S3) Gametogenesis Flashcards
What are the starting cells for spermatogenesis and how long are they available for?
- Spermatogonia
- 70 years
Spermatogonia divide by mitosis to give rise to two types of cells.
Identify and describe them
- Ad spermatogonium: reserve stock (resting)
- Ap spermatogonium: maintain stock (active)
What do ap spermatogonia do?
From puberty onwards, ap spermatogonia produce type B spermatogonia which give rise to primary spermatocytes
Briefly describe the process of spermatogenesis
What is spermiation?
Spermiation is the release of spermatids into the lumen of seminiferous tubules
What is spermiogenesis?
Spermiogenesis is when a haploid spermatid differentiates into a spermatozoon
Outline the steps involved in spermiogenesis
⇒ Spermiation
⇒ Spermatid remodelling (seminiferous tubule)
⇒ Spermatid moves through rete testis and ductuli efferentes and into the epididymis
What is the spermatogenic cycle?
Spermatogenic cycle is the time taken for reappearance of the same stage (of spermatogenesis) within a given segment of the seminiferous tubule
How long is the spermatogenic cycle in adult humans?
16 days
What is the spermatogenic wave?
Spermatogenic wave is the distance (in the tubule) between the same stage of spermatogenesis
How does the spermatogenic wave move?
Waves move in corkscrew-like spirals towards the inner part of the lumen
How do spermatids reach the epididymis in the process of spermiogenesis?
- Spermatids are non-motile
- Transported by Sertoli cell secretions assisted by peristaltic contraction
Identify the different structures in the pathway for the delivery of sperm
Seminiferous tubules → Rete testis → Ductuli efferentes → Epididymis → Vas deferens → Urethra
Identify the contents of seminal vesicle secretions (~70%)
- Amino acids
- Citrate
- Fructose
- Prostaglandins
Identify the contents of prostate gland secretions (~25%)
- Proteolytic enzymes
- Zinc
What is the function of the bulbourethral gland (<1%)?
Secrete mucoproteins to help lubricate and neutralise acidic urine in distal urethra
What is the overall contribution of sperm to semen?
2-5% of overall volume
How many sperm are there per ejaculate?
200-500 million
Which glands secrete into the urethra?
- Prostate gland
- Bulbourethral gland
Which gland secretes into the vas deferens?
Seminal vesicle
What is sperm capacitation?
- Sperm capacitation refers to the physiological changes spermatozoa must undergo in order to fertilise an egg
- The final maturation step for sperm
Which three processes are stimulated by the female genital tract?
- Removal of glycoproteins and cholesterol from sperm membrane
- Activation of sperm signalling pathways
- Allow sperm to bind to zona pellucida of oocyte
When does oocyte maturation begin?
Before birth
What are oogonia and what do they do?
- Germ cells arise from yolk sac and colonise the gonadal cortex, differentiating into oogonia
- Oogonia proliferate rapidly by mitosis
Describe the arrangement and actions of oogonia by the end of 3rd month
- Arranged in clusters surrounded by flat epithelial cells
- Majority continue to divide by mitosis but some enter meiosis
What are primary oocytes?
Primary oocytes are oogonia that have arrested in prophase of meiosis I
What is the maximum number of germ cells in the female?
When is this value reached?
- ~7 million germ cells
- Reached by mid gestation
What are primordial follicles?
Primordial follicles are surviving primary oocytes which are individually surrounded by a layer of follicular cells
What are follicular cells?
Follicular cells are the layer of flat epithelial cells that surround individual primary oocytes in late gestation
Define follicular atresia
Atresia is the degeneration of those ovarian follicles which do not ovulate
Describe atresia in females
- After mid gestation, many oogonia and primary oocytes degenerate
- By 7th month gestation, ~5 milion oogonia have degenerated
How many oocytes are left by the start of puberty?
~40,000 (most oocytes undergo atresia during childhood)
How many oocytes mature each month from puberty onwards?
~15-20 oocytes start to mature each month
Identify the 3 stages in oocyte maturation
- Preantral
- Antral
- Preovulatory
What happens in the pre-antral stage?
- Primordial follicle grows to form the primary follicle
- Follicular cells change and proliferate to form a stratified cuboidal epithelium of granulosa cells
Describe the function of granulosa cells
Granulosa cells secrete layer of glycoprotein on oocytes forming the zona pellucida
What happens in the antral stage?
- Fluid-filled spaces appear between granulosa cells
- These spaces coalesce to form the antrum
What induces the preovulatory growth phase?
Surge in LH
What happens during the preovulatory stage?
- Meiosis I completion results in 2 haploid cells of unequal size
- One cell receives most of cytoplasm the first polar body receives practically none
When does meiosis II occur in oogenesis?
- Before ovulation, cell then enters meiosis II but arrests in metaphase
- After ovulation, meiosis II is only completed if oocyte is fertilized, otherwise cell degenerates
Which substances stimulate the rapid growth of the follicle?
- Follicle stimulating hormone (FSH)
- Luteinising hormone (LH)
What is a Graafian follicle?
- A Graafian follicle is a mature follicle in the ovary
- It is ~2.5cm in diameter
What role does LH have in ovulation?
- LH surge increases collagenase activity
- Prostaglandins increase response to LH & cause local muscular contractions in ovarian wall
What happens to the oocyte in ovulation?
Oocyte is extruded & breaks free from ovary
How is the corpus luteum formed?
- After ovulation, remaining granulosa and theca interna cells become vascularized
- They develop a yellowish pigment and change into lutein cells, which form the corpus luteum
What does the corpus luteum do?
- Secrete oestrogen & progesterone
- Stimulates uterine mucosa to enter secretory stage in preparation for embryo implantation
- Dies after 14 days if no fertilisation occurs
How is the oocyte transported from the ovary?
- Oocyte carried into tube by sweeping movements of fimbriae and mucosal cilia
- Oocyte then propelled by peristaltic muscular contractions of uterine tube and by mucosal cilia
What happens to the corpus luteum if no fertilisation occurs?
- Corpus luteum degenerates
- Forms corpus albicans – mass of fibrotic scar tissue
- Progesterone production decreases
- Menstrual bleeding occurs
The corpus luteum does not degenerate if fertilisation occurs.
Explain why
Degeneration prevented by human chorionic gonadotropin, secreted by developing embryo
What role does the corpus luteum have post-fertilisation?
The corpus luteum continues to grow and forms the corpus luteum of pregnancy (corpus luteum graviditatis)
When does the corpus luteum stop secreting progesterone and why?
Cells continue to secrete progesterone until ~ 4th month until the placenta takes over
Outline the four steps involved in the ovarian cycle
⇒ Hypothalamic GnRH stimulates anterior pituitary to release FSH and LH
⇒ FSH stimulates follicle growth
⇒ FSH and LH stimulate follicle maturation
⇒ LH surge triggers ovulation & promotes development of the corpus luteum
Compare and contrast oogenesis and spermatogenesis in terms of the following:
- Commencement
- Completion
- Gamete motility
- Daughter cells
- Stages
- Overall yield
