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    Sexual differentiation in humans

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    Sexual differentiation in humans

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    The human Y chromosome showing the SRY gene which codes for a protein regulating sexual differentiation.

    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.[1] Sexual differentiation includes development of different genitalia and the internal genital tracts and body hair plays a role in sex identification.[2]

    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.[3] Females typically have two X chromosomes, and males typically have a Y chromosome and an X chromosome. At an early stage in embryonic development, both sexes possess equivalent internal structures. These are the mesonephric ducts and paramesonephric ducts. The presence of the SRY gene on the Y chromosome causes the development of the testes in males, and the subsequent release of hormones which cause the paramesonephric ducts to regress. In females, the mesonephric ducts regress.

    Divergent sexual development, known as intersex, can be a result of genetic and hormonal factors.[4]

    Contents

    1 Sex determination

    2 Reproductive system

    2.1 Internal genital differentiation

    2.2 External genital differentiation

    3 Secondary sexual characteristics

    3.1 Breast

    4 Psychological and behavioral differentiation

    5 Intersex variations

    6 Timeline 7 See also 8 Further reading 9 References 9.1 Sources

    Sex determination[edit]

    Main article: XY sex-determination system

    Most mammals, including humans, have an XY sex-determination system: the Y chromosome carries factors responsible for triggering male development. In the absence of a Y chromosome, the fetus will undergo female development. This is because of the presence of the sex-determining region of the Y chromosome, also known as the SRY gene.[5] Thus, male mammals typically have an X and a Y chromosome (XY), while female mammals typically have two X chromosomes (XX).

    Chromosomal sex is determined at the time of fertilization; a chromosome from the sperm cell, either X or Y, fuses with the X chromosome in the egg cell. refers to the gonads, that is the testis or ovaries, depending on which genes are expressed. refers to the structures of the external and internal genitalia.[6]

    6 weeks elapse after fertilization before the first signs of sex differentiation can be observed in human embryos.[7] The embryo and subsequent early fetus appear to be sexually indifferent, looking neither like a male or a female. Over the next several weeks, hormones are produced that cause undifferentiated tissue to transform into either male or female reproductive organs. This process is called sexual differentiation. The precursor of the internal female sex organs is called the Müllerian system.

    Reproductive system[edit]

    Main article: Development of the reproductive system

    This article needs additional citations for verification. Please help improve this article by adding citations to reliable sources. Unsourced material may be challenged and removed.

    "Sexual differentiation in humans" – news · newspapers · books · scholar · JSTOR

    By 7 weeks, a fetus has a genital tubercle, urogenital groove and sinus, and labioscrotal folds. In females, without excess androgens, these become the clitoris, urethra and vagina, and labia.

    Differentiation between the sexes of the sex organs occurs throughout embryological, fetal and later life. This includes both internal and external genital differentiation. In both males and females, the sex organs consist of three structures: the gonads, the internal genitalia, and the external genitalia. In males, the gonads are the testes and in females they are the ovaries. These are the organs that produce gametes (egg and sperm), the reproductive cells that will eventually meet to form the fertilized egg (zygote).

    As the zygote divides, it first becomes the embryo (which means 'growing within'), typically between zero and eight weeks, then from the eighth week until birth, it is considered the fetus (which means 'unborn offspring'). The internal genitalia are all the accessory glands and ducts that connect the gonads to the outside environment. The external genitalia consist of all the external reproductive structures. The sex of an early embryo cannot be determined because the reproductive structures do not differentiate until the seventh week. Prior to this, the child is considered bipotential because it cannot be identified as male or female.

    Source : en.wikipedia.org

    Sexual Differentiation

    The chromosomal sex of the embryo is established at fertilization. However, 6 weeks elapse in humans before the first signs of sex differentiation are noticed. Sex differentiation involves a series of events whereby the sexually indifferent gonads and genitalia progressively acquire male or female characteristics. Believed initially to be governed entirely by the presence or absence of the SRY gene on the Y chromosome, gonadal determination has proven to rely on a complex network of genes, whose balanced expression levels either activate the testis pathway and simultaneously repress the ovarian pathway or vice versa. The presence or absence of primordial germ cells, of extragonadal origin, also has a sexually dimorphic relevance. Subsequently, internal and external genitalia will follow the male pathway in the presence of androgens and anti-Müllerian hormone (AMH), or the female pathway in their absence. Here we review the sexually undifferentiated stage of embryonic development, and the anatomic, histologic, physiologic and molecular aspects of the fetal sexual differentiation of the gonads, the internal reproductive tract and the external genitalia. For complete coverage of all related areas of Endocrinology, please visit our on-line FREE web-text, WWW.ENDOTEXT.ORG.

