Reproductive system

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This article is about the sexual reproductive system. For non-sexual reproduction, see Asexual reproduction.
Reproductive system
Male pelvic structures.svg
Latin systema reproductionis
Anatomical terminology

The reproductive system or genital system is a system of sex organs within an organism which work together for the purpose of sexual reproduction. Many non-living substances such as fluids, hormones, and pheromones are also important accessories to the reproductive system.[1] 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.[2]

Plants[edit]

Fungi[edit]

Main article: Fungi § Reproduction

Fungal reproduction is complex, reflecting the differences in lifestyles and genetic makeup within this diverse kingdom of organisms.[3]

Animals[edit]

In mammals, the major organs of the reproductive system include 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.[4]

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.

Vertebrates[edit]

Vertebrate animals all share key elements of their reproductive systems. They all have gamete producing organs or gonads. In females, these gonads are then connected by oviducts to an opening to the outside of the body, typically the cloaca, but sometime to a unique pore such as a vagina or intromittent organ.

Mammals[edit]

A newborn joey suckles from a teat found within its mother's pouch

Most mammal reproductive systems are similar, however, there are some notable differences between the "normal" mammal and humans. For instance, most mammalian males have a penis which is stored internally until erect, and most have a penis bone or baculum. Additionally, males of most species do not remain continually sexually fertile as humans do. Like humans, most groups of mammals have descended testicles found within a scrotum, however, others have descended testicles that rest on the ventral body wall, and a few groups of mammals, such as elephants, have undescended testicles found deep within their body cavities near their kidneys.[5]

The reproductive system of marsupials is unique in that the female has two vaginae, both of which open externally through one orifice but lead to different compartments within the uterus; males usually have a two-pronged penis which corresponds to the females' two vaginae. Marsupials typically develop their offspring in an external pouch containing teats to which their newborn young (joeys) attach themselves for post uterine development. Also, marsupials have a unique prepenial scrotum.[6] The 15mm (5/8 in) long newborn joey instinctively crawls and wriggles the several inches (15 cm), while clinging to fur, on the way to its mother's pouch.

The uterus and vagina are unique to mammals with no homologue in birds, reptiles, amphibians, or fish.[citation needed] In place of the uterus the other vertebrate groups have an unmodified oviduct leading directly to a cloaca, which is a shared exit-hole for gametes, urine, and feces. Monotremes (i.e. platypus and echidnas), a group of egg-laying mammals, also lack a uterus and vagina, and in that respect have a reproductive system resembling that of a reptile.

Canine reproductive system[edit]

In domestic canines, sexual maturity (puberty) occurs between the ages of 6 to 12 months for both males and females, although this can be delayed until up to two years of age for some large breeds.

Equine reproductive system[edit]

The mare's reproductive system is responsible for controlling gestation, birth, and lactation, as well as her estrous cycle and mating behavior. The stallion's reproductive system is responsible for his sexual behavior and secondary sex characteristics (such as a large crest).

Human reproductive system[edit]
Further information: Human reproduction

The human reproductive system usually involves internal fertilization by sexual intercourse. During this process, the erect penis of the male is inserted into the female's vagina until the male ejaculates semen, which contains sperm, into the female's vagina. The sperm then travels through the vagina and cervix into the uterus or fallopian tubes for fertilization of the ovum. Upon successful fertilization and implantation, gestation of the fetus then occurs within the female's uterus for approximately nine months, this process is known as pregnancy in humans. Gestation ends with birth, the process of birth is known as labor. Labor consists of the muscles of the uterus contracting, the cervix dilating, and the baby passing out the vagina (the female genital organ). Human's babies and children are nearly helpless and require high levels of parental care for many years. One important type of parental care is the use of the mammary glands in the female breasts to nurse the baby.[7]

The female reproductive system has two functions: The first is to produce egg cells, and the second is to protect and nourish the offspring until birth. The male reproductive system has one function, and it is to produce and deposit sperm. Humans have a high level of sexual differentiation. In addition to differences in nearly every reproductive organ, numerous differences typically occur in secondary sexual characteristics.

