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5BBA2300 Essentials of Embryology > Germ & Sex Cell Determination > Flashcards

Germ & Sex Cell Determination Flashcards

1
Q

what are germ cells?

A
  • 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

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2
Q

distinction between germ cells and somatic cells

A

germ cells produce gametes (male - sperm, female - egg)
somatic cells produce all other cells of the organisms (won’t contribute to next generation)

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3
Q

what role does the egg play in organisms with external development such as the Xenopus?

A

the egg provides all the necessary resources for early embryonic development (until the organisms can feed for itself) e.g. Xenopus egg

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4
Q

what does the sperm contribute during fertilisation?

A

its genetic material/chromosomes and a centrosome

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5
Q

what is germplasm?

A

special cytoplasm in eggs of fish, nematode, frogs

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6
Q

Describe the Drosophila experiment that demonstrated that germplasm contains factors necessary for germ cell formation and specification

A
  • 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
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7
Q

what is the role of Blimp1/Prdm1?

A
  • 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)
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8
Q

what effect does UV irradiation have on Posterior Pole cells (PGCs)?

A
  • disrupts their normal development
  • when posterior pole of embryo exposed to UV irradiation it can lead to loss or improper formation of PGCs
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9
Q

PGCs express what transcription factor?

A

Oct4, a TF, associated with stem cell pluripotency

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10
Q

what is the role of the Oskar gene in PGC development in Drosophila?

A
  • 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)
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11
Q

What is Oogenesis?

A
  • process of egg cell (ova) formation in females
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12
Q

What is Spermatogenesis?

A
  • process of sperm cell formation in males
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13
Q

in the embryo when is meiosis stopped?

A
  • In oogenesis, meiosis is initiated during embryonic development but arrested in prophase I
  • in spermatogenesis, meiosis begins at puberty and continues throughout life
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14
Q

why is reduction division in meiosis important?

A
  • 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
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15
Q

for what period are primordial germ cells called primordial germ cells?

A
  • from time of formation until they reach the gonad
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16
Q

what are gonads?

A

Primary reproductive gland that produces reproductive cells (gametes)
males = testes, females = ovaries

17
Q

what is primordial germ cell migration?

A

-process by which primordial germ cells move from their site of origin to the developing gonads

18
Q

primordial germ cells have what receptor and ligand?

A
  • 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
19
Q

what is sex determination?

A

the decision to develop as male or female

20
Q

what is sex differentiation?

A
  • process by which an organism develops into either a male or a female
  • with distinct reproductive structures and secondary sexual characteristics
21
Q

describe the two main mechanisms of sex determination

A
  1. 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
  2. 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)
22
Q

what are the two main mechanisms of sex determination?

A
  • environmental
  • genetic/chromosomal
23
Q

Discuss what abnormalities may occur in genetic/chromosomal sex determination

A

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

24
Q

what is the SRY gene?

A
  • located on the Y chromosome
  • involved in sex determination in mammals
  • activates male development during embryogenesis
25
Q

what is the SRY protein?

A
  • encoded by the SRY gene
  • characterised by an HMG box DNA binding domain
  • triggers formation of testes and male development
26
Q

when does sex determination occur?

A
  • 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
27
Q

what is the role of the HMG Box Domain?

A
  • found in the SRY protein
  • enables binding to specific regions of DNA
  • initiates a cascade of gene expression leading to male development
28
Q

why is timing crucial for sex determination?

A
  • 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
29
Q

What is the role of SF1 in gonadal development?

A

SF1 is a Transcription Factor (TF) involved in gonadal development in both sexes
- it regulates male-specific gene expression of SRY, Sox9 and Fgf9

30
Q

what happens when SRY is present in male embryos?

A
  • 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
31
Q

what happens in the absence of SRY during development?

A
  • 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
32
Q

what are the consequences of loss-of-function mutations in ovary development genes?

A

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

33
Q

what are the consequences of gain-of-function mutations in testes development genes?

A
  • 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
34
Q

what determines ovary development in the absence of SRY?

A
  • the default development pathway represses testes-promoting genes like Sox9
  • repression of Sox9 promotes differentiation of granulosa cells and ovary development
35
Q

what is the cause of sex-reversal in hornless goats?

A

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

36
Q

what are the characteristics of gots with PIS?

A
  • 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

5BBA2300 Essentials of Embryology (12 decks)

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