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| + | == Genetic Sex == |
| + | * Established at the time of '''fertilisation'''. |
| + | * The foetus receives its genetic endowment in two equal portions from the male gamete ('''Spermatozoon''') and female gamete ('''Oocyte'''). |
| + | * The X sex chromasome is larger than the Y sex chromasome. |
| + | * '''Females''' have XX chromasomes. All oocytes contain one X chromasome, they are the '''homogametic''' sex. |
| + | * '''Males''' have XY chromasomes. Spermatozoa form two distinct populations, one containing X chromasomes and one containing Y chromasomes. They are the '''heterogametic''' sex. |
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| + | == Internal Genetalia == |
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| + | == External Genetalia == |
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− | [[Image:Overview of Sexual Differentiation.jpg|thumb|right|150px|Overview of Sexual Differentiation,Copyright RVC 2008]]
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− | = Genetic Sex =
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− | * Established at the time of '''fertilisation'''.
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− | * The foetus receives its genetic endowment in two equal portions from the male gamete ('''spermatozoon''') and female gamete ('''oocyte''').
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− | * The X sex chromosome is larger than the Y sex chromosome.
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− | * '''Females''' have '''XX''' chromosomes. All oocytes contain one X chromosome, they are the '''homogametic''' sex.
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− | * '''Males''' have '''XY''' chromosomes. Spermatozoa form two distinct populations, one containing X chromosomes and one containing Y chromosomes. They are the '''heterogametic''' sex.
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− | * The '''Y''' chromosome is required to '''initiate male development'''.
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− | * In absence of the Y chromosome, female gonads (ovaries) will develop.
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− | * The prime male determining gene is '''SRY''' (sex determining region of the Y chromosome).
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− | * SRY is a very small piece of DNA that acts as a molecular switch to regulate structural genes and autosomal chromosomes, determining if and when they are to be activated.
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− | * SRY encodes the protein '''Testes Determining Factor''' (TDF)
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− | * The presence of SRY alone is sufficient to ensure development of male gonads (testes).
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− | * Female is the default sex, in absence of SRY female gonads (ovaries) will develop.
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− | * Subsequent gonadal development is dependent on a normal population of '''germ cells'''.
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− | * Normal oocyte growth requires the activity of both X chromosomes.
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− | * Normal spermatogenesis requires the activity of no more than one X chromosome.
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− | * Early development of gonads proceeds similarly in both sexes. The gonads are derived from two distinct tissues:
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− | ** '''Somatic mesenchymal tissue''' forms the matrix of the gonad
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− | ** '''Primordial germ cells''' form the gametes
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− | * '''Genital ridge''' forms from somatic mesenchyme.
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− | * Primordial germ cells migrate to the genital ridge.
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− | * At this stage it is not possible to discriminate between male and female gonads.
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− | * Y chromosomal determination of gonadal sex is only visible when primordial germ cell colonisation is complete.
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− | == Male ==
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− | * Sex cord cells proliferate to form the testes cord. During this period '''SRY''' is expressed for the first time within '''Sertoli cells''' (SRY is NOT expressed in primordial germ cells).
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− | * Sex cords with a basement membrane become '''semniferous cords''' which give rise to '''semniferous tubules'''.
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− | * '''Within the semniferous cords''' are two cell populations:
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− | ** '''Primordial germ cells''' - Prospermatogonia which give rise to '''spermatozoa'''
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− | ** '''Mesodermal cord cells''' which give rise to '''Sertoli cells'''
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− | * '''Between cords''', the loose mesenchyme undergoes '''vascularisation''' and develops as '''stromal tissue'''.
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− | * Within the stromal tissue, cells condense into clusters to form specialized endocrine units - the interstitial '''Leydig's cells'''
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− | == Female ==
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− | * At the time the male gonad is undergoing marked changes under the direction of SRY, the female gonad continues to appear indifferent and does not express SRY.
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− | * Small clusters of mesenchyme surround the germ cells to initiate formation of '''primary follicles''', characteristic of the ovary.
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− | * In follicles
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− | ** '''Mesenchymal cells''' give rise to '''granulosa cells'''
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− | ** '''Germ cells''' give rise to '''oocytes'''
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− | == Foetal sex differentiation ==
| + | ::[[Fertilisation, Implantation and Early Embryonic Development -Sexual Differentiation - Anatomy & Physiology|Turner's Syndrome]] |
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− | * Endocrine activity of the ovaries is NOT essential for sex differentiation during foetal life, abnormal ovarian development will have no effect until puberty.
| + | ::[[Fertilisation, Implantation and Early Embryonic Development -Sexual Differentiation - Anatomy & Physiology|Kleinfelter's Syndrome]] |
− | * Testes secrete two essential hormones:
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− | ** '''Steroidal hormones''' (mainly C19 androgens) from '''leydig's cells'''
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− | ** '''Mullarian Inhibiting Hormone''' (MIH) also known as '''Anti-Mullarian Hormone''' (AMH) from '''Sertoli cells'''
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− | * In absence of these testicular hormones, female differentiation occurs.
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− | * Thus, sexual differentiation must be actively diverted down the male route, the female route requires no active intervention.
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| + | ::[[Fertilisation, Implantation and Early Embryonic Development -Sexual Differentiation - Anatomy & Physiology|Testicular Feminisation]] |
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| + | ::[[Fertilisation, Implantation and Early Embryonic Development -Sexual Differentiation - Anatomy & Physiology|Adrenogenital Syndrome]] |
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− | [[Category:Pregnancy and Parturition]] | + | ::[[Fertilisation, Implantation and Early Embryonic Development -Sexual Differentiation - Anatomy & Physiology|Bovine Freemartinism]] |
− | [[Category:Bullet Points]]
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