WK 3 (Spermatogenesis & controlled breeding) Flashcards
Function of gene SrY
Y chromosome contains male determining gene
This switches on ‘structural genes’ in autosomal chromosomes that cause development of male genital system
The testis
Has 2 functions:
- Production and transmission of male genes (spermatozoa)
- Production of reproductive hormones (androgens)
Testis consist of seminiferous tubules (sperm maturation occurs)
Testicular parenchyma
Consists of 2 discrete compartments:
- Within seminferous tubules
- Sertoli cells present
- Sperm development - Between seminferous tubules
- Leydig cells present - where testosterone is made under influence of LH
- Androgens synthesised
Blood testis barrier
Blocks blood and associated immune cells from getting in to where the sperm are developing.
This is important because at a certain point in the spermatogenic cycle the cells become different than self and if immune cells could see them they would attack them
Damage to blood testis barrier affects spermatogenesis as immune cells kill developing sperm so ejaculate contain less sperm until damage is resolved
Sertoli cell
Sertoli cell influences spermatogenesis:
- Spermatocytes receive testicular proteins via sertoli cell gap junctions
- Spermatocytes and spermatids are physically anchored to the sertoli cell
- Sertoli cell removes material from the elongating spermatid during cytoplasmic condensation
All sertoli cells linked to each other by gap junctions to provide network for communication throughout tubule
Sertoli cells play critical role in mediating the actions of hormones on spermatogenesis
3 phases of spermatogenesis:
- Mitotic proliferation
- Meiotic division
- Cytodifferntiation (spermiogenesis)
Spermatogenesis - Phase 1: Mitotic proliferation
Produces large numbers of cells (spermatogonia A0-A4)
Spermatogonia are diploid and genetically identical
Occurs in basal compartment of tubule
Doesn’t need to occur in adluminal compartment (blood testis barrier) as cells are still self-cells
There is a point at which some of these cells (after a couple of divisions), revert back to an earlier version of themselves
- go from being spermatogonia (A4) to (A1)
- In order for there to constantly be sperm produced, the testes has to maintain a population of these (A1) spermatogonia - does this by a subset of the divided spermatogonia regenerating and reverting back to (A1) spermatogonia
Spermatogenesis - Phase 2: Meiotic division
No longer referred to as spermatogonia but spermatocytes
Generates genetic diversity (chromatids exchange genetic material)
Halves chromosome number (haploid)
Spermatocytes to spermatids
Occurs in adluminal compartment of tubule (non-self cells)
Spermatogenesis - Phase 3: Cytodifferentiation
Packages genes for delivery to oocyte
Elongating spermatids –> spermatozoa
Final stage of spermatogenesis
Re-packaging of this cell from something round and passive (just floats) to what we see as a sperm (motile – Head- Tail)
Sertoli cell removes material from the elongating spermatid during cytoplasmic condensation (cytoplasmic droplet)
Occurs right at the top near the lumen
This process is happening from the basement membrane pushing up towards the lumen
- Most immature cell types can be found at the basement membrane and then as they get more and more developed they make their way further up to the top, closer to the lumen before they get released
Process isn’t perfect – things go wrong - apoptosis
- Reduction in efficiency can be targeted by treatments later on down the track to increase number of sperm generated
What percentage of normal sperm is expected in production animals used for breeding?
90%
The rate of spermatogenesis is…..
The rate of spermatogenesis is CONSTANT
Spermatogenic wave
Refers to how there are different stages of division occurring along the seminiferous tubule
How is a constant supply of sperm maintained?
Different stages of spermatic division are occurring at different sites along the seminiferous tubule .