Male reproductive system[edit]

The male reproductive system is a series of organs located outside of the body and around the pelvis region of a male that contribute towards the reproduction process. The primary direct function of the male reproductive system is to provide the male sperm for fertilization of the ovum.

The major reproductive organs of the male can be grouped into three categories. The first category is sperm production and storage. Production takes place in the testes which are housed in the temperature regulating scrotum, immature sperm then travel to the epididymis for development and storage. The second category are the ejaculatory fluid producing glands which include the seminal vesicles, prostate, and the vas deferens. The final category are those used for copulation, and deposition of the spermatozoa (sperm) within the male, these include the penis, urethra, vas deferens, and Cowper's gland.

Major secondary sexual characteristics includes: larger, more muscular stature, deepened voice, facial and body hair, broad shoulders, and development of an adam's apple. An important sexual hormone of males is androgen, and particularly testosterone.

The testes release a hormone that controls the development of sperm. This hormone is also responsible for the development of physical characteristics in men such as facial hair and a deep voice.

Female reproductive system[edit]

The human female reproductive system is a series of organs primarily located inside of the body and around the pelvic region of a female that contribute towards the reproductive process. The human female reproductive system contains three main parts: the vagina, which leads from the vulva, the vaginal opening, to the uterus; the uterus, which holds the developing fetus; and the ovaries, which produce the female's ova. The breasts are involved during the parenting stage of reproduction, but in most classifications they are not considered to be part of the female reproductive system.

The vagina meets the outside at the vulva, which also includes the labia, clitoris and urethra; during intercourse this area is lubricated by mucus secreted by the Bartholin's glands. The vagina is attached to the uterus through the cervix, while the uterus is attached to the ovaries via the fallopian tubes. Each ovary contains hundreds of egg cells or ova (singular ovum).

Approximately every 28 days, the pituitary gland releases a hormone that stimulates some of the ova to develop and grow. One ovum is released and it passes through the fallopian tube into the uterus. Hormones produced by the ovaries prepare the uterus to receive the ovum. The lining of the uterus, called the endometrium, and unfertilized ova are shed each cycle through the process of menstruation. If the ovum is fertilized by sperm, it attaches to the endometrium and the fetus develops.

Production of gametes[edit]
Main articles: Spermatogenesis and Oogenesis

The production of gametes takes place within the gonads through a process known as gametogenesis. Gametogenesis occurs when certain types of germ cells undergo meiosis to split the normal diploid number of chromosome(n=46) into haploids cells containing only 23 chromosomes.[8]

In males, this process is known as spermatogenesis and takes place only after puberty in the seminiferous tubules of the testes. The immature spermatozoon or sperm are then sent to the epididymis where they gain a tail and motility. Each of the original diploid germs cells or primary spermatocytes forms four functional gametes which is each forever young. The production and survival of sperms require a temperature that is lower than the normal core body temperature. Since the scrotum, where the testes is present, is situated outside the body cavity, it provides a temperature about 3 °C below normal body temperature.

In females, gametogenesis is known as oogenesis which occurs in the ovarian follicles of the ovaries. This process does not produce mature ovum until puberty. In contrast with males, each of the original diploid germ cells or primary oocytes will form only one mature ovum, and three polar bodies which are not capable of fertilization It has long been understood that in females, unlike males, all of the primary oocytes ever found in a female will be created prior to birth, and that the final stages of ova production will then not resume until puberty.[8] However, recent scientific data has challenged that hypothesis.[9] This new data indicates that in at least some species of mammal oocytes continue to be replenished in females well after birth.[10]

Development of the reproductive system[edit]

