Early in embryogenesis, germ cells are specified as distinct from somatic cells.
Search term Sex determination in mammals: An XXY human is phenotypically male, with a syndrome of moderate abnormalities Klinefelter syndrome ; see Figure XO humans have a number of abnormalities Turner syndrome ; see Figureincluding short stature, mental retardation, and mere traces of gonads, but they are clearly female in morphology.
These data are consistent with a mammalian sex-determination mechanism based on the presence or absence of a Y chromosome. Without a Y, the person develops as a female; with it, as a male. Mammalian reproductive development and endocrine organ control In contrast with flies, each individual human cell does not make an independent determination of its sex.
Humans mosaic for XX and XY tissues typically have a generalized appearance characteristic of one or the other sex. The observation of nonautonomy in mammalian sex determination can be understood in view of the biology of the reproductive system: The gonad forms within the first 2 months of human gestation.
Primordial germ cells migrate into the genital ridge, which sits atop the rudimentary kidney. The chromosomal sex of the germ cells determines whether they will migrate superficially or deeply into the gonadal ridge and whether they will organize into a testis or an ovary Figure If they form a testis, the Leydig cells of the testis secrete testosterone, an androgenic male-determining steroid hormone.
Recall the discussion of steroid hormone recep-tor transcription factors in Chapter This hormone binds to androgen receptors. These receptors function as transcription factors; their transcription-factor activity, however, depends on binding to their cognate hormone.
Thus, the androgen— receptor complex binds to androgen-responsive enhancer elements, leading to the activation of malespecific gene expression.
In chromosomally female embryos, no Leydig cells form in the gonad, no testosterone is produced, androgen receptor is not activated, and the embryos continue along the default female pathway of development.
Hence, it is the presence or absence of a testis that determines the sexual phenotypethrough the endocrine release of testosterone. Indeed, in XY embryos lacking the androgen receptor, development proceeds along a completely female pathway even though the embryos have testes.
Figure Development of the mammalian urogenital system. Female germ cells migrate into the cortex and become organized into an ovary. Male germ cells migrate into the medulla and become more Molecular genetic analysis has focused on identifying the locus on the Y chromosome that drives testis formation.
Mutational analysis of mammalian sex determination The Y chromosome testis-determining gene was identified through mapping and characterization by Robin Lovell-Badge and Peter Goodfellow of a genetic syndrome common to mice and humans that almost certainly affects this factor see the molecular map in Figure This syndrome is called sex reversal.
Sex-reversed XX individuals are phenotypic males and have been shown to carry a fragment of the Y chromosome in their genomes.
In general, these Y-chromosome duplications arise by an illegitimate recombination between the X and Y chromosomes that fuses a piece of the Y chromosome to a tip of one of the X chromosomes.
The part of the Y chromosome that includes these duplications was cloned; by subsequent molecular analysis, Lovell-Badge and Goodfellow identified from this region a transcript that is expressed in the appropriate location of the developing kidney capsule.
Figure A molecular map of the distal part of the short arm of the human Y chromosome. The gene for testis-determining factor TDF was localized to a kb region on the basis of the following logic: Lovell-Badge and Goodfellow used a transgene to provide spectacular evidence in support of this identity Figure A cloned kb genomic fragment of the mouse Y chromosomeincluding the Sry gene, was inserted into the mouse genome by germ- line transformation.
An XX offspring containing this inserted Sry DNA the transgene was completely male in external and internal phenotype and, as predicted, possessed the somatic tissues of the testis Figure bincluding the Leydig cells that make testosterone.
It should be noted, however, that this mouse was sterile. The sterility is probably a consequence of having two X chromosomes in a male germ cell, because XXY male mice are similarly sterile.
Figure A transgenic mouse that proves that Sry can cause the sex-reversal syndrome. The role of the androgen receptor in receiving the testosterone signal and establishing the male secondary sexual characteristics was elucidated through the study of rare Tfm mice lacking this receptor.
Chromosomally XY mice hemizygous for the X-linked Tfm Testicular feminization mutation develop as phenotypic females see Figure except that they are infertile and are typically diagnosed at puberty because of their failure to menstruate.
Tfm XY mice have testes, but the target cells that must decide between alternative pathways regarding sexually dimorphic characteristics lack androgen receptors and so are completely insensitive to the presence of testosterone.
Thus, these mice develop along the default developmental pathwaywhich leads to phenotypic feminization. The testis serves as a command organ that, through testosterone release, directs male phenotypic development throughout the body. By agreement with the publisher, this book is accessible by the search feature, but cannot be browsed.Gonads were isolated from all-female and all-male tilapia before (5–15 days after hatching [dah]) and after (25–70 dah) morphological sex differentiation.
Introduction. Sex determination, unlike many developmental processes, is characterized by a lack of conservation throughout the vertebrates. the two somatic gonads are clear variants of each other. 1. Introduction to sex determination 2 In his chapter, Michael Herman focuses on cell fate specifications that occur only in hermaphrodites.
Both All WormBook content, except where otherwise noted, is licensed under a Creative. 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, breasts, body hair, and plays a role in gender identification.
To determine the affects of aphidicolin on gonad morphology, somatic cells were detected with a dilution of an antibody against steroidogenic factor 1 (SF1), a rabbit polyclonal antibody donated by Ken-ichirou Morohashi. Introduction.
Sex determination is the fundamental step that an animal takes to become male or female. while equated with the differentiation of gonads into either ovaries or more generally referred to as environmental sex determination, or ESD) is widespread in reptiles, for example tuataras—both species of Sphenodon; crocodylians.
Introduction to the germ line * Comparative size of gonads at different stages is not to scale. Germline color scheme: yellow mitotic region, light green transition (early prophase of meiosis I), dark green pachytene, dark blue spermatogenesis, and pink oogenesis. Germline sex determination and meiotic prophase also begin in this stage.