Week 1.1 - Origin of the gametes Flashcards
How many gametes are produced in a lifetime by oogenesis/spermatogenesis?
- approx 400 in oogenesis
- approx 200 million a day in spermatogenesis
Which gamete production is continuous and which is intermittent?
- Spermatogenesis is continuous
- Oogenesis is intermittent (once a month)
What are the two main functions of meiosis?
- Reduce chromosome number in gamete to 23
- Ensure every gamete is genetically unique
How many gametes produced from meiosis?
-4
How does genetic variation arise in meiosis?
- Crossing over (exchange of DNA between to homologous chromosomes)
- Random segregation (Distribution of chromosomes among four gametes)
- Independent assortment (Two homologous chromosomes of a pair go to separate gametes
What is the sequence during differentiation in spermatogenesis?
-Germ cell -> spermatogonium -> primary spermocyte -> secondary spermocyte -> Spermatid -> spermatozoon
What is spermiogenesis?
-Maturation of a spermatid into spermatozoon
How long does spermatogenesis take?
-64 days
What is the function of ad/ap spermatogonium?
- Ad spermatogonium replenishes germ cell stock
- Ap spermatogonium progress to spermatozoa
Where does spermatogenesis take place?
-Seminiferous tubules in testis
What are sertoli cells and where are they found?
- Cells which nourish the developing sperm
- Found lining the seminiferous tubules and interwoven between spermatogonia
Within the tubules, how is differentiation arranged?
-Germ cells line the basal lamina of a tubule and become more differentiated towards the lumen
What is the sertoli cell barrier (blood testis barrier)? Why is it important?
- Tight junction barrier between germline cells and spermatozoa
- Cellular environment for differentiation of sperm not suitable for germ cells
- Also prevents autoimmune response due to genetic differences in spermatozoa
Do all spermatogonia mature at the same time?
- No, different areas of the seminiferous tubule mature at different times
- Different groups mature together because cytoplasmic bridges join some spermatogonia together. This produces a spiral pattern of differentiation along the seminiferous tubule towards the lumen
What is spermatogenic wave?
-The distance between two identical stages of maturation along a tubule
What is a spermatogenic cycle?
- The time taken for the reappearance of the same stage in a given segment
- About 16 days
What is spermiation?
-Release of spermatids into the lumen of seminiferous tubules
Describe when and where spermatids mature into spermatozoa?
-when they are Released into lumen of seminiferous tubules they are carried to the rete testis and into the epididymis. They are remodelled as they do, finally becoming spermatozoa in the epididymis
When do the spermatozoa become motile?
-When they reach the epididymis
If sperm are non-motile until the epididymis, how do they get there?
-They are carried by sertoli cell secretions and peristaltic action
How much semen is produced per ejaculate?
-2ml
What are the constituents of semen?
- Seminal vesicle secretions (70%)
- Secretions of prostate (25%)
- Sperm (2-5%)
- Bulbourethral gland secretions
What are seminal vesicle secretions comprised of?
- Amino acids
- Cirtate
- Fructose
- Prostaglandins
What are prostatic secretions comprised of?
- Proteolytic enzymes
- Zinc
How many sperm per ejaculate?
-200-500 million
What is the purpose of bulbourethral gland secretions?
- Neutralise acid in distal urethra
- Lubricate urethra
State the structures sperm passes on its way to ejaculation
- Epididymis
- Vas deferens
- Pass seminal vesicles
- Passes through prostate
- Passes bulbourethreal glands
- urethra
What comprises the ejaculatory ducts?
-The union of the vas deferens with the seminal vesicles
What is sperm capacitation?
- Final step before sperm become fertile
- Under the conditions of the female genitalia tract glycoproteins and cholesterol are removed from the plasma membrane of the sperm and there is activation of sperm signalling pathways
- Allows sperm to bind to zona pellucida of oocyte and initiate acrosome reaction
What must sperm be incubated in in IVF?
-Capacitation media
What is the acrosome reaction?
-Breaking through the zona pellucida by enzymatic reactions
Are female gametes produced throughout life?
-No, entire stock of potential gametes developed before birth
In females, what happens to gamete stock during age?
