LECTURE 27 - testicular function

Question 1

What does the presence of the Y chromosome lead to?

Answer 1

• leads to testis formation under hormonal regulation (androgen) in embryo
• has a controller gene
  > sex-determingin region of Y gene (SRY)
  > produced testis determine factor from week 6-7

Question 2

What is puberty?

Answer 2

• dependent on attainment of critical body weight/fat
• complex process involving secondary sexual characteristic development and accelerated growth
• initiation of gametogenesis (spermatogenesis - haploid sperm)
• caused by initiation of pulsatile release of GnRH from hypothalamus

Question 3

What is the structure of the male reproductive organs?

Answer 3

• within testis there are seminiferous tubules, within lumens spermatogenesis will occur)
• sperm released in lumen will join in epididymis
• small proportion of sperm travels along vas deferens gets mixed with secretion from seminal vesicles and prostate and come out of urethra
• scrotum provides cooler environment compared to body

Question 4

What are the 2 products of post-pubertal testis?

Answer 4

1. spermatozoa

2. hormones

Question 5

What are the 2 compartments of the post-pubertal testis?

Answer 5

1. Within seminiferous tubules (90%)
   - Sertoli cells: nurture germ cells in testis
   - developing germ cells
   > spermatogenesis (formation of mature spermatozoa)
   > spermiogenesis (differentiation from round to elongated cell)
2. Between seminiferous tubules (10%)
   - Leydog cells: androgen (mainly testosterone) producing cell

Question 6

What are Sertoli cells?

Answer 6

• critical for all aspects of spermatogenesis - nurse cells
• secrete inhibit and androgen binding proteins to regulate and maintain spermatogenesis
• maintenance of spermatogonial stem cell niche
• form intimate associations with germ cells - tight junctions
• form a syncytium-like epithelial monolayer in which germ cell embedded - gap junctions (exchange molecules and communication)
• form blood testis barrier - tight junction with adjacent Sertoli cells
• final process, phagocytosis - spermination
• secrete fluid into the lumen of seminiferous tubules which flush spermatozoa into rete testis and epididymis

Question 7

What is the blood-testis barrier?

Answer 7

• between basal and apical compartments of tubule
• gap and tight junctions link each sertoli cell to its neighbour
• develops during puberty prior to onset of spermatogenesis

Question 8

What are the functions of the blood-testis barrier?

Answer 8

1. Separates developing germ cells from immune system preventing immune response (anti-sperm antibodies and autoimmune orchitis - sub fertility)
2. Controlled chemical microenvironment for spermatogenesis (selective transport of ions and small molecules)

Question 9

What is spermatogenesis?

Answer 9

• in humans starts at puberty, takes 74 days, produce 1000 spermatozoa/ heartbeat
• 1 primary spermatocyte (diploid) –> 4 round spermatids (haploid)

Question 10

What are the 3 phases of spermatogenesis?

Answer 10

1. Proliferation (mitosis)
   - expand in cell number (diploid cells)
2. Division (meiosis)
   - spermatogonia become spermatocytes to produce haploid spermatid
3. Differentiation
   - spermiogenesis

Question 11

What are prospermatogonia and what happens to them at puberty?

Answer 11

• neonatal male germ cells
• at puberty, prospermatogonia reactivated and undergo mitosis in basal compartment of seminiferous tubule
• AS spermatogonia are reservoir of self renewal stem cells (maintain germ cell line) and remain in outer edge of tubule as undifferentiated spermatogonia
• some AS spermatogonia will form type A spermatogonia and start process of spermatogenesis

Question 12

Describe the mitotic proliferation of spermatogonium

Answer 12

• each A spermatogonium undergoes mitosis to form type B spermatogonia which divide again to form primary spermatocytes which will undergo meiosis
• all primary spermatocytes are genetically identical to spermatogonia
• primary spermatocytes move towards lumen and through blood testis barrier
• primary spermatocytes enter a resting phase where chromosomes duplicate ready for first meiotic division

Question 13

Describe briefly sperm development

Answer 13

PGC –> prospermatogonia –> AS spermatogonia –> A spermatogonia –> B spermatogonia –> primary spermatocyte –> meiosis to form spermatids –> spermiogenesis to form spermatozoa in lumen

Question 14

What happens in the meiotic divisions?

Answer 14

Meiosis I: primary spermatocytes divide to form secondary spermatocyte

Meiosis II: secondary spermatocytes divide to form haploid spermatid (round)

–> spermiogenesis forms actual sperm (spermatozoa)

Question 15

What is spermiogeneis/ cytodifferentiation?

Answer 15

• round spermatids elongate to form elongated spermatozoa
• sperm are highly specialised although very simplistic
• sperm made up of:
  acrosome, centriole, flagellum, mitochondria, nucleus

Question 16

What is the acrosome?

Answer 16

• formed from Golgi apparatus which migrates to one end of nucleus
• contains hydrolytic enzymes which are released upon binding to the zona pellucida of the egg and aid in penetration
• has two membranes that fuse when egg plasma membrane is met to allow contents to be released

Question 17

What is the flagellum?

Answer 17

• centrioles migrate to opposite end of nucleus to acrosome and form axoneme
• form sperm movement through the female tract and penetration of the egg vestments
• sperm centrioles also important for 1st division of the zygote

Question 18

What are mitochondria?

