Germ & Sex Cell Determination Flashcards
what are germ cells?
- specialised type of cell that develops into egg/sperm [gametes], passes on genetic information
- germ cells are totipotent = can give rise to all cell types incl. more germ cells
nb: germ cells specified & set aside in developing embryo although mature eggs and sperm are functional only in the adult
distinction between germ cells and somatic cells
germ cells produce gametes (male - sperm, female - egg)
somatic cells produce all other cells of the organisms (won’t contribute to next generation)
what role does the egg play in organisms with external development such as the Xenopus?
the egg provides all the necessary resources for early embryonic development (until the organisms can feed for itself) e.g. Xenopus egg
what does the sperm contribute during fertilisation?
its genetic material/chromosomes and a centrosome
what is germplasm?
special cytoplasm in eggs of fish, nematode, frogs
Describe the Drosophila experiment that demonstrated that germplasm contains factors necessary for germ cell formation and specification
- germ cells normally form posteriorly during syncitium (before cellularisation)
- posterior pole plasms (posterior cytoplasm) removed and transplanted into anterior ⟶ germ cells form anteriorly
- indicating germplasm contains factors necessary for germ cell formation & specification
what is the role of Blimp1/Prdm1?
- acts as a suppressor of somatic cell development and plays a role in promoting formation & specification of germ cells
- a mechanism is mammals and chicks (they have no evidence germplasm)
what effect does UV irradiation have on Posterior Pole cells (PGCs)?
- disrupts their normal development
- when posterior pole of embryo exposed to UV irradiation it can lead to loss or improper formation of PGCs
PGCs express what transcription factor?
Oct4, a TF, associated with stem cell pluripotency
what is the role of the Oskar gene in PGC development in Drosophila?
- mutations in the Oskar gene can lead to the loss or impaired formation of PGCs
- Offspring from Oskar mutant individuals typically lack functional germ cells, resulting in infertility
(it helps establish & maintain specialized cytoplasmic region containing germplasm, which is essential for proper PGC development)
What is Oogenesis?
- process of egg cell (ova) formation in females
What is Spermatogenesis?
- process of sperm cell formation in males
in the embryo when is meiosis stopped?
- In oogenesis, meiosis is initiated during embryonic development but arrested in prophase I
- in spermatogenesis, meiosis begins at puberty and continues throughout life
why is reduction division in meiosis important?
- germ cells must undergo reduction division in meiosis to produce gametes
- essential halving of chromosome number at meiosis means that when 2 gametes come together at fertilisation the diploid number is restored
- occuring in gonads
for what period are primordial germ cells called primordial germ cells?
- from time of formation until they reach the gonad
what are gonads?
Primary reproductive gland that produces reproductive cells (gametes)
males = testes, females = ovaries
what is primordial germ cell migration?
-process by which primordial germ cells move from their site of origin to the developing gonads
primordial germ cells have what receptor and ligand?
- PGCs have the receptor CXCR4 and are attracted to the ligand SDF1 so migrate to where ligand is secreted (at genital ridge)
- When the PGCs reach the genital ridge, they form the gonads
what is sex determination?
the decision to develop as male or female
what is sex differentiation?
- process by which an organism develops into either a male or a female
- with distinct reproductive structures and secondary sexual characteristics
describe the two main mechanisms of sex determination
- Environmental – same genetic makeup, environment is influencing factor. E.g. temperature dependent, in certain reptiles higher incubation temps of eggs tend to result in development of a certain temp
- Chromosomal/genetic – presence/ absence of specific sex chromosomes (X and Y) determines the development of male or female characteristics; males/females differ based on:
a. Number of chromosomal sets (haploid etc.) [XX or XY in humans]
b. ratio of X chromosomes to autosomes (presence of Y chromosome and SRY gene triggers development of male characteristics, absence of Y and SRY triggers female characteristics)
what are the two main mechanisms of sex determination?
- environmental
- genetic/chromosomal
Discuss what abnormalities may occur in genetic/chromosomal sex determination
a. Turner Syndrome (X): individual born with a single X chromosome (45,X) ⟶ results in female characteristics but may lead to certain physical & reproductive challenges
b. Triple X Syndrome (XXX): female born with extra X chromosome (47,XXX), may not show noticeable physical effects but can lead to certain developmental & learning challenges in some cases
c. Klinefelter Syndrome (XXY): male is born with an extra X chromosome (47,XXY) ⟶ result in physical, hormonal, and developmental differences
what is the SRY gene?
- located on the Y chromosome
- involved in sex determination in mammals
- activates male development during embryogenesis
what is the SRY protein?
- encoded by the SRY gene
- characterised by an HMG box DNA binding domain
- triggers formation of testes and male development
when does sex determination occur?
- during days 10.5-12.5 of embryonic development
- both males & females initially undifferentiated (bipotential stage)
- SRY gene switched on in males, directing testes development
what is the role of the HMG Box Domain?
- found in the SRY protein
- enables binding to specific regions of DNA
- initiates a cascade of gene expression leading to male development
why is timing crucial for sex determination?
- SRY gene expression occurs in short window
- if SRY not activated, default pathway leads to female development
- critical for determining sex of individual during embryogenesis
What is the role of SF1 in gonadal development?
SF1 is a Transcription Factor (TF) involved in gonadal development in both sexes
- it regulates male-specific gene expression of SRY, Sox9 and Fgf9
what happens when SRY is present in male embryos?
- SRY activates Sox9 expression in the gonads
- Sox9 expression increases until it reaches a threshold level
- autoregulatory mechanism triggers to further increase Sox9 expression
- this leads to Sertoli cell differentiation and development of testes
what happens in the absence of SRY during development?
- without SRY, the Wnt signalling and B-catenin repress Sox9 expression
- repression of Sox9 leads to granulosa cell differentiation in the gonads
- results in ovary development, supporting female sexual development
what are the consequences of loss-of-function mutations in ovary development genes?
loss-of-function mutations may cause XX individuals to develop as males (XX males).
- improper ovary development favours the default developmental pathway leading to testes formation
what are the consequences of gain-of-function mutations in testes development genes?
- Gain-of-function mutations may cause XY individuals to develop as females (XY females)
- Improper testes development leads to development of ovarian structures in XY individuals
what determines ovary development in the absence of SRY?
- the default development pathway represses testes-promoting genes like Sox9
- repression of Sox9 promotes differentiation of granulosa cells and ovary development
what is the cause of sex-reversal in hornless goats?
sex-reversal in hornless goats is associated with a condition called polled intersex (PIS) syndrome
PIS is caused by abnormalities in the FoxI2 transcription factor
what are the characteristics of gots with PIS?
- goats with PIS may have internal genitilia of one sex and external genitalia of another or both sexes
- XX PP goats are sterile males with abnormal epididymis
- XX PP goats may also be intersexes or sterile males with testicular hypoplasia