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    Sexual Differentiation

    Rodolfo Rey, MD, PhD, Nathalie Josso, MD, PhD, and Chrystèle Racine, PhD.

    Author Information

    Last Update: May 27, 2020.

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    ABSTRACT

    The chromosomal sex of the embryo is established at fertilization. However, 6 weeks elapse in humans before the first signs of sex differentiation are noticed. Sex differentiation involves a series of events whereby the sexually indifferent gonads and genitalia progressively acquire male or female characteristics. Believed initially to be governed entirely by the presence or absence of the gene on the Y chromosome, gonadal determination has proven to rely on a complex network of genes, whose balanced expression levels either activate the testis pathway and simultaneously repress the ovarian pathway or vice versa. The presence or absence of primordial germ cells, of extragonadal origin, also has a sexually dimorphic relevance. Subsequently, internal and external genitalia will follow the male pathway in the presence of androgens and anti-Müllerian hormone (AMH), or the female pathway in their absence. Here we review the sexually undifferentiated stage of embryonic development, and the anatomic, histologic, physiologic and molecular aspects of the fetal sexual differentiation of the gonads, the internal reproductive tract and the external genitalia. For complete coverage of all related areas of Endocrinology, please visit our on-line FREE web-text, WWW.ENDOTEXT.ORG.

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    INTRODUCTION

    Genital sex differentiation involves a series of events whereby the sexually indifferent embryo progressively acquires male or female characteristics in the gonads, genital tract and external genitalia. Sex development consists of several sequential stages. Genetic sex, as determined by the chromosome constitution, drives the primitive gonad to differentiate into a testis or an ovary. Subsequently, internal and external genitalia will follow the male pathway in the presence of specific testicular hormones, or the female pathway in their absence. Since the presence of the fetal testis plays a determining role in the differentiation of the reproductive tract, the term "sex determination" has been coined to designate the differentiation of the gonad during early fetal development.

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    THE BIPOTENTIAL GONAD

    No sexual difference can be observed in the gonads until the 6th week of embryonic life in humans and 11.5 days post-coitum (dpc) in mice. Undifferentiated gonads of XX or XY individuals are apparently identical and can form either ovaries or testes. This period is therefore called indifferent or bipotential stage of gonadal development.

    The Gonadal Ridge

    The urogenital ridges are the common precursors of the urinary and genital systems and of the adrenal cortex (1). In the human, they develop during the 4th week post-fertilization at the ventral surface of the cranial mesonephroi, and are formed by intermediate mesoderm covered by coelomic epithelium. Each urogenital ridge divides into a urinary and an adreno-gonadal ridge in the 5th week (Table 1). The adreno-gonadal ridge is the common precursor of the gonads and adrenal cortex. The gonadal ridge is bipotential and can develop into an ovary or a testis. Gonads are subsequently colonized by the primordial germ cells, of extra-gonadal origin. The mesonephroi also give rise to components of the internal reproductive tract and of the urinary system.

    The molecular mechanisms underlying the specific location of the gonads on the surface of the mesonephroi begin to be unveiled in chicken embryos, where Sonic hedgehog (SHH) signaling mediated by the bone morphogenetic protein 4 (BMP4) establishes the dorsoventral patterning of the mesoderm and induces coelomic epithelium cell ingression, thus probably initiating gonadal development (2). However, since there are significant differences in gonadal development between birds and mammals, these mechanisms need to be explored to establish whether they are conserved amongst vertebrates.

    TABLE 1.

