Endocrine control of spermiogenesis

• Spermiogenesis requires stimulation by FSH and LH, which are controlled by the pulsatile secretion of GnRH from hypothalamus

• Production of ABP (Androgen Binding Protein)
• Conversion of testosterone to dihydrotestosterone and estrogen
• Stimulation of spermatocytogenesis
• Completion of sperm release
• Secretion of inhibin, which has a negative feedback action on FSH release, GnRH

• Results in hypertrophy of Leydig cells
• Stimulates production of testosterone by Leydigs

• Major action is on Sertoli cell
• Produced by Leydig
• Diffuses into Sertoli and is secreted into blood - has effects on brain, muscle, etc.
• Causes negative feedback at hypothalamus and pituitary which inhibits LH release
• If administer exogenous testosterone, increased systemic conc. will result in negative feedback, reducing LH and reducing endogenous testosterone production, thereby inhibiting spermatogenesis
• If administer LH (or hCG) or GnRH, stimulate endogenous testosterone production and don't upset balance of androgen within tubule:androgen outside
• Relatively high level of testosterone (compared to systemic circulation) required for spermatid migration

• Role is not to concentrate testosterone in tubule
• Most likely role is to transport testosterone to head of epididymis

• From Sertoli cell
• Paracrine, autocrine and endocrine action
• Negative action on FSH, GnRH

• Produced in Sertoli cells by conersion of testosterone
• Paracrine, autocrine and endocrine action
• Negative action on FSH
• Positive action on LH

Spermatogenesis

• Mitosis of spermatogonia and meiosis with differentiation into sperm takes 5-7 weeks in most mammals
• Renewal of stem cells
• Five mitotic divisions produce 16 primary spermatocytes
• Two meiotic divisions produce 64 spermatids
• Spermiogenesis (spermateliosis)
• Spermatids differentiate from compact spherical cell into spermatozoa without cell division
• Golgi granules coalesce to form acrosome
• One centriole gives rise to flagellum, other gives rise to basal plate
• Nucleus moves to periphery and chromatin condenses
• Spermatid rotates so tail sticks out into lumen
• Mitochondria congregate around flagellum and become localized in midpiece
• Most of cytoplasmic mass is engulfed by Sertoli cells when spermatozoa are released into lumen of seminiferous tubule, small amount remains at neck (proximal cytoplasmic droplet)
• Proximal droplet moves to distal end of midpiece during epididymal passage
• Distal droplet is lost during storage in the epididymis or during ejaculation

• First described approx. 300 yrs ago
• Realized essential for fertilization around 1830
• Covered by plasma membrane
• Acrosome contains hydrolytic enxzymes (acrosin, hyaluronidase, etc.)
• Postacrosomal region is where attachment and fusion to oolemma takes place
• Tail Responsible for motility
• Midpiece (mitochondria)
• Principle piece
• End piece
• 9:2 arrangement of doublets
• Kartagener's syndrome - lack 9:2 arrangement; immotile cilia syndrome (infertility,respiratory infections, situs inversus)

• Changes with time in the appearance of one section of a tubule; series of changes in a given area of seminiferous epithelium
• Based on cell associations of different stages of spermatogonia, spermatocyte, spermatid, spermatozoa within one section of a seminiferous tubule
• In any given area of a tubule, stem cells initiate series of mitotic divisions preceding meiosis at extremely regular intervals
• Time from spermatogonium to spermatozoa is remarkably constant, cannot speed up process
• Each step of spermatogenesis has a fixed and constant duration
• Before spermatozoa are fully formed as a result of one cycle, three new cycles have started, four different stages of cells are associated with each other in one section of tubule at any one time

• Sequential change in stage of cycle along the length of the tubule
• In each sequential segment of tubule, a sequential stage of the cycle is found

