Male Reproductive System II

Question 1

What is the typical sequence of spermatogenesis?

Answer 1

• Starting at the basement mb and moving towards the lumen, basally, Spermatogonia (A”1”) -> 2 Spermatogonia (A”2”) -> 4 Spermatogonia (A”3) & so on during PROLIFERATION (# of divisions depends on sp)
  Spermatogonia (A”3”) -> Spermatogonia (A”4”) -> Spermatogonia (I) -> Spermatogonia (B) -> then adluminally -> primary spermatocytes -> meiosis I -> secondary spermatocytes -> meiosis II -> Spermatids -> undergo DIFFERENTIATION

Question 2

What is another name for proliferation?

Answer 2

spermatocytogenesis

Question 3

what is another name for differentiation?

Answer 3

spermiogenesis

Question 4

What do male germline stem cells do?

Answer 4

• thread 1 generation to the next
• include PGCs, gonocytes, SPERMATOGONIAL STEM CELLS (SSCs)

Question 5

What are spermatogonial stem cells?

Answer 5

• can self-renew & produce differentiated germ cells
• are the only adult stem cells that can pass on genes to the next generation

Question 6

What are the two proposed models for the formation of spermatogonial stem cells (SSCs)?

Answer 6

1. all gonocytes are the same but some (randomly) transition to SSCs, which can renew & give rise to progenitors, while others directly give rise to differentiating spermatogonia
   - PGC -> gonocyte -> (SSC -> progenitor -> A”1” -> sperm) OR (A”1” -> sperm)
2. different types of gonocytes are responsible for forming the SSC pool, initial progenitor population, & initial differentiating spermatogonial population
   - PGC -> Gonocyte -> (SSC -> progenitor -> A”1” -> sperm) OR (progenitor -> A”1” -> sperm) OR (A”1” -> sperm)

Question 7

What are the possible outcomes of SSC division?

Answer 7

• self-renewal capability is important in ensuring SSC pool maintenance
• self-renewal & maintenance must be kept in balance, or it may lead to either germ cell tumour or depletion of SSCs
• DURING NEONATAL DEVELOPMENT & FOR REGENERATION of spermatogenesis after cytotoxic insult, SYMMETRICAL SELF RENEWAL may predominate
  Symmetrical self renewal:
  SSC -> SSC & SSC
• DURING STEADY-STATE SPERMATOGENESIS, BALANCE OF SYMMETRICAL SELF-RENEWAL & SYMMETRICAL DIFFERENTIATION may occur @ defined frequencies
  Symmetrical differentiation:
  SSC -> committed progenitor spermatogonia
• RECENT EVIDENCE SUPPORTS ASYMMETRIC DIVISION of an SSC to produce 1 new SSC & 1 transient amplifying progenitor
  Asymmetric division:
  SSC -> (SSC) OR (transient amplifying progenitor spermatogonia -> committed progenitor spermatogonia) OR (transient amplifying progenitor spermatogonia -> transient amplifying progenitor spermatogonia -> committed progenitor spermatogonia)

Question 8

Process of A”1” to spermiation?

Answer 8

Spermatogenic stem cell (pool of undifferentiated spermatogonia -> transit amplifying progenitor cells ) -> differentiating spermatogonia (8.6 days) -> preleptotene -> meiosis -> spermiogenesis -> spermiation
(approximately 35 days from A”1” to spermiation)

Question 9

What happens at the undifferentiated spermatogonia stage?

Answer 9

• spermatogonia classified into undifferentiated & differentiating
• in rodents & most domestic spp, spermatogenesis starts w/ division of SPERMATOGONIA TYPE A”single” (A”s”)
• divide to form 2 new A”s” for self-renewal or a connected pair called A”pair” (A”pr”) for differentiation
• in rodents, further divisions lead to formation of type A”aligned” (A”al”) which remain as chains of 4, 8, or 16 cells
• UNDIFFERENTIATED SPERMATOGONIA (TYPES A”s”, A”pr”, & A”al”) HAVE SIMILAR CELLULAR MORPHOLOGY
• A”s” are rare & relatively quiescent & represent ‘TRUE SSC’
• A”pr” & A”al” cells may have some SSC potential but are on way toward differentiation & called ‘TRANSIT AMPLIFYING PROGENITOR CELLS’

Question 10

What are differentiating spermatogonia?