The development of the reproductive system and urinary systems are closely tied in the development of the human fetus. Despite the differences between the adult male and female reproductive system, there are a number of homologous structures shared between them due to their common origins within the fetus. Both organ systems are derived from the intermediate mesoderm. The three main fetal precursors of the reproductive organs are the Wolffian duct, Müllerian ducts, and the gonad. Endocrine hormones are a well known and critical controlling factor in the normal differentiation of the reproductive system.[11]

The Wolffian duct forms the epididymis, vas deferens, ductus deferens, ejaculatory duct, and seminal vesicle in the male reproductive system and essentially disappears in the female reproductive system. For the Müllerian Duct this process is reversed as it essentially disappears in the male reproductive system and forms the fallopian tubes, uterus, and vagina in the female system. In both sexes the gonad goes on to form the testes and ovaries, because they are derived from the same undeveloped structure they are considered homologous organs. There are a number of other homologous structures shared between male and female reproductive systems. However, despite the similarity in function of the female fallopian tubes and the male epididymis and vas deferens, they are not homologous but rather analogous structures as they arise from different fetal structures.

Examples of homologous human reproductive organs
Male organ Female organ Shared function
Cowper's gland Bartholin's glands Lubrication secretions
Penis Clitoris Erectile tissue and sensation
Testes Ovary Gamete production
Prostate gland Skene's gland Ejaculatory fluid and sensation
Diseases of the human reproductive system[edit]

Like all complex organ systems the human reproductive system is affected by many diseases. There are four main categories of reproductive diseases in humans. They are: 1) genetic or congenital abnormalities, 2) cancers, 3) infections which are often sexually transmitted diseases, and 4) functional problems cause by environmental factors, physical damage, psychological issues, autoimmune disorders, or other causes. The best known type of functional problems include sexual dysfunction and infertility which are both broad terms relating to many disorders with many causes.

Specific reproductive diseases are often symptoms of other diseases and disorders, or have multiple, or unknown causes making them difficult to classify. Examples of unclassifiable disorders include Peyronie's disease in males and endometriosis in females. Many congenital conditions cause reproductive abnormalities but are better known for their other symptoms, these include: Turner syndrome, Klinefelter's syndrome, Cystic fibrosis, and Bloom syndrome.[12]

Birds[edit]

Male and female birds have a cloaca, an opening through which eggs, sperm, and wastes pass. Intercourse is performed by pressing the lips of the cloacae together, which is sometimes knowna intromittent organ which is known as a phallus that is analogous to the mammals' penis. The female lays amniotic eggs in which the young fetus continues to develop after it leaves the female's body. Unlike most vertebrates female birds typically have only one functional ovary and oviduct.[13] As a group, birds, like mammals, are noted for their high level of parental care.

Reptiles[edit]

Reptiles are almost all sexually dimorphic, and exhibit internal fertilization through the cloaca. Some reptiles lay eggs while others are viviparous (animals that deliver live young). Reproductive organs are found within the cloaca of reptiles. Most male reptiles have copulatory organs, which are usually retracted or inverted and stored inside the body. In turtles and crocodilians, the male has a single median penis-like organ, while male snakes and lizards each possess a pair of penis-like organs.

A male common frog in nuptial colors waiting for more females to come in a mass of spawn

Amphibians[edit]

Most amphibians exhibit external fertilization of eggs, typically within the water, though some amphibians such as caecilians have internal fertilization.[14] All have paired, internal gonads, connected by ducts to the cloaca.

Fish[edit]

Fish exhibit a wide range of different reproductive strategies. Most fish however are oviparous and exhibit external fertilization. In this process, females use their cloaca to release large quantities of their gametes, called spawn into the water and one or more males release "milt", a white fluid containing many sperm over the unfertilized eggs. Other species of fish are oviparous and have internal fertilization aided by pelvic or anal fins that are modified into an intromittent organ analogous to the human penis.[15] A small portion of fish species are either viviparous or ovoviviparous, and are collectively known as livebearers.[16]

Fish gonads are typically pairs of either ovaries or testes. Most fish are sexually dimorphic but some species are hermaphroditic or unisexual.[17]

Invertebrates[edit]

Invertebrates have an extremely diverse array of reproductive systems, the only commonality may be that they all lay eggs. Also, aside from cephalopods, and arthropods, nearly all other invertebrates are hermaphroditic and exhibit external fertilization.

