Sexual differentiation
Sexual differentiation | |
---|---|
Anatomical terminology |
Sexual differentiation is the process of development of the sex differences between males and females from an undifferentiated zygote.[1][2] Sex determination is often distinct from sex differentiation; sex determination is the designation for the development stage towards either male or female, while sex differentiation is the pathway towards the development of the phenotype.[3]
In many species, testicular or ovarian differentiation begins with appearance of Sertoli cells in males and granulosa cells in females.[4] [citation needed]
As male and female individuals develop from embryos into mature adults, sex differences at many levels develop, such as genes, chromosomes, gonads, hormones, anatomy, and psyche. Beginning with determination of sex by genetic and/or environmental factors, humans and other organisms proceed down different pathways of differentiation as they grow and develop.
Sex determination systems
Other chromosomal systems exist in other taxa, such as the
Environmental sex determination refers to the determination (and then differentiation) of sex via non-genetic cues like social factors, temperature, and available nutrients. In some species, such as the hermaphroditic clownfish, sex differentiation can occur more than once as a response to different environmental cues,[9] offering an example of how sex differentiation does not always follow a typical linear path.
There have been multiple transitions between environmental and genetic sex determination systems in reptiles over time,[10] and recent studies have shown that temperature can sometimes override sex determination via chromosomes.[11]
Humans
The early stages of human differentiation appear to be quite similar to the same biological processes in other mammals and the interaction of genes, hormones and body structures is fairly well understood. In the first weeks of gestation, a fetus has no anatomic or hormonal sex, and only a karyotype distinguishes male from female. Specific genes induce gonadal differences, which produce hormonal differences, which cause anatomic differences, leading to psychological and behavioral differences, some of which are innate and some induced by the social environment.
Various processes are involved in the development of
The development of sexual differences begins with the XY sex-determination system that is present in humans, and complex mechanisms are responsible for the development of the phenotypic differences between male and female humans from an undifferentiated zygote.[13] Atypical sexual development, and ambiguous genitalia, can be a result of genetic and hormonal factors.[14]
The differentiation of other parts of the body than the sex organ creates the secondary sex characteristics. Sexual dimorphism of skeletal structure develops during childhood, and becomes more pronounced at adolescence.
Other animals
The first genes involved in the cascade of differentiation can differ between taxa and even between closely related species. For example: in zebrafish the first known gene to induce male differentiation is the amh gene, in tilapia it is tDmrt1, and in southern catfish it is foxl2.[15]
In fish, due to the fact that modes of reproduction range from
In birds, thanks to research on
Flexibility
The most intensively studied species, such as fruit flies, nematodes, and mice, reveal that evolutionarily, sex determination/differentiation systems are not wholly conserved and have evolved over time.[10] Beyond the presence or absence of chromosomes or social/environmental factors, sexual differentiation can be regulated in part by complex systems like the ratio of genes on X chromosomes and autosomes, protein production and transcription, and specific mRNA splicing.[10]
Differentiation pathways can be altered at many stages of the process. Sex reversal, where the development of a sexual phenotype is redirected during embryonic development, happens in the initiation phase of gonadal sex differentiation. Even in species where there is a well-documented master regulator gene, its effects can be overridden by a downstream gene.[17]
Furthermore, hermaphrodites serve as examples of the flexibility of sexual differentiation systems.
