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A depiction of the migration of the cells which will give ride 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] 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.[3] If there is no Y chromosome present the opposite will occur, developing ovarian cords .[4] 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

Once the genital ridge has committed to becoming male sex cords, Sertoli cells develop.[3] 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.[5] These hormones drive the formation of the other male sex characteristics, and induce testicular descent out of the abdomen.[3] These hormones also cause the development of the male reproductive tract. Embryos are formed with Wolffian and Mullerian ducts, which will either become the male or female reproductive tract, respectively.[5] 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.[3] The other male sex organs (ex. the prostate) as well as external genitalia are also formed under the influence of testosterone.[3]

Female

Female sex cord development depends on differential gene expression, where multiple pro-ovarian genes (including Wnt4, FoxL2, and RSP01) and the lack of SRY gene expression are responsible.[2] The lack of testosterone allows the Müllerian ducts to proliferate, while the Wolffian ducts are repressed.[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, it is the coordination between multiple genes that is required for Müllerian duct development.[2] Once the Müllerian ducts have been determined, homeobox genes (more specifically, Hox genes) play a key role in developing female reproductive structures; their differential expression gives rise to the fallopian tubes, uterus, and upper region of the vagina.[2][6]

The development of the internal female genitalia, from the Müllerian ducts, occurs in three phases. First, cells are directed to continue proliferating on the female reproductive structure development pathway.[6] Phase two is invagination, referring to the ducts folding in on themselves to form the openings of the fallopian tubes.[6] In the final phase, Müllerian ducts proliferate and elongate to meet the urogenital sinus, subsequently forming the uterus and upper region of the vagina.[6] From this process, fallopian tubes have formed at the cranial end (the end closer to the head of the body) and the uterus and upper portion of the vagina have formed at the opposite end.[6]

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.[7][8] In contrast, amniotes have single-structure gonads. Sex-specific development is dependent on the fate of the primary sex cord. [7] There are also species-specific anomalies in sex cord development. Freemartin cattle are one notable phenomenon of abnormal gonad development.[9][10] 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).[9][10]

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. ^ a b c d e f g h i 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.
  3. ^ a b c d e 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.
  4. ^ 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.
  5. ^ a b 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.
  6. ^ a b c d e P A, Aatsha; Arbor, Tafline C.; Krishan, Kewal (2023), "Embryology, Sexual Development", StatPearls, Treasure Island (FL): StatPearls Publishing, PMID 32491533, retrieved 2023-11-19
  7. ^ a b 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. doi:10.1111/j.0022-1112.2006.00897.x. ISSN 0022-1112.
  8. ^ Costello, D. P. (1957-09-27). "Development of Vertebrates . Emil Witschi. Saunders, Philadelphia, Pa., 1956. xvi+ 588 pp. Illus. $8.50". Science. 126 (3274): 616–616. doi:10.1126/science.126.3274.616.b. ISSN 0036-8075.
  9. ^ a b Jost, ALFRED; Vigier, BERNARD; Prépin, JACQUES; Perchellet, JEAN PIERRE (1973-01-01), Greep, ROY O. (ed.), "Studies on Sex Differentiation in Mammals11The Gregory Pincus Memorial Lecture.", Proceedings of the 1972 Laurentian Hormone Conference, Recent Progress in Hormone Research, vol. 29, Boston: Academic Press, pp. 1–41, ISBN 978-0-12-571129-6, retrieved 2023-10-23
  10. ^ a b Lillie, Frank R. (1916-04-28). "The Theory of the Free-Martin". Science. 43 (1113): 611–613. doi:10.1126/science.43.1113.611. ISSN 0036-8075.
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