    Chronology of Human Sex Differentiation*

    Age from conception CR length (mm) Event

    22 days 2-3 Intermediate mesoderm becomes visible

    Primordial germ cells in the yolk sac

    24 days 2.5-4.5 Formation of solid Wolffian ducts

    Primordial germ cells migrate to the hindgut

    26 days 3-5 Wolffian ducts develop a lumen

    Primordial germ cells in the hindgut

    28 days 4-6 Primordial germ cells migrate to the urogenital ridges

    32 days 5-7 Gonadal primordia develop

    Growth of Wolffian ducts

    33-37 days 7-11 Primordial germ cells reach gonadal ridge

    Urogenital sinus is distinguishable

    Differentiation of Müllerian ducts

    Genital tubercle and urethral folds are visible

    41-44 days 11-17 Seminiferous cord differentiation

    Differentiation between pelvic and phallic parts of the urogenital sinus

    44-50 days 15-20 Seminiferous cords with germ cells

    50-60 days 30 Beginning of secretion of AMH

    Leydig cell differentiation

    Cranial part of Müllerian ducts begins to regress

    Source : www.ncbi.nlm.nih.gov

    sex chromosome

    sexual differentiation, in human embryology, the process by which the male and female sexual organs develop from neutral embryonic structures. The normal human fetus of either sex has the potential to develop either male or female organs, depending on genetic and hormonal influences. In humans, each egg contains 23 chromosomes, of which 22 are autosomes and 1 is a female sex chromosome (the X chromosome). Each sperm also contains 23 chromosomes: 22 autosomes and either one female sex chromosome or one male sex chromosome (the Y chromosome). An egg that has been fertilized has a full complement of 46 chromosomes,

    sex chromosome

    genetics

    By The Editors of Encyclopaedia Britannica • Edit History

    Key People: Nettie Stevens Clarence E. McClung

    Related Topics: Klinefelter syndrome Turner syndrome Barr body Y chromosome X chromosome

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    Learn how sex is determined by sex chromosomes inherited from parents' DNA through meiosis

    A video about sex chromosomes and the inheritance of biological sex.

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    sex chromosome, either of a pair of chromosomes that determine whether an individual is male or female. The sex chromosomes of human beings and other mammals are designated by scientists as X and Y. In humans the sex chromosomes consist of one pair of the total of 23 pairs of chromosomes. The other 22 pairs of chromosomes are called autosomes.

    Individuals having two X chromosomes (XX) are female; individuals having one X chromosome and one Y chromosome (XY) are male. The X chromosome resembles a large autosomal chromosome with a long and a short arm. The Y chromosome has one long arm and a very short second arm. This path to maleness or femaleness originates at the moment of meiosis, when a cell divides to produce gametes, or sex cells having half the normal number of chromosomes. During meiosis the male XY sex-chromosome pair separates and passes on an X or a Y to separate gametes; the result is that one-half of the gametes (sperm) that are formed contains the X chromosome and the other half contains the Y chromosome. The female has two X chromosomes, and all female egg cells normally carry a single X. The eggs fertilized by X-bearing sperm become females (XX), whereas those fertilized by Y-bearing sperm become males (XY).

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    sex: Sex chromosomes

    In most species of animals the sex of individuals is determined decisively at the time of fertilization of the egg, by means of chromosomal...

    Unlike the paired autosomes, in which each member normally carries alleles (forms) of the same genes, the paired sex chromosomes do not carry an identical complement of genetic information. The X chromosome, being larger, carries many more genes than does the Y. Traits controlled by genes found only on the X chromosome are said to be sex-linked (see linkage group). Recessive sex-linked traits, such as hemophilia and red–green colour blindness, occur far more frequently in men than in women. This is because the male who inherits the recessive allele on his X chromosome has no allele on his Y chromosome to counteract its effects. The female, on the other hand, must inherit the recessive allele on both of her X chromosomes in order to fully display the trait. A woman who inherits the recessive allele for a sex-linked disorder on one of her X chromosomes may, however, show a limited expression of the trait. The reason for this is that, in each somatic cell of a normal female, one of the X chromosomes is randomly deactivated. This deactivated X chromosome can be seen as a small, dark-staining structure—the Barr body—in the cell nucleus.

    The effects of genes carried only on the Y chromosome are, of course, expressed only in males. Most of these genes are the so-called maleness determiners, which are necessary for development of the testes in the fetus.

    Several disorders are known to be associated with abnormal numbers of sex chromosomes. Turner’s syndrome and Klinefelter’s syndrome are among the most common of these. See also X trisomy; XYY-trisomy.

    The Editors of Encyclopaedia BritannicaThis article was most recently revised and updated by Kara Rogers.

    Source : www.britannica.com

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