Spermatogenesis Flashcards
What is spermatogenesis?
The process whereby spermatozoa are formed
What are the 3 phases of spermatogenesis?
- Proliferation (mitosis)
- Meiosis
- Differentiation
Where does spermatogenesis take place?
In the seminiferous tubules
What happens in the proliferation phase of spermatogenesis?
- Mitotic divisions of spermatogonia
- Large number of B spermatogonia produced from A spermatogonia
- Start as A up against basement membrane, A1-4
- Push between Sertoli cells towards lumen
- then form intemediate spermatogonia
- Final mitotic division from I to B-spermatogonia
Describe the I-spermatogonia
- Linked by intercellular bridges
- Allows cohorts to develop in synchornicity
- Some are not joined and will go back to replenish stem cell pool and start again at A1
How long does spermatogenesis take?
5-9 weeks depending on species
Describe the meiosis stage of spermatogenesis
- Mitotic division from B-spermatogonia to primary spermatocytes
- Then meiotic division from primary to secondary spermatocytes
- DNA repication adn crossing over occurs
Why is crossing over in spermatogenesis meiosis important?
Ensures genetic heterogeneity
Describe the differentiation phase of spermatogenesis
- Spermatids from secondary spermatocytes via meiosis
- Spermatids differentate into spermatozoa (no division in this phase)
What stage of spermatogenesis has the longest life span and why?
- Primary spermatocyte
- Prophase of meiosis I is long process
What stages can differentiation be split into?
- Golgi
- Cap
- Acrosomal
- Maturation phase
Describe the golgi phase of differentiation
- Active golgi in spermatid
- Forms golgi vesicles that fuse to form acrosomic vesicles
- Centrioles migrate to base of nucleus
- Proximal centriole anchors tail to nucleus, distal forms central part of flagellum
- Dense acrosomic granules as important enzymes are being produced
Describe the cap phase of differentiation
- Acrosome forms cap
- Golgi apparatus migrates and disappears
- Primitive flagellum forms (axoneme)
- Change in morphology
Describe the acrosomal phase of differentiation
- Sperm start to take on familiar shape
- Acrosome spreads out, nucleus longates
- Acrosome ocvers 2/3rds of nucleus
- Manchette tubules form
- Neck and annulus formed
- Proximal tubule lodged at base of nucleus
- More elongation, mitochondria appearing
Describe the maturation phase of differentiation
- Manchette tubules formm postnuclear cap, pick up where acrosomal cap stops
- mitochondria form spiral assembly in middl epiece of sperm
- Annulus forms junction between middle and principle piece of sperm
What is meant by the cycle of the seminiferous epithelium?
- Progression through a complete series of stages (cellular associations) ato one location along the serminiferous tubule
- At any onne time, different sections will be at different stages of development
What is the importance of he seminiferous tubule having different stages of development?
Different sections at different stages so not all sections will be releasing sperm at any one time
In what direction do spermatozoa develop
- Start close to basement membrane
- Move towards lumen as develop
In the cycle of the seminiferous tubules, what is meant by a stage?
Specific cellular associations at any cross section at any moment in time e.g. stage 1, which contains A’gonia, 1’cyte, 2’cyte and spermatids
- Go up to stage 8
What is meant by stage duration?
The time taken to progress from one stage to the next
What is meant by cycle duration?
The time taken to complete any one cycle (i.e. to go through all stages)
Explain why the time to complete spermatogenesis in the bull is 61 days, or 4.5 cycles
- Each cycle is 13.5 days, and this contains each stage once
- Each cell has to pass through each stage more than once as it develops through its different forms, each time going through the stage in a different form of development
- In the bull, each cycle lasts 13.5 days, and each cel has to go through the cycel 4.5 times
- the half cycle is because the earliest A-spermatogonia start their development in the middle of stage III
- The duration of each stage and length of cycle varies between species
What is the spermatogenic wave?
- Different stages along each section of the tubule
- Some sections will be releasing sperm while others will not
- Allows steady trickle of sperm to avoid overload
Where do the stem cells for the earliest A-spermatogonia come from?
are from A4-spermatogonia that do not develop into the linked I-spermatogonia and instead go back to form stem cells. This occurs half way through stage III
What morphological changes occur in the head of sperm in the differentiation to spermatozoa?
- Nucleus, acrosome and post-nuclear cap develop
- Shape varies across species
What is contained in the acrosome?
Hydrolytic enzymes
What is the function of the acrosome?
release hydrolytic enzymes for the penetration of the zona pellucida
describe the morphological features in the tail of the sperm during differentiation to spermatozoa
- Self powered flagellum composed of 4 parts
- Capitulum (part of middle piece that fits into the posterior nucleus)
- Middle piece (containing mitochondria)
- Principle piece
- Terminal piece
What hormones is spermatogenesis dependent on?
- GnRH to stimulate secretion of FSH and LH
- Testosterone and oestrogen from gonad
describe the hypothalamic-pituitary-gonadal axis
- Hypothalamus secrete GnRH
- Stimulates FSH and LH release from anterior pituitary
- LH stimulates testosterone from the Leydig cells
- FSH stimulates conversion of testosterone to oestradiol in the Sertoli cells, also stimulates release of inhibin
Describe the nature of GnRH release
Pulsatile, every 4-8 hours
Describe the action of LH
- Binds to Leydig cells
- Stimulates testosterone production
- Prolonged LH secretion shuts down expression of own receptors on Leydig cells, stops production of testosterone (negative feedback)
What is the importance of the pulsatile secretion of GnRH
- Leads to pulsatile secretion of LH
- In order to continue production of testosterone, need pulsatile secretion of LH (more would shut dpwn receptors)
Describe the next steps for testosterone following release from the Leydig cells
- Back to GnRH to inhibit release (if too high)
- Crosses to Sertoli cells for conversion to oestradiol
What is the action of FSH?
- Binds to receptors on Sertoli cells
- Enables conversion of testosterone to oestradiol and dihydrotestosterone
- Also stimulates inhibin
What is the action of dihydrotestosterone?
- Inhibit GnRH release from hypothalamus
- More potent than testosterone alone
What is the action of inhibin?
Inhibits action of FSH and LH
What is the effect of the oestradiol produced in the Sertoli cells?
Negative feedback to stop production of GnRH
Outline the synthetic pathway for androgen production
- All from cholesterol
- Series of enzymatic conversions
- Cholesterol to progesterone to androstenedione to testosterone to dihydrotestosterone
What is the function of the epididymis?
Maturation and storage of spermatozoa
How long is the epididymis in
a. cats
b. bulls
A: 2m long
B: 60m long
Define spermiogenesis
The final stage of spermatogenesis
Define spermiation
The process by which mature spermatocytes are released into the seminiferous tubules
How does frequent ejaculation affect semen quality?
- Production occurs at a fixed rate (although may decrease)
- If the stores are used up, then replacement takes time
- very frequent ejaculation will decrease semen quality by decreasing the sperm density
