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BGDA Lectures > Gametogenesis, fertilization and preimplantation development - BGDA > Flashcards

Gametogenesis, fertilization and preimplantation development - BGDA Flashcards

1
Q

What are the four phases of gametogenesis?

A

1) Extraembryonic origin of the germ cells and their migration into the developing gonads
2) Increase in number of germ cells by mitosis
3) Reduction in chromosomal number by meiosis
4) Structural and functional maturation of the eggs and spermatozoa

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2
Q

Where are PGCs formed and when?

A

Epiblast in 2nd week

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3
Q

What is the initial movement of PGCs?

A

They move through the primitive streak during gastrulation and migrate to the wall of the yolk sac.

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4
Q

What happens to PGCs in week 4?

A

Begin to migrate from the yolk sac to the wall of the gut tube. Continue migration from gut tube via the mesentery of the gut to the dorsal body wall.

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5
Q

What happens to PGCs in the 6th week?

A

Come to rest on either side of the midline. Most PGCs populate the region of the body wall at the level that will participate in the formation of the gonads.

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6
Q

What does the interphase of the cell cycle involve?

A

a) G1: a period of active RNA and protein synthesis
b) S: DNA duplication
c) G2: prepare for mitosis
d) G0: as new postmitotic cells specialise and differentiate, cell cycle activities may be temporarily or permanently suspended

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7
Q

What are the 4 phases of the division stage of cell cycle?

A

Prophase, metaphase, anaphase, telophase

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8
Q

Mitosis occurs in all cells except?

A

Germ cells

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9
Q

Which cells undergo miosis?

A

Only the cells that will form sperm and egg cells involving two unique and closely associated cell division.
* Miosis I
* Miosis II

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10
Q

Outline the key phases of mitosis.

A

A) Prophase
* nucleolus disappears
* replicated chromatin condenses into chromosomes

B) Metaphase
* chromosomes line up in the equatorial plane
* each chromosome is attached by microtubules extending from the centomere to the centriole, forming the mitotic splindle

C) Anaphase
* the centomere of each chromosome divides
* migration of chromatids to opposite poles of the splindle

D) Telophase
* chromosomes uncoil and lengthen
* the nuclear envelop reforms
* cytoplasm divides

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11
Q

Outline the process of miosis.

A

Miosis I:
a) Prophase I:
- chromosomes of each homologous pair match up, centomere to centomere, to form a joint structure called a chiasma
- crossing over occurs

b) Metaphase I
- the four-stranded chiasma structures are organised on the equator of a spindle apparatus

c) Anaphase I
- each homologous pair is distributed to one of the two daughter nuclei

d) Telophase I
- sister chromatids remain joined

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12
Q

What happens during Miosis II?

A

No DNA replication occurs here.
a) Metaphase II
- chromosomes line up single file in haploid cells
b) Anaphase II/Telophase II
- sister chromatids separate into nonidentical haploid cells

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13
Q

Which hormones regulate spermatogenesis?

A

LH - stimulates testosterone production by Leydig cells
FSH - binds to sertoli cells to stimulate testicular fluid production and synthesis of intracellular androgen receptor proteins

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14
Q

What is oogenesis?

A

The process whereby oogonia differentiate into mature oocytes.

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15
Q

What are the two stages of oogenesis?

A

Prenatal Maturation of oocytes and Postnatal Maturation of oocytes

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16
Q

Outline key features of prenatal maturation of oocytes.

A
  • By 12 weeks of development, oogonia in the genital ridges enter the first meiotic prophase and then almost immediately become dormant (arrested in prophase I and do not finish their first meioitic division before puberty is reached)
  • By 5 months, when all oogonia have initiated the first meiotic division to become primary oocytes
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17
Q

During which month of development have germ cells in the ovary reached maximum?

A

Fifth month

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18
Q

What happens near the time of birth during postnatal maturation of oocytes?

A

Near the time of birth, all primary oocytes have started prophase of meiosis I and arrested in diplotene stage
- no oogonia develop after birth

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19
Q

What happens at puberty during postnatal maturation of oocytes?

A
  • Each month, 15-20 follicles selected from follicular pool begin to mature
  • Some of these die, while others begin to accumulate fluid in a space called the antrum (antral or vesicular stage)
  • Dominant follicle continues to become mature vesicular follicles or graafian follicles approx 37 hours prior to ovulation
  • Completion of meiosis I and releasing of one secondary oocyte and first polar body
  • Secondary oocyte enters meiosis II but arrests in metaphase approx 3 hours before ovulation
  • Meiosis II is completed only if the oocyte is fertilised
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20
Q

What are the effects of FSH on ovulation?

A
  • Promotes the development of follicles in each cycle
  • Stimulates maturation of follicular (granulosa) and theca folliculi
  • Production of estrogen due to the cooperation of theca interna and granulosa cells
21
Q

What is the result of estrogen production during ovulation?

A
  • the uterine endometrium enters the follicular or proliferative phase
  • thinning of the cervical mucus occurs to allow passage of sperm
  • the anterior lobe of the pituitary gland is stimulated to secrete LH
22
Q

What does the midcyle LH surge during ovulation do?

