Spermatogenesis and fertilisation Flashcards
what happens to males in phase II of gametogenesis
- Spermatogonia undergo mitosis in early embryonic testes
- Spermatogonia undergo periodic waves of mitosis form puberty onwards throughout life
describe phase IV of gametogenesis for males
- Much more rapid than oogenesis, begins in seminiferous tubules of testes after puberty
- Type A spermatogonia are mitotically active throughout reproductive life
- Give rise to type B spermatogonia which enter meiosis
- Spermatogonia are kept at base of seminiferous epithelium by interlocking sertoli cells
- Connected by intercellular cytoplasmic bridges
what are the two types of spermatogonia
type A and type B
what id the difference between type A and type B spermatogonia
Type A spermatogonia are mitotically active throughout reproductive life
• Give rise to type B spermatogonia which enter meiosis
where are spermatogonia kept
At Base Of Seminiferous Epithelium By Interlocking Sertoli Cells
where does spermatogenesis happen
- the development the germ cells begins with the spermatogonia at the periphery of the seminiferous tubule and they advances towards the lumen they mature and they become primary spermatocytes I, and secondary spermatocytes, spermatids and finally mature sperm.
what do sertoli processes form
- they form an immunological barrier
- if this is broken than autoimmune infertility can occur
what is another word for type B spermatogonia
primary spermatocytes
- this happens when they enter meiosis
what do type B spermatogonia cells do during early meiosis I
- they become immunologically district from other cells
- they move through the barrier to interior of seminiferous tubule
how long do primary spermatocytes take to complete meiosis I
- primary spermatocytes take 24 days to complete meiosis I
what do primary spermatocytes do in meiosis I
- making mRNA for later protein production.
- mRNA stored until required late
what happens after completion of meiosis I what is produced
- secondary spermatocytes are produced
- these immediately enter meiosis II and produce spermatids
what do spermatids become
- Spermatids undergo spermiogenesis to become spermatozoa
what do spermatids need to have happen to them in order to become spermatozoa
• Reduction in nuclear size
• Golgi apparatus condenses to form acrosome
• Flagellum grows out of centriole
• Cytoplasm streams away from nucleus
• Mitochondria in spiral arrangement around flagellum
• Head partitioned into domains
- Cytoplasm moves to residual body which is phagocytosed by Sertoli cells
in order to fertilise the egg what does a sperm need to undergo
capacitiation
where does capacitation happen
• It takes place within the female genital tract and requires contact with the secretions of the oviduct.
what happens in capacitation
• It is part of a series of physiological processes which allows the sperm to fertilise the egg.
• Only capacitated sperm are capable of the acrosome reaction and fusion with the oocyte.
- they become more sensitive to signalling from the uterus
what does GnHR do
stimulates FSH and LH -release from the anterior pituitary
what does LH do
• LH stimulates testosterone production by Leydig cells (which surround the seminiferous tubules) – they are not inside the seminiferous tubules
what is the main target of testosterone and pituitary FSH
• Main target of testosterone and pituitary FSH are Sertoli cells
what do Sertoli cells secrete
• Sertoli cells then secrete androgen binding protein (abp) and tubular fluid
what does Abg do
- Abg binds to testosterone & carries it to area of seminiferous tubule where it stimulates spermatogenesis
what do you need in order for fertilisation to take place
- Functioning hypothalamic-pituitary-gonadal axis
- Normal oogenesis & spermatogenesis
- Normal structure and function of reproductive tract
- Trouble free transport of gametes
- Gamete fusion
- Trouble free transport of embryo
where is the sperm deposited
- in the cervical os
how does the sperm travel towards the cervics and into the uterus
The ciliated surface of the cervical os help the sperm towards the cervical canal (over 99% spermatozoa do not get there)
• Transport into uterus & oviducts by sperm’s own propulsion & fluid currents caused by uterine cilia. It’s a distance of about 13-15cm & takes about 2-7 hours.
• During this journey sperm undergo capacitation
• Sperm become hyperactive & sensitive to surrounding signals.
what is capacitation
– the removal of its glycoprotein coat.
what happens to the cervix when ovulation happens
the properties of the cervix mucus changes from a sperm hostile to sperm friendly
how is an egg fertilised
- Sperm finds egg
- Sperm recognises egg.
- Sperm has acrosome reaction to penetrate extracellular layer
- Sperm cell membrane fuses with egg cell membrane and triggers Ca2+ wave in egg.
- Polyspermy is blocked.
- Fertilisation cone forms around sperm head.
- Movement and fusion of pronuclei.
how does the acrosome reaction and egg sperm fusion happen
- The acrosome membrane fuses with the overlying plasma membrane
- Enzymes released that allow passage through zona pellucida
- Enzymes include hyaluronidase, which dissolves the intercellular matrix between the cumulus cells, and other enzymes dissolve the zona pellucida
- Sperm finishes its journey between the zp and the oocyte membrane (oolemma)
- Sperm becomes enveloped
- Fusion occurs - mechanism unknown
- Large increase in intracellular Ca2+ which forms a wave across the egg (mechanism uncertain – few candidates)
How do the sperm and egg find each other
- Sperm attracted to egg via chemotaxis
- Only found in mature eggs and sperm
- Mechanisms vary depending on species
how do cells fuse with each other in mammals
- A glycoprotein called ZP3
- Found in the zona pellucida
- Binds to β1, 4 galactosyl transferase receptor on the sperm plasma membrane
- Capacitated sperm are species-specific in their binding to ZP3
- Binding triggers changes in Ca2+ and pH in sperm and acrosome reaction
how is the number of cell fusing restricted
Production of second messengers on sperm/egg binding triggers polyspermy blocking.
Slow block:
- release of calcium in a wave
- calcium triggers cortical granule
- release and activation of cell division
where does the fertilised ovum travel
- travels down the oviduct
- it travels down the fallopian tube the fertilised oocyte or zygote will begin to divide and it undergoes cleavage to form two cells
what happens at the 16-32 stage
- The outer cells form one population and are trophoblast precursors.
- The inner cells are the pluriblast cells.
This is called a morula.
what happens at the 32-64 stage
(4-5 days after ovulation) the cells which have been polarised start to differentiate into different cell types.
This is called the blastocyst.
– there is something stoping it from developing further, the zona pellucida has harderend therefore if it wants to grow it has to free itself from the zona pellucida
Around day 5 the blastocyst frees itself from the zona pellucida.
It does this through a series of expansion-contraction cycles and with the help of enzymes that dissolve the zona pellucida.
This is called ‘hatching’.
The blastocyst will attach to the uterine wall about 7-9 days post ovulation
what stops the blastocysts from developing further
zona pellucida has harderend therefore if it wants to grow it has to free itself from the zona pellucida
how does the blastocysts free itself
It does this through a series of expansion-contraction cycles and with the help of enzymes that dissolve the zona pellucida.
This is called ‘hatching’