Answer 10

• Retinoic acid causes A”al” to transition (w/o cell division) to A”1” to start differentiation
• involves major changes in morphology & mitotic behaviour
• then 5 synchronized cell divisions to form A”2”, A”3”, A”4” intermediate (In), & B-spermatogonia
• DIFFERENTIATING SPERMATOGONIA include A”1-4”, I, & B-spermatogonia
• B-spermatogonia must physically pass through the blood-testis barrier before further germ cell differentiation

Question 11

What are primary & secondary spermatocytes?

Answer 11

• once in adluminal compartment, B- spermatogonia divide again to produce 2 PRIMARY SPERMATOCYTES that immediately enter the 1st phase of meiosis
• during the long prophase of 1st meiosis, DNA undergoes complete replication to form tetrads, followed by crossing over of homologous chromosomes to ensure genetic heterogeneity of gametes
• primary spermatocytes go through 5 stages, PRELEPTOTENE, LEPTOTENE, ZYGOTENE, PACHYTENE, & DIPLOTENE
• @ end of 1st meiosis, each primary spermatocyte has produced 2 SECONDARY SPERMATOCYTES (haploid, N), which are relatively short-lived & immediately undergoes the second meiotic division

Question 12

How are round & elongating/elongated spermatids formed?

Answer 12

• @ end of Meiosis II, each secondary spermatocyte has produced 2 haploid ROUND (spherical) SPERMATIDS
• no further cell division will occur
• testosterone drives round spermatids to undergo extensive morphological differentiation known as SPERMIOGENESIS
• which leads to formation of ELONGATING/ELONGATED SPERMATIDS
• retinoic acid causes spermatozoa to be released into lumen of the seminiferous tubule during SPERMIATION process

Question 13

What is differentiation or spermiogenesis?

Answer 13

• extensive morphological changes occur to turn a round spermatid into a sperm
• these changes include: formation of acrosome, condensation of nuclear material, outgrowth of a motile tail, & loss of excess cytoplasm, organelles, water (volume in later maturation phase is 20-30 % of a cap phase spermatid)
• can be divided into 4 phases (golgi phase -> cap phase -> acrosomal phase -> maturation phase)

Question 14

What is the golgi phase?

Answer 14

1. newly formed spermatid is spherical & has a well-developed Golgi apparatus
2. small golgi vesicles fuse to form “PROACROSOMIC” secretory granules & centrioles start migrating to opposite side
3. ACROSOMAL VESICLE (granule is formed), PROXIMAL CENTRIOLE (PC; will give rise to attachment point of tail), DISTAL CENTRIOLE (DC; will give rise to axoneme)

Question 15

What is the cap phase?

Answer 15

1. golgi migrates to caudal pole & distal centriole (DC) forms AXONEME (AX) or flagellum
2. acrosomic vesicle flattens & begins to form a cap & CAP has an outer acrosomal membrane (OAM) & an inner acrosomal membrane (IAM) & enzymes

Question 16

What is the acrosomal phase?

Answer 16

1. nucleus begins to elongate & acrosome covers most of its anterior & MANCHETTE forms from caudal half & extends down
2. neck & annulus are formed, all components still w/in cytoplasm

Question 17

What is the maturation stage?

Answer 17

1. mito form a spiral assembly around the flagellum that defines the middle piece
2. POSTNUCLEAR CAP is formed from manchette microtubules
3. ANNULUS forms the junction btwn the middle piece & the principal piece

Question 18

How do sperm vary in length btwn spp?

Answer 18

• 50 microm in humans
• 60 in boar & stallion
• 75 in Ru
• 150-250 rodents

Question 19

What 2 parts do the sperm seem to only consist of under a light microscope?

