65 Spermatogenesis Flashcards
What are the two major compartments of the testes?
- Seminiferous tubules
- Interstitial spaces
• Both compartments separate: “blood-testis barrier”
What are the seminiferous tubules in testes?
- ~ 250 m total length
- Developing germ cells
- Sertoli (sustentacular) cells
What are the interstitial spaces in testes?
- Leydig cells (synthesise androgens
* Blood and lymph vessels
Role of blood-testis barrie?
- Prevents immune reaction to spermatozoa
* Separates fluids of different composition
What is spermatogenesis?
Production of mature spermatozoa from undifferentiated germ cells (primordial germ cell)
What are the 3 stages of spermatogenesis?
- Mitotic proliferation
- Meiotic (reduction) division
- Cell modelling (spermiogenesis)
How long is the spermatogenic cycle?
~74 days (from 1st mitotic division to release of spermatozoa)
Spermatogenesis occurs in waves, how often is its initiated?
Every 16 days
Spermatogenic cycle stages (length of time)
- Spermatogonium —> 1 ̊ spermatocytes = 25 days
- Meiotic division 1 —> 2 ̊ spermatocytes = 9 days
- Development —> spermatids = 19 days
- Differentiation –> spermatozoa = 21 days
= ~ 74 days
Structure of sperm
- Flagella: End piece, principal piece, middle piece
* Head (nucleus covered by acrosome)
How much sperm produced a day?
200 million/ day (~ 2300/sec)
What happens in spermiation at the end of differentiation?
- Cytoplasmic links are broken
- Spermatozoa released into tubule lumen
- Sperm virtually immobile
Role of Sertoli cells in spermiation?
Fluid secreted by Sertoli cells flushes spermatozoa from seminiferous tubules, through the rete testis into the epididymis:
- Capacity for motility by the time they reach the tail of the epididymis
- Motility is suppressed by epididymal fluid
- Instead movement through reproductive tract is aided by peristaltic muscle contractions
What is capacitation?
- If ejaculated spermatozoa are placed with oocytes in vitro fertilisation doesn’t occur immediately
- Need to undergo capacitation - occurs in female reproductive tract (2-6 h)
- Glycoprotein coat gained in the epididymis is stripped
- Results in 2 changes:
- Head acquires the capacity to initiate the acrosome reaction
- Hyperactivation (increased flagellar beats)
Human sperm quality?
- Relatively poor
* ~ 30% show morphological abnormalities or abnormal function
Normal sperm count?
50-150 million/ml
Sub fertility sperm count?
< 15 million/ml
What is oligozoospermia?
Low sperm count (<15 million/ml)
What is azoospermia?
Absence of sperm in the ejaculate
What is asthenozoospermia?
Low sperm motility (< 50% moving)
What is teratozoospermia?
High proportion of abnormally-shaped sperm
What is antiserum antibodies?
Abnormal immune response to semen
What stimulates the anterior pituitary gonadotrophs?
Pulsatile secretion of Gonadotrophin-releasing hormone (GnRH) from the hypothalamus
What pituitary hormones control testicular functions?
- Gonadotrophins
- Luteinizing hormone (LH)
- Follicle stimulating hormone (FSH)
Blood supply between hypothalamus and pituitary gland
Hypothalamo-hypophysial portal system - allows the hypothalamus to communicate with the anterior pituitary via the release of neurotransmitters into the bloodstream
Control of gonadotrophin secretion
- LH and FSH secretion is controlled by GnRH secreted by the hypothalamus
- GnRH is released into the portal blood in pulses every hour
- GnRH must be pulsatile or it’s ineffective
- High GnRH pulse amplitude and frequency preferentially stimulates LH synthesis and secretion
- Low GnRH pulse amplitude and frequency stimulates FSH synthesis and secretion
Hormone production in the anterior pituitary gland
- Gonadotrophs
• Secrete LH and FSH (glycoproteins)
• Most cells secrete one or other, but some secrete both - Lactotrophs
• Secrete prolactin (protein)
Hormone production in the posterior pituitary gland
Neurosecretory neurones
• Secrete arginine vasopressin (AVP) and oxytocin (peptides)
Functions of LH and FSH
- Leydig cells in the testes have receptors for LH - stimulates synthesis and secretion of testosterone
- If LH secretion is too low, testosterone is low - spermatogenesis halts
- However, LH and testosterone cannot maintain spermatogenesis at its normal level without FSH
- FSH is required for maximum sperm production - acts on Sertoli cells
Functions of FSH on Sertoli cells
- ↑ RNA and protein synthesis
- ↑ Energy metabolism
- ↑ cAMP
- ↑ Inhibit secretion
- ↑ ABP secretion
- ↑ Fluid secretion
- ↑ Androgen receptors —> ↑ FSH receptors
What hormones are produced by the testes?
- Testosterone (steroid) by Leydig cells
• In some tissues do not exert direct effects, but are converted to dihydrotestosterone or oestrogens - Oestrogens (steroid) by Sertoli and Leydig cells
- Inhibits (peptide hormone) by Sertoli cells
• Feedback loop to control hormone levels - Oxytocin (peptide hormone) by Leydig cells
• Contraction of smooth muscle of the genital tract
Steroid hormone biosynthesis
Acetate –> Cholesterol –> (Progesterone —> Testosterone) OR (Testosterone) —> Oestogens
Role of testosterone
• Essential for spermatogenesis
– If production prevented spermatogenesis ceases
– Blocked when primary spermatocytes enter meiotic prophase
• If blood [testosterone] is low then fewer stem cells will begin cell division but the whole process will still take 74 days
What does inhibin do?
Hormone secreted by Sertoli cells that inhibit the production of follicle-stimulating hormone (FSH) by anterior pituitary
