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REPRO > Lecture 5: Gametogenesis > Flashcards

Lecture 5: Gametogenesis Flashcards

1
Q

How are mature gametes made?

A
  • germ cells colonise the gonad in the embryo (male germ cells colonise the medulla of developing gonads, and the female germ cells colonise the cortex)
  • proliferate by mitosis
  • reshuffle genetically (genetic diversity) ad reduce to haploid by meiosis
  • cytodifferentiate into mature gametes

Timing and scale of this varies between sexes

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2
Q

Where is the SRY gene found?

A

On the Y chromosome

-codes for a transcription factor that leads to the development of the testis

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3
Q

How does oogenesis differ from spermatogenesis?

A 
Oogenesis: 
-very few gametes (400 in lifetime)
-intermittent production (1/month)
Spermatogenesis:
-huge number made (200 million)
-continuous production
-essentially 'disposable'
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4
Q

What is formed during the process of fertilisation?

A

Zygote (diploid)- from fertilisation of two haploid gametes

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5
Q

What determines the sex of the offspring?

A

The sperm, because it contains XY, whereas female contains only XX

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6
Q

What are the 2 main functions of meiosis?

A

-reduce chromosome number in gamete to 23
-ensures every gamete is genetically unique
Used only in production of sperm and eggs

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7
Q

What does meiosis produce?

A

4 daughter cells

-however in a female only one develops into a mature oocyte, others form polar bodies

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8
Q

How does genetic variation occur?

A

Crossing over: exchanging DNA b/w 2 homologous chromosomes
Independent assortment: random orientationof each bivalent along metaphase plate)
Random segregation: random distribution of alleles amoung the 4 gametes

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9
Q

Where does spermatogenesis occur?

A

Seminiferous tubules, in spaces between adjacent sertoli cells

  • spermatogensis occurs in the walls in the basal compartment
  • spermatids emerge into lumen from the adluminal compartment
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10
Q

Where do the seminiferous tubules coalesce?

A

Rete testis: concentrates the sperm
-it anastomoses then with the ductili efferentes, where the sperm then enter the head of the epididymis (storage tube for sperm)

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11
Q

What is the function of sertoli cells?

A

Nuture the spermatids

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12
Q

What are the compartments in the seminiferous tubules?

A

Compartments are created due to tight junctions between the sertoli cells which form the blood testes barrier
Basal compartment
Adluminal compartment

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13
Q

Why is the blood testes barrier important?

A
  • Different environments are required for the germ cell line (spermatogonium) and the development of the spermatids
  • sperm could be recognised as non self generating an immune response
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14
Q

What other cells are found in the seminiferous tubule?

A

Leydig cells are also found in the seminiferous tubules, and produce testosterone to stimulate spermatogenesis

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15
Q

What are the male germ cells ‘raw material’ called?

A

Spermatogonia

-available for up to 70 years

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16
Q

What are the 2 different forms of spermatogonia?

A
  • Ad spermatogonium: reserve stock (resting)
  • Ap spermatogonium: maintain stock, and from puberty onwards they produce type B spermatogonia which give rise to primary spermatocytes
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17
Q

What are the steps in spermatogenesis?

A
  • spermatogonium divide by mitosis giving rise to Ad or Ap spermatogonium
  • Ad are reserve stock to regenerate numbers
  • Ap go down a meiosis pathway producing type B spermatogonia which give rise to primary spermatocytes
  • primary spermatocytes divide by meiosis giving rise to secondary spermatocytes and then to spermatids which are haploid
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18
Q

What is the difference between spermatogenesis and spermiogenesis?

A

Spermatogenesis is describing the entire process of gamete production in males
Spermiogenesis is describing the differentiation of spermatids into spermatozoa

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19
Q

How long is the spermatogenic cycle?

A

16 days

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20
Q

How have males evolved to increase chances of fertilisation?

A

If spermatogenesis was in synchrony, men would only be fertile every 16 days.
We have adapted so that men are fertile all the time

21
Q

How do cells appear in a cross section of the seminiferous tubule?

A

In groups with the same maturation stage, in a spiral like pattern

22
Q

What is the spermatogenic cycle?

A

The time taken for reappearance of the same stage within a given segment of the tube

23
Q

What is the spermatogenic wave?

A

The distance between the same stage of maturation

24
Q

What is spermiation?

A

Release of spermatids into the lumen of the seminiferous tubule

  • spermiogenesis needs to occur (differentiation) to remodel the sperm
  • they are non-motile until they reach the epididymis (they are transported via sertoli cell secretions assissted by peristaltic contraction)
25
Q

Where in the testis do the spermatids remodel?

A
  • seminferous tubule
  • rete testis
  • ductuli efferentes
  • into the head of the epididymis where spermatozoa are formed
26
Q

What facilitates movement of the sperm tail?

A

Dynein

27
Q

How much semen is produced per ejaculate?

A

2 ml

28
Q

What is the composition of semen?

