Difference between revisions of "Sexual Differentiation - Anatomy & Physiology"

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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).  SRY is a very small piece of DNA that acts as a molecular switch to regulate structural genes and autosomal chromosomes
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* The presence of SRY alone is sufficient to ensure development of male gonads (Testes).
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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 =
 
−
* 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 chromosome is larger than the Y sex chromosome.
 
−
* '''Females''' have '''XX''' chromosomes.  All oocytes contain one X chromosome, they are the '''homogametic''' sex.
 
−
* '''Males''' have '''XY''' chromosomes.  Spermatozoa form two distinct populations, one containing X chromosomes and one containing Y chromosomes.  They are the '''heterogametic''' sex.
 
  
  
−
* The '''Y''' chromosome is required to '''initiate male development'''.
 
−
* In absence of the Y chromosome, female gonads (ovaries) will develop.
 
−
* 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 ==
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::[[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.
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::[[Fertilisation, Implantation and Early Embryonic Development -Sexual Differentiation - Anatomy & Physiology|Kleinfelter's Syndrome]]
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* 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]]
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::[[Fertilisation, Implantation and Early Embryonic Development -Sexual Differentiation - Anatomy & Physiology|Bovine Freemartinism]]
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[[Category:Bullet Points]]
 

Revision as of 09:20, 3 July 2008

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 chromosome is larger than the Y sex chromosome.
  • Females have XX chromosomes. All oocytes contain one X chromosome, they are the homogametic sex.
  • Males have XY chromosomes. Spermatozoa form two distinct populations, one containing X chromosomes and one containing Y chromosomes. They are the heterogametic sex.
  • The Y chromosome is required to initiate male development.
  • In absence of the Y chromosome, female gonads (Ovaries) will develop.
  • The prime male determining gene is SRY (sex determining region of the Y chromosome). SRY is a very small piece of DNA that acts as a molecular switch to regulate structural genes and autosomal chromosomes
  • The presence of SRY alone is sufficient to ensure development of male gonads (Testes).

Internal Genetalia

External Genetalia

Turner's Syndrome
Kleinfelter's Syndrome
Testicular Feminisation
Adrenogenital Syndrome
Bovine Freemartinism
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