A depiction of the migration of the cells which will give rise to the sex cords into the genital ridge where they will become the gonads of the embryo

Sex cords are embryonic structures which eventually will give rise (differentiate) to the adult gonads (reproductive organs).[1] They are formed from the genital ridges - which will develop into the gonads - in the first 2 months of gestation (embryonic development) which depending on the sex of the embryo will give rise to male or female sex cords.[2] These epithelial cells (from the genital ridges) penetrate and invade the underlying mesenchyme to form the primitive sex cords.[3] This occurs shortly before and during the arrival of the primordial germ cells (PGCs) to the paired genital ridges.[3] If there is a Y chromosome present, testicular cords will develop via the Sry gene (on the Y chromosome): repressing the female sex cord genes and activating the male.[4][5] If there is no Y chromosome present the opposite will occur, developing ovarian cords.[6][7] Prior to giving rise to sex cords, both XX and XY embryos have Müllerian ducts and Wolffian ducts.[2] One of these structures will be repressed to induce the other to further differentiate into the external genitalia.[2]

Male sex cord development

Sex cords
Details
Precursorgenital ridge
Gives rise totestis cords, cortical cords
SystemReproductive system
Identifiers
Latinchorda sexualis primordialis gonadalis
TEcords_by_E5.7.1.1.0.0.7 E5.7.1.1.0.0.7
Anatomical terminology

Once the genital ridge has committed to becoming male sex cords, Sertoli cells develop.[4] These cells then induce the production and organisation of cells making up the testicular cords.[2] These cords will eventually become the testes, which in turn produce hormones, in particular testosterone.[8] These hormones drive the formation of the other male sex characteristics, and induce testicular descent out of the abdomen.[4] These hormones also cause the development of the male reproductive tract.[4] Embryos are formed with Wolffian and Mullerian ducts, which will either become the male or female reproductive tract, respectively.[8] In a male embryo, the testicular cords will induce the development of the Wolffian duct into the vas deferens, epididymis and the seminal vesicle and cause the repression and regression of the Mullerian duct.[4] The other male sex organs (ex. the prostate) as well as external genitalia are also formed under the influence of testosterone.[4]

Female sex cord development

Female sex cord development depends on specific genes being expressed, where multiple pro-ovarian genes (including Wnt4, FoxL2, and Rsp01)[9][10][11] and the lack of Sry gene expression are responsible.[2] The lack of testosterone allows for Müllerian duct proliferation, and Wolffian duct repression.[2] The lack of male sex hormones gives rise to female sex cords and subsequent genitalia differentiation, rather than a presence of female sex hormones.[2] After inducing female sex cord formation, coordination between multiple genes (Bmp, Pax2, Lim1, and Wnt4 in mice) is required for Müllerian duct development.[2] Once the Müllerian ducts are determined, genes contributing to cell identity and positioning (specifically, Hox genes) play a key role in developing female reproductive structures.[12][2] The Hox genes are expressed in specific combinations to give rise to the fallopian tubes, uterus, and upper region of the vagina.[2][13]

Developing internal female genitalia, from the Müllerian ducts, occurs in three phases. First, cells are directed to proliferate on the female reproductive structure development pathway.[13] Phase two is invagination: referring to the ducts folding in on themselves, forming openings of the fallopian tubes.[13] In phase three, Müllerian ducts proliferate and elongate, subsequently forming the uterus and upper region of the vagina.[13] Fallopian tubes form at the end closer to the head of the body, and the uterus and upper portion of the vagina form at the opposite end.[13]

Unusual development/different species

In early prenatal development, amphibians and elasmobranchs have gonads with a dual structure; A gonadal cortex, associated with ovarian differentiation, and a gonadal medulla, associated with testicular differentiation.[14][15][16] In contrast, amniotes have single-structure gonads.[13] Sex-specific development is dependent on the fate of the primary sex cord.[14] There are also species-specific anomalies in sex cord development. Freemartin cattle are one notable phenomenon of abnormal gonad development.[17][18] These are genetically female cattle that develop testicle-like structures in replacement of ovaries due to exchange of blood during development in parabiosis with male twin(s).[17][18]