A

Seminal vesicle (70%)- amino acids, citrate, fructose- energy source for glycolysis, prostaglandins

Prostate (25%)- proteolytic enzymes, zinc

Sperm (2-5%)- 200-500 million per ejaculate

Bulbourethal gland secretions (Cowper gland)- mucoproteins to help lubricate and neutralise acidic urine in distal urethra

29
Q

What controls the motility of sperm?

A

Zinc
-as the amount of zinc lowers, the motility of the sperm increases, so when they enter a female reproductive tract, conc of zinc falls, stimulating a greater motility

30
Q

What is sperm capacitation?

A

Final maturation step required before the sperm becomes fertile
Conditions in female genital tract stimulate:
-removal of glycoproteins and cholesterol from sperm membrane
-activation of sperm signalling pathways
= allows sperm to bind to zona pellucida of oocyte and initate the acrosome reaction

31
Q

What has to be done to sperm in IVF?

A

The sperm are infertile as they haven’t travelled in the womans reproductive tract
-the sperm have to be incubated in capacitation media which mimics the environemnt of female reproductive tract

32
Q

When does maturation of oocytes begin?

A

Before birth

  • germ cells (arise from yolk sac), colonise the gonadal cortex and differentiate into oogonia
  • oogonia then proliferate rapidly by mitosis
  • at end of 3rd month oogonia arranged in clusters surrounded by flat epithelial cells (which eventually differentiate into follicular cells)
  • majority continue to divide by mitosis but some enter meiosis which arrest in prophase of meiosis 1: they are now called primary oocytes
33
Q

How many chromatids fo oogonium and primary oocytes have?

A

Oogonium: 46 chromatids

Primary oocyte: 92 chromatids

34
Q

What is the max number of germ cells reached is females?

A

7 million, and this is reached by mid gestation

  • cell death begins and many oogonia and primary oocytes die: atresia (quality control mechanisms as you want the best 400)
  • at 7th month gestation majority of oogonia have degenerated and 2 million primary oocytes survive
  • these 2 million have now entered meiosis 1 and are individually surrounded by layer of flat epithelial cells now called follicular cells: now called PRIMORDIAL FOLLICLES
35
Q

How many primordial follicles are left at puberty?

A

40,000 remain

36
Q

What happens during each mestrual cycle?

A 
15-20 oocytes start to mature each month (but only 1/2 complete maturation each month)
3 stages of maturation
-preantral
-antral
-preovulatory
37
Q

What happens in the preantral stage?

A

As primordial follicles begin to grow, the surrounding follicular cells change from flat to cuboidal and proliferate to produce stratified epithelium of granulosa cells (granulosa cells are follicular cells)
-granulosa cells secrete a layer of glycoprotein on oocyte called the ZONA PELLUCIDA

38
Q

What happens in the antral stage?

A

Fluid filled spaces appear between the granulosa cells and these coaelsce to form the ANTRUM
-outer fibrous layer (theca externa) and an inner secretory layer (theca interna) surrounds the outside of the granulosa cells

39
Q

What is the cumulus oophorus?

A

The granulosa cells around the oocyte

40
Q

What is the function of theca interna?

A

Interna: receptors to LH, this stimulates production of androgens

41
Q

What happens in the preovulatory stage?

A
  • surge in LH induces this stage
  • meiosis 1 is now complete resulting in 2 haploid daughter cells of unequal size
  • one cell receives most of the cytoplasm, the other receives none (first polar body)
  • the cell then enters meiosis 2 but arrests in metaphase 3 hours before ovulation
  • meiosis 2 is only completed if the oocyte is fertilised, otherwise cells degenerates around 24 hours after ovulation
42
Q

What happens during ovulation?

A
  • FSH and LH stimulate rapid growth of the follicle several days before ovulation occurs
  • mature follicle is now 2.5 cm in diameter and is called the graafian follicle
  • LH surge increases collagenase activity (help rupture of follicle)
  • prostaglandins increase response to LH and cause local muscular contractions in ovarian wall
  • oocyte is extruded and breaks free from ovary
43
Q

What is the corpus luteum?

A

Remaining granulosa and theca interna cells become vascularised and secrete oestrogen and progesterone
-this stimulates the uterine mucosa to enter secretory stage in prep for embryo implantation

44
Q

When does the corpus luteum die if not fertilised?

A

14 days

45
Q

Where does the sperm meet the oocyte?

A

Ampulla

46
Q

How is the oocyte transported?

A
  • fimbriae sweep over surface of ovary
  • uterine tube contracts rhythmically
  • oocyte carried into the tube by sweeping of fimbriae and cilia on epithelial lining
  • oocyte then propelled by peristaltic muscular contractions of the tube and cilia
47
Q

What happens to the corpus luteum if fertilisation doesn’t occur?

A
  • degenerates
  • forms a mass of fibrotic scar tissue called the corpus albicans
  • progesterone production decreases, precipitating menstrual bleeding
48
Q

What happens to the corpus luteum if fertilisation does occur?

A
  • degeneration of CL prevented by human chorionic gonadotropin secreted by developing embryo
  • CL continues to grow and forms the corpus luteum of pregnancy (corpus luteum of pregnancy)
  • cells continue to secrete progesterone until 4th month, secretion of progesterone by placenta then becomes adequate

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