-Declines by atresia
How does a germ cell in the female develop into a primary oocyte?
- Germcells differentiate into oogonia
- Proliferate rapidly by mitosis
- Oogonia become surounded by flat epithelial cells
- Majority continue to divide by mitosis but some enter meiosis
- Meiosis arrests in prophase I -> these are primary oocytes
At what point in gestation is the maximum number of germ cells reached? Approximately how many?
- Mid gestation
- 7 million
What happens once the maxmum number of oogonia are reached?
- Atresia begins and many oogonia and primary oocytes degenerate until majority have degenerated (by 7th month) (selection)
- Approximately 2 million remain, all primary oocytes
What is a primordial follicle?
-A single primary oocyte, halted in prophase I (also known as diplotene stage), surrounded by follicular cells.
What happens to the primordial follicles between birth and puberty?
-Atresia continues until around 40,000 remain at puberty
When puberty begins, what happeds to the primordial follicles?
-15-20 ooytes begin to mature each month passing through three stages of maturation:
Preantral
Antral
Preovulatory
If 15-20 oocytes begin maturation each month, why is only 1 (sometimes 2) ova ovulated?
-Atresia continues
Describe the preantral stage of oocyte maturation
- Primordial follicle begins to grow
- Surrounding follicular cells change from flat to cuboidal and proliferate to produce a stratified epithelium of granulosa cells
What is the function of the granulosa cells in a maturing oocyte?
-Secrete a layer of glycoprotein on the oocyte producing zona pellucida
Describe the antral state of oocyte maturation
-As development of the primordial follicle continues, fluid filled spaces appear between the granulosa cells which coalesce to produce the antrum
What is the name of the granulosa cells which directly surround the oocyte? What is their function?
- Cumulus oophorus
- Protect and nature oocyte
Describe a secondary follicle
- Primary oocyte at one pole of follicle surrounded by cumulus oophorus
- Large antrum formed surrounded by granulosa cells, theca interna and theca externa
Describe the preovulatory stage of oocyte maturation
- Meiosis I completed resulting in two haploid daughter cells of unequal size
- One cell received most of the cytoplasm (secondary oocyte), the other (first polar body) receives practically none
- Secondary oocyte (and polar body) enters meiosis II but arrests in meiosis II, approx 3 hours before ovulation
What induces the preovulatory stage of oocyte maturation?
-LH surge
When is meiosis II completed?
-Only if the oocyte is fertilised
What happens if the oocyte is not fertilised?
-Oocyte degenerates approx 24 hours after ovulation
What stimulates rapid growth of the follicle several days before ovulation?
-FSH and LH
What is a graafian follicle?
-A mature follicle ready for ovulation
How is the secondary oocyte released from the mature follicle and ovary?
- LH surge increases collagenase activity to break down collagen matrix
- Prostaglandins increase in response to LH and cause local muscular contractions in ovarian wall
- Oocyte extruded and breaks free from ovary
What happens to the remnants of the follicle after ovulation?
- Remains in the ovary and forms corpus luteum
- Granulosa and theca interna cells become vasularized
- Develop yellowish pigment and change into lutein cells
What is the purpose of corpus luteum?
-Secrete progesterone and oestrogen to stimulate uterine mucosa to enter secretory stage in preparation for embryo implantation
What happens to the corpus luteum of fertilisation doesnt occur?
- Regresses and degenerates
- Forms a mass of scar tissue (corpus albicans)
- Progesterone production decreases precipitating menstrual bleeding
How does the secondary oocyte travel from the ovary to the uterus?
- Fimbrae sweep over surface of ovary shortly before ovulation
- Uterine tubes begin to contract rhythmically
- Oocyte carried into tube by sweeping movements of fimbrae and by motion of cilia on epithelial lining
- Propelled by peristaltic contractions of tube and cilia on lining towards uterus
What happens to the corpus luteum if fertilisation occurs?
- Degeneration prevented by hCG
- Corpus luteum continues to grow and forms corpus luteum graviditatis
- Continues to secrete progesterone until 4th month of gestation until progesteone secretion from placenta is adequate
