Early development

Questions for Unit

Question 1 (Objective 2)

Figure 19 is a graph showing how the viscosity (thickness and stickiness) of a woman's cervical mucus changes with time. Day 0 is the start of her menstrual period.

  1. Can you suggest when ovulation might be occurring?
  2. What would be the effect on mucus viscosity if the woman took a daily dose of progestogen?

Figure 19, Relationship between viscosity of cervical mucus and time over the course of one menstrual cycle. M=menstruation.

Answer

  1. This woman is probably ovulating around days 12–16 of her cycle. At this time the cervical mucus is of low viscosity. This would allow easier entry by sperm, and fertilization might occur if sperm were present when an egg was ovulated.
  2. Progestogen increases the viscosity of cervical mucus. Therefore if it were taken every day, the drop in viscosity would not occur (or at least not to the same extent). Thus it might prove impossible for sperm to pass through the cervix, and fertilization would not occur. Furthermore, the persistent high level of progestogen would prevent the wall of the uterus from becoming receptive to an implanting embryo.

Question 2 (Objective 3)

State two main differences in the meiotic divisions that take place in males and females.

Answer

  1. In females, mciosis starts before birth, then stops until puberty, and from then on until the menopause a few divisions are completed each month, resulting in the production of (usually) just one egg. In males, all the stages of meiosis occur after puberty, and the process then carries on continuously during adulthood, resulting in a constant and copious supply of sperm (millions per day).
  2. In females, of the four products of meiosis only one will become an egg. In males, all of the meiotic products can become sperm.

Question 3 (Objectives 4 and 5)

Describe the first four divisions undergone by a fertilized egg. In what way are the first three different from the fourth?

Answer

The first four divisions are cleavage divisions, taking place without net cell growth. The first three (1 to 2, 2 to 4, and 4 to 8 cells) all involve symmetrical divisions across the centre of the cells, yielding daughter cells of equal sizes. But the fourth, 8- to 16-cell, division is asymmetric, yielding two populations of cells, one made up of large cells (which will give rise to the trophoblast), and the other of smaller cells (which will give rise to the inner cell mass).

Question 4 (Objective 5)

A sponge is a simple animal consisting of only two cell types, one of which encloses the other. If the cells are separated by passing the sponge through a sieve, and thoroughly mixed, in time they will reaggregate and the two cell types will resume their correct respective positions. Suggest how this may happen.

Answer

By differential adhesion. The sponge cell membranes carry molecules which can recognize and stick to some surface molecules but not others. Thus the inner cells will stick tightly to each other, and less tightly to the outer cells. These can also stick to each other, but more loosely still. Over time the tightly stuck cells will find themselves in the centre of the mass, and the more loosely stuck cells will surround them.

Question 5 (Objectives 3 and 6)

Is it correct to say that an ejaculate consists of two populations of sperm? Is this important?

Answer

Yes – the two populations arc X-bearing sperm and Y-bcaring sperm. This relates to the sex chromosome they receive at meiosis. There are equal numbers of X- and Y-bearing sperm, so equal chances of getting male or female embryos. The X chromosome is larger and therefore heavier than the Y chromosome, so it was suggested that this might be a basis for separating the populations of sperm, for use in selecting the sex of an embryo in IVF or GIFT. But the random swimming paths taken by sperm largely negate any effects the weight difference might have.

Question 6 (Objective 6)

The Roman Catholic Church is currently strongly opposed to abortion, yet 200 years ago it was not. Why has this change come about?

Answer

It used to be thought that the soul did not enter the fetus until its movements could be felt (quickening). Before this, the fetus was not considered to be alive, so could be aborted without sin. But in 1869 Pope Pius VI decreed that the soul entered the embryo at fertilization. This meant that the embryo was alive from this moment onwards, so abortion was seen to be taking a life.

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