This figure shows the process of spermatogenesis. The panel on the left indicates the formation of spermatozoa through meiosis. The micrograph on the right shows a section of the testis where spermatogenesis occurs. Image credit: Openstax, Anatomy and Physiology. Distributed under the Creative Commons Attribution 3.0 Unported license.

The egg cell

Egg cells are among the largest cells. They are produced in the ovaries of females - diploid oocytes undergo meiotic divisions to produce haploid ova (singular: ovum). However, unlike spermatogenesis, oogenesis is more complex. During early fetal development, oogonia cells begin to develop into primary oocytes. Each oocyte is surrounded by cells called the granulosa. Together, the oocyte and the surrounding granulosa cells are referred to as a follicle. The early follicle is called the primary follicle. The primary oocyte (i.e., the egg cell within the primary follicle) begins meiosis I but does not complete it (this is referred to as the prophase arrest). Each female baby has, on average, two million primary oocytes in each ovary at birth. Many of these primary oocytes will die off during early development: at the start of puberty, each ovary will contain about 300,000 to 400,000 primary follicles. One primary follicle will be recruited for maturation at each menstrual cycle from puberty onwards, under the influence of Leutinising Hormone (LH, secreted by the anterior pituitary gland). The primary oocyte will complete meiosis I and begin meiosis 2 during maturation. However, it will not complete meiosis II but stop at metaphase 2 (metaphase arrest). At this stage, the follicle is referred to as a secondary follicle. The oocyte then becomes detached from the rest of the follicle and is surrounded by a group of cells called the corona radiata. Under the influence of the LH hormone (LH surge), the secondary follicle will rupture, releasing the ovum (egg) into the Fallopian tube. This process is ovulation. The remains of the follicle become another structure in the ovary - the corpus luteum. The corpus luteum becomes an endocrine tissue and secretes the hormone progesterone, preparing the endometrium for implantation.

This diagram shows the main stages of oogenesis, ovulation and post-ovulation development. The descriptions of these processes are indicated in the text. Image credit: Alexandra Garcia. Distributed under the Creative Commons Attribution-Share Alike 4.0 International license.

Fertilisation

Fertilisation is the union of gametes to produce a zygote. The basic processes of fertilisation are common to sexually reproducing animals. As indicated above, sperm cells move chemotactically towards egg cells. When they encounter the egg cells, they initially attach loosely to the corona radiata (cells surrounding the oocyte). Next, the sperm cells will push through this layer and adhere to the zona pellucida. The zona pellucida contains various transmembrane proteins, collectively called ZP proteins. Proteins on the sperm cell surface bind to the ZP proteins (specifically the ZP3 protein). ZP3 is highly species-specific, so it is impossible for gametes from different species to fertilise each other). The attachment to the ZP3 protein triggers the acrosome reaction in the sperm cell: this causes hydrolytic enzymes to be released from the sperm head. The enzymes digest the zona pellucida layer near the attached sperm. Once the zona pellucida has been breached, the sperm cell membrane fuses with the egg cell's plasma membrane. This causes the sperm cell's nucleus to be released into the egg cell's cytoplasm. After that, the sperm nucleus will fuse with the egg nucleus, thus creating a diploid cell. This completes fertilisation, and the resultant cell is called a zygote.

Of the many sperm cells that initially attach to the corona radiata, only one will successfully attach to a ZP3 protein and initiate the acrosome reaction. The acrosome reaction also initiates a cortical reaction in the egg, where the ovum retracts from the zona pellucida. This prevents other sperm cells from fertilising it (i.e. preventing polyspermy).

Diagram showing the process of fertilisation. 1. Initial attachment of sperm to the corona radiata. 2. Penetration of the corona radiata and attachment to the zona pellucida. 3. a closeup of the attachment to the zona pellucida, followed by the acrosome reaction and the entry of the sperm nucleus into the egg cell's cytoplasm. Image credit: OpenStax Anatomy and Physiology. Distributed under the Creative Commons Attribution 3.0 Unported license.

An electron micrograph of a sperm cell attached to an egg cell. Image credit: This image is in the public domain.

Further thoughts on fertilisation

In many species, male and female gametes differ widely in shape and structure (as discussed above). Furthermore, they differ in size, and females produce fewer gametes than males. The differences between male and female gametes are referred to as anisogamy. Scientists think anisogamy evolved because males and females faced different selective pressures to increase their reproductive success. Males seek to maximise their number of offspring by producing small gametes in large quantities. However, females produce fewer, larger gametes. This is because the egg carries all the nutrients and developmental factors the embryo needs during the early stages of life. Both strategies are directed at increasing the chances of survival of the progeny.

