spermatogenesis and fertilisation Flashcards
where does stage 4 of gametogenesis take place in boys?
seminiferous tubules of testes after puberty
what are Type A spermatagonia?
mitotically active throughout reproductive life
what are Type B spermatagonia made from?
Type A spermatagonia
where are spermatagonia found?
the base of seminiferous epithelium by interlocking Sertoli cells connected by intercellular cytoplasmic bridges
how is the blood testes barrier formed?
by sertoli processes forming an immunological barrier
what happens if the blood testes barrier is broken?
autoimmune infertility can occur
what is spermatogenesis?
the production or development of mature spermatozoa
explain the process of spermatogenesis
type a spermatognia –> type b spermatagonia –> primary spermatocyte –> secondary spermatocyte –> spermatids –> mature sperm
where does spermatogenesis occur?
from periphery of seminiferous tubule to the lumen
what are primary spermatocytes?
Type B spermatogonia when they enter meiosis
how are primary spermatocytes made?
o During early meiosis I, become immunologically distinct from other cells
o Move through barrier to interior of seminiferous tubule
what phase of cell division converts primary spermatocytes to secondary spermatocytes?
meiosis I
what phase of cell division converts secondary spermatocytes to spermatids?
meiosis II
what do primary spermatocytes do during meiosis I?
o During this time they make mRNA for later protein production
o mRNA is stored until its required later
what can early translation into protein cause?
sterility
when do secondary spermatocytes enter meiosis II?
immediately after meiosis I is completed
what is spermiogenesis?
Spermatids undergo spermiogenesis to become spermatozoa
what occurs during spermiogenesis?
- 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
what is the final step of sperm maturation?
capacitation
where does capacitation take place?
in the female genital tract
why does capacitation occur in the female genital tract?
requires contact with the secretions of the oviduct
why is capacitation needed?
Only capacitated sperm are capable of the acrosome reaction + fusion with oocyte
what does GnRH stand for?
Gonadotrophin releasing hormone
where is GnRH produced?
hypothalamus
what does GnRH stimulate?
FSH and LH release from the anterior pituitary
what does LH stimulate?
testosterone production by Leydig cells (which surround the seminiferous tubules)
where are FSH and LH released from?
anterior pituitary
what is the main target of testosterone and FSH?
sertoli cells
what do sertoli cells secrete when stimulated by testosterone and FSH?
androgen binding protein (ABP)
Tubular fluid
how is spermatogenesis stimulated?
ABP binds to testosterone & carries it to area of seminiferous tubule where it stimulates spermatogenesis
what are the essential “ingredients” for fertilisation?
o Functioning hypothalamic-pituitary-gonadal axis
o Normal oogenesis & spermatogenesis
o Normal structure and function of reproductive tract
o Trouble free transport of gametes
o Gamete fusion
o Trouble free transport of embryo
where is sperm deposited?
cervical OS
what helps sperm towards the cervical canal?
ciliated surface of the cervical os
what helps transport of sperm into the uterus and oviducts?
o Sperm’s own propulsion
o Fluid currents caused by uterine cilia
how long does it take sperm to travel 13-15cm?
2-7 hours
what happens to sperm during the journey into the uterus and oviduct?
sperm undergo capacitation
what is capacitation
removal of the glycoprotein coat so sperm become hyperactive and sensitive to surrounding signals
at the time of ovulation, what happens to the properties of the cervix mucus?
changes from a sperm-hostile to sperm-friendly environment
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
describe what happens in the acrosome reaction
- acrosome membrane fuses with the overlying plasma membrane
- enzymes released
- sperm finishes journey between zona pellucida and oocyte membrane
- sperm becomes enveloped and fusion occurs
- large increase in intracellular Ca2+ which forms a wave across the egg
what is the oolemma?
the oocyte membrane
what enzymes are released during the acrosome reaction and what do they do?
hyaluronidase – dissolves the intercellular matrix between the cumulus cells
Other enzymes dissolve the zona pellucida
describe how egg/sperm fusion occurs
- sperm undergoes acrosome reaction and binds with egg receptor
- Cortical vesicles fuse w plasma membrane –> exocytosis -> release contents into peri-vitelline space
• Modified fertilisation membrane detaches + rises
• Sperm is pulled into egg w help from fertilisation cone
• Microtubules radiating from centrosome associated w male pronucleus helps guide nucleus to centre of egg where it fuses w female pro-nucleus
how do the sperm and egg find each other?
• They need to recognise each other from a distance;
o Sperm attracted to egg via chemotaxis
o Only found in mature eggs and sperm
o Mechanisms vary depending on species
how do the cells fuse with each other in a species-specific way?
- in mammals there’s a glycoprotein called ZP3 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 cells fusing restricted?
• Production of second messengers on sperm/egg binding triggers polyspermy blocking.
• Slow block:
o release of calcium in a wave
o calcium triggers cortical granule
o release and activation of cell division
what happens to the fertilised ovum as it travels down the oviduct?
- As it travels down the Fallopian tube (oviduct) the fertilised oocyte or zygote will begin to divide.
- It undergoes cleavage to form 2 cells
what happens at the 16-32 cell stage of a fertilised ovum?
polarisation of 2 cell populations to form a morula;
o The outer cells form one population and are trophoblast precursors.
o The inner cells are the pluriblast cells
when does the 32-64 cell stage occur?
4 to 5 days after ovulation
what occurs at the 32-64 cell stage?
the cells which have been polarised start to differentiate into different cell types - known as a blastocyst
when does hatching occur?
around day 5
how does hatching occur?
o Around day 5 the blastocyst frees itself from the zona pellucida.
o It does this through a series of expansion-contraction cycles and with the help of enzymes that dissolve the zona pellucida
when does the blastocyst attach itself to the uterine wall?
7-9 days post ovulation
