Video Transcript
Which process in egg development
occurs in the ovary of a female embryo? (A) Multiplication and growth, (B)
multiplication and maturation, (C) maturation only, (D) multiplication only, or (E)
growth and maturation.
Egg cells, which are sometimes
called ova, or a singular ovum, are the gametes, or sex cells, of biological
females. The development of mature ova in
biological females is called oogenesis, and it occurs within the ovaries. Oogenesis starts before the female
has even been born while she is still an embryo and continues during fetal life. It can be divided into three key
phases: multiplication, growth, and maturation. Let’s take a look at each of the
different phases to work out which occur while the female is still an embryo,
starting with the multiplication phase.
When a biological female is an
embryo, or more specifically a fetus, between eight to 20 weeks old, lots of her
cells are dividing and multiplying by mitosis. At this stage, the immature ova are
called primary ova, primary germ cells, or primordial germ cells. Primary germ cells are diploid
cells. You may recall that a diploid cell
is often represented as 2n. This means that they have two sets
of chromosomes like most other body cells, which is 46 chromosomes in total in
humans. For simplicity, let’s represent the
chromosome number in these diploid primary germ cells as just four chromosomes in
this diagram.
Primary germ cells are those that
eventually give rise to mature gametes in both males and females. During the multiplication phase in
biological females, primary germ cells are converted into oogonia, which are also
diploid. The multiplication phase continues
until a few weeks before birth.
The next phase of oogenesis is
called the growth phase, which also occurs in the ovaries before the female is
born. Each oogonium increases in size to
become a primary oocyte in the growth phase. And the follicle that contains the
oocyte becomes enriched with nutrients, such as proteins and hormones. During this phase, the primary
oocytes also replicate their chromosomes to be ready for meiosis.
You may recall that meiosis halves
the number of chromosomes in a diploid cell to form four haploid cells with half the
number of chromosomes of most other body cells, which is often represented as n. Haploid cells in humans contain 23
chromosomes in total. It achieves this through two
cellular divisions: meiosis I and meiosis II.
The primary oocytes begin meiosis
I, but this is halted, or arrested, during prophase, early in the process. As the primary oocytes have not
completed meiosis I yet, they are also diploid cells. Primary oocytes remain dormant
until puberty begins around 12 years later. Then, the maturation phase can
occur, during which a primary oocyte undergoes most of the other phases of meiosis
and is eventually converted into a haploid cell called a secondary oocyte and a
smaller haploid cell called a polar body.
Meiosis only completes when the
haploid secondary oocyte is both released from the ovaries in ovulation and
successfully fertilized by a haploid sperm cell, the male gamete. This forms a mature, fertilized
ovum, which is a diploid cell, as it contains genetic material from both the sperm
cell and the ovum and can now be referred to as a zygote.
It’s worthwhile noting that meiosis
II also produces three haploid polar bodies, one from the splitting of the secondary
oocyte and two from the first polar body dividing. All three of these polar bodies
eventually degrade.
The maturation phase is sometimes
referred to as a postnatal phase of oogenesis, as it occurs after the female is born
and the word “postnatal” literally means after birth. In contrast, the multiplication and
growth phases are sometimes referred to as prenatal, which means before birth, and
they occur before the female is born. The fact that the maturation phase
only starts during puberty gives us a clue to our answer, which asks us to identify
the processes in egg development that occur in the ovary of a female embryo, before
birth.
The only stages that occur before
birth in a biological female are the multiplication phase and the growth phase. Therefore, the correct answer must
be option (A), multiplication and growth.