• Primary component is tight junctions between Sertoli cells which divide tubules into 2 compartments
  • Basal compartment - contains spermatogonium and preleptotene spermatocytes (2n)
  • Adluminal compartment - communicates with lumen of tubule and contains more advanced stages (1n)
• Demonstrates wide range of permeability from complete exclusion of some substances to nearly free transfer of others
• Not only excludes some substances (such as antibodies) but functions in retaining others (such as Androgen Binding Protein, inhibin)
• Functions in keeping haploid cells from initiating immune reaction
• Incomplete or partial barrier of myoid cells surrounding tubule also a component
• Fluid
  • Sertoli cells secrete fluid which transports immotile sperm from tubules
  • Rete apparently secretes fluid also (although some claim evidence is lacking)
  • Sperm movement is by fluid secretion, cilia movement and contractile elements (myoid layer)

Sperm Maturation

• Maturation and storage of sperm
• Head
  • Not motile - swim in circular motion
  • Not fertile
  • Proximal droplet
  • Low disulfide linkage
  • Fluid absortion
• Body 
  • First see some forward motility in body
  • Some fertility
  • Droplet moving down
  • More disulfide linkage
  • Binds oocytes
• Tail
  • Normal motility
  • Fertile
  • Distal droplet
  • Hiigh disulfide linkage
  • Binds oocytes
• Transport takes 9-13 d depending on species - mandatory
  • Control of this????

Sperm Delivery

Accessory glands 

• Contributions not needed for fertility
• Coating of gland secretion removed during capacitaion
• Ampulla 
  • - thickened glandular portion of ductus deferens as it connects with urethra  

• Large in stallion, bull
•   Present in dog but not always grossly visible  
• Absent in boar, tom  

• Vesicular glands (seminal vesicles) - sac like organs with lobulated surface  

• Provide volume, protein, sugars, salts  
• Generally fill before copulation and empty during ejaculation
•   Boar - store large volume of fluid
•   Bull - more a solid mass of tissue, produces only small volume of fluid  
• Absent in tom, dog
•   Stallion - fills during sexual stimulation  

• Prostate - solid tissue mass which secretes during copulation  

• Animals with extended copulation times (boar and dog) have high volume contribution from prostate  
• Secretion mainly ionic compounds (NaCl, Na citrate, and variety of enzymes)  
• Present in all domestic species
• Bull and boar - anterior mass is body of prostate, disseminated portion is pars disseminata  

• Bulbourethral (Cowper's) - small, except in boar; muscular gland associated with caudal pelvic urethra  

• Boar - large gland containing large volume of stored gel (sialic acid) which is emitted during ejaculation  
• Can differentiate castrated from cryptorchid pig on basis of rectal palpation. In ruminant and boar, ducts open into urethral recess, situated dorsally, which may prevent passage of catheter  

Erection  

• Arteriole expansion  
• Venule contraction
• Relaxation of retractor
• Contraction of ischiovavernous muscle
• Filling of coprpus cavernosum 
• Parasympahthetic  
• Emission  - Sympathetic Movement of sperm to pelvic urethra  
• Viagra (Sildenafil), Cialis (tadalafil)

   

  • Erection
    • The physiologic mechanism of erection of the penis involves release of nitric oxide (NO) in the corpus cavernosum during sexual stimulation. 
    • NO then activates the enzyme guanylate cyclase, which results in increased levels of cyclic guanosine monophosphate (cGMP) producing smooth muscle relaxation in the corpus cavernosum and allowing inflow of blood. 
  • Sildenafil has no direct relaxant effect on isolated human corpus cavernosum, but enhances the effect of nitric oxide (NO) by inhibiting phosphodiesterase type 5 (PDE5)
    • Responsible for degradation of cGMP in the corpus cavernosum. 
  • When sexual stimulation causes local release of NO, inhibition of PDE5 by sildenafil causes increased levels of cGMP in the corpus cavernosum, resulting in smooth muscle relaxation and inflow of blood to the corpus cavernosum. 
  • Sildenafil at recommended doses has no effect in the absence of sexual stimulation.