1- Fertilization Flashcards
(28 cards)
Epigenesis Definition
Individuals acquire their final forms through a series of progressive changes.
Aristotle: what did he think
looked at chicken embryos came up with epigenesis as a theory
Homunculus
- little man
- humans are present in a little farm in the gametes
- a very small human or humanoid creature.
- a supposed microscopic but fully formed human being from which a fetus was formerly believed to develop.
Preformationtionism
theory that organisms develop from miniature versions of themselves. Instead of assembly from parts, preformationists believed that the form of living things exist, in real terms, prior to their development
Steps of Epigenesis
- production of gametes (gametogenesis)
- fertilization (fusion)
- cell division (mitosis)
- generation of diverse cell types
- cell fate specification
- differentiation
- tissue organization and patterning
- axis specification
- gastrulation
- organogenesis
- post embryonic development
- metamorphessi
- regeneration
- aging
Sperm Structure
Virtually no cytoplasm, small and motile. Deliver DNA, no baggage
- Head:
- Midpiece: mitochondria (NEG for locomotion)
- Tail: flagella that emanates from pair of centrioles at the base of the nucleus
Sperm Head
- haploid nucleus
- acrosome (made from golgi complex) has enzymes from protein degradation (digest extracellular egg coat)
- globular proefins (between acromomes and nucleus)
Egg cell Glycoprotein-rich extracellular layer: vertebrates vs. invertebrates
vert: zona pellucida
invert: vitelline envelope
Egg Cell
- massive cytoplasm (biochemical determinate for early devo)
- haploid nucleus
- no centrioles (in many species)
- Under plasma membrane is gel-like cortex. It is stiff due to presence of large quantities of globular actin molecules.
- Cortex houses large numbers of cortical granules (have digestive enzymes) which contain proteolytic enzymes (prevent sperm from access after fert).
Functions of massive egg cytoplasm - 5
- nutritive proteins (yolk proteins and a.a)
- ribosome and tRNA
- mRNA
- morphogenetic factors (transcription factors and paracrine factors)
- protective chemicals (protection from environment [sun shield screens] and predators)
Microfilaments:
location, formation and Role,
- sea urchin egg cell
- Actin molecules polymerize to form microfilaments
- aid in fertilization
- form microvilli that may help in sperm entry.
Mammalian extracellular coats
- zona pellucida.
- Cummulus (where sperm embeds)
- The cumulus cells adjacent zone pellucida called corona radiata
Molecular Biology Dogma
info from DNA–> RNA –> proteins
Sperm path
goes through cummulus–> zone pellucid –>plasma membrane –>ovum (cytoplasm)
what is the response to certain chemicals released by same species
chemotaxis
Events of fertilization
1) species specific chemotaxis of sperm to egg
2) exocytosis of sperm acrosomal vesicle to facilitate entry into egg
3) binding and passage of sperm across the extracellular matrix of egg
4) fusion of egg and sperm cell membranes
5) fertilization - fusion of haploid pronuclei (to restore diploid chromo number)
In the natural habitat how do sperms identify
and eggs of the same species?
- Sperms are attracted towards egg by chemotaxis.
- Eggs release a chemical that diffuses and a gradient is built in the habitat.
- Sperms swim up the concentration gradient of sperm activating peptides such as resact.
- Resact is secreted by egg jelly of Arbacia punctulata.
Explain Resact and how it works (the steps)
- molecule secreted & diffuses = gradient
- increased [ ] near egg
- sperm cell has receptor
- binds and activates cyclase
- converts GTP –> cGMP (2nd messenger)
- opens Ca2+
- [ ] Ca increases in cell due to influx
- Ca involved with membrane fusion
- sperm is active and is ready to start swimming towards resact
- chemotaxis = moment of molecule due to certain chemical
Fusion of egg and sperm 5 steps
1) sperm contacts jelly layer
2) acrosomal reaction
3) digestion of jelly layer
4) binding to vitelline envelope
5) fusion of acrosomal process membreane and egg membrane
Acrosomal reaction
• Contact between sperm and egg jelly results in acrosome reaction.
• receptor on sperm recognize polysaccharides on the egg jelly
• polysaccharides are sulphated and imbedded into jelly layer
• if wrong sperm comes it falls the sp. specific binding
• Ca2+ increases further
• Na increases in sperm
• H+ decreases and are thrown out of sperm cell
• pH becomes basic
triggers phospholipase3 which generates IP3 (PLC converts PIP2 –> IP3 & DAG)
•!P# and DAG 2nd messengers
• Ca2+ induces exocytosis of digestive enzymes to digest the path, though which acrosomal process adhere to egg envelope.
• Ca2+ influx also associates with this process by activating RhoB (A G protein that can bind to guanine nucleotides) causes polymerization of actin molecules
•actin = globular proteins
•polyactin –> you get microfilliments
•g (globular) actin –> f (fillimentious) actin
Acrosomal reaction is mediated by (3)
1) Initial Ca2+ entry into sperm head
2) Na+/H+ exchanger pumps in Na+ while pump out H+.
3) PLC generates IP3, which induce Ca2+ release from the reservoirs to stimulate the fusion of acrosomal membrane to plasma membrane.
The acrosomal reaction in a sea urchin sperm & Binding of sea urchin sperm to eggs
-Initiated by the egg’s jelly coat which causes a calcium influx and exocytosis.
-microfilliments start growing and create acrosomal process that has binding proteins (the dots)
-Exposes bindin-an egg binding protein-on acrosomal membrane
-Species-specific interactions of bindin with vitelline
envelopes.
• Bindin binds to a large transmembrane glycoprotein i.e., the bindin receptor of the egg plasma membrane which extends out into the vitelline envelope.
• Probably hundreds of bindin receptors are needed to capture a sperm.
3 - Species specific barriers
to overcome during fertilization
- Sperm attraction: Chemotaxis
- Sperm adhesion to egg surface: Binding of sulfate containing polysaccharide on egg jelly to sperm cell membrane located near acrosomal vesicle.
- Sperm activation and acrosome reaction: Bindin on acrosomal process on sperm and bindin receptor located on egg cell membrane.
Fusion of Egg-Sperm cell membranes
- Fusion of sperm-egg membranes creates fertilization cone, which is formed due to polymerization of actin in egg. (g-actin –> f-actin)
- outer cortical region of egg is gel like (under PM due to increased of large globular proteins
