Oogenesis is the process in which the female reproductive system produces gametes from the primordial germ cells. The step of oogenesis occurs in both prenatal and postnatal development. The step of oogenesis up to the point of the primary oocyte formation is done prenatally. This means that the females are born with primary oocytes. These oocytes after puberty either form secondary oocyte in the case of fertilization or degenerates in the case of the menstrual cycle. As we know that oogenesis occurs in the follicles, therefore, oogenesis and folliculogenesis occur in conjugation interacting via reciprocal induction.
Stages of oogenesis
The oogenesis normally completes in five steps.
Stage1. In stage 1 the primordial germ cells undergo mitosis and produce two Oogonia. Oogonia are diploid cells. This stage occurs prenatally.
Stage 2. In this stage, the oogonia that are produced in stage 1 also undergo mitosis and produce two diploid primary oocytes. This stage also occurs prenatally.
Stage 3. In this stage, the primary oocytes that are formed at stage 2 undergo meiosis I replicating their DNA, but then they are arrested at the prophase of the first meiotic division. This stage also occurs prenatally.
Stage 4. In this stage, the block is removed from the oocytes by the luteinizing hormones surge and puberty. After the removal of the block, the primary oocytes complete the first meiotic division and produce a secondary oocyte and a polar body. The secondary oocyte that is formed as a result of the first meiotic division enters meiosis II and is arrested at the metaphase of meiosis II. The secondary oocyte is a diploid cell. The oocyte enlarges.
Ovulation occurs at this stage. This stage occurs after puberty.
Stage 5. The secondary oocyte complete meiosis II and transformed into the mature gamete called ova. Ova is a haploid cell. This stage occurs only when the egg is fertilized by the sperm means fertilization occurs.
Structure of the secondary oocyte
The secondary oocyte is a very large and diploid cell. It’s diameter ranges up to 100 micrometers. It is surrounded by the zona pellucida which in turn surrounded by the cells of the corona radiata. Space is present between the cell membrane and zona pellucida called perivitelline space.
What is corpus luteum?
After ovulation, the wall of the ruptured follicle that contains the granulosa and theca Interna cells collapses and gets transformed into a glandular structure called corpus luteum. The corpus luteum secretes progesterone and estrogen.
Under the influence of progesterone and estrogen, the endometrium of the uterus starts preparation for the implantation of the embryo.
Corpus luteum is a yellowish structure. If pregnancy does not occur then the corpus luteum degenerates and changed into fibrous tissue called corpus albicans. Corpus albicans is a white body. Corpus albicans is also called the corpus luteum if menstruation.
If the egg is fertilized the corpus luteum will stay for 3 to 4 months. Then it is called the corpus luteum of pregnancy. The progesterone secreted by the corpus luteum for initial 3-4 months maintains the pregnancy and thereafter the pregnancy is maintained by the progesterone secreted by the placenta.
The fate of ovarian follicles
1. In each ovarian cycle, a lot of ovarian follicles start to develop but only one gets to maturity.
2. The follicles that fail to reach maturity do not exist in the next ovarian cycle but they degenerate.
3. Once the follicle reaches maturity there are two possible pathways for them when they produce secondary oocyte.
a. The secondary oocyte may complete its second meiotic division and convert it into a fertilized egg.
b. The secondary oocyte does not complete their second meiotic division and degenerates.
What is ovulation?
Ovulation is the process in the prenatal development in which an egg is released from the female ovary. The Graafian follicles become enlarges and big that if form a bulge on the surface of the ovary. Eventually, the follicle rupture and ova are released from the ovary.
The expelled secondary oocyte is surrounded by the zona pellucida and the number of layers of follicular cells.
The secondary oocyte is picked up by the fimbriated end of the fallopian tube and placed into the lumen of the fallopian tube. The wall of the ruptured follicle with some of the granulosa cell collapses and converts into the corpus luteum.
If the ovum is not fertilized then the corpus luteum will last for 10-12 days and then degenerates and convert into a fibrous tissue called corpus albicans.
If the ovum is fertilized then the corpus luteum will last for 2 to 3 months and are called the corpus of pregnancy. The corpus of pregnancy secrets estrogen and progesterone that maintains the pregnancy. After 2 or 3 months the pregnancy is maintained by the progesterone secreted by the placenta.
The occurrence of an abnormal oocyte is rare when it is compared with sperms. There are many types of oocyte abnormalities some of them are
1. Binucleated or trinucleated oocyte. Although these oocytes may give rise to twins or triplets, they usually degenerate before reaching maturity.
2. Oocytes with irregular chromosomal contents. Irregular chromosomal contents in the oocytes occur due to nondisjunction of chromosomes in meiosis I or meiosis II. The oocytes with irregular chromosomal contents may contain twenty-four chromosomes or twenty-two chromosomes.
If an oocyte that contains twenty-four chromosomes is fertilized by a sperm that contains twenty-three chromosomes then a zygote with forty-seven chromosomes is produced (i.e, trisomy).
The trisomy 21 or Down, s syndrome is the most typical type of trisomy.
Likewise, if an ovum with twenty-two chromosomes is fertilized by a standard sperm that contains twenty-three chromosomes a zygote with forty-five chromosomes will be produced. (i.e, monotony).
An example of monosomy is Turner syndrome.
Summary of oogenesis
Oogenesis is the procedure of the formation of oocytes from the primordial germ cells. This process starts long before birth. Ovulation is the process of shedding an ovum from the ovary. After ovulation, the wall of the follicle that ruptured collapses and form a structure called corpus luteum. There are two types of corpus luteum that are corpus albicans and corpus luteum of pregnancy.