See also

References

  1. ↑ Kanai, Yoshiakira; Kurohmaru, Masamichi; Hayashi, Yoshihiro; Nishida, Takao (1989). "Formation of male and female sex cords in gonadal development of C57BL/6 mouse". The Japanese Journal of Veterinary Science. 51 (1): 7–16. doi:10.1292/jvms1939.51.7. ISSN 0021-5295.
  2. 1 2 3 4 5 6 7 8 9 10 Reyes, Alejandra P.; León, Nayla Y.; Frost, Emily R.; Harley, Vincent R. (2023). "Genetic control of typical and atypical sex development". Nature Reviews Urology. 20 (7): 434–451. doi:10.1038/s41585-023-00754-x. ISSN 1759-4812. PMID 37020056. S2CID 257984306.
  3. 1 2 Sadler, T.W. (2015). Langman's medical embryology (13th ed.). Philadelphia: Wolters Kluwer. ISBN 9781469897806. OCLC 885475111.
  4. 1 2 3 4 5 6 Wilhelm, Dagmar; Koopman, Peter (2006). "The makings of maleness: towards an integrated view of male sexual development". Nature Reviews Genetics. 7 (8): 620–631. doi:10.1038/nrg1903. ISSN 1471-0056. PMID 16832429. S2CID 20339526.
  5. ↑ Gubbay, John; Collignon, Jérôme; Koopman, Peter; Capel, Blanche; Economou, Androulla; Münsterberg, Andrea; Vivian, Nigel; Goodfellow, Peter; Lovell-Badge, Robin (1990). "A gene mapping to the sex-determining region of the mouse Y chromosome is a member of a novel family of embryonically expressed genes". Nature. 346 (6281): 245–250. Bibcode:1990Natur.346..245G. doi:10.1038/346245a0. ISSN 0028-0836. PMID 2374589. S2CID 4270188.
  6. ↑ Fouquet, J. P.; Dang, D. C. (1980). "A comparative study of the development of the fetal testis and ovary in the monkey (Macaca fascicularis)". Reproduction Nutrition Développement. 20 (5A): 1439–1459. doi:10.1051/rnd:19800804. ISSN 0181-1916. PMID 7349493.
  7. ↑ Chassot, Anne-Amandine; Gillot, Isabelle; Chaboissier, Marie-Christine (2014). "R-spondin1, WNT4, and the CTNNB1 signaling pathway: strict control over ovarian differentiation". Reproduction. 148 (6): R97–R110. doi:10.1530/REP-14-0177. ISSN 1470-1626. PMID 25187620.
  8. 1 2 Coward, Kevin; Wells, Dagan, eds. (2013-10-31). Textbook of Clinical Embryology (1 ed.). Cambridge University Press. doi:10.1017/cbo9781139192736. ISBN 978-1-139-19273-6.
  9. ↑ Chassot, Anne-Amandine; Gillot, Isabelle; Chaboissier, Marie-Christine (December 2014). "R-spondin1, WNT4, and the CTNNB1 signaling pathway: strict control over ovarian differentiation". Reproduction. 148 (6): R97–R110. doi:10.1530/REP-14-0177. ISSN 1470-1626. PMID 25187620.
  10. ↑ Ottolenghi, Chris; Omari, Shakib; Garcia-Ortiz, J. Elias; Uda, Manuela; Crisponi, Laura; Forabosco, Antonino; Pilia, Giuseppe; Schlessinger, David (2005-06-08). "Foxl2 is required for commitment to ovary differentiation". Human Molecular Genetics. 14 (14): 2053–2062. doi:10.1093/hmg/ddi210. ISSN 1460-2083. PMID 15944199.
  11. ↑ Chassot, A.-A.; Ranc, F.; Gregoire, E. P.; Roepers-Gajadien, H. L.; Taketo, M. M.; Camerino, G.; de Rooij, D. G.; Schedl, A.; Chaboissier, M.-C. (2008-01-18). "Activation of -catenin signaling by Rspo1 controls differentiation of the mammalian ovary". Human Molecular Genetics. 17 (9): 1264–1277. doi:10.1093/hmg/ddn016. ISSN 0964-6906.
  12. ↑ Duverger, Olivier; Morasso, Maria I. (2008). "Role of homeobox genes in the patterning, specification, and differentiation of ectodermal appendages in mammals". Journal of Cellular Physiology. 216 (2): 337–346. doi:10.1002/jcp.21491. ISSN 0021-9541. PMC 2561923. PMID 18459147.
  13. 1 2 3 4 5 6 P A, Aatsha; Arbor, Tafline C.; Krishan, Kewal (2023), "Embryology, Sexual Development", StatPearls, Treasure Island (FL): StatPearls Publishing, PMID 32491533, retrieved 2023-12-04
  14. 1 2 Komatsu, T.; Nakamura, S.; Nakamura, M. (2006). "A sex cord-like structure and some remarkable features in early gonadal sex differentiation in the marine teleost Siganus guttatus (Bloch)". Journal of Fish Biology. 68 (1): 236–250. Bibcode:2006JFBio..68..236K. doi:10.1111/j.0022-1112.2006.00897.x. ISSN 0022-1112.
  15. ↑ Costello, D. P. (1957-09-27). "Development of Vertebrates . Emil Witschi. Saunders, Philadelphia, Pa., 1956. xvi+ 588 pp. Illus. $8.50". Science. 126 (3274): 616. doi:10.1126/science.126.3274.616.b. ISSN 0036-8075.
  16. ↑ Kempton, Rudolf T. (1968). "Sharks, Skates, and Rays. Perry W. Gilbert , Robert F. Mathewson , David P. Rall". Physiological Zoology. 41 (1): 125–126. doi:10.1086/physzool.41.1.30158491. ISSN 0031-935X.
  17. 1 2 JOST, ALFRED; VIGIER, BERNARD; PRÉPIN, JACQUES; PERCHELLET, JEAN PIERRE (1973), "Studies on Sex Differentiation in Mammals", Proceedings of the 1972 Laurentian Hormone Conference, Elsevier, vol. 29, pp. 1–41, doi:10.1016/b978-0-12-571129-6.50004-x, ISBN 9780125711296, PMID 4584366, retrieved 2023-12-04
  18. 1 2 Lillie, Frank R. (1916-04-28). "The Theory of the Free-Martin". Science. 43 (1113): 611–613. Bibcode:1916Sci....43..611L. doi:10.1126/science.43.1113.611. ISSN 0036-8075. PMID 17756274.
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