10: Spermatogenesis

Hormones needed for spermatogenesis

All of them

• GnRH

• FSH and LH

• T₂

• DHT

• E₂

• AMH

Source of GnRH in the male

Tonic center → no surge center present!

Source of FSH and LH in the male

Anterior pituitary

Source of T₂ in the male

LH binding to Leydig cells → production of T₂

Source of DHT in the male

FSH binding to sertoli cells → conversion of T₂ to DHT

Enzyme that converts T₂ → DHT

5α-reductase

Primary function of DHT

Responsible for the formation of ASGs and external genitalia

Source of E₂ in the male

FSH binding to Sertoli cells → conversion of T₂ to E₂

Enzyme that converts T₂ → E₂

Aromatase

Why is spermatogenesis in the male a constant process

Tonic/basal release of GnRH from the hypothalamus means there is constantly a basal level of gonadotropins present

Hormones that negatively feedback to the GnRH centers in the hypothalamus in the male

T₂, DHT, and E₂

If Leydig cells are constantly producing T₂ (thanks tonic center) how is there not negative feedback shutting it off

Most of the T₂ is kept in the testes, so there is only a low concentration in the blood. This allows there to be enough T₂ for spermatogenesis, but not enough in the blood to shut off the supply of gonadotropins

What keeps T₂ sequestered in the testes

Androgen binding protein

Why does GnRH release in the male cause unequal production of FSH and LH

Sertoli cells are the male analog of granulosa cells, and they produce inhibin to directly inhibit FSH release from the anterior pituitary

Where does spermatogenesis happen

Seminiferous tubules

Phases of spermatogenesis

• Proliferation

• Meiotic

• Differentiation phase

Proliferation phase of spermatogenesis

Diploid spermatogonia mitotically divide, renewing the pool of sperm stem cells

Meiotic phase of spermatogenesis

Primary spermatocytes enter meiosis, producing genetically diverse haploid spermatids

Differentiation phase of spermatogenesis

Spermatids develop into highly specialized spermatozoa

Parts of a differentiated spermatozoon

Head, midpiece, and principal piece

Where are the most immature sperm cells located

Along the basement membrane of the seminiferous tubules

Where are the most mature sperm cells located

At the top of sertoli cells and extending into the lumen of the seminiferous tubules

Four phases of spermatid → spermatozoon differentiation

• Golgi phase

• Cap phase

• Acrosomal phase

• Maturation phase

Why do primary spermatocytes have to be behind the blood testes barrier

They become haploid secondary spermatocytes and are highly antigenic

What powers a mature, flagellated spermatozoon

Mitochondrial helix

Golgi phase of differentiation

Golgi apparatus produces the acrosomal vesicle on the top of the nucleus while centrioles/microtubules migrate to other side of the nucleus

Cap phase of differentiation

Acrosome spreads over the top of the nucleus, and the centrioles start growing into the thail

Acrosomal phase of differentiation

Acrosomal contents cover most of the elongating into the nucleus. At this point the almost-spermatozoon is embedded in the top of the sertoli cell with the developing flagellum in the lumen of the seminiferous tubules

Maturation phase of differentiation

Post-nuclear cap forms and mitochondria form the midpiece around the flagellum

Microtubule arrangement in the flagellum

9 + 2 arrangement

Process where sperm are released from sertoli cells

Spermiation (analogous to ovulation)

Morphology of the nucleus in the mature spermatozoon

Flattened oval nucleus with highly condensed, keratinized chromatin

When does the spermatozoon nucleus decondense

During fertilization, when it comes in contact with glutathione in the cytoplasm of the nucleus

Function of the acrosome

Contains lysosomes that degrade a spot in the zona pellucida so the spermatozoon can make it through during fertilization

T/F: male gamete formation is continual

True; except for the seasonal breeder

Cycle of the seminiferous epithelium

The progression through all the stages of sperm development in one section of the seminiferous tubule epithelium

T/F: mature sperm are released together along the length of the seminiferous tubules

False; each segment releases them at their own time, resulting in waves of spermiation

Daily sperm production

The number of sperm produced by both testes in a day

Daily sperm output

Indirect measurement of DSP that doesn’t require removing the testicles

Efficiency of sperm production

Amount of sperm per day per gram of testicular parenchyma

Primary factor that effects efficiency of sperm production

Number of sertoli cells

Ways to measure sperm production capacity

• Collecting ejaculates

• Testicular circumference

What do we care most about when evaluating a semen sample

Viability and motility

Normal sperm motility

Forward, progressive movement

Best way to asses if sperm are alive

Look for motility under a microscope at 37 C

Threshold amount of abnormal sperm that will decrease fertility

>20%

Sites where problems can result in abnormal sperm morphology

In the testes or in the epididymis

Morphology changes seen if there are problems in the epididymis

Cytoplasmic droplets

Morphology change seen if there are problems in the testes

Head and tail abnormalities

Parameters to evaluate before diluting an ejaculate

• Volume

• Concentration

• Progressive motility

Function of seminal extenders

• Prolong sperm lifespan

• Increase volume for AI doses

• Prevent cell damage

Seminal extender components that prevent cellular damage

• Buffers

• Cryoprotectants

• Fructose and glucose

• Antibiotics

How do you test Leydig cell function

Look at T₂ levels

How do you test sertoli cell function

Look at estrone sulfate, a metabolite of E₂

Why is it relevant to know the seminiferous epithelial cycle length

Gives you the time needed for recovery after a reproductive tract injury in the male

Where does final maturation of spermatozoa happen

Epididymis from head → tail

Compensable sperm defects

Morphology and motility defects that can be overcome by increasing the breeding dose

Uncompensable sperm defects

Sperm defects of the nucleus that don’t prevent the sperm from reaching the oocyte, but prevent fertilization

How are sperm moved from the testes to the ductus deferens

Epididymal tube contraction

Hormone that is needed for epididymal function

Testosterone

What does scrotal circumference indicate

• Indirect measure of scrotal volume and sperm production

• Correlated to how quickly offspring mature