Gametogenesis and fetilisation Flashcards
(21 cards)
Describe the developmental sequence of a sperm
- spermatogonia (multiply by mitosis)
- primary spermatocyte (multiply by meiosis)
- secondary spermatocyte
- spermatids
- spermatazoa
Describe the developmental sequence of the female egg
- oogonia
- primary oocyte
- large primary oocyte
- secondary oocyte
- ovum
Describe the migration of primordial germ cells (PGCs) to give rise to gametes
- PGCs migrate from their site of origin (the extra-embryonic tissue) into the gonad during early development
- they migrate into the embryo via the gut and into the genital ridge
- the genital ridge gives rise to the somatic cells of the gonad (testes or ovary)
- The PGCs divide by mitosis to produce the gamtes
List the processes that occur during spermiogenesis
- golgi apparatus develops into acrosomal cap
- flagellum develops
- cytoplasm extruded
- mitochondria coalesce near base of flagellum
- arginine-rich protamines replace histones
- nucleus condenses
- cytoplasmic bridges are lost
Describe the features of a mature spermatozoon
- The acrosome contains lytic enzymes for protein digestion, carbohydrate digestion and lipid digestion
- the flagellum is specialised for motility
How is the flagellum specialised for motility?
- the axoneme contains 2 central singlet microtubules (MTs) surrounded by 9 doublet microtubules
- dynein is attached to the microtubules
- dynein uses the energy of ATP hydrolysis to slide the MTs past one another
- MT sliding causes the flagellum to bend
Describe the process of oogenesis
- PGCs multiply by mitosis during migration to generate oogonia that continue to divide by mitosis
- diploid oogonia enter meiosis and arrest in prophase of meiosis I as primary oocytes
- upon ovulation, meiosis I is completed, secondary oocytes arrest in metaphase II
- meiosis II i completed after fertilisation
How are eggs specialised?
- nutritive yolk proteins
- protein synthesis machinery
- mRNAs encoding proteins needed for early development
- morphogenetic factors to direct early development
- protective chemicals –> UV filter, enzymes for DNA repair
- extracellular glycoprotein coat
What is the zona pellucida?
- surrounds the oocyte
- consists of three glycoproteins –> ZP1, ZP2, ZP3
What is ZP3?
- the sperm receptor
- its O-linked polysaccharide determines species specificity
What are cortical granules?
- derived from the golgi apparatus
- contain proteases and glycosidases
Describe the stages of fertilisation of a mammalian egg
- binding of the sperm to the zona pellucida
- acrosome reaction
- penetration through zona pellucida
- fusion of plasma membranes
- sperm nucleus and contents enter egg cytoplasm
How does the sperm penetrate the egg?
- the oocyte is surrounded by cumulus cells from the follicle, in a matrix of hyaluronic acids
- hyaluronidase activity on the sperm head enables it to penetrate this layer
Describe the cascade that occurs from sperm galactosyltransferase (GaIT)
- GaIT-ZP3 crosslinking causes GaIT proteins to cluster, triggering G protein activation
- the change in membrane potential opens voltage-gated calcium channels, increasing intracellular Ca2+
- calcium-mediated exocytosis of the acrosomal vesicle is initiated –> acrosomal reaction
- acrosomal enzymes (B-N-acetylglucosaminidase which digests oligosaccharide side chains) and acrosin (serine protease), lyse the zona pellucida
- sperm Izumo binds oocyte Juno, recruiting oocyte CD9, causing the plasma membrane to fuse and the sperm enters the oocyte
What does calcium release within the oocyte trigger?
- cortical reaction –> exocytosis of cortical granules
- completion of the second meiotic division, producing the definitive oocyte and second polar body and initiating development
How does cortical reaction cause modification of the ZP
- actin polymerises into microfilaments, which transport cortical granules to the plasma membrane
- cortical granules contents released by exocytosis
- enzymes partially digest ZP2 and remove carbohydrates from ZP3
- the ZP hardens, and further sperm cannot bind, blocking polyspermy
How is development initiated after meiosis II is completed?
- Ca2+ activates a kinase that leads to proteolysis of cyclin and the metaphase II arrested oocyte completes meiosis
- a centrosome forms around the sperm centriole, becoming the MT organising centre for the sperm aster (for the first mitotic spindle)
How do the sperm and egg pronuclei prepare for the first mitotic division of the zygote?
- both pronuclei undergo DNA replication as they migrate along microtubules towards one another
- the pronuclear envelopes break down
- the centrosome replicates and organises a mitotic spindle
- the chromosomes align on a common metaphase plate
What do the sperm and egg provide during fertilisation?
The sperm provides:
- a haploid genome
- a centriole
The egg provides:
- a haploid genome
- mitochondria and other organelles
- mRNAs and proteins needed for early development
What are the main results of fertilisation?
- restoration of the diploid number of chromosomes
- sex determination of new individual
- initiation of cleavage
What does successful fertilisation require?
- Sperm penetration of the cumulus cell layer
- Sperm binding to the zona pellucida (GalT-ZP3)
- Exocytosis of the acrosomal vesicle
- Passage of the sperm through the zona pellucida
- Fusion of sperm and oocyte membranes (Izumo-Juno)
- Intracellular Ca2+ spiking
- Exocytosis of cortical granules
- Enzymatic modification of the zona pellucida to block
polyspermy - Pronuclear DNA replication
- Chromosome alignment on a single mitotic spindle
