• In both spermatogenesis and oogenesis, a haploid nucleus is formed in preparation for fertilization.
  
• However, a major task of spermatogenesis is to prepare the cell for motility during fertilization whereas a prime objective of oogenesis is to construct a relatively large cell with enough resources in the form of mRNAs, organelles, and metabolic substrates for construction of the embryo before it can make its own resources or obtain them from its environment.
• Spermatogenesis also differs from oogenesis in that the mechanisms of oogenesis vary more between species than for spermatogenesis. For example, in most mammals only a few eggs are produced during an individual's lifetime whereas in other species such as frogs or sea urchins, thousands of eggs can be produced on a routine basis. In the species that produce thousands of eggs, the oogonia are self-renewing stem cells that proliferate throughout the lifespan of the organism. In species that produce fewer eggs, the oogonia divide to form a limited number of egg precursor cells.

In the human ovary the oogonia rapidly divide up to the 6th or 7th month of gestation to form about 7 million germ cells after which time the number of germ cells decreases rapidly.

• The remaining cells enter prophase of the first meiotic division at this time.
• The primary oocytes that are produced progress through the first prophase part of meiosis but the signal to resume meiosis is not given until puberty.
• Overall, only about 400 oocytes mature during a woman's lifetime from the millions of primary oocytes present at birth.

The process of meiosis also differs between spermatogenesis and oogenesis.

• During oogenesis, the primordial germ cell produces an oogonium with a diploid nucleus.
• Prophase of meiosis I ensues and the primary oocyte is formed.
• At this point however, the cell divides but most of the cytoplasm goes to one of the cells which is the secondary oocyte.
• The other cell produced from the first stage of meiosis is very small and called the first polar body.
• The secondary oocyte then undergoes the second meiotic division and produces a second polar body and a mature ovum.
• After ovulation, the cell migrates through the fallopian tubes for fertilization.
• During spermatogenesis at the end of the two meiotic divisions, there are four haploid sperm produced but after oogenesis only one haploid fertilized egg is produced.
• The reason for the differences is that during the first metaphase in oogenesis, the spindle of the primary oocyte migrates to the periphery of the cell allowing almost all of the cytoplasm to go to one cell.
• Thus, oogenesis serves its task of producing a haploid cell which is prepared for fertilization and has sufficient cytoplasm to support development of the zygote.

Amphibians:

During the prophase I stage of meiosis in amphibians the oocyte cytoplasm accumulates cellular components that are needed for early embryogenesis.

• In amphibians the prophase I meiotic stage is subdivided into previtellogenic and vitellogenic or yolk-forming phases.
• The stem cell oogonial population in fishes and amphibians can generate new oocytes each year.
• The yolk material of the eggs consists largely of a nutrient-rich substance which is referred to as vitellogenin.
• Vitellogenin is synthesized in the liver and carried by the bloodstream to the ovary where it breaks down into compounds such as carbohydrates and lipids as sources of energy for the cell.
• The oocyte in prophase (diplotene stage) is also very active in RNA synthesis for translation to proteins needed for growth. Certain chromosomes stretch out large loops of DNA which are referred to as lampbrush chromosomes. These lampbrush chromosomes are very active in RNA synthesis for synthesizing the nutrients of the oocyte.
• Amphibian oocytes can remain for years in meiotic prophase until progesterone is secreted by the follicle cells in response to the gonadotropin hormones secreted by the pituitary gland. Meiotic division ensues and the mature ovum is released in its second meiotic metaphase from the ovary through a process called ovulation.
• Fertilization of the egg then enables the cell to complete meiosis to form a haploid genome.

Insects:

There are many types of oogenesis in insects, but probably the best studied is meroistic oogenesis that occurs in the fruit fly or Drosophila.

• In Drosophila, each oogonium divides 4 times to produce a clone of 16 cells which are connected to each other. Only those two cells having four interconnections are capable of developing into oocytes. Only one of these two cells develops into an egg.
• Thus, only one of 16 cells can become an oocyte. All of the other cells become what is referred to as nurse cells which nourish and protect the growing oocyte.
• The cytoskeleton consisting of microtubles controls the movement of organelles and RNAs between nurse cells and the oocyte during development.
• In fact, unlike amphibian oogenesis, most of the RNA is made in the nurse cells and transferred to the ooctye.

Mammals:

There are two basic patterns of ovulation in mammals; stimulated ovulation and periodic ovulation.

• In some mammals such as rabbits, the sexual activity results in physical stimulation of the cervix which triggers the release of gonadotropins from the pituitary.
• The gonadotropins signal the resumption of meiosis in the ovum which eventually results in ovulation.
• This process assures that most mating in these mammals leads to fertilization and is considered a highly successful type of replication in mammals.

Most mammals have a periodic type of ovulation.

In many mammals, the female ovulates only at specific times of the year which is called esterus.

• Esterus is usually caused by environmental cues such as the amount of light during the day.
• Gonadotropin-releasing factor is released from the hypothalamus which stimulates the pituitary to release gonadotropin.
• The gonadotropins are FSH or follicle-stimulating hormone and LH or leuteinizing hormone. The gonadotropins also stimulate ovulation so that esterus and ovulation occur at about the same time which increases the likelihood of fertilization.
• Thus, the pituitary, hypothalamus, and ovary all participate in oogenesis.

Humans and other primates also have periodic ovulation but lack esterus.

• They instead have a cyclic ovulation averaging about 30 days which is referred to as the menstrual cycle.
• Most of the oocytes in the adult human ovary are maintained in the first meiotic prophase.
• Each oocyte is enveloped in a primordial follicle consisting of a single layer of granulosa cells and a layer of thecal cells.
• Some primordial follicles enter a stage of follicular growth during which the oocyte greatly increases in volume.
• The fully grown follicle or Graffian follicle contains a large oocyte surrounded by several layers of granulosa cells collectively called the cumulus.
• An antrum or cavity also forms which becomes filled with fluids that provide nutrients to the oocyte.
• Between the oocyte and the granulosa cells is the acellular zona pellucida which aids in transfer of nutrients from the antrum to the oocyte.
• The layer of granulosa cells immediately surrounding the zona pellucida and the oocyte is the corona radiata.
• Over 99% of all Graffian gollicles die and the rest reach maturity and ovulate.

The first day of the menstrual cycle is when uterine bleeding begins.

• This bleeding is due to sloughing off of uterine tissue and blood vessels that would have aided the implantation of the blastocyst. This is the menstrual flow phase.
• Also at this time the blood levels of the ovarian hormones (estrogen and progesterone) and gonadotropins (FSH and LH) are relatively low.
• During the proliferative phase of the uterine cycle, the ovary secretes increased estrogen and FSH and LH secretion from the pituitary also increase.
• The oocyte begins meiotic divisions and ovulation begins.
• Following ovulation the uterine secretory phase begins and the oocytes enter a leuteal phase as progesterone is secreted from the corpus luteum or the remaining cells of the ruptured follicle.
• If the ovum is not fertilized, the corpus luteum degenerates, progesterone levels decrease, and the uterine wall is sloughed off and the cycle is renewed again to begin another 30 days of preparing the egg, uterus, and female body for fertilization