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Sexual Differentiation - Anatomy & Physiology (view source)
Revision as of 14:47, 13 August 2009
, 14:47, 13 August 2009→Genetic Sex
= Genetic Sex =
= Genetic Sex =
* Established at the time of '''fertilisation'''.
* 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 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.
* 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.
* '''Females''' have '''XX''' chromosomes. All oocytes contain one X chromosome, they are the '''homogametic''' sex.
* The '''Y''' chromosome is required to '''initiate male development'''.
* The '''Y''' chromosome is required to '''initiate male development'''.
* In absence of the Y chromosome, female gonads (Ovaries) will develop.
* 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).
* 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, determining if and when they are to be activated.
* 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.
* SRY encodes the protein '''Testes Determining Factor''' (TDF)
* SRY encodes the protein '''Testes Determining Factor''' (TDF)
* The presence of SRY alone is sufficient to ensure development of male gonads (Testes).
* The presence of SRY alone is sufficient to ensure development of male gonads (testes).
* Female is the default sex, in absence of SRY female gonads (Ovaries) will develop.
* Female is the default sex, in absence of SRY female gonads (ovaries) will develop.
* Subsequent gonadal development is dependent on a normal population of '''germ cells'''.
* Subsequent gonadal development is dependent on a normal population of '''germ cells'''.
* Normal Oocyte growth requires the activity of both X chromosomes.
* Normal oocyte growth requires the activity of both X chromosomes.
* Normal Spermatogenesis requires the activity of no more than one X chromosome.
* Normal spermatogenesis requires the activity of no more than one X chromosome.
* 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).
* 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).
* Sex cords with a basement membrane become '''Semniferous Cords''' which give rise to '''Semniferous Tubules'''.
* Sex cords with a basement membrane become '''semniferous Cords''' which give rise to '''semniferous Tubules'''.
* '''Within the semniferous cords''' are two cell populations:
* '''Within the semniferous cords''' are two cell populations:
** '''Primordial germ cells''' - Prospermatogonia which give rise to '''Spermatozoa'''
** '''Primordial germ cells''' - Prospermatogonia which give rise to '''spermatozoa'''
** '''Mesodermal cord cells''' which give rise to '''Sertoli cells'''
** '''Mesodermal cord cells''' which give rise to '''Sertoli cells'''
* '''Between cords''', the loose mesenchyme undergoes '''vascularisation''' and develops as '''stromal tissue'''.
* '''Between cords''', the loose mesenchyme undergoes '''vascularisation''' and develops as '''stromal tissue'''.
* Within the stromal tissue, cells condense into clusters to form specialized endocrine units - the interstitial '''Leydig Cells'''
* Within the stromal tissue, cells condense into clusters to form specialized endocrine units - the interstitial '''Leydig's cells'''
* 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.
* 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.
* Small clusters of mesenchyme surround the germ cells to initiate formation of '''Primary Follicles''', characteristic of the Ovary.
* Small clusters of mesenchyme surround the germ cells to initiate formation of '''primary follicles''', characteristic of the ovary.
* In follicles:
* In follicles
** '''Mesenchymal cells''' give rise to '''Granulosa cells'''
** '''Mesenchymal cells''' give rise to '''granulosa cells'''
** '''Germ cells''' give rise to '''Oocytes'''
** '''Germ cells''' give rise to '''oocytes'''
== Foetal sex differentiation ==
== Foetal sex differentiation ==
* Endocrine activity of the Ovaries is NOT essential for sex differentiation during foetal life, abnormal ovarian development will have no effect until puberty.
* Endocrine activity of the ovaries is NOT essential for sex differentiation during foetal life, abnormal ovarian development will have no effect until puberty.
* Testes secrete two essential hormones:
* Testes secrete two essential hormones:
** '''Steroidal hormones''' (mainly C19 androgens) from '''Leydig cells'''
** '''Steroidal hormones''' (mainly C19 androgens) from '''leydig's cells'''
** '''Mullarian Inhibiting Hormone''' (MIH) also known as '''Anti-Mullarian Hormone''' (AMH) from '''Sertoli Cells'''
** '''Mullarian Inhibiting Hormone''' (MIH) also known as '''Anti-Mullarian Hormone''' (AMH) from '''Sertoli cells'''
* In absence of these testicular hormones, female differentiation occurs.
* In absence of these testicular hormones, female differentiation occurs.
* Thus, sexual differentiation must be actively diverted down the male route, the female route requires no active intervention.
* Thus, sexual differentiation must be actively diverted down the male route, the female route requires no active intervention.