Lecture 6- Gametogenesis Flashcards
1
Q
- After colonising the gonad, germ cells:
A
- Proliferate by mitosis
- Reshuffle genetically and reduce to haploid by meiosis
- Cytodifferentiation into mature gametes
- Timing and scale varies between sexes
2
Q
oogenesis
A
- female: ovum
- very few gametes (around 400 in lifetime)
- intermittent production (1/month)
- 1/400th of reproductive potential
3
Q
spermatogenesis
- male: sperm
- huge number (200 million/ day)
- continous production
- essentially disposable cells
A
4
Q
2 main functions of meisosis
A
- Reduce the chromosomes number in the game to 23
- Ensures every gamete is genetically unique
5
Q
- MEIOSIS HAS …….. successive cells division:
A
2
- Meiosis 1 and 2
6
Q
meiosis produces
A
Produces 4 daughter cells
In females only one develops into a mature oocyte, others form polar bodies
7
Q
Genetic variation
Arises from:
A
-
Crossing over (meiosis 1)
- Exchange of regions of DNA between 2 homologous chromosomes
-
Independent assortment
- Random orientation of each bivalent along the metaphase plate with respect to other bivalents
- 8.4 million different varieties
- Random segregation
8
Q
spermatogensis describes
A
the production of male gametes
9
Q
spermatogensis overview
A
- Occurs in the testicle
- Sperm is formed in the seminiferous tubule (spermatogensis)
- Process of maturation of the sperm happens from the outside to the inside of the seminiferous tubules
- Diploids spermatogonium are found closest to the interstitium
- These become primary spermatocytes, then secondary spermatocytes and then the haploid spermatid is found at the centre of the seminiferous tubule
- These are then transferred through the lumen
- Leydig cells are found surrounding the seminiferous tubules in the interstitium
- Produce testosterone to promote spermaturation
- Process of maturation of the sperm happens from the outside to the inside of the seminiferous tubules
- Sperm is formed in the seminiferous tubule (spermatogensis)
10
Q
- Spermatids that emerge from the walls of the seminiferous tubules
A
- swim down the lumen of the seminiferous tubules and these tubules convalesce at the rete testis
- The rete testis concentrates the sperm and then pass onto the ductili efferentes to do the same
- The sperm emerge from the ductili efferentes into the head of the epididymis (storage tube for sperm)
- Upon ejaculation the sperm are released into the Vas deferens
11
Q
epididymis
A
(storage tube for sperm)
12
Q
Cells that aid spermatogenesis
A
- Sertoli cells
- Tight junctions form the blood testes barrier
- Leydig cells
13
Q
Sertoli cells
A
- spermatogenesis occurs supported by Sertoli cells.
- Nurture developing sperm
14
Q
- Tight junctions form the blood testes barrier
A
- Separates basal and adluminal compartment
- Provides correct environment for sperm to mature
- Prevent the immune system from recognising the genetic diversity of the sperm cell
15
Q
A
16
Q
raw material for spermatogenesis
A
spermatogonia
- Available for up to 70 years
17
Q
spermatogonia divide to
A
- Divide by mitosis to give rise to
- Ad spermatogonium (resting; reserve stock)
- Ap spermatogonium (active: maintain stock and from puberty onwards produce Type B spermatogonia which give rise to primary spermatocytes)
18
Q
Primary spermatocytes divide by meiosis giving rise to
A
- Secondary spermatocytes and then spermatids
- Each primary spermatocyte forms 4 haploid spermatids which differentiate (spermiogenesis) into spermatozoa
19
Q
spermiation
A
- release of sperm into seminiferous tubules
20
Q
Spermatogenic cycle (time) and waves (space)
A
- Not all stages in spermatogenesis are visible in a single cross section of seminiferous tubule
- Cells tend to appear in groups with same maturation stage
-
Spermatogenic cycle defined as time taken for reappearance of the same stage within a given segment of tubule (16 days)
- Diff stages in spermatogenesis are also ordered in space as well as time
- Each stage follows in an orderly sequence along the length of the tubule
- The distance between the same stage is called the spermatogenic waves
21
Q
outline steps of spermatogenesis
A