In every species, if you multiply the cycle of the seminiferous epithelium by 4.5 you will get the number of days for complete spermatogenic cycle
E.g. Rams
Each site along the seminiferous tubule is at a different stage of division so each site takes 10.5 days to release sperm but the complete spermatogenic cycle of 1 cell through all the divisions and re-packaging takes 47 days or 4.5 seminiferous epithelium cycles (as at each site, at each stage there is layers of cells at different stages of maturation – basement membrane up to seminiferous epithelium)
I.e. spermatogonia is at one of 8 stages of maturation – different spermatogonia are in different phases so every 10.5 days sperm is being released from a particular site of the tubule, (each site has layers of cells underneath that are not ready to be released yet) but the journey of those cells up until the point of release would have taken 47 days
As each site is at a different stage of division, one site is always going to be at stage 8 at any given point in time meaning there is ALWAYS a constant supply of sperm
if each site was at the same stage of division sperm release would be pulsatile and it would take 10.5 days for new release of sperm (not practical)
10.5 days of seminiferous epithelium cycle refers to the very top layer of this picture – i.e. it takes 10.5 days for the top layer of spermatids to mature and be released BUT it takes 47 days for the most basic cell to go work its way up from the basement membrane and be released into the lumen
Complete spermatogenic cycle = spermatogonia (A1) spermatozoa
Cycle of seminiferous epithelium = just the very edge/outer layer (right near lumen) = the releasing of sperm into seminiferous lumen
Endocrine control of spermatogenesis
GnRH pulse generator is located in hypothalamus
GnRH pulses elicit release of FSH and LH from the anterior pituitary gland, which stimulates release of steroid hormones from the testes
Both LH (or T) and FSH are required to initiate spermatogenesis
- Puberty
- Seasonal anoestrous
But only LH or Testosterone (and DHT) are required to maintain spermatogenesis
Hypothalamus releases GnRH feeds back to pituitary –> Anterior lobe of pituitary produces LH –> LH acts on the Leydig cells inside of testes (in interstitial space) —> LH is converted into testosterone testosterone –> acts on sertoli cells –> developing germ cells and supports spermatogenesis
FSH promotes B spermatogonia indirectly via Sertoli cells, feedback via differentiating germ cells to Sertoli cells, affecting Inhibin production
DHT (dihydrotestosterone)
Potent form of testosterone
Testosterone is transformed into DHT by an enzyme called 5 reductase
Inhibin in males
Produced by the sertoli cells
Feedback to the pituitary which regulates the amount of FSH produced
Can the timing or rate of spermatogenesis be changed?
NO
However the efficiency of spermatogenesis CAN be changed
- So the rate of sperm production can be altered
The rate of sperm production and efficiency of spermatogenesis is governed by:
- Length of cycle (CONSTANT)
- Weight of the testes
- Sperm production/unit weight of testis
- Number of spermatogonia feeding into cycle (can be altered
- Extent of cell loss at each stage of the cycle (can be altered)
Factors effecting the efficiency of spermatogenesis
Breakdown of the blood-testis barrier
Irradiation
Heat & cryptorchidism
Diet
Drugs/toxic agents
Disease
Endocrines
i.e. factors that effect: weight of testes & sperm produced/unit weight of testes
Efficiency of spermatogenesis - Breakdown of blood-testis barrier
Caused by:
Mechanical injury - lowers sperm count
Autoimmune orchitis
Aspermatogenesis
Breakdown leads to auto-immune attack on sperm cells - infertility
Efficiency of spermatogenesis - Irradiation
Dividing cells are susceptible to irradiation damage causing wave of maturation depletion in adult, possibly complete loss of spermatogonia in fetus
Efficiency of spermatogenesis - Heat & cryptorchidism
Spermatogenesis only occurs 4-7 °C below body core temperature
Spermatocytes, spermatids particularly sensitive to local heating
Temperature controlled by:
- Cremaster muscle (regulates height of testes)
- Scrotal sweat glands
- Pampiniform plexus (hot arterial blood exchanging blood with the cooler venous blood before reaching the testes)
Cryptorchids:
Undescended testis – needs to be removed or can cause cancer
No spermatogenesis
Efficiency of spermatogenesis - Diet
Deficiencies causing testicular degeneration: Vitamin A Essential fatty acids Some amino acids Zinc Vitamin B via pituitary
High energy or protein food can stimulate testes via action on hypothalamus and pituitary gland, e.g. lupin grains
- Feeding lupins increases the pulsatility of GnRH pulse generator, making more GnRH –> more LH –> more testosterone
Of no additional use when diet is adequate
Efficiency of spermatogenesis - Drugs/toxic agents
Agents causing testicular degeneration:
- Cadmium salts
- Cytotoxic drugs
- Some antibacterial drugs
- Corticosteroids (via pituitary gland)
- Alcohol
Dividing cells are susceptible to cytotoxic or antibacterial drug damage causing wave of maturation depletion in adult , possibly complete loss of spermatogonia during prolonged treatment
Efficiency of spermatogenesis - Disease
Disease causing oligospermia
- Genetic abnormalities
- Mumps (adults)
- Local Inflammation
Temporary or permanent oligiospermia may arise from local over-heating during inflammation or breakdown of blood-testis barrier
Efficiency of spermatogenesis - Endocrines
Both LH and FSH are required to initiate spermatogenesis
LH or testosterone (and DHT) maintain spermatogenesis
Affect of breeding season on GnRH (males)
Less GnRH in non-breeding season –> less Lh –> less testosterone –> reduced spermatogenesis
Affects some species more than others (merinos not so affected)
Male effect - GnRH pulses
GnRH pulses dramatically increase
Nutrition stress - Testosterone release
Reduces testosterone through reduced LH secretion
Effect of stress on testosterone production
Reduced testosterone through reduced LH via generalised negative feedback of corticosteroids
Post spermatogenesis and spermiogenesis
When elongated spermatids are released from sertoli cells its is known as spermiation
- At this point they change from spermatids to spermatozoa – they are still not motile
Once sperm have detached from the sertoli cell they are transported to the epididymis
Transport of sperm to the epididymis
When sperm is released from Sertoli cell it is immotile
Tubular fluid secreted by sertoli cells:
- High in K+ (renders sperm immotile)
- Low in Na+
- High in inositol, glutamic acid, some amino acids
- Inhibitor of acrosin
- Androgen binding protein (some spp.)
