Question
Question: What happens to the primary oocyte when the ovarian follicles mature?...
What happens to the primary oocyte when the ovarian follicles mature?
Solution
An oocyte is a female gametocyte that is involved in reproduction. It is produced during female gametogenesis in the ovary. The female germ cells produce a primordial germ cell which undergoes mitosis to form oogonia. The oogonia during oogenesis become primary oocytes. The primary oocytes are arrested at an early stage of meiosis-I i.e., prophase-I. They complete the cell division just the day before ovulation.
Complete answer:
In females, the total number of eggs produced in a lifetime are present in the newborn female. They are presently arrested at the diplotene stage of meiosis I until puberty. During puberty, increase in the luteinizing hormone (LH) stimulates the resumption of meiosis.
The arrested egg cells are called primary oocytes or primordial follicles. The primary oocyte is diploid in nature. After puberty, during each menstrual cycle, completion of meiosis-I occurs just the day before ovulation. This process is stimulated by pituitary gonadotropin hormone.
In meiosis-I, the diploid oocyte becomes two haploid daughter cells with 23 chromosomes. Here, each chromosome is made up of two chromatids. Out of two, one cell becomes the secondary oocyte and the other cell forms the polar body. The polar body eventually will die and deteriorate by apoptosis.
The secondary oocyte then undergoes meiosis-II but arrested at the metaphase stage. The oocyte develops into an ootid which is released from the follicle during ovulation. The ootid will not continue to develop further without fertilization. The meiosis-II completed only during fertilization.
Note:
During fertilization, the ootid attains its final stage of maturation and becomes an ovum. Ovum is the fully mature human egg cell. It combines with the sperm cell to form the zygote. Both the egg and the sperm contain 23 chromosomes each. These chromosomes fuse together to create a full set of chromosomes. The newly formed zygote will develop into an embryo and into a new born baby in nine months.