Answer 19

head & tail

Question 20

What does the post-nuclear cap consist of & what does it help evaluate?

Answer 20

fibrous sulfur-rich proteins (stain intense w/ eosin-bromophenol blue in dead sperms, helps in evaluating the quality of the ejaculate)

Question 21

What is the structure of the sperm head?

Answer 21

• head differs in different spp (generally spatula shaped in most but in rodents is sickle shaped)
• nucleus is oval, flat, & covered by a nuclear membrane
• chromatin is compacted & inactive due to high keratinoid proteins
• anterior 2/3rds of nucleus is covered by acrosome

Question 22

what is an acrosome?

Answer 22

membrane covered lysosome containing hydrolytic enzymes (ex: acrosin, hyaluronidase, zona lysin, esterases, acid hydrolase)

Question 23

what is an acrosome reaction?

Answer 23

during fertilization acrosome undergoes a specialized exocytosis

Question 24

What is the post-acrosomal sheath?

Answer 24

region contains receptors for recognition of homologous oocyte

Question 25

What is the structure of the sperm tail?

Answer 25

• tail is composed of capitulum, middle piece, principal piece, & terminal piece

Question 26

What is the capitulum of the sperm tail?

Answer 26

fits into the implantation socket of the head

Question 27

What is the middle piece of the sperm tail?

Answer 27

• has laminated columns, gives side-to-side flexibility to neck
• 9 course outer fibers
• axoneme composed of 9 pairs of microtubules (DOUBLETS) around 2 central filaments
• all covered by a helical mitochondrial sheath

Question 28

What is the principal piece of the sperm tail?

Answer 28

forms majority of tail & continues to almost the end of the flagellum

Question 29

What is the terminal piece of the sperm tail?

Answer 29

is where only microtubules remain

Question 30

When do the stages of spermatogenesis take place?

Answer 30

• sperm production takes time & in non-seasonal males should be continuous
• before 1 spermatogenic series is completed, generally 4 or more new series are initiated
• all descendants of B-spermatogonia develop synchronously
• successive generations follow each other
• so spermatogenic cycle is divided into stages or cellular associations
• these are arbitrarily defined associations that transition 1 to the next at predictable intervals
• identical associations are found in cross sections
• ex: in bulls, rams, & sw there are 8 stages

Question 31

What are the stages along a seminiferous tubule?

Answer 31

• @ any given cross section along a seminiferous tubule a different stage of cycle can be seen
• each stage follows or precedes adjacent stages
• complete sequence of stages (I through VIII) along a length of tubule is called a spermatogenic wave
• spermatogenic wave in bulls is ~10 mm long

Question 32

How are LH & testosterone linked?

Answer 32

• each LH pulse drives a pulse of testosterone release by Leydig cells
• each remains high in serum for ~0.5-1 hr

Question 33

How is spermatogenesis regulated by hormones?

Answer 33

• involves interplay btwn endocrine & paracrine/autocrine systems
• FSH acts on Sertoli cells to promote their function & facilitate spermatogenesis
• LH acts on Leydig cells to stimulate production of testosterone
• testosterone & inhibin act through a negative feedback mechanism to temporarily inhibit further release of GnRH, FSH, & LH
• FSH increases proliferation of spermatogonia
• w/in tubules, FSH receptors are only found on Sertoli cells, suggesting indirect FSH effects on germ cells
• FSH has little or no direct effect on completion of meiosis & spermatogenesis
• testosterone is crucial for transition of spermatocytes to spermatids
• owing to countercurrent system of pampiniform plexus, testosterone in testis is 100-500x higher than in circulation
• both testosterone withdrawal & lack of androgen receptors can result in lack of post-meiotic germ cells
• similar to FSH, any effect of testosterone on germ cells is indirect & mediated through somatic cells
• androgen receptors are absent in germ cells but abundant in Sertoli, peritubular myoid, & Leydig cells
• E”2” is converted from testosterone by aromatase in Sertoli cells
• lack of ERalpha or aromatase leads to spermatogenesis impairment in mice