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
delivery of sperm
1. seminiferous tubules
2. rete testis
3. ductili efferntes
4. epidiymis
5. vas ferens (seminal vesicle secretion)
6. urethra (prostate and bulbourethral gland)
26
how much of ejaculate is sperm
2-5%
* Ejaculate is mainly seminal vesicle secretions
27
what do sperm need for movement
* **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
seminal vesicle secretions make up
70% of secretions
* aa
* citrate
* fructose
* prostaglandins
29
secretions of the prostate make up
25%
- proteolytic enzymes, zinc
30
secretions of the sperm via vas deferens make up
2-5%
200-5000 million per ejaculate
31
secretions of the bulbourethral gland
also called Cowper gland
- mucoproteins help lubricate and neutralise acidic urine in the distal urethra (\<1% total vol)
32
* Final maturation step required before sperm are fertile
**Sperm capacitation**
33
sper capacitation
* 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
**Maturation of oocytes begins....**
*
**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
the female has developed he rentire stock of potential gametes...
before birth
36
* Max number germ cells reach by
* 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
All surviving (2 mill) primary oocytes have now entered meiosis I and are individually surrounded by layers of flat epithelial cells called follicular cells--\>
primordial follicle
38
**Maturation of oocytes continues at puberty**
*
* 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
**1) Preantral stage**
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
**2) Antral stage**
* 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
**3) Preovulatory stage**
* 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
overview of ovum formation
* 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
* 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
ovulation
**Ovulation**
1. FSH and LH stimulate rapid growth of follicle several days before ovulation occurs
2. Mature follicle now 2.5cm In diameter and called graafian follicle
3. LH surge increases **collagenase** activity
4. **Prostaglandins** increase response to LH and cause local **muscular contractions** in ovarian wall
5. Oocyte extruded and break free from ovary
44
corpus luteum
* 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
45
oocyte transport
* 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
if no fertilisation occurs
* corpus luteum degernates
* forms mass of fibrotic scar tissue- corpus albicans
* progesterone production decreases, precipitating menstrual bleeding
47
if fertilisation occurs
* 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
follicles stimulated to grow by ......... and to amture by ....... and .......
follicles stimulated to grow by FSH and to amture by FSH and LH
49
ovulation occurs on
LH surge
50
LH also promotes
developemnt of the corpus luteum
51
the ovarian cycle
52
overview of gametogenesis