Cephalopods[edit]

All cephalopods are sexually dimorphic and reproduce by laying eggs. Most cephalopods have semi-internal fertilization, in which the male places his gametes inside the female's mantle cavity or pallial cavity to fertilize the ova found in the female's single ovary.[18] Likewise, male cephalopods have only a single testicle. In the female of most cephalopods the nidamental glands aid in development of the egg.

The "penis" in most unshelled male cephalopods (Coleoidea) is a long and muscular end of the gonoduct used to transfer spermatophores to a modified arm called a hectocotylus. That in turn is used to transfer the spermatophores to the female. In species where the hectocotylus is missing, the "penis" is long and able to extend beyond the mantle cavity and transfer the spermatophores directly to the female.

Insects[edit]

Arachnids[edit]

See also: Opiliones penis

See also[edit]

References[edit]

  1. ^ Introduction to the Reproductive System. , Epidemiology and End Results (SEER) Program.[dead link]
  2. ^ Reproductive System 2001 Body Guide powered by Adam
  3. ^ Alexopoulos et al., pp. 48–56.
  4. ^ STD's Today National Prevention Network, Center for Disease Control, United States Government, retrieving 2007
  5. ^ Werdelin L, Nilsonne A (January 1999). "The evolution of the scrotum and testicular descent in mammals: a phylogenetic view". J. Theor. Biol. 196 (1): 61–72. doi:10.1006/jtbi.1998.0821. PMID 9892556. 
  6. ^ Renfree, Marilyn; Tyndale-Biscoe, C. H. (1987). Reproductive physiology of marsupials. Cambridge, UK: Cambridge University Press. ISBN 0-521-33792-5. 
  7. ^ Sexual Reproduction in Humans. 2006. John W. Kimball. Kimball's Biology Pages, and online textbook.
  8. ^ a b Development of sex cells in Reproductive system, Body Guide. Adam.
  9. ^ Tilly JL, Niikura Y, Rueda BR (August 2008). "The Current Status of Evidence for and Against Postnatal Oogenesis in Mammals: A Case of Ovarian Optimism Versus Pessimism?". Biol. Reprod. 80 (1): 2–12. doi:10.1095/biolreprod.108.069088. PMC 2804806. PMID 18753611. 
  10. ^ Johnson J, Canning J, Kaneko T, Pru JK, Tilly JL (March 2004). "Germline stem cells and follicular renewal in the postnatal mammalian ovary". Nature 428 (6979): 145–50. doi:10.1038/nature02316. PMID 15014492. 
  11. ^ EDRI Federal Project Inventory: Cellular and Molecular Mechanisms of Abnormal Reproductive Development US EPA. Dr. William R. Kelce. 2006.
  12. ^ Genetic Conditions > Reproductive system. 2007. Genetics Home Reference. U.S. National Library of Medicine.
  13. ^ Ritchison. BIO 554/754 Ornithology. Eastern Kentucky University.
  14. ^ Grzimek, B. (1974). Grzimek's Animal Life Encyclopedia: Volume 5 Fishes II & Amphibians. New York: Van Nostrand Reihnhold Co. pp. 301–302. ASIN B000HHFY52. 
  15. ^ Fish Reproduction
  16. ^ Science, Biology, and Terminology of Fish reproduction: Reproductive modes and strategies-part 1. 2002. MARTIN MOE. THE BREEDER'S NET Online Magazine
  17. ^ Bony Fish Reproduction 2002. SeaWorld/Busch Gardens Animal Information Database.
  18. ^ Cephalopods. The Living World of Molluscs. Robert Nordsieck.

External links[edit]