OOGENESIS: PHASES, CHARACTERISTICS IN ANIMALS AND PLANTS - BIOLOGY - 2024

The oogenesis or gamete is the development process of the female gamete in animals and flowering plants (a "mature egg" occurs in animals and a "megagametofito" in plants). This event occurs once female individuals reach maturity, thus beginning their reproductive cycle.

In women, oogenesis begins in the prenatal period, where the oogonia multiplies through mitotic divisions. The oogonia thus produced enlarge to form the primary oocytes before the birth of the fetus and, finally, it is at puberty in women that mature ovules develop.

Oogenesis process in humans and other animals (Source: Henry Vandyke Carter via Wikimedia Commons)

The development of primary oocytes is regulated by two hormones of the pituitary: follicle-stimulating and luteinizing, and these, in turn, are regulated by gonadotropin-releasing hormone that is excreted in the hypothalamus.

In most cases, when the egg is not fertilized, it is eliminated from the body through bleeding from the female genital organ of animals. This event is called "menstruation", menstrual cycle or heat, among others.

In flowering plants or angiosperms, the megagametophyte (female gamete) and the microgametophyte (male gamete) in addition to developing in the same plant, also develop in the same structure, which is a flower with bisexual characteristics.

The stamens of a flower produce the microgametophyte, while the carpels produce the megagametophyte. However, some plants have flowers only with stamens and other flowers only with carpels, and these species are known as monoecious.

In plants, female gametogenesis comprises two main processes known as megasporogenesis and megagametogenesis, which have to do with the formation of the megaspore within the nucela and with the development of the megaspore to become a megagametophyte, respectively.

Oogenesis in animals

Oogenesis, itself, is the production of ovules and occurs in the ovaries of female mammalian animals. A part of the ovaries is formed by the ovarian follicles, since the ovule primordia are fused to these until they mature.

When juvenile female mammals reach puberty, the ovaries enter an active phase characterized by the growth and cyclical maturation of small groups of follicles.

The common thing is that in each cycle a single primary follicle reaches full maturity and the oocyte is released from the ovary to the uterus. It has been counted that of the 400 thousand oocytes that a woman presents at birth, only 400 mature during the fertile period.

This maturation process from the primary follicles to the end of the mature ovum is known as "folliculogenesis", and involves different steps of division and differentiation for the follicular cells before transforming into a mature ovum.

Gametogenesis occurs continuously in female mammals until the permanent cessation of the menstrual cycle, a period known as "menopause" in humans.

Scientists estimate that the ideal age for human reproduction is between 20 and 35 years of age, since in this period the ovules develop with full viability and the probability of chromosomal abnormalities in an embryo increases as women they grow old.

- Characteristics

- Female eggs are formed during embryonic development, new egg primordia do not originate after birth.

- The mature ovum is detached from the ovary and goes to the uterus, where it is maintained until fertilization by a male gamete.

Electron micrograph of an ovum, the female gamete (Source: TheBloxter446 via Wikimedia Commons)

- At the end of each fertility cycle, the eggs that are not fertilized are discarded and expelled through bleeding known as “menstruation”.

- All the steps of oogenesis take place inside the ovaries.

- During female gametogenesis, three polar bodies are created that are not viable or fertile.

- In the first meiotic process, the cell cytosol is not divided equally, one of the resulting cells is left with most of the cytoplasmic volume and the others are considerably smaller.

- Phases

Prenatal development

During the early stages of development of the female embryo, cells known as oogonia multiply by mitosis. The oogonia, product of the mitotic process, grow in size to originate the primary oocytes before birth.

During the development of primary oocytes, the surrounding connective tissue cells form a single layer of flat follicular cells. The primary oocyte enclosed by this layer of cells constitutes a primordial follicle.

At puberty, the primary oocyte enlarges, the follicular epithelial cells change to a cubic and later columnar shape, and their fusion gives rise to a primary follicle.

The primary oocyte is surrounded by a covering of an amorphous, acellular, glycoprotein-rich material known as the “zona pellucida”. This has a mesh shape with many “fenestrations”.

Primary oocytes begin to divide by meiosis before the birth of the fetus. However, the completion of prophase does not occur until the individual reaches puberty.

Postnatal development

After puberty begins, ovulation occurs each month. This means that the release of an oocyte occurs from the ovarian follicle to the uterus.

The primary oocytes that were suspended in the prophase of the first meiotic cycle are activated during this period and, as the follicle matures, the primary oocyte completes the first meiotic division to give rise to a secondary oocyte and a first polar body.

