Lecture 5: Gametogenesis Flashcards
How are mature gametes made?
- germ cells colonise the gonad in the embryo (male germ cells colonise the medulla of developing gonads, and the female germ cells colonise the cortex)
- proliferate by mitosis
- reshuffle genetically (genetic diversity) ad reduce to haploid by meiosis
- cytodifferentiate into mature gametes
Timing and scale of this varies between sexes
Where is the SRY gene found?
On the Y chromosome
-codes for a transcription factor that leads to the development of the testis
How does oogenesis differ from spermatogenesis?
Oogenesis: -very few gametes (400 in lifetime) -intermittent production (1/month) Spermatogenesis: -huge number made (200 million) -continuous production -essentially 'disposable'
What is formed during the process of fertilisation?
Zygote (diploid)- from fertilisation of two haploid gametes
What determines the sex of the offspring?
The sperm, because it contains XY, whereas female contains only XX
What are the 2 main functions of meiosis?
-reduce chromosome number in gamete to 23
-ensures every gamete is genetically unique
Used only in production of sperm and eggs
What does meiosis produce?
4 daughter cells
-however in a female only one develops into a mature oocyte, others form polar bodies
How does genetic variation occur?
Crossing over: exchanging DNA b/w 2 homologous chromosomes
Independent assortment: random orientationof each bivalent along metaphase plate)
Random segregation: random distribution of alleles amoung the 4 gametes
Where does spermatogenesis occur?
Seminiferous tubules, in spaces between adjacent sertoli cells
- spermatogensis occurs in the walls in the basal compartment
- spermatids emerge into lumen from the adluminal compartment
Where do the seminiferous tubules coalesce?
Rete testis: concentrates the sperm
-it anastomoses then with the ductili efferentes, where the sperm then enter the head of the epididymis (storage tube for sperm)
What is the function of sertoli cells?
Nuture the spermatids
What are the compartments in the seminiferous tubules?
Compartments are created due to tight junctions between the sertoli cells which form the blood testes barrier
Basal compartment
Adluminal compartment
Why is the blood testes barrier important?
- Different environments are required for the germ cell line (spermatogonium) and the development of the spermatids
- sperm could be recognised as non self generating an immune response
What other cells are found in the seminiferous tubule?
Leydig cells are also found in the seminiferous tubules, and produce testosterone to stimulate spermatogenesis
What are the male germ cells ‘raw material’ called?
Spermatogonia
-available for up to 70 years
What are the 2 different forms of spermatogonia?
- Ad spermatogonium: reserve stock (resting)
- Ap spermatogonium: maintain stock, and from puberty onwards they produce type B spermatogonia which give rise to primary spermatocytes
What are the steps in spermatogenesis?
- spermatogonium divide by mitosis giving rise to Ad or Ap spermatogonium
- Ad are reserve stock to regenerate numbers
- Ap go down a meiosis pathway producing type B spermatogonia which give rise to primary spermatocytes
- primary spermatocytes divide by meiosis giving rise to secondary spermatocytes and then to spermatids which are haploid
What is the difference between spermatogenesis and spermiogenesis?
Spermatogenesis is describing the entire process of gamete production in males
Spermiogenesis is describing the differentiation of spermatids into spermatozoa
How long is the spermatogenic cycle?
16 days
How have males evolved to increase chances of fertilisation?
If spermatogenesis was in synchrony, men would only be fertile every 16 days.
We have adapted so that men are fertile all the time
How do cells appear in a cross section of the seminiferous tubule?
In groups with the same maturation stage, in a spiral like pattern
What is the spermatogenic cycle?
The time taken for reappearance of the same stage within a given segment of the tube
What is the spermatogenic wave?
The distance between the same stage of maturation
What is spermiation?
Release of spermatids into the lumen of the seminiferous tubule
- spermiogenesis needs to occur (differentiation) to remodel the sperm
- they are non-motile until they reach the epididymis (they are transported via sertoli cell secretions assissted by peristaltic contraction)
Where in the testis do the spermatids remodel?
- seminferous tubule
- rete testis
- ductuli efferentes
- into the head of the epididymis where spermatozoa are formed
What facilitates movement of the sperm tail?
Dynein
How much semen is produced per ejaculate?
2 ml
What is the composition of semen?
Seminal vesicle (70%)- amino acids, citrate, fructose- energy source for glycolysis, prostaglandins
Prostate (25%)- proteolytic enzymes, zinc
Sperm (2-5%)- 200-500 million per ejaculate
Bulbourethal gland secretions (Cowper gland)- mucoproteins to help lubricate and neutralise acidic urine in distal urethra
What controls the motility of sperm?