- Spermatogonia (2N, 2C)- found towards the edge of the seminiferous tubule
- Spermatogonia replicates genetic info and moves towards the interior- primary spermatocyte (2N, 4C)
- Primary spermatocye is sanwiched between adjacent Sertoli cells and are separated by from the exterior of the tube by tight junctions
- Tight junctions between Sertoli cells help to form the blood testis barrier
- To prevent autodestruction of gametes
- Primary spermatocyte –>secondary spermatocytes- meosisis 1
- 1N, 2C
- Secondary spermacotcye undergo meosisiss 2 forming haploid spermatid (1N, 1C)
- Haploid spermatids undergo final maturation
- Sperms are transported out of the testes via the epididymis
22
Q
spermiation
A
Spermatids released in lumen of seminiferous tubule
23
Q
spermiogenesis
A
remodeling of sperm as they pass down seminiferous tubule, through rete testis and ductuli efferentes and into the epididymis to finally form spermatozoa
24
Q
spermatids are
A
- Non- motile (transport via Sertoli cell secretions assisted by peristaltic contraction) until they reach epididymis
25
Q
delivery of sperm
A
- seminiferous tubules
- rete testis
- ductili efferntes
- epidiymis
- vas ferens (seminal vesicle secretion)
- urethra (prostate and bulbourethral gland)
26
Q
how much of ejaculate is sperm
A
2-5%
- Ejaculate is mainly seminal vesicle secretions
27
Q
what do sperm need for movement
A
- Fructose is the energy source to power movement
- Zinc- important for motility (the lower the conc, the more motile the sperm are e.g. in vagina)
28
Q
seminal vesicle secretions make up
A
70% of secretions
- aa
- citrate
- fructose
- prostaglandins
29
Q
secretions of the prostate make up
A
25%
- proteolytic enzymes, zinc
30
Q
secretions of the sperm via vas deferens make up
A
2-5%
200-5000 million per ejaculate
31
Q
secretions of the bulbourethral gland
A
also called Cowper gland
- mucoproteins help lubricate and neutralise acidic urine in the distal urethra (<1% total vol)
32
Q
- Final maturation step required before sperm are fertile
A
Sperm capacitation
33
Q
sper capacitation
A
- Conditions in female genital tract stimulate
- Removal of glycoproteins and cholesterol from sperm membrane
- Activation of sperm signalling pathways (atypical soluble adenylyl cyclase and PKA involved)
- Allow sperm to bind to zona pellucida of oocyte and initiate acrosome reaction
34
Q
Maturation of oocytes begins….
*
A
before birth (1)
- Germ cells (arise from yolk sac) colonise the gonadal cortex and differentiation into oogonia (single = oogonium)
- Oogonia then proliferate rapidly by mitosis
- By end of 3rd month oogonia arranged in clusters surrounded by flat epithelial cells
- Majority continue to divide by mitosis, but some enter meiosis (these arrest in prophase of meiosis I and are called oocytes )
- This happens at around 4 month gestation befor ehte baby is born
35
Q
the female has developed he rentire stock of potential gametes…
A
before birth
36
Q
- Max number germ cells reach by
A
- mid gestation (7 million)
- Cell death then begins and many oogonia and primary oocytes degenerate (atresia)
- By the 7th month gestation majority of oogonia have degenerated
37
Q
All surviving (2 mill) primary oocytes have now entered meiosis I and are individually surrounded by layers of flat epithelial cells called follicular cells–>
A
primordial follicle
38
Q
Maturation of oocytes continues at puberty
*
A
- Most oocytes undergo atresia during childhood so around 40,000 remain by puberty
- From pubery onwards 15-20 oocytes start to mature each month passing through 3 stages
- Preantral
- Antral
- preovulatory
39
Q
1) Preantral stage
A
Primary follicles begin to form the surrounding follicular cells change from flat to cuboidal and proliferate to produce a stratified epithelium of granulosa cells
- Secrete glycoprotein coating= zona pellucida- protective
40
Q
2) Antral stage
A
- As the development continues, fluid filled spaces appear between granulosa cells- forming antrum
- Several follicles begin to develop with each ovarian cycle
- Usually only one reaches maturity (rest become atretic)
- Cumulus oophorus cells will remain with the ovum during ovulation, the other cells will form corpus luteum
