Video Transcript
In which of the following stages
can cells be found in a healthy infant ovary? (A) Polar bodies, (B) primary germ
cells, (C) primary oocytes, (D) secondary oocytes, or (E) oogonia.
This question concerns the
production of mature egg cells in biological females, which occurs in the
ovaries. And it provides us with answer
choices that describe several different cells that would be present in the ovaries
in the various stages of this process.
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’ ovaries is called oogenesis. And it starts before the female has
even been born while she is still an embryo. It can be divided into three key
phases: multiplication, growth, and maturation. Let’s take a look at each of the
different phases and the cells they produce to work out which cells will be found in
a healthy infant ovary.
Let’s begin with the multiplication
phase. When a biological female is an
embryo and then 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 germ cells. Primary germ cells are diploid
cells. This means that they have two sets
of chromosomes, like most other body cells, which is often represented as 2n. Primary germ cells are those that
eventually give rise to mature gametes in both males and females.
Human diploid cells have 46
chromosomes in total. But for simplicity, let’s represent
the chromosome number in diploid cells as just four chromosomes in this diagram. 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 that
follows the multiplication phase 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. A haploid cell is often represented
as n, and in humans, each contains 23 chromosomes in total. Meiosis converts a diploid cell
into a haploid cell through two cellular divisions: meiosis I and meiosis II.
The primary oocytes begin meiosis
I, but this is halted 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 in
their follicles until puberty begins, approximately 12 years later. Then, the maturation phase can
occur, during which a primary oocyte will undergo 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.
It’s important to note that meiosis
only completes when the haploid secondary oocyte is both released from its ovary and
successfully fertilized by a haploid sperm cell, the male gamete.
Fertilization 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. This also produces another polar
body, and the first polar body divides in meiosis II as well to form two more polar
bodies. 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.
This means that when the female is
an infant, her developing egg cells will have undergone both the multiplication and
growth phases, and a large number of primary oocytes will most likely be present in
her ovaries. There are unlikely to be any
primary germ cells or oogonia present in a healthy infant ovary, as they will have
all been converted into primary oocytes by the time she has been born. So options (B) and (E) are
incorrect.
Remember, meiosis I halts in
prophase just before the female is born. So the primary oocytes formed in
the growth phase will not develop into secondary oocytes or polar bodies until
approximately 12 years later when she reaches puberty. Therefore, options (A) and (D) must
also be incorrect, as these cells will not be present in an infant’s ovary who has
not yet reached puberty.
This means we can confirm the cells
that can be found in a healthy infant ovary and as a result the correct answer to
this question. The best answer to this question is
(C), primary oocytes.