Zinc
-as the amount of zinc lowers, the motility of the sperm increases, so when they enter a female reproductive tract, conc of zinc falls, stimulating a greater motility
What is sperm capacitation?
Final maturation step required before the sperm becomes fertile
Conditions in female genital tract stimulate:
-removal of glycoproteins and cholesterol from sperm membrane
-activation of sperm signalling pathways
= allows sperm to bind to zona pellucida of oocyte and initate the acrosome reaction
What has to be done to sperm in IVF?
The sperm are infertile as they haven’t travelled in the womans reproductive tract
-the sperm have to be incubated in capacitation media which mimics the environemnt of female reproductive tract
When does maturation of oocytes begin?
Before birth
- germ cells (arise from yolk sac), colonise the gonadal cortex and differentiate into oogonia
- oogonia then proliferate rapidly by mitosis
- at end of 3rd month oogonia arranged in clusters surrounded by flat epithelial cells (which eventually differentiate into follicular cells)
- majority continue to divide by mitosis but some enter meiosis which arrest in prophase of meiosis 1: they are now called primary oocytes
How many chromatids fo oogonium and primary oocytes have?
Oogonium: 46 chromatids
Primary oocyte: 92 chromatids
What is the max number of germ cells reached is females?
7 million, and this is reached by mid gestation
- cell death begins and many oogonia and primary oocytes die: atresia (quality control mechanisms as you want the best 400)
- at 7th month gestation majority of oogonia have degenerated and 2 million primary oocytes survive
- these 2 million have now entered meiosis 1 and are individually surrounded by layer of flat epithelial cells now called follicular cells: now called PRIMORDIAL FOLLICLES
How many primordial follicles are left at puberty?
40,000 remain
What happens during each mestrual cycle?
15-20 oocytes start to mature each month (but only 1/2 complete maturation each month) 3 stages of maturation -preantral -antral -preovulatory
What happens in the preantral stage?
As primordial follicles begin to grow, the surrounding follicular cells change from flat to cuboidal and proliferate to produce stratified epithelium of granulosa cells (granulosa cells are follicular cells)
-granulosa cells secrete a layer of glycoprotein on oocyte called the ZONA PELLUCIDA
What happens in the antral stage?
Fluid filled spaces appear between the granulosa cells and these coaelsce to form the ANTRUM
-outer fibrous layer (theca externa) and an inner secretory layer (theca interna) surrounds the outside of the granulosa cells
What is the cumulus oophorus?
The granulosa cells around the oocyte
What is the function of theca interna?
Interna: receptors to LH, this stimulates production of androgens
What happens in the preovulatory stage?
- surge in LH induces this stage
- meiosis 1 is now complete resulting in 2 haploid daughter cells of unequal size
- one cell receives most of the cytoplasm, the other receives none (first polar body)
- the cell then enters meiosis 2 but arrests in metaphase 3 hours before ovulation
- meiosis 2 is only completed if the oocyte is fertilised, otherwise cells degenerates around 24 hours after ovulation
What happens during ovulation?
- FSH and LH stimulate rapid growth of the follicle several days before ovulation occurs
- mature follicle is now 2.5 cm in diameter and is called the graafian follicle
- LH surge increases collagenase activity (help rupture of follicle)
- prostaglandins increase response to LH and cause local muscular contractions in ovarian wall
- oocyte is extruded and breaks free from ovary
What is the corpus luteum?
Remaining granulosa and theca interna cells become vascularised and secrete oestrogen and progesterone
-this stimulates the uterine mucosa to enter secretory stage in prep for embryo implantation
When does the corpus luteum die if not fertilised?
14 days
Where does the sperm meet the oocyte?
Ampulla
How is the oocyte transported?
- fimbriae sweep over surface of ovary
- uterine tube contracts rhythmically
- oocyte carried into the tube by sweeping of fimbriae and cilia on epithelial lining
- oocyte then propelled by peristaltic muscular contractions of the tube and cilia
What happens to the corpus luteum if fertilisation doesn’t occur?
- degenerates
- forms a mass of fibrotic scar tissue called the corpus albicans
- progesterone production decreases, precipitating menstrual bleeding
What happens to the corpus luteum if fertilisation does occur?
- degeneration of CL prevented by human chorionic gonadotropin secreted by developing embryo
- CL continues to grow and forms the corpus luteum of pregnancy (corpus luteum of pregnancy)
- cells continue to secrete progesterone until 4th month, secretion of progesterone by placenta then becomes adequate