Rete testes absorbs fluid
- Fluid going in at one end out the other –> sperm floating along
Rete testes –> Efferent ducts –> Epididymis
Once sperm reach the efferent ducts there is more active movement via cilia and smooth muscle contraction
Secretions of tubules, and possible active ion transport from rete testis, draw soerm to:
Rete testis (absorption of water) –> Efferent ducts (Epithelial cilia, smooth muscle contraction) –> Epididymis
Epididymal Structure
Caput (Head)
- Fluid from sertoli cells reabsorbed
- Thus sperm are concentrated 100-fold
- Sperm subsequently transported by muscles in epididymis
Corpus (body)
- Modification of environment and sperm maturation
Cauda (tail)
- Sperm storage
Passage through epididymis takes 6-14 days
When sperm leave the testes are they fertile?
No - They have to go through epididymal maturation
Sperm in the caput (head) of the epididymis
Sperm are:
Immotile
Infertile
Proximal cytoplasmic droplet (result of repackaging by sertoli cell - round cell into elongated cell, a little bit of cytoplasm from the round cell remains
(When cytoplasmic droplet is called proximal = close to the head, distal = close to the Tail)
Sperm in the cauda (tail) of the epididymis
Sperm are:
Forward progressive motile
Fertile
Will either have:
- A distal cytoplasmic droplet
OR
- It will have lost its cytoplasmic droplet
Androgen
Supports sperm maturation via rete testis or blood
Epididymal maturation (brief)
Sperm structure
Sperm membranes
Sperm metabolism
Sperm motility
Epididymal maturation - Sperm structure
Loss of surplus cytoplasm (removal of cytoplasmic droplet)
Condensation of nuclear chromatin by disulphide bridges – less susceptible to damage from reactive oxygen species found in female tract
Epididymal maturation - Sperm membranes
Addition of surface glycoproteins to stabilize plasma membrane
Membrane fluidity and lipid composition change - more robust and enables it to undergo a further maturation event called capacitation
- Cholesterol selectivity metabolized shifting balance towards diaglycerol and unsaturated fatty acids
Epididymal maturation - Sperm metabolism
Depression of metabolic activity to prolong life of cell
Increased dependence on external fructose for energy production
Epididymal maturation - Sperm motility
cAMP content of tail increases allowing acquisition of motility
Anatomy of the penis - Shaft
Corpus spongiosum (urethra) and corpus cavernosum (main body) act as blood reservoirs during erection
Os penis in dog
Anatomy of the penis - Glans penis
Glans penis = End part
Tom covered in androgen dependent spines (if you castrate spines disappear)
Alpaca has a stiff spine
Boar corkscrew shape
Fibroelastic penis
Bull, boar, ram
Limited erectile tissue
Presence of sigmoid flexure
- Allows the penis to be retracted into the body until erection
- Sigmoid flexure held by retractor penis muscles
- Contract retractor muscles - penis held in sheath
- Relax retractor muscles - penis protrudes
Musculovascular Penis
Stallion, Human
Large corpus cavernosum fills with blood during erection
No sigmoid flexure
Retractor penis muscle in stallion
Erection - Psychogenic stimuli
Visual cues
- Mating in others – induces erection and can stimulate the release of oxytocin which pushes some sperm from the tail of the epididymis up into the urethra = high sperm counts in ejaculates and helps animal to jump/mount
- Lordosis (female exhibiting standing heat)
Olfactory cues
- Sniffing of vulva
- Female urination
- Pheromones
- Androgens, Boar mate (nasal spray
product for pigs – causes ejaculation)
Penile erection - Nervous Stimulation
Erection induced by:
- Visual cues
- Olfactory cues
- Tactile stimulation of penis and perineum
- Pudendal nerve (afferent somatic
innervation)
- Pudendal nerve (afferent somatic
Complex series of neural signals:
- Efferent innervation of penis controls erection
Pelvic nerve (parasympathetic) promotes erection - Non-andrenergic, cholinergic + non-andrenergic, non-cholinergic
Hypogastric nerve (sympathetic) suppresses erection - Andrenergic innervation
Pudendal nerve (somatic) promotes
