Sex cords

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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 Gonad Precursor Migration.png
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]

Contents

Male sex cord development

Sex cords
Details
Precursor Genital ridge
Gives rise to Testis cords, cortical cords
System Reproductive system
Identifiers
Latin chorda sexualis primordialis gonadalis
TE cords_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

Related Research Articles

<span class="mw-page-title-main">Gonad</span> Gland that produces sex cells

A gonad, sex gland, or reproductive gland is a mixed gland that produces the gametes and sex hormones of an organism. Female reproductive cells are egg cells, and male reproductive cells are sperm. The male gonad, the testicle, produces sperm in the form of spermatozoa. The female gonad, the ovary, produces egg cells. Both of these gametes are haploid cells. Some hermaphroditic animals have a type of gonad called an ovotestis.

<span class="mw-page-title-main">Androgen</span> Any steroid hormone that promotes male characteristics

An androgen is any natural or synthetic steroid hormone that regulates the development and maintenance of male characteristics in vertebrates by binding to androgen receptors. This includes the embryological development of the primary male sex organs, and the development of male secondary sex characteristics at puberty. Androgens are synthesized in the testes, the ovaries, and the adrenal glands.

<span class="mw-page-title-main">Mesonephric duct</span> Paired organ in mammals

The mesonephric duct, also known as the Wolffian duct, archinephric duct, Leydig's duct or nephric duct, is a paired organ that develops in the early stages of embryonic development in humans and other mammals. It is an important structure that plays a critical role in the formation of male reproductive organs. The duct is named after Caspar Friedrich Wolff, a German physiologist and embryologist who first described it in 1759.

<span class="mw-page-title-main">XY gonadal dysgenesis</span> Medical condition

XY complete gonadal dysgenesis, also known as Swyer syndrome, is a type of defect hypogonadism in a person whose karyotype is 46,XY. Though they typically have normal vulvas, the person has underdeveloped gonads, fibrous tissue termed "streak gonads", and if left untreated, will not experience puberty. The cause is a lack or inactivation of an SRY gene which is responsible for sexual differentiation. Pregnancy is often possible in Swyer syndrome with assisted reproductive technology. The phenotype is usually similar to Turner syndrome (45,X0) due to a lack of X inactivation. The typical medical treatment is hormone replacement therapy. The syndrome was named after Gerald Swyer, an endocrinologist based in London.

<span class="mw-page-title-main">Paramesonephric duct</span> Paired ducts in the embryo in the primitive urogenital structures

The paramesonephric ducts are paired ducts of the embryo in the female reproductive system that run down the lateral sides of the genital ridge and terminate at the sinus tubercle in the primitive urogenital sinus. In the female, they will develop to form the fallopian tubes, uterus, cervix, and the upper one-third of the vagina.

<span class="mw-page-title-main">Persistent Müllerian duct syndrome</span> Medical condition

Persistent Müllerian duct syndrome (PMDS) is the presence of Müllerian duct derivatives in what would be considered a genetically and otherwise physically normal male animal by typical human based standards. In humans, PMDS typically is due to an autosomal recessive congenital disorder and is considered by some to be a form of pseudohermaphroditism due to the presence of Müllerian derivatives. PMDS can also present in non-human animals.

<span class="mw-page-title-main">Anti-Müllerian hormone</span> Mammalian protein found in humans

Anti-Müllerian hormone (AMH), also known as Müllerian-inhibiting hormone (MIH), is a glycoprotein hormone structurally related to inhibin and activin from the transforming growth factor beta superfamily, whose key roles are in growth differentiation and folliculogenesis. In humans, it is encoded by the AMH gene, on chromosome 19p13.3, while its receptor is encoded by the AMHR2 gene on chromosome 12.

The development of the urinary system begins during prenatal development, and relates to the development of the urogenital system – both the organs of the urinary system and the sex organs of the reproductive system. The development continues as a part of sexual differentiation.

<span class="mw-page-title-main">Male reproductive system</span> Reproductive system of the human male

The male reproductive system consists of a number of sex organs that play a role in the process of human reproduction. These organs are located on the outside of the body, and within the pelvis.

