Fertilization

Fertilization

process, gamete meet and fuse, combine gene, begin creation of new organism

6 Major steps of fertilization

Movement of gametes, Contact/Recognition, Fusion of cell membranes, Regulation of sperm entry, Fusion of genetic material, activation of egg metabolism/start development

Similarity of sperm and egg

Haploid genome, cell membrane that recognize and fuse with other

Difference in sperm and egg

Egg, cytoplasm, Nucleus and propulsion system vs Nucleus and enriched cytoplasm (ribosomes/mitochondria/mRNA/Enzymes)

4 sperm part

Head, Midpiece, Tail, End piece

Sperm anatomy - Head

streamline haploid nucleus, tightly compressed DNA, Acrosomes

Acrosomes

caplike organelle, derived from Golgi apparatus, contain enzymes for digesting protein/sugar in egg outer covering

LINC (Linker of nucleoskeleton and cytoskeleton) protein complex

Sperm midpiece, anchors acrosomes to nuclear envelope, shapes head, joins head to mitochondrial midpiece

Sperm anatomy - End piece

Function unknown but need for movement (coordination?)

Sperm anatomy - Tail

travel by whipping flagellum, Axoneme(microtubules) emanate from 1 centriole, Dynein (ATPase) convert ATP to mechanical energy

Modification of sperm

Centriole duplicates, Microtubules spread from Centriole (form Axoneme), Golgi becomes acrosome, Mitochondria collects around flagellum, eliminate cytoplasm and condense nucleus

3 Egg parts

Envelope, Cytoplasm, Nucleus

Egg cytoplasm

all material to begin growth stored in, Nutritive protein, Ribosomes, tRNA, mRNA, Morphogenetic factor, Protective chemical (UV/Distasteful)

Egg Extracellular envelope

Fibrous mat, species specific recognition, Vitelline (See urchin with Jelly) vs Zona pellucida (mammals)

Egg envelope parts

Extracellular, Cortical granules, Cumulus (supporting/follicular cells), Corona radiata, Polar body

Cortical granules

membrane bound Golgi derived, has proteolytic enzymes, homologous to acrosomal, role in polyspermy prevention

Corona Radiata

innermost layer of cumulus cell, adjacent to zona pellucida, hormones to regulate translocation

Events leading to fusion

egg/sperm translocation, Acrosome reaction, Digestion of extracellular surrounding egg, Binding and recognition

Translocation of oocyte

Released and surrounded by ECM/cumulus cells → COC, Fimbria waft COC into oviducts, Cilia of Fimbria and muscle contraction move to site of fertilization

Translocation of sperm

200-300 million ejaculated but only 200 reach egg, in oviduct within 30 min

translocation of sperm from vagina to oviduct

Sperm motility → Flagellar action, Uterine muscle contraction, Sperm rheotaxis → directional cues from flow of liquid from oviduct (sperm specific calcium ions)

Capacitation

when sperm bound epithelial cells in oviduct, gain ability to recognize cues (Thermotaxis/Chemotaxis), gain ability to undergo acrosome reaction, ability to fuse with egg cell membrane, become hyperactivated

model of sperm capacitation

sperm exposure to Albumin and bicarbonate ions, HCo3-/Ca2” activates adenylate kinase (SACY → internal membrane), Elevate cAMP, increase PKA activation, protein phosphorylation

Sperm Hyperactivation

at end of capacitation, swim faster and generate more fore, shift from symmetrical to asynchronous beat with greater bending, to detach from epithelial cell and continue journey, activated by Progestogen and CatSper calcium pumps

Thermotaxis

capacitated sperm sense temperature difference between isthmus and warmer ampulla, swim to warmer sites mediated through G protein coupled receptors (GPCRs)

Chemotaxis

capacitated sperm detect and respond to progesterone in ampulla

Progesterone

secreted by cumulus cell, activates CatSper calcium pumps for hyperactivation, in ampulla

Acrosome reaction in sea urchin

Exocytosis of enzymes, digest through jelly to egg surface, Globular actin protein elongate into fibrous microfilaments → Acrosomal process, species specific recognition molecules for binding to vitelline →Bindin

Capacitated acrosome reaction near COC

sperm in COC recognize zona pellucida glycoproteins (ZP1-4), Acrosome intact sperm recognize ZP3, Acrosome reacted sperm recognize ZP2, egg encounter heterogeneous sperm population (reacted or not)

Gamete fusion in sea urchin

Formation fertilization cone, membrane fusion → cytoplasmic bridge, Actin (from acrosomal/cone) forms connection → guide sperm, Bindin promotes

Bindin

Sea urchin, binds sperm to vitelline envelope, promotes fusion

Gamete fusion in mammals

Izumo binds Juno, side of sperm contacts egg, equatorial membrane fusion (no drilling)

Monospermy

2 haploid = 1 Diploid, restore chromosome number for species

Fast block

Change in electric potential(to pos), in sea urchin, fusing sperm chemical alter Sodium ion channels, for a minute (until slow), not in mammals

Slow block

minute after fusion, sea urchin/most mammals, cortical granules release enzyme, cleave protein connecting membrane and envelope, clip bindin receptors, cortical granules bind extracellular envelope (fertilization envelop)

Mammal fast block equivalent

Ovastacin (cortical graules protease) cleave ZP2 (can’t bind sperm), Juno release from membrane, Zinc ion bind Zona → change to mechanical shield

Activation mammalian egg

Spern phospholipase PLCzeta digest PIP2, PIP2 into IP3/DAG, IP3 release Ca2+ into cytoplasm, increase [Ca2+] needed for activation/meiosis

genetic material fusion

Methyl groups removed (totipotency), Dynein regulate pronuclei movement along microtubular belt, pronuclear envelope breaks down, chromosomes organized by sperm centriole