertaken universally by industrialized nations, and by the later 20th century, increasingly by developing nations, building bigger and bigger dams. More recently the debate over the environmental impact of big dams and their questionable cost-benefit analysis has been universally recognized. In the 1990s the Narmada scheme caused the World Bank to rethink its
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3.4.6 Soil salinization

The change from annual flooding by a river to perennial irrigation that can be provided from a reservoir can cause soil salinization, if salts normally present in the river water accumulate in the soil as the water evaporates. These salts were previously washed away by the flooding, but the reduced supply of water by irrigation leaves them in the soil. The water is taken in by plants, or evaporated by the sun, leaving the salts behind. This causes a decline in crop yields until eventually the
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3.4.5 Sediment loss to agriculture

The trapping of sediment behind dams may also affect agriculture. The Nile Valley, for example, used to flood naturally once a year, and the sediment in the waters was deposited on the land, forming a fertile soil. These floodwaters and the sediment they carry are now trapped behind the Aswan Dam, and artificial fertilizers must be used down river in the valley. Without the yearly supply of sediment in floodwater, the banks of the Nile are eroding downstream of the dam, and the Nile delta is
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3.4.2 Ecological changes

Creation of a reservoir produces ecological changes not only to the area of the reservoir itself, by destroying the natural vegetation, but also upstream and downstream of the reservoir. The gradient of a river upstream of a reservoir may be reduced, so the water will slow down, changing the character of the river, causing deposition of sediment, and changes to the natural vegetation and animal life. Downstream of the reservoir the discharge will change, as well as the sediment load, also aff
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