<span class="mw-page-title-main">Human reproductive system</span> Organs involved in reproduction

The human reproductive system includes the male reproductive system which functions to produce and deposit sperm; and the female reproductive system which functions to produce egg cells, and to protect and nourish the fetus until birth. Humans have a high level of sexual differentiation. In addition to differences in nearly every reproductive organ, there are numerous differences in typical secondary sex characteristics.

<span class="mw-page-title-main">XX male syndrome</span> Congenital condition where an individual with a 46,XX karyotype is male

XX male syndrome, also known as de la Chapelle syndrome, is a rare congenital intersex condition in which an individual with a 46,XX karyotype develops a male phenotype. Synonyms include 46,XX testicular difference of sex development, 46,XX sex reversal, nonsyndromic 46,XX testicular DSD, and XX sex reversal.

<span class="mw-page-title-main">Intermediate mesoderm</span> Layer of cells in mammalian embryos

Intermediate mesoderm or intermediate mesenchyme is a narrow section of the mesoderm located between the paraxial mesoderm and the lateral plate of the developing embryo. The intermediate mesoderm develops into vital parts of the urogenital system.

<span class="mw-page-title-main">Gonadal dysgenesis</span> Congenital disorder of the reproductive system

Gonadal dysgenesis is classified as any congenital developmental disorder of the reproductive system in humans. It is atypical development of gonads in an embryo. One type of gonadal dysgenesis is the development of functionless, fibrous tissue, termed streak gonads, instead of reproductive tissue. Streak gonads are a form of aplasia, resulting in hormonal failure that manifests as sexual infantism and infertility, with no initiation of puberty and secondary sex characteristics.

<span class="mw-page-title-main">Sexual differentiation in humans</span> Process of development of sex differences in humans

Sexual differentiation in humans is the process of development of sex differences in humans. It is defined as the development of phenotypic structures consequent to the action of hormones produced following gonadal determination. Sexual differentiation includes development of different genitalia and the internal genital tracts and body hair plays a role in sex identification.

Pseudohermaphroditism is an outdated term for when an individuals gonads was mismatched with their internal reproductive system and/or external genitalia. The term was contrasted with "true hermaphroditism", a condition describing an individual with both female and male reproductive gonadal tissues. Associated conditions includes Persistent Müllerian duct syndrome and forms of androgen insensitivity syndrome.

The development of the reproductive system is the part of embryonic growth that results in the sex organs and contributes to sexual differentiation. Due to its large overlap with development of the urinary system, the two systems are typically described together as the genitourinary system.

<span class="mw-page-title-main">WNT4</span> Protein-coding gene in the species Homo sapiens

WNT4 is a secreted protein that, in humans, is encoded by the WNT4 gene, found on chromosome 1. It promotes female sex development and represses male sex development. Loss of function may have consequences, such as female to male sex reversal.

<span class="mw-page-title-main">Leydig cell hypoplasia</span> Medical condition

Leydig cell hypoplasia (LCH), also known as Leydig cell agenesis, is a rare autosomal recessive genetic and endocrine syndrome affecting an estimated 1 in 1,000,000 genetic males. It is characterized by an inability of the body to respond to luteinizing hormone (LH), a gonadotropin which is normally responsible for signaling Leydig cells of the testicles to produce testosterone and other androgen sex hormones. The condition manifests itself as pseudohermaphroditism, hypergonadotropic hypogonadism, reduced or absent puberty, and infertility.

The fetal endocrine system is one of the first systems to develop during prenatal development of a human individual. The endocrine system arises from all three embryonic germ layers. The endocrine glands that produce the steroid hormones, such as the gonads and adrenal cortex, arise from the mesoderm. In contrast, endocrine glands that arise from the endoderm and ectoderm produce the amine, peptide, and protein hormones.

Müllerian duct anomalies are those structural anomalies caused by errors in Müllerian duct development during embryonic morphogenesis. Factors that precipitate include genetics, and maternal exposure to teratogens.

References

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  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.
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