gametogenesis & sex determination Flashcards
where are sperm produced
seminiferous tubules
where are oocytes produced
cortex of the ovary
what are the two ways to form an early germ cell
predetermine/ autonomous
and induced formation
what do BMPs do and how do they do it
Prevent germ cell differentiation into somatic cells.
Smad signalling to the nucleus, makes fragilis and Blimp1 proteins
what is Blimp1 and what cells does it act on
a transcription factor activating Nanos and maintaining Nanog expression.
it acts on Primordial Germ Cells
what does Nanos do?
limits differentiation to somatic cells and stops apoptosis
the SRY gene is a member of which family of transcription factors? which protein does it turn on?
Sox family. Sox9
what is Sox9, give three things it does
an autosomal transcription factor expressed on in a positive feedback loop with its own gene (gonad formation)
it blocks ovary formation by the genital ridge - stops function of the paracrine Wt/beta catenin pathway
activates expression of anti-mullerian hormone
activates expression of FGF9 (in +ve feedback loop with sox9) in the genital ridge
what three genes are involved in the positive feedback loop of male development
SRY -> SOX9 -> FGF9
what does Wnt4 do in females
a paracrine signal that remains high in the female genital ridge but is lost in males
where are oocytes contained
follicles
what are the molecular components that define germ cells
Vasa, Nanos, Tudor, Piwi
what do Vasa proteins do?
bind mRNA and increase translation efficiency of germ cell specific messages
what do Nanos proteins do
bind mRNA and decreases translation efficiency of germ cell specific messages - present in somatic cells (mesoderm/ ectoderm/ endoderm)
what is predetermined (autonomous) formation of germ cells
egg is fertilised, embryo divides, at some point in early development a small group of cells is earmarked- here the germ cells are defined by the segregation of Vasa/ Nanos/ Tudor/ Piwi which become concentrated in the cytoplasm of the egg (the germ plasm) (no external signal)
what animals have predetermined formation of germ cells
Nematodes, Insects, Fish, Frogs
what type of germ cell formation do mammals have
induced formation
what structure of the embryo forms the testis/ovary
the genital ridge
what evidence is there against predetermination in humans
germ cells appear after gastrulation (day 13-19)
what are P granules
a class of perinuclear RNA granules specific to the germ line - Complexes of RNA and proteins that help to form the gonads. "germ granules"
what do P granules do
they contain inhibitors of gene transcription and prevent the germ cells differentiation into somatic cells - concentrated into P cells
what is the location of PGCs in the human embryo (day24)
found in yolk sac (endodermal)
what does every cell in our body start life as
pluripotent epiblast cells
what are the pluripotent markers in PGCs
Nanog and Sox2
what causes posterior epiblast cells to become PGCs
BMPs
why do PGCs migrate to areas outside the embryo proper (humans&mouse to the yolk sac)
To separate these cells from paracrine differentiative signals in the rapidly forming embryo
what does E-cadherin do in germ cell development
it keeps the PGCs together as a group and stops their migration out of the yolk sac and being lost
what is fragilis
a plasma membrane receptor for adhesion with E cadherin (and others)
Induction of mouse germ cell development is done by what group of proteins
Bone Morphogenetic Proteins BMPs - BMP4, BMP8b, BMP2
where do BMPs act and on what cells
extraembryonic epiblast cells
what protein is responsible for specification in mouse germ cell development
Blimp1
what does Blimp1 do and how
Prevent germ cell differentiation into somatic cells.
By transcriptional regulation - activating Nanos and maintaining Nanog
if you remove what protein you fail to get PGCs (in an induced formation)
BMP(4)
what proteins are responsible for PGCs migration to the genital ridge
Nanos3, Kit and Dead end1
what do Nanos3 and Dead end1 do during migration
prevent apoptosis (by binding to mRNAs)
what does Kit do during migration of PGCs?
prevents apoptosis, stimulates proliferation, and may help line path to gonad (no kit = sterile)
what defines the outcome of the PGCs (male/ female)
the Y chromosome ‘gene’ expression by the genital ridges and surrounding tissues (ie the somatic cells)
What is the SRY gene? - major function?
sex determining region of the Y chromosome. (a transcription factor
- The mammalian Testis Determining Factor TDF
which is the shared evolutionary male gene
Sox9
what is Fgf9? give three functions
- Paracrine signalling molecule formed by the somatic cells of the genital ridge
causes proliferation and differentiation of some genital ridge cells to form Sertoli cells and formation of chords of cells - gives the tubular structure of the testis later
Represses (with Sox9) the Wnt/B-catenin path so blocks ovary formation
Coordinates differentiation pathway of PGCs
where is testosterone produced
The interstitial cells between the seminiferous tubules in the testes
What does the Wolffian duct develop into?
