63 Development of the reproductive system Flashcards
What does sexual reproduction create?
- Creates genetic variety - useful for adapting to constantly changing and challenging environments
- Some gene variants may become advantageous in future under some environmental constraint or insult
Sexual reproduction requires the formation of sperm and oocytes that are ___ ?
Haploid
What does fertilisation re-establish?
Re-establishes the diploid state and chromosomal sex is determined
Definition of sexual differentiation?
Process by which male and females become structurally and functionally dissimilar
Definition of sex determination?
Initiation of the male differentiation pathway by SRY
What happens if the developing testes (or ovaries) are removed before sexual differentiation?
Female differentiation ensues
What does the development of sexually differentiated gonads drive?
Further sexual differentiation of the reproductive tracts
What are specialised structures of the female reproductive tract needed for?
To nurture the growing embryo and infant after birth
How many chromosomes in human somatic cells? (female vs male)
- 22 pairs of autosomes
- 1 pair fo sex chromosomes (XX or XY)
- Female (46, XX) - homogametic
- Male (46, XY) - heterogametic
What occurs to the number of chromosomes during meiosis? (female vs male)
Germ cells produce gametes with half the number of chromosomes (haploid)
- Oocytes (23, X)
- Spermatozoa (23, X or 23, Y)
What chromosomes do not normally influence gonad differentiation?
Autosomes or “X” chromosomes
The genetic determinant of sex is the presence or absence of the _____ ? Leads to?
Y chromosome
- Presence of Y chrom –> male gonads (testes)
- Absence of Y chrom –> female gonads (ovaries)
What happens once gonadal differentiation is initiated?
The developing gonad directs further development events that produce sexual dimorphism
or
Gonadal hormones trigger the cascades that produce sexual dimorphic development of the reproductive tract
Features of Y chromosome
- Small and most of its DNA is condensed
- Encodes only ~ 48 genes - involved in skeletal growth, tooth development, with few genes involved with testes development
- Regulatory gene - controls other developmental genes and so indirectly controls the formation of testes
Where are many of the genes required for testes development located?
- Autosomes
- X chromosome
• Y chromosome has regulatory gene that control these other developmental genes and so indirectly controls the formation of testes
How does non-disjunction occur between the X and Y chromosome?
Y chromosome retains regions of homology with the X chromosome that permits pairing during meiosis but error can occur (non-disjunction)
What is the non-disjunction in XY females?
Part of the short arm of the Y chromosome is missing or mutated in XY females
What is the non-disjunction in XX males?
- A section of the Y chromosome translocated to an autosome or X chromosome
- This region contain the gene SRY (sex-determining region of the Y chromosome)
What is SRY?
• Sex-determining region of the Y chromosome
• Encodes a DNA binding protein (transcription factor)
that regulates expression of genes on other chromosomes responsible for testes differentiation
Testes and ovaries form from which two distinct cell types?
Gonadal differentiation
- Somatic mesenchyme
2. Primordial germ cells
What is the genital ridge formed by?
Gonadal differentiation
Proliferation of the surface epithelium and condensation of mesenchyme forming sex chords
Gonads are identical in both sexes until which week of development?
(Gonadal differentiation)
7th week
Where do primordial germ cells (PGCs) originate from?
Gonadal differentiation
Epiblast
Where and when are the primordial germ cells (PGCs) first identifiable?
(Gonadal differentiation)
- In the wall of the yolk sac
* ~ 2 weeks
How many primordial germ cells first appear and what do they undergo?
(Gonadal differentiation)
- ~ 10 PGCs appear
* Undergo mitotic divisions en route to the genital ridge (~ 2000 arrive)
Where do the primordial germ cells (PGCs) migrate into? Driven by?
(Gonadal differentiation)
- Migrate into genital ridge
* Driven by chemotaxis
What happens in week 6 with the primordial germ cells (PGCs)?
(Gonadal differentiation)
- Invade the genital ridges
* Become surrounded by the primitive medullary sex cords
What happens if the migration of primordial germ cells (PGCs) fails to enter the genital ridge?
(Gonadal differentiation)
Gonads do not develop