Socially-determined
In some species, such as sequentially hermaphroditic
Alternative morphs
Brain differentiation
In many animals, differences in the exposure of a fetal brain to sex hormones are correlated with significant differences of brain structure and function, which correlate with adult reproductive behavior.[5] The causes of differences between the sexes are only understood in some species. Fetal sex differences in human brains coupled with early differences in experience may be responsible for sex differences observed in children between 4 years old and adolescence.[25]
Many individual studies in humans and other primates have found statistically significant sex differences in specific brain structures; however, some studies have found no sex differences, and some meta-analyses have called into question the over-generalization that women and men's brains function differently.[26] Males and females statistically differ in some aspects of their brains, but there are areas of the brain which appear not to be sexually differentiated at all. Some scholars describe human brain variation not as two distinct categories, and not even a maleness-femaleness continuum, but as mosaics.[27]
In birds, hypotheses of male-female brain sex differences have been challenged by recent findings that differences between groups can be at least partially explained by the individual's dominance rank.[28] Furthermore, the behavioral causes of brain sex differences have been enumerated in studies of sex differences between different mating systems. For example, males of a polygynous vole species with intrasexual male competition have better spatial learning and memory than the females of their own species, but also better spatial learning and memory than all sexes of other closely related species that are monogamous; thus the brain differences commonly seen as "sex differences" have been instead linked to competition.[29] Sexual selection does play a role in some species, though, as males who display more song behaviors are selected for by females—so some sex differences in bird song brain regions seem to have been evolutionarily selected for over time.[29]
References
- ISBN 978-0199657148.
- ISBN 978-0080914558.
- ISBN 978-0-19-965714-8.
- ^ ISBN 978-0-429-10222-6.
- ^ PMID 17237341.
- ^ Gilbert, Scott F. (2000). "Chromosomal Sex Determination in Mammals". Developmental Biology. 6th Edition.
- S2CID 24751510.
- PMID 27543823.
- ^ PMID 27748421.
- ^ PMID 20145384.
- ISSN 0024-4082.
- ^ "Human sexual differentiation".
- S2CID 25732504.
- PMID 16160410. Archived from the originalon 2014-04-06. Retrieved 2014-05-22.
- ^ ISBN 978-0-429-08641-0.
- ^ S2CID 24751510.
- S2CID 4313871.
- S2CID 84279130.
- S2CID 1769706.
- PMID 19812079.
- PMID 29806019.
- ISBN 978-3-319-16999-6
- OCLC 850824972.
- ISSN 0024-4066.
- PMID 21802808.
- S2CID 9909395.
- PMID 26621705.
- PMID 21687671.
- ^ PMID 27870407.
Bibliography
- Baum, Michael J. (2006). "Mammalian animal models of psychosexual differentiation: When is 'translation' to the human situation possible?". Hormones and Behavior. 50 (4): 579–88. S2CID 7465192.
- Crouch, RA (1998). "Betwixt and between: The past and future of intersexuality". The Journal of Clinical Ethics. 9 (4): 372–84. S2CID 43549952.
- Hughes, I A; Houk, C; Ahmed, S. F.; Lee, P. A. (2005). "Consensus statement on management of intersex disorders". Archives of Disease in Childhood. 91 (7): 554–63. PMID 16624884.
- Phoenix, C. H.; Goy, R. W.; Gerall, A. A.; Young, W. C. (1959). "Organizing Action of Prenatally Administered Testosterone Propionate on the Tissues Mediating Mating Behavior in the Female Guinea Pig". Endocrinology. 65 (3): 369–382. PMID 14432658.
- Wallen, Kim (2005). "Hormonal influences on sexually differentiated behavior in nonhuman primates". Frontiers in Neuroendocrinology. 26 (1): 7–26. S2CID 10236292.
- Wilson, BE; Reiner, WG (1998). "Management of intersex: A shifting paradigm". The Journal of Clinical Ethics. 9 (4): 360–9. S2CID 27104390.
External links
- Human Sexual Differentiation by P. C. Sizonenko
- The Ciba Collection of Medical Illustrations: Vol.2, Reproductive System by Frank H. Netter, M.D. comparing female and male reproductive systems development and anatomy
- Development of the Female Sexual & Reproductive Organs – illustrations comparing female and male genitalia during the early development
Overview | |
---|---|
Genetic basis |
|
See also | |
More articles related to sexual differentiation | ||||||||||||||||||||||||||||||||||||||||||||||||||
---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
|
Authority control databases: National |
---|