Answer 18

• helically arranged around first part of flagellum (midpiece)
• energy for motility

Question 19

What is the nucleus?

Answer 19

• sex-determination (X or Y)
• reshaping and elongation of nucleus
• sperm DNA becomes highly condensed - histones replaced by protamines
• transcriptionally and translationally inactive

Question 20

What is the cytoplasm?

Answer 20

• superfluous cytoplasm forms residual body (cytoplasmic droplet) which is phagocytosis by Sertoli cells, loss of organelles such as ER (cannot translate any proteins)
• sperm have very little cytoplasm

Question 21

What is a syncytium and what is its function?

Answer 21

• during mitosis and meiosis cytokinesis is incomplete - forms intercellular bridges
• syncytia persist until final stages of spermatogenesis

Function:
- allows sharing of cytoplasm components e.g. developing sperm that carry a Y chromosome, can be supplied with essential proteins encoded by genes on the X chromosome (axoneme and acrosome)

Question 22

What is spermiation?

Answer 22

• last step of spermatogenesis
• facilitated by Sertoli cell cytoskeleton
• cytoplasmic bridges rupture (syncytium)
• loss of spermatid cytoplasm - anchor with the Sertoli cell
• sperm are released into lumen of seminiferous tubule in testis

Question 23

How do sperm move into seminiferous tubules and rete testis?

Answer 23

• spermatozoa released form testis are immotile until transverse epididymis (acquired caput)
• no cilia but fluid secretions from Sertoli cells and smooth muscle-like cells in testicular tunic and lamina propria (collagen and elastin) of seminiferous tubules
• periodic powerful contraction - peristalsis

Question 24

Describe the endocrine control of testicular function

Answer 24

• at puberty hypothalamus releases GnRH, levels only controlled if testis is working (controlling via hormones)
• LH and FSH released by ant. pit.
  FHS acts on Sertoli cells to maintain spermatogenesis
• LH acts on leydig cells

Question 25

How does FSH act on Sertoli cells?

Answer 25

- binds to FSH receptor on sertoli cells - induces expression of androgen receptors increasing responsiveness to androgens - stimulates production of ABP (androgen binding protein) by serotli cells - stimulates production of inhibit by Sertoli cells

Question 26

How does LH act?

Answer 26

- binds to LH receptors mainly on Leydig cells but also sertoli cells - induce Leydig cells to produce androgen (testosterone)

Question 27

What does testosterone do?

Answer 27

- development of testis (embryo) - sexual function and characteristics (puberty) - reproductive function (adult) - produced by Leydig cells (between tubules) - carried by ABP in testicular fluid - converted to dihydrotestosterone (by 5a-reductase) - bind and effect Sertoli cells function - -ve feedback on hypothalamus and ant. pit. by repressing LH and FHS production

Question 28

What does inhibin do?

Answer 28

- produced by sperm cells - acts on pit. gland - inhibits FSH production

Question 29

What does the androgen binding protein (ABP) do?

Answer 29

- produced by Sertoli cells under testosterone stimulation - binds and carries testosterone in testicular fluid of seminiferous tubules - maintains high levels of testosterone (100x more concentrated than blood) - testosterone is lipid soluble and diffuses across plasma membrane - binding to ABP prevents diffusion and increases concentration

Question 30

What can a high/low FSH/LH levels show?

Answer 30

Found in blood test FSH affects sperm production LH stimulates testes to secrete testosterone High FSH and LH - indicates failure of testicular function - acquired testicular damage Low FSH and LH - indicate that testes not receiving adequate stimulatory message from hypothalamus and pit. (dysfunction) - congenital hypogonadism

Question 31

What can high/low testosterone levels indicate?

Answer 31

Shown in blood test Testosterone measured with LH/FSH test Low - hypothalamic and pit. disease - damage to testis High - testicular tumour

Question 32

How do mature sperm reach the oocyte?

Answer 32

- human sperm are 10-15μm - have to traverse 30-40cm of male and female reproductive tract to reach oocyte - ~1 million complete the journey Post testis: sperm need to undergo sperm maturation in the male tract (epididymis) and capacitation in the female tract - important for the acquisition of motility and fertility potential

Question 33

What is the epididymus?

Answer 33

- highly segmented organ that is divided into 3 main anatomic segments 1. Caput - close testis (motility) 2. Corpus (fertilising ability) 3. Cauda/ vas deferens (storage) Each segment - displays differential expression of genes - maintains luminal ions concentrations - -> essential to regulate steps of sperm maturation

Question 34

What is epididymal maturation (sperm maturation)?

Answer 34

Sperm changes during epididymal transit Sperm interact with epididymal epithelial cells - movement of cytoplasmic droplet - retention of cytoplasmic droplet associated with reduced fertility Change sperm proteome - protein, lipid and sugar content (incorporated into sperm cells) Change in sperm surface - removal and/or modification of external proteins, sugars and lipid sperm membrane Intracellular signalling pathways activation during transit - addition or removal of protein phosphate group (phosphorylation)

Question 35

What is ejaculation?

Answer 35

- sperm (2-5%) + seminal plasma = semen - approx. 3ml in human ~100 million sperm - seminal plasma is secreted by the accessory sex glands (seminal vesicles, prostate, Bulbourethral gland) - transport, nutrition, buffering, antioxidants