Structural Biochemistry/Cell Signaling Pathways/Reproductive System

Overview
The reproductive system or genital system is a system of organs within an organism which work together for the purpose of reproduction. Many non-living substances such as fluids, hormones, and pheromones are also important accessories to the reproductive system. Unlike most organ systems, the sexes of differentiated species often have significant differences. These differences allow for a combination of genetic material between two individuals, which allows for the possibility of greater genetic fitness of the offspring.

The major organs of the reproductive system includes, the external genitalia (penis and vulva) as well as a number of internal organs including the gamete producing gonads (testicles and ovaries). Diseases of the human reproductive system are very common and widespread, particularly communicable sexually transmitted diseases.

Most other vertebrate animals have generally similar reproductive systems consisting of gonads, ducts, and openings. However, there is a great diversity of physical adaptations as well as reproductive strategies in every group of vertebrates.

Two Types of Reproduction
In the animal kingdom, there are two type of reproduction. Sexual reproduction occurs when haploid gametes form a zygote, a diploid cell. Both the male and females have gametes. The gamete in males is called the sperm which is motile. Female gamete is the egg which remains stationary, it is not mobile like the sperm. The other type is asexual reproduction which does not involve the sperm and egg. Asexual reproduction occurs on mitotic cell division. Majority of animals reproduce sexually however there are invertebrates that undergo asexual reproduction.

Sexual Reproduction vs. Asexual Reproduction
Sexual reproduction is advantageous because it allows for the production of varied genotypes that result from recombination during meiosis. The varied genotypes increases reproductive success because in the case of pathogens or other environmental factors, this will not wipe out the entire population. Asexual reproduction makes genetically identical copies that live better in stable, unchanging environments.

Asexual Reproduction
Fission and budding are two forms of asexual reproduction. Fission occurs when a parent organism divides into two daughter organisms. Sea anemones undergo fission as the parent organism divides equally into two smaller organisms. These two daughter organisms are genetic copies of the parent. Budding happens when an organism develops from the outgrowth of an existing one. Budding is observed in corals where buds are formed. There's another form of asexual reproduction that requires two steps, fragmentation and regeneration. Fragmentation is when the body splits into several parts and then regeneration takes place as the body parts that got released become restored and grow back. Reproduction occurs when the parts that spilt off grow and are capable of evolving into a complete animal. Many animals including sponges, bristle worms, sea squirts and cnidarians undergo asexual reproduction in two steps. One example are n annelid worms, they are able to break their body into several parts and each part is able to undergo regeneration and become a complete worm in a very short amount of time. Parthenogenesis is another form of asexual reproduction where eggs develop without the need for fertilization. The offspring for parthenogenesis may be either haploid or diploid.

Cycles of Reproduction
Hormones that control the reproductive cycle are regulated by environmental cues such as seasonal temperature, lunar cycles or the length of days. Ovulation is a part of the reproduction cycle that takes place midway during which mature eggs are discharged. These cycles occur in both sexual and asexual reproduction. The Daphnia water flea produces two kinds of eggs. One egg undergoes fertilization while the other uses the process of parthenogenesis where no fertilization is necessary for the development. However, asexual reproduction can only happen if environmental settings are agreeable.

Female Reproductive Anatomy
The two sets of labia make up the external part of the female reproductive system. The labia covers the opening of the vagina and clitoris. The gonads, where reproductive hormones and eggs are produced, and system of ducts make up the internal part. These ducts store the embryo and fetus as well as transporting gametes. Gonads in the female are a set of ovaries that are made up of follicles. The ovaries are also where the oocytes, an egg cell that has not been fully developed yet. The oocyte are protected by supporting cells when eggs are being developed. During the menstruation cycle, ovulation occurs where a follicle reaches maturation and releases an egg. After this has occurred, a follicular tissue develops and forms the corpus luteum in the ovary. The corpus luteum releases the hormones, estradiol and progesterone. The corpus luteum will deteriorate if the egg is not fertilized. The vagina is a tubular chamber through which the penis enters during sexual intercourse. It also serves as the birth canal through which babies are delivered. The opening to the vagina is the vulva which is protected by the labia majora.

Male Reproductive Anatomy
The scrotum and penis make up the external part of the reproductive system. The gonads, where reproductive hormones and sperm are produced, accessory glands and ducts make up the interna; part. Gonads in the male are the testes where sperm is produced. Sperm can only be produced in mammals when the testes are cooler than body temperature. When ejaculation occurs, the sperm travels from the epididymis duct through the vas deferens. Semen is the result of ejaculation and this is made up of secretions from the accessory glands that mix with sperm to ultimately make this fluid.

Mammary Glands
The mammary glands are not part of the reproductive system but they play an important role in reproduction. Mammary glands in female produce milk. There are epithelial tissues inside the glands that secrete milk. This milk goes through several ducts and leaves through the nipple. Female breasts are made up of fatty tissues that are connected to one another. Male breasts usually are small because they have the hormone, estradiol which reduces the development of connective adipose tissues.

Unusual cases
In most animals, individuals are either definite males or definite females. However, in some species, individual organisms are both male and female. Hermaphroditism is when one organism has both sexes. Earthworms and garden snails always have both male and female organs, and when, for example, two earthworms mate, they fertilize each other. A special variation on the theme is sequential hermaphroditism, in which an organism changes sex during its life. If an organism is female first and later changes to male, that organism is protogynous, and if the organism is male first and changes to female, it is said to be protandrous. In different species, sequential hermaphroditism can be influenced by the organism’s age or size or by various environmental/climatic factors.

Reference
http://biology.clc.uc.edu/courses/bio105/reproduc.htm