A
  • Causes oocytes to complete meiosis I and initiate meiosis II
  • Stimulates production of progesterone by follicular stromal cells (luteinization)
  • Causes follicular rupture and ovulation
23
Q

How is the corpus luteum formed?

A

Formed under the influence of LH
- granulosa cells remaining in the wall of the ruptured follicle
- theca interna

24
Q

What is the function of the corpus luteum?

A

Secretion of estrogens and progesterone - causes the uterine mucosa to enter the pro-gestational or secretory stage in preparation for implantation of the embryo

25
Q

What happens to the corpus luteum if fertilisation occurs?

A
  • Degeneration of the corpus luteum is prevented by hCG
  • continues to grow and forms the corpus luteum of pregnancy (corpus luteum graviditatis)
  • this structure regresses slowly as secretion of progesterone by the trophoblastic component of the placenta becomes adequate for maintenance of pregnancy
26
Q

How does oocyte transport occur?

A

Carried into the tube by sweeping movements of the fimbriae and by motion of cilia on the epithelial lining
Reaches uterine lumen in 3-4 days

27
Q

What is fertilization?

A

The process by which male and female gametes fuse

28
Q

Where does fertilization occur?

A

In the ampullary region of the uterine tube

29
Q

What is capacitation?

A

A period of conditioning in the oviduct that lasts about 7 hours. Involves the epithelial interactions between the sperm and the mucosal surface of the tube. Removal of a glycoprotein coat and seminal plasma proteins from the plasma membrane that overlies the acrosomal region of the spermatozoa.

30
Q

What is the acrosome reaction?

A

Occurs after binding to the zona pellucida, induced by zona proteins. Culminates in the release of enzymes needed to penetrate the zona pellucida.

31
Q

What is the 1st phase of fertilization?

A

Penetration of the corona radiata - when capacitated sperm pass freely through corona cells

32
Q

What is the 2nd phase of fertilization?

A

Penetration of the zona pellucida:
* Both binding and the acrosome reaction are mediated by the ligand ZP2/ZP3, a zona protein
* Plasma membrane covering the acrosomal head cap disappears
* Release of acrosomal enzymes (acrosin) allows sperm to penetrate the zona

33
Q

What is the 3rd phase of fertilization?

A

Fusion of the oocyte and sperm cell membranes
* Actual fusion is accompanied between the oocyte membrane and the membrane that covers the posterior region of the sperm head.

34
Q

What are the consequences of membrane fusion?

A
  • Formation of a calcium wave that radiates over the surface of the egg from the point of sperm contact
  • Release of the contents of thousands of small cortical granules, located just beneath the oocyte cell membrane, into the perivitelline space between the oocyte and zona pellucida
35
Q

How does the egg respond once the spermatozoon has entered the oocyte?

A

1) Cortical and zona reactions
* prevent polyspermy

2) Resumption of the second meiotic division

3) Metabolic activation of the egg
* the activating factor is probably carried by the spermatozoon
* activation encompasses the initial cellular and molecular events associated with early embryogenesis

36
Q

What are the main results of fertilization?

A

1) Restoration of the diploid number of chromosomes

2) Determination of the sex of the new individual
* an XX carrying sperm produces a female embryo
* a Y-carrying sperm produces a male embryo

3) Initiation of cleavage
* without fertilization, the oocyte usually degenerates 24 hours after ovulation

37
Q

What is cleavage?

A

A series of mitotic divisions that increase the number of cells - daughter cells become smaller with each cleavage division (known as blastomeres)

38
Q

What happens after the third cleavage?

A
  • form a compact ball of cells held together by tight junctions
  • compaction: segregates inner cells, which communicate extensively by gap junctions from outer cells
39
Q

What is the morula?

A

16-cell stage

40
Q

What do the inner cells of the morula give rise to?

A

Tissues of the embryo proper

41
Q

What do the surrounding cells of the morula give rise to?

A

Trophoblast, which later contributes to the placenta

42
Q

How does the blastocyst form?

A

When the morula enters the uterine cavity, fluid begins to penetrate through the zona pellucida into the intercellular spaces of the inner cell mass, forming a single cavity the blastocele

43
Q

What is hatching?

A

When the zona pellucida disappears on day 7

44
Q

What are the roles of the zona pellucida?

A

Prevents polyspermy, allows fertilization, induces meiosis II and prevents ectopic implantation

45
Q

How does gametogenesis become defective?

A
  • Some PGCs may become stranded during their migration
  • Come to rest at extragonadal sites
  • Occasionally give rise to a type of tumour called a teratoma (germ cell tumour)
46
Q

How do sperm abnormalities present?

A
  • Double head/tails
  • Double flagella
  • Loss of acrosome
  • Loss of motility
47
Q

What is oligospermia?

A

Low sperm count - less than 20 million sperm after 72 hour abstinence from sex

48
Q

What is azoospermia?

A

Absent sperm due to blockage of duct network

49
Q

What is immotile cilia syndrome?

A

Lack of sperm motility

BGDA Lectures (38 decks)

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