gametogenesis and fertilisation Flashcards
Define primordial germ cells and explain their role in the lifecycle.
Describe spermatogenesis and oogenesis, and compare and contrast these processes.
Primordial germ cells (PGCs) migrate from their site of origin into the gonad during early development
PGCs originate in the extra- embryonic tissue.
They migrate into the embryo via the gut and into the genital ridge.
PGCs multiply by mitosis as they migrate.
The genital ridge gives rise to the somatic cells of the gonad (testis or ovary).
The PGCs divide by meiosis to produce the gametes.
Spermatogenesis: meiosis occurs before differentiation
Meiosis reduces the chromosome number by half and introduces genetic variability
Embryo: PGCs multiply by mitosis during migration to generate spermatogonia that arrest in G1
After birth: diploid spermatogonia multiply by mitosis to generate spermatocytes
Spermatocytes enter meiosis
Incomplete cytokinesis yields syncytia: haploid spermatids connected by cytoplasmic bridges
->Allows synchronisation of maturation and sharing of gene products
Spermatids differentiate into spermatozoa
->4 gametes (n) from each primary spermatocyte
Spermiogenesis
Differentiation to produce mature sperm cell
- 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
The sperm head is specialised to aid penetration of the outer coverings of the egg
The sperm flagellum (tail) is specialised for motility
A mature spermatozoon (plural: spermatozoa)
The acrosome contains lytic enzymes for:
* protein digestion, e.g. acrosin
* carbohydrate digestion, e.g. b-N-acetylglucosaminidase
* lipid digestion, e.g. phospholipase C
The axoneme contains 2 central singlet microtubules (MTs) surrounded by 9 doublet MTs. Dynein is attached to the MTs.
dyenin uses the energy of ATP hydrolysis to slide the MTs past one another
Oogenesis: meiosis occurs after differentiation
Embryo: 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
Adult: Primary oocytes grow and mature
Upon ovulation, meiosis I is completed; secondary oocytes arrest in metaphase II
Meiosis II is completed after fertilisation
->1 ovum/egg (n) and 2 polar bodies from each primary oocyte
Eggs are specialised to generate a new individual, with nutrient reserves and an elaborate coat
- nutritive yolk proteins
- protein synthesis machinery (ribosomes, tRNAs)
- mRNAs encoding proteins needed for early development
- morphogenetic factors to direct early development
- protective chemicals e.g. UV filters, enzymes for DNA repair
- extracellular glycoprotein coat
the mammalian oocyte is surrounded by the zoma pellucida, a translucent layer consisting of 3 glycoproteins-ZP1, ZP2, ZP3
cortical granules derived from the golgi apparatus contain protease and glycosidases
ZP3 is the sperm receptor; its O-linked polysaccharide determines species specificity
- ZP3 ‘knockout’ mice produce oocytes lacking the zona pellucida, and are therefore infertile
- Human ZP3 gene ‘knock-in’ rescues zona formation and fertility
- Human sperm cannot bind human ZP3 ‘knock-in’ because it cannot recognise the murine polysaccharide
Penetration of the cumulus cell layer
The oocyte is surrounded by cumulus cells from the follicle, in a matrix of hyaluronic acid.
Hyaluronidase activity on the sperm head enables it to penetrate this layer.
Sperm galactosyltransferase (GalT) recognizes N- acetylglucosamine residues on ZP3
GalT-ZP3 crosslinking causes GalT 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: the acrosomal reaction.
Acrosomal enzymes, including b-N- acetylglucosaminidase (digests oligosaccharide side chains) and acrosin (serine protease), lyse the zona pellucida.
Sperm Izumo binds oocyte Juno, recruiting oocyte CD9, causing the plasma membranes to fuse, and the sperm enters the oocyte
Sperm entry triggers calcium release from the egg ER
Calcium release within the oocyte (egg) triggers:
* cortical reaction: exocytosis of cortical granules
* completion of the second meiotic division, producing the definitive oocyte and second polar body and initiating development
A wave of Ca2+ release crosses the egg at 5-10 μm s-1, followed by Ca2+ oscillations.
The sharp increase in free Ca2+ is essential for egg activation and for the initiation of development.
Cortical reaction causes modification of the ZP
Actin polymerises into microfilaments, which transport cortical granules to the plasma membrane.
Cortical granule contents released by exocytosis
Enzymes partially digest ZP2 and remove carbohydrate from ZP3. The ZP hardens, and further sperm cannot bind, blocking polyspermy.
Oocyte completes meiosis II, initiating development
- 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)
Sperm and egg pronuclei approach each other and 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.