In this first meiosis, cytoplasmic division is uneven, the resulting secondary oocyte receives almost all of the cell's cytoplasm, while the polar body receives very little cytoplasm.

During ovulation, the nucleus of the secondary oocyte begins the second meiotic division until metaphase, where cell division stops. If at that time a sperm enters the secondary oocyte, the second meiotic division is completed.

After this second meiotic division, a cell with high cytoplasmic content (the fertilized secondary oocyte) and another smaller cell, which represents the second polar body, is formed again, which ends up degenerating. The maturation of the oocyte ends with the degeneration of the two polar bodies as a result of division.

Oogenesis in plants

In flowering plants, the synthesis of megagametophytes takes place inside the flower, in a structure called the ovary. The ovaries are found inside the carpels, each carpel is made up of an ovary, a style and a stigma.

The set of carpels of a flower is called "gynoecium" and these can be united or separated inside the flower, depending on the species.

Inside the ovaries, one or multiple ovules can be found. The shape, the number of carpels and the number of ovules and their arrangement vary with the species, so much so that these characteristics are used as taxonomic characters for classification.

In plants, each ovule is a very complex structure, it is made up of a foot called a funiculus, which holds the entire nucela inside. The nucela, in turn, is surrounded by one or two layers called integuments (the number of integuments varies depending on the species).

The integuments meet at one end, leaving a small opening called a micropyle. The micropyle is the space through which the pollen tube passes through to fertilize the egg cell.

Inside the nucela is where the synthesis process of megagametophytes takes place.

The megagametophyte is also called an embryo sac, since the embryo develops inside it once fertilization occurs.

- Characteristics

- The ovocell or female gamete in plants comprises eight different cells, 7 form the embryo sac and one the ovocell, oosphere or female gamete itself.

- The ovary in most plants contains several ovules, which can be fertilized during the same fertilization event.

- The ovules can be “self pollinated”, that is, the pollen of the same flower in which the ovule and anthers are found can fertilize the ovules inside the carpel.

- Within the egg cells there are two polar nuclei that fuse to create the endosperm, which is the substance that the embryo feeds on during the first stages of its development.

- The megaspore divides three times in a mitotic way, originating an embryo sac with 8 nuclei.

- There are cells that are housed at the ends of the nucela, they are known as the synergists and antipodes.

- Phases

In principle, a single female gamete or megasporocyte develops inside the nucela. Within this structure, a diploid stem cell of the megasporocytes undergoes meiosis (meiosis I) and forms four haploid cells, called megaspores.

The four megaspores are arranged in a linear fashion. In theory, at this point megasporogenesis is complete; three of the megaspores eventually disintegrate and only one survives to mature and transform into the megagametophyte.

However, in most flowering plants, the developing megagametophyte begins to feed on the nucela and divides mitotically (mitosis I), resulting in two new nuclei.

Each of the two new nuclei divides mitotically once more (mitosis II) to give rise to four new nuclei. Eventually the resulting four nuclei divide again by mitosis (mitosis III), forming eight nuclei.

The eight nuclei are divided into two groups of four nuclei, one is located at the end of the micropyle, while the other is located at the opposite end. One nucleus from each group of the four migrates towards the center of the megagametophyte, giving rise to the polar nuclei.

The three remaining cells at the micropillar end are the synergists and those at the opposite end are the antipodes. The synergids will be part of the fertilization process once the flower is pollinated.

The entire structure of the mature female gamete is called the embryo sac and is built by the central binucleate cell and the six nuclei that make up the synergistic and antipode cells.

References

1. Desai, N., Ludgin, J., Sharma, R., Anirudh, RK, & Agarwal, A. (2017). Female and male gametogenesis. In Clinical reproductive medicine and surgery (pp. 19-45). Springer, Cham.
2. Evans, HM, & Swezy, O. (1932). Ovogenesis and the normal follicular cycle in adult mammalia. California and western medicine, 36 (1), 60.
3. Lindorf, H., De Parisca, L., & Rodríguez, P. (1985). Botany Classification, structure and reproduction.
4. Moore, KL, Persaud, TVN, & Torchia, MG (2018). The Developing Human-E-Book: Clinically Oriented Embryology. Elsevier Health Sciences.
5. Raven, PH, Evert, RF, & Eichhorn, SE (2005). Biology of plants. Macmillan.
6. Wang, JH, Li, Y., Deng, SL, Liu, YX, Lian, ZX, & Yu, K. (2019). Recent Research Advances in Mitosis during Mammalian Gametogenesis. Cells, 8 (6), 567.