Development of the Reproductive System and Sex Determination Flashcards
Why is sexual reproduction advantageous ?
- Sexual reproduction creates genetic variety, which in turn is useful for adapting to constantly changing and challenging environments
- Some gene variants may become advantageous in the future under some environmental constraint or insult
What are the requirements of sexual reproduction ? What is the result of sexual reproduction ?
- Sexual reproduction requires formation of sperm and oocytes that are haploid
- Fertilization re-establishes the diploid state and chromosomal sex is determined
Define sexual differentiation.
- Process by which male and females become structurally and functionally dissimilar.
- If the developing testes (or ovaries) are removed before sexual differentiation, then female differentiation ensues.
- The development of sexually differentiated gonads drives further sexual differentiation of the reproductive tracts
Define sexual determination.
The term sex determination is used to describe the initiation of the male differentiation pathway by SRY.
What is the aim of specialised structures of the female reproductive tract ?
Specialized structures of the female reproductive tract are needed to nurture the growing embryo and infant after birth.
Describe the chromosomal composition of human somatic cells.
Human somatic cells have 22 pairs of autosomes and 1 pair of sex chromosomes (XX or XY)
Distinguish between males and females’ chromosomes.
- Female (46, XX) – homogametic
* Male (46, XY) – heterogametic
Identify the end result of meiosis of germ cells.
During meiosis germ cells produce gametes with half the number of chromosomes (haploid):
• Oocytes are 23, X
• Spermatozoa are 23, X or 23, Y
How can we distinguish between male and female karyotype visibly ?
Male, XY (X smaller than Y)
Female, XX
Identify the genetic determinant of sex.
The genetic determinant of sex is the presence or absence of the Y chromosome;
• Presence of “Y” Chrom à male gonads (testes)
• Absence of “Y” Chrom à female gonads (ovaries)
Which chromosomes do not impact gonad differentiation ?
Autosomes or “X” chromosomes do not normally influence gonad differentiation
Once gonadal differentiation is initiated by the presence or absence of the Y chromosome, what controls further sexual events ?
Once gonadal differentiation is initiated, the developing gonad directs further developmental events that produce sexual dimorphism
Describe the structural feature, and genes present of the Y chromosome.
The Y chromosome is 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
Which chromosomes are involved with testes development ?
Y chromosome encodes only few genes involved with testes development.
Many of the genes required for testes development are located on autosomes or on the X chromosome. Instead, Y chromosome has a regulatory gene that controls these other developmental genes and so indirectly controls the formation of testes.
How was the location of the regulatory gene on the Y chromosome that controls other developmental genes on the X chromosome and autosomes, and so indirectly controls the formation of testes.
The location of this regulatory gene was established in animal studies and by studying subjects with genetic abnormalities.
E.g. Phenotypic women with “XY” genotypes or phenotypic men with “XX” genotypes
What happens, to the chromosomes of an XY female ?
Part of the short arm of the Y chromosome is missing or mutated in XY females (due to non-disjunction)
This region contains a gene known as SRY (Sex-determining region of the Y chromosome).
What happens to the chromosomes of an XX male ?
XX males have a section of the Y chromosome translocated to an autosome or X chromosome (due to non-disjunction)
This region contains a gene known as SRY (Sex-determining region of the Y chromosome).
Given that X and Y chromosomes are different, how can they pair ?
Because the Y chromosome retains regions of homology with the X chromosome that permits pairing during meiosis but errors can occur called non-disjunction
Describe the function of the SRY gene. What happens following SRY’s action ?
SRY – encodes a DNA binding protein (transcription factor) that regulates expression of genes on other chromosomes responsible for testes differentiation.
Once gonadal differentiation is initiated, gonadal hormones trigger the cascades that produce sexual dimorphic development of the reproductive tract
Identify the steps of gonadal differentiation which are common to both genders.
• Primordial germ cells (PGCs) originate from the epiblast and are first identifiable in the wall of the yolk sac (~2 wks)
• ~10 primordial germ cells first appear but undergo mitotic divisions en route to the genital ridge (~2000 arrive)
• They migrate into the genital ridge driven by chemotaxis
—–
• By 6 wks the PGCs invade the genital ridges and become surrounded by the primitive medullary sex cords
• If the migration of PGCs fails to enter the genital ridge the gonads do not develop
• PGCs have an inductive influence on development of the gonad into ovaries and testes but no role driving sexual dimorphism
• The path of development is determined by the presence of Y chromosome (SRY)
Which cell types do ovaries and testes develop from ?
• Testes and ovaries form from two distinct cell types
– Somatic mesenchyme (immature unspecialised CT)
– Primordial germ cells (cells destined to be
How long are male and female gonads identical ?
• Gonads are identical in both sexes until the 7th wk of dev (indifferent gonads)
Describe the steps of gonadal differentiation in the male (AFTER steps common to both).
1) Under the influence of the SRY gene, primitive sex cords proliferate and penetrate the medulla forming testis cords
2) Testis cords become looped –contact with the ingrowing mesonephric tubule called Rete testis
3) Primordial germ cells come to reside within the developing testis cords
4) Mesodermal cells differentiate into Sertoli cells in the cords
5) Mesenchyme tissue in interstitial spaces develop into Leydig
cells and start to secrete testosterone by the 8th wk
6) Testosterone then influences the development of the genital ducts and external genitalia
7) By 20 wks, the testis cords are horseshoe-shaped, composed of germ cells and Sertoli cells
Describe the steps of gonadal differentiation in the female (AFTER steps common to both).
1) Female development (past 6th week) due to the absence of Y chromosome, leads to degeneration of medullary cords/primitive sex cords
2) Cortical cords form from proliferating surface epithelium forming distinct cell clusters around germ cells by 12 wks
3) Cells proliferate and surround each oogonium (germ cell) with epithelial layer of follicular cells – forming primordial follicles
4) Germ cells are absolutely required for the formation of the ovarian follicles
