Difference between revisions of "Sexual Differentiation - Anatomy & Physiology"
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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'''. | * Established at the time of '''fertilisation'''. | ||
| − | * The foetus receives its genetic endowment in two equal portions from the male gamete (''' | + | * 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. | ||
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* 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 ( | + | * 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. | ||
| − | * The presence of SRY alone is sufficient to ensure development of male gonads ( | + | * SRY encodes the protein '''Testes Determining Factor''' (TDF) |
| − | * Female is the default sex, in absence of SRY female gonads ( | + | * 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. | ||
* 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 | + | * Normal oocyte growth requires the activity of both X chromosomes. |
| − | + | * 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: | ||
| + | ** '''Somatic mesenchymal tissue''' forms the matrix of the gonad | ||
| + | ** '''Primordial germ cells''' form the gametes | ||
| + | * '''Genital ridge''' forms from somatic mesenchyme. | ||
| + | * Primordial germ cells migrate to the genital ridge. | ||
| + | * At this stage it is not possible to discriminate between male and female gonads. | ||
| + | * Y chromosomal determination of gonadal sex is only visible when primordial germ cell colonisation is complete. | ||
| + | == Male == | ||
| + | * 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'''. | ||
| + | * '''Within the semniferous cords''' are two cell populations: | ||
| + | ** '''Primordial germ cells''' - Prospermatogonia which give rise to '''spermatozoa''' | ||
| + | ** '''Mesodermal cord cells''' which give rise to '''Sertoli cells''' | ||
| + | * '''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's cells''' | ||
| + | == Female == | ||
| + | * 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. | ||
| + | * In follicles | ||
| + | ** '''Mesenchymal cells''' give rise to '''granulosa cells''' | ||
| + | ** '''Germ cells''' give rise to '''oocytes''' | ||
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| − | + | == 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. |
| + | * Testes secrete two essential hormones: | ||
| + | ** '''Steroidal hormones''' (mainly C19 androgens) from '''leydig's cells''' | ||
| + | ** '''Mullarian Inhibiting Hormone''' (MIH) also known as '''Anti-Mullarian Hormone''' (AMH) from '''Sertoli cells''' | ||
| + | * 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. | ||
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| − | : | + | [[Category:Pregnancy and Parturition]] |
| + | [[Category:Bullet Points]] | ||
Latest revision as of 18:17, 3 July 2012
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, determining if and when they are to be activated.
- SRY encodes the protein Testes Determining Factor (TDF)
- 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.
- Subsequent gonadal development is dependent on a normal population of germ cells.
- Normal oocyte growth requires the activity of both X chromosomes.
- Normal spermatogenesis requires the activity of no more than one X chromosome.
- Early development of gonads proceeds similarly in both sexes. The gonads are derived from two distinct tissues:
- Somatic mesenchymal tissue forms the matrix of the gonad
- Primordial germ cells form the gametes
- Genital ridge forms from somatic mesenchyme.
- Primordial germ cells migrate to the genital ridge.
- At this stage it is not possible to discriminate between male and female gonads.
- Y chromosomal determination of gonadal sex is only visible when primordial germ cell colonisation is complete.
Male
- 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.
- Within the semniferous cords are two cell populations:
- Primordial germ cells - Prospermatogonia which give rise to spermatozoa
- Mesodermal cord cells which give rise to Sertoli cells
- 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's cells
Female
- 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.
- In follicles
- Mesenchymal cells give rise to granulosa cells
- Germ cells give rise to oocytes
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.
- Testes secrete two essential hormones:
- Steroidal hormones (mainly C19 androgens) from leydig's cells
- Mullarian Inhibiting Hormone (MIH) also known as Anti-Mullarian Hormone (AMH) from Sertoli cells
- 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.