Animal Reproduction and Development

Fertilization

In sexual reproduction, fertilization must occur for new organisms to form.
In order for a new organism to be formed through sexual reproduction, the male sperm cell and female egg cell must join together. Fertilization, the union of sperm and egg, is the fusion of the two haploid (1n) gametes. Each of these has half of the number of chromosomes (a structure within the nucleus that contains DNA, the genetic material that is passed from one generation to the next) of the entire organism. When these haploid gametes join, they form a diploid (2n) zygote that will develop into an embryo. This has the entire complement of chromosomes the organism needs to be viable and develop. In humans, each sex cell has 23 chromosomes. When these two cells come together, the resulting zygote has 46 chromosomes. The zygote will divide, copying each of those chromosomes every time so that the adult human has 46 individual chromosomes in every one of his or her cells.

There are several steps that occur during fertilization:

Step 1. Sperm produce special chemicals that, once activated, allow them to gain entry through the membrane of the egg cell.

Step 2. The membranes of both the sperm and egg cells fuse together.

Step 3. Chemical reactions in the egg cell membrane produce a barrier that prevents other sperm cells from entering the outer membrane

Step 4. Entry of the sperm cell initiates the development of the zygote.

Step 5. The nuclei of the sperm and egg cells fuse to form the diploid nucleus of the zygote.

While millions of sperm are produced and could potentially come into contact with the egg, only one can fertilize it. Sperm compete with each other as they move toward the egg cell and can even attempt to block each other or thwart progress as they get close to the egg. Specific recognition molecules that identify the egg cell are present that guide the sperm towards the egg instead of other body cells. This function is particularly important to species that release their gametes into the environment with hopes of the sex cells finding each other. Sea urchins are an example of this because their eggs release a special attractant that draws the sperm to them. When the sperm detect this biochemical, they increase their energy levels and become much more active.
Externally fertilized egg cells (sea urchin) have additional mechanisms to ensure proper fertilization. The sperm-binding receptors recognize sperm cells from the correct species and allow it to penetrate the egg cell membrane. Internally fertilized eggs (mouse) allow the first sperm that contacts the egg cell membrane to penetrate it.

External Fertilization

Some organisms have eggs that are fertilized externally.
In many organisms, fertilization takes place outside of the female's body. Called external fertilization, it is quite common in aquatic environments. Spawning is the release of sperm and eggs into the environment. Organisms such as corals, fish, amphibians, and sea stars spawn. This practice does not occur at random. Many aquatic organisms that reproduce via external fertilization have reproductive cycles triggered by light intensity, phases of the moon, and even water temperature to best guarantee that the other gametes of the opposite sex will be present in the water at the same time. For example, trout spawn when water temperatures increase, while catfish rely on both changes in temperature and increased periods of daylight. Producing far more gametes than needed helps to ensure reproductive success. Although only one sperm fertilizes one egg, many sperm do not ever reach the egg. Sperm may be carried away in the current, encounter detrimental environment conditions, or succumb to other difficulties. Additionally, if fertilization does occur and new offspring are formed, most of them may be eaten by predators long before they are mature enough to find protection. Behavior of the parents also helps ensure that fertilization occurs. Many species, such as salamanders and salmon, travel for miles to a certain spawning location to meet up and mate with others of their species. When the males and females reach the same area, there is a coordinated release of sperm and egg.

Internal Fertilization

Some organisms have eggs that are fertilized internally.
Most animals reproduce via internal fertilization. Here, the sperm cells are transferred directly into the female through the use of reproductive structures, such as the penis and vagina. This method is very common in animals on land; however, marine mammals also rely on it. Animals have evolved a great diversity of structures and methods to transfer the sperm from the male to the female. Most animals have some form penis to transfer the sperm to the female's vagina. This is usually carried out through the process of copulation—the act of animal reproduction that results in sperm being transferred to a receptive female. There are many adaptations of these organs that exist in nature. For example, dogs have a specialized penile foreskin that locks it inside the female until the transfer of sperm is complete. Many insect species have specific shapes to their sex organs that only allow the males of one species to copulate with females of that same species. This prevents interbreeding from other species. There are even some species, such as the damselfly, in which the male has an adaptation on its penis that removes the sperm deposited into the female from a prior mate and replaces it with his own.

Animals with internal fertilization are classified by where their embryos develop. Oviparous is a reproductive strategy in which eggs are released by the female and the embryos develop inside the eggs outside the female's body. Insects, reptiles, and birds are animals that lay eggs outside their bodies. These eggs have some form of protection against the elements (such as drying out). The eggs have abundant amounts of nutrients within them to nourish the developing embryo. Viviparous animals keep the embryo within the mother's body during the early stages of development. Mammals and marsupials (animals with pouches) are viviparous. Female mammals have a special part of their reproductive tract that holds the embryo, called the uterus. Here, there is an exchange of blood, oxygen, and other nutrients between the mother and the embryo. In other animals, such as the garter snake, the fertilized eggs remain in the mother until they are ready to hatch. Ovoviviparous is the reproductive strategy in which fertilized eggs develop and hatch within the mother's body but are not attached to her. For example, basking sharks are able to nourish their young off the yolks of their eggs. This allows the young to develop within the mother to a point where they are better suited for survival in the outside environment.
Oviparous development, such as that which happens in birds, is a reproductive strategy in which eggs are released by the female and develop outside her body. Viviparous animals, such as mammals, keep the embryo within the mother's body during the early stages of development. Ovoviviparous development, such as that which happens in sharks, is the reproductive strategy in which fertilized eggs develop and hatch within the mother's body, but are not attached to her.