Video Transcript
How many eggs, ova, are produced
from each cell that begins meiosis?
Egg cells, which are otherwise
known as ova, are produced and developed via a process called oogenesis. Let’s walk through the steps of
oogenesis to determine how many egg cells it produces.
The process of oogenesis begins
before birth and then pauses until puberty when the process resumes. In a biological female embryo,
primordial germ cells are formed. They are diploid cells, which is
often represented as 2n, and each contains 46 chromosomes, 23 from the biological
father and 23 from the biological mother. Before birth, the primordial germ
cells divide by mitosis to form a large number of oogonia. Each oogonium then enlarges to form
a primary oocyte. The primary oocyte is surrounded by
a follicle, which becomes rich with nutrients. Primary oocytes then remain dormant
in the ovaries until puberty begins.
Once the biological female has
begun her menstrual cycle, the primary oocytes begin their maturation phase. During this phase, the primary
oocyte undergoes meiosis I and is converted into the secondary oocyte. Half of the primary oocyte’s
genetic material goes into the secondary oocyte, while the other half goes into a
much smaller nonfunctional cell called a polar body. For this reason, the secondary
oocyte and polar body both contain only 23 chromosomes and are described as haploid
cells, which is often represented as n.
Meiosis II is completed upon
fertilization of the secondary oocyte by a sperm cell. At this stage, two cells are
produced: the haploid mature ovum and another polar body that will not become an
ovum. The other polar body that was
formed in meiosis I also divides into two more polar bodies, and these three
nonfunctional cells tend to degrade. So, although meiosis produces four
cells in total, only one of these is an egg cell. Therefore, at the end of oogenesis,
from each primary oocyte that enters meiosis, the number of egg cells produced is
one egg.