-
Theca interna- receptors to luteinizing hormone- can produce androgens on stimulation
- Follicular cells can convert androgen to oestrogen
41
Q
3) Preovulatory stage
A
- Surge in LH induces preovulatory growth phase
- Meiosis I is now completed resulting 2 haploid daughter cells of unequal size
- Each daughter cell now has 23 chromatids and 46 chromatids
- One cell receives most of the cytoplasm and the other (first polar body) receives practically none
- Cells then enter meiosis II but arrest in metaphase 3 hours before ovulation
- Meiosis II is only completed if oocyte is fertilised, otherwise cell degenerates 24 hours after ovulation
42
Q
overview of ovum formation
A
- Immature egg starts off as an oogonium
- Diploid set
- Full setoff chromosomes
- 2 sets of chromatids – one from mum and one from dad
- Follicle protects the oocyte and mature together
- Primordial follicle
- Flat squamous follicular cells
- Primary follicle is large
- 1-2 cuboidal follicular cell layers
- Secondary follicles
- Cluster of granulosa cell layers
- Small irregular antral space
- Internal and external theca layers
- Matura follicle
- Single large antrum
- Within the follicle the remaining granulosa cells surrounding the follicle are called the corona radiata
- Stalk of cells anchoring the oocyte to the follicle wall is the cumulus oophorus
- prominent theca interna and theca externa layers
- Primordial follicle
- During follicle development the oocytes stays in the primary oocyte up until the mature follicle
- Primary oocyte (we are born with) stay in a state of arrested prophase (still diploid) until puberty
- During the menstrual cycle the primary oocyte finishes prophase I
- At ovulation- secondary oocyte
- Continues into meiosis 2- becomes haploid
- Arrested in metaphase 2
- This happens right before the secondary oocyte is released from the follicle out of the ovary
- On releasing the secondary oocyte the follicle becomes the corpus luteum
- CL- secrete progesterone and androgens which prepares the uterus for implantation
- If the egg is not fertilised the corpus luteum degenerated into the corpus albicans
- Following ovulation the secondary oocyte is released and guided by the fimbria toward the uterus down the fallopian tubes
43
Q
ovulation
A
Ovulation
- FSH and LH stimulate rapid growth of follicle several days before ovulation occurs
- Mature follicle now 2.5cm In diameter and called graafian follicle
- LH surge increases collagenase activity
- Prostaglandins increase response to LH and cause local muscular contractions in ovarian wall
- Oocyte extruded and break free from ovary
44
Q
corpus luteum
A
- Remaining granulosa and theca internal cells become vascularised
- Develop yellowish pigment and change into lutein cells which form the corpus luteum
- Secrete oestrogen and progesterone
- Stimulate uterine mucosa to enter secretory phase in prep for embryo implantation
- Dies after 14 days if no fertilisation
- Secrete oestrogen and progesterone
45
Q
oocyte transport
A
- Shortly before ovulation fimbriae sweep over the surface of the ovary
- Uterine tube begins to contract rhythmically
- Oocyte carried into tube by sweeping movements of fimbriae (cilia)
- Oocyte then propelled by peristaltic muscular contractions of the tube and by ciulia in the mucosa
- If fertilised, oocyte reaches uterine lumen in 3 to 4 days
46
Q
if no fertilisation occurs
A
- corpus luteum degernates
- forms mass of fibrotic scar tissue- corpus albicans
- progesterone production decreases, precipitating menstrual bleeding
47
Q
if fertilisation occurs
A
- degeneration of CL prevented by human chorionic gonadotrophin secreted by the developing embryo
- CL continues to grow and and forms the CL of prpeganncy (corpus luteum graviditatis)
- cells continue to secrete progesterone until 4th month
- secretion of progesterone by placenta then becomes adequate
48
Q
follicles stimulated to grow by ……… and to amture by ……. and …….
A
follicles stimulated to grow by FSH and to amture by FSH and LH
49
Q
ovulation occurs on
A
LH surge
50
Q
LH also promotes
A
developemnt of the corpus luteum
51
Q
the ovarian cycle
A
52
Q
overview of gametogenesis
A