Because of genetic changes that occur during meiosis, not all sperm cells from a male organism are genetically identical. Since only one of many millions of sperm cells will fertilise an egg cell and pass on its genetic legacy to the progeny, scientists believe that sperm cells compete with each other. This is referred to as sperm competition. Sperm competition may have evolved due to non-monogamous mating (where a female may be impregnated by multiple males - polygyny and polygynandry). Sperm competition is a form of sexual selection, a potent driver of evolutionary change in species.

Locating fertilisation

Now that we have explored fertilisation, we will look at where it occurs in sexually reproducing organisms. Biologically, fertilisation is internal if it occurs within an individual's body. Otherwise, it is external. Both refer to fertilisation in the natural setting: if fertilisation is conducted in a laboratory, it is called in vitro fertilisation.

Internal fertilisation

In internal fertilisation, gametes are released into the reproductive tissues of the female parent. Therefore, the female parent possesses reproductive structures that produce eggs (ovaries) and allow for the transfer of the eggs to the sites of fertilisation (oviduct or Fallopian tube). If the embryo develops internally within the parent, then there will be reproductive tissues that support it (uterus). The male parent contains reproductive tissues that produce sperm (testis) and various structures for delivering the sperm into a female mate's reproductive system.

In species that perform internal fertilisation, there are variations in the way the embryos develop:

• Oviparity: After fertilisation, females lay the eggs containing the undeveloped embryo. The embryo develops externally within the egg and will hatch once it attains developmental maturity.

• Viviparity: In viviparity, the embryo develops within the parent. Live young progeny are delivered at birth.

• Ovoviviparity: In this mode of reproduction, embryos develop within eggs, but those eggs remain within the parent until birth.

Advantages of internal fertilisation

1. Mate choice: Females generally can choose the best mate for copulation and progeny production. Elaborate mating rituals are common in species where internal fertilisation is the norm.

2. Protection: both gametes and the developing embryos are protected from environmental factors, such as heat and water availability (dehydration). Also, in many species, the young progeny are protected from predation (until they have attained developmental maturity).

Disadvantages of internal fertilisation

1. Carrying the undeveloped embryo within her body can be dangerous for the female parent. She must provide nutrients to the developing embryo (energy costs), and the growing embryo (during gestation) can cause physiological and other damage to her.

2. Only a few offspring are produced at each reproductive cycle, which can significantly impact population sizes when conditions fluctuate.

External fertilisation

In this mode of reproduction, male and female parents release their gametes into the environment where fertilisation occurs. Once the gametes are released, the parents have little control over how and if fertilisation will occur. Fertilisation success depends on favourable environmental factors that allow gametes to contact each other. Therefore, many gametes are produced in species that employ external fertilisation.

Broadcast spawning

Sessile aquatic invertebrates largely carry out broadcast spawning. The males and females of these species release their gametes into the water column. Water currents transport the gametes some distance away from the parents. Fertilisation occurs in the water column, and after early development, the embryos settle on some surface to continue their development into their juvenile and adult forms. For this method of reproduction to be successful, male and female animals must release their gametes at the same time. Various environmental variables may act as triggers. For example, some organisms spawn seasonally, while others may respond to changes in temperature, pH or environmental chemicals.

Cauliflower coral spawning, French Frigate Shoals. Image credit: USFWS - Pacific Region. Distributed under a Creative Commons licence (CC BY-NC 2.0).

Motile aquatic organisms engage in mating behaviours to enhance fertilisation success. For example, in amphibians, mating ensures that the males release their sperm directly onto the eggs as soon as the latter are laid in the water. Other species exhibit brooding behaviours to maximise the survival of the progeny after external fertilisation.

Advantages of external fertilisation

1. Species that reproduce through external fertilisation usually produce many gametes and, hence, potentially many offspring. However, many of those progeny will not survive to adulthood.

2. Mixing of the gene pool for externally fertilising species may occur to a greater extent than that for internally fertilising species. This is because of the large numbers of gametes released into the environment during breeding.

3. Because they use various abiotic and biotic media for gamete dispersal, these species may be better colonisers (that species that adopt internal fertilisation) and occupy larger habitats.

4. The female parent does not have to face the dangers of gestation.

Disadvantages of external fertilisation

1. Reproductive success is highly dependent on environmental and other factors (i.e. biotic and abiotic). Generally, fertilisation rates are low.

2. As early development occurs externally (relative to the parent organism), the chances of the immature young making adulthood are low.