Vas deferens, epididymis, seminal vesicle
loss of the androgen receptor results in what
female appearance but with retained testes
what three proteins lead to female development
Wnt, beta-catenin, follistatin
in some reptiles expression of Sox9 is related to what
temperature
– Alligator eggs incubated at 33degreesC 100% male, 30degreesC 100% female
in birds and some reptiles high levels of what drives expression of Sox9 -
high levels of DMRT1 present on the Z chromosome drive expression of Sox9 - a gene dosage system (male ZZ, female ZW)
what does the Mullerian duct develop into
the ovarian ducts, uterus cervix and top of vagina
the fate of the PGCs depend upon the gender of the ______ not that of the ______
The fate of the PGCs depends upon the gender of the ridge not that of the PGCs
when do quiescent PGCs restart meiosis (in males)
puberty
what is induced formation of early germ cells
the germ cell formation occurs after implantation into the uterus and Vasa/ Nanos/ Tudor and Piwi are not present in the egg but their expression is induced by signalling molecules (paracrine molecules) from neighbouring cells
describe the germ cell creation in C.elegans
Vasa/ Nanos/ Tudor and Piwi are concentration in P granules, these are retained in the P cells (P1-P4) in the germ plasm - p granules are markers for the germ plasm to segregate to the p1 then p2 then p3 then p4 cell during each division
how was induction (formation of germ cells) proved
- The PGCs form in the posterior (back) part of the epiblast – if you replace these cells in a embryo with anterior epiblast cells (front) from a embryo expressing GFP - these new cells will go on to form GFP expressing PGCs
- If you remove BMP(4) the inducer – by gene knockout- you fail to get PGCs
what are the main factors for sexual differentiation in males and females
Nanos2 in males
Stra8 in females
how many cells are Blimp1 positive in mouse at embryonic day 6.5
6
in you inject a DNA solution containing the SRY gene into a XX female mouse what happens
you get a male mouse
where is SRY expressed
in somatic cells of the genital ridge
what does Sox9 do at the genital ridge
blocks ovary formation by stopping function of the paracrine Wnt/beta-catenin pathway
in mammals what drives Sox9 expression
SRY gene
sertoli cells induce differentiation of what in males
differentiation of mesenchymal cells in the genital ridge to form interstitial cells- which produce testosterone
what are two things does testosterone do in the embryo
- Allows growth of the Wolffian (mesonephric) duct to form the epididymis and the vas deferens
- Drives the secondary sex determinants
what remains high in the female genital ridge but is lost in males
Wnt4
what does follistatin do in the genital ridge
induces the genital ridge to form an epithelium (granulosa cells) and surrounding the PGCs forming follicles
over expression of beta catenin in males causes what
an ovary to form
what has high levels in the male genital ridge but is reduced in the female
FGF9
what causes the female PGCs to enter meiosis
Retinoic acid diffuses into the PGCs and causes expression of Stra8 (the ‘master switch’ for meiosis)
where is retinoic acid made
in the mesonephros
what is the the principal excretory organ during early human embryonic life (4—8 weeks).
Mesonephros
where does the gonad develop
on the medial surface of the mesonephros
what does the mesonephros form in males and females
forms parts of the epididymis and vas deferens in males, but regresses, in females
What stops PCGs in male fetal testis entering meiosis?
Stra8 levels do not rise in male PCGs because fiboblast growth factor 9 (FGF9) is produced by male gonadal ridge cells and
a) Induces Cyp26b1 which degrades RA before it reaches the PCGs
b) Represses Stra8 expression – so inhibits meiosis entry
what is Cytochrome P26b1 (Cyp26b1)
an enzyme involved in the degradation of retinoic acid - absent in females, only present in male gonads. driven by Sox9/FGF9
both sexes make retinoic acid but it is _______________ in male gonads
- Both sexes make RA but is only metabolised in male gonads
Loss of Cyp26B1 enzyme in males results in what
male PGCs entering meiosis in utero this results in an infertile male (has testis but no sperm)
