Fertilization Flashcards
In ruminants, primates, rodents and cats, Upon intromission and ejaculation, semen is deposited into…
The Vagina
In pigs and equids, Upon intromission and ejaculation, semen is deposited into….
The cervix to into the uterus
In dogs, Upon intromission and ejaculation, semen is deposited into…
The uterus
Retrograde transport of semen
60% lost in cattle 12 hours post-insemination
Gel fraction reduces in some species (pig, horse, primates, rodents)
forms a visible vaginal “plug” in rodents
Major barrier to sperm transport in the female tract
Cervix (except in pigs and dogs)
Cervical sperm transport description
Rapid phase:
Injection to uterus by ejaculatory force (especially mares)
Slow phase:
Sperm swim upgrade through “channels” in cervical crypts to avoid retrograde mucous flow
In the slow phase of cervical transport sperm swim through…
“channels” in cervical crypts to avoid retrograde mucous flow
In the uterus sperm is transported anterograde towards the…
Utero-tubule junction by peristaltic contraction (estrogen induced)
The uterus has peristaltic contractions induced by ________ that move sperm anterograde towards the UTJ
Estrogen
In the female tract the _____-________ _________ acts as a reservoir for sperm
Utero-tubule junction
Sperm accumulate and bind tot he epithelium in the folds and are released slowly to the oviduct
Sperm Capacitation definition
physiological changes that sperm undergo in the female tract to gain the capacity to undergo the acrosome reaction (AR) (gaining the capacity to fertilize)
Capacitation normally occurs in the female tract (uterus, uterine tube) but can also be induced in vitro
Capacitation of sperm normally occurs in the….
Female tract (uterus, uterine tube) but can also be induced in vitro
Sperm undergo the acrosome reaction when…
Triggered by binding to the zona pellucida (oocyte is present)
Fertilizing capacity of an ejaculate can be stretched out over many hours, thereby increasing the odds of fertilization since….
It will not undergo acrosome reaction unless bound to zona pellucida of oocyte
(Once sperm are capacitated and/or undergo AR, their lifespan is dramatically shortened)
Requirement for ____________ stretches out the fertilizing lifespan of an ejaculate
Capacitation
Sperm Changes in Capacitation
Removal of surface factors (DF) (seminal plasma):
-ASF, Caltrin, CRISP-1
-Changes surface charge
-Unmasking of binding sites
Binding of surface factors
-Modulators of AC/cAMP: Calcitonin, FPP, adenosine, angiotensin II
-Modulators of Ca++ influx: progesterone, GABA
-Adhesion molecules: SPAM1 from female tract, via clusterin
Ability to undergo membrane fusion event (acrosome reaction):
-Membrane cholesterol efflux
-Increased plasma membrane fluidity and movement of some surface protein (e.g., dopamine receptors)
Motility changes (faster!!!!)
-Progressive motility
-Hypermotility (aka hyperactivation)
Metabolic changes
-Influx of Ca++
-Increased metabolism
Acrosome reaction steps
Sperm binds to zona pellucida via ligand-receptor interaction (ZP3)
Receptor on sperm triggers calcium influx
Intracellular [Ca++] (rises F-actin -> G-actin)
Ca++ induces membrane fusion
-Outer acrosomal and sperm cell membrane fuse
-Resulting pores permit escape of soluble acrosomal contents
-Exposure of inner acrosomal membrane, which binds to ZP2
Zona penetration by sperm
Hyaluronidase digests hyaluronic acid secreted by cumulus cells
Acrosin digests a pathway through zona pellucida (ZP2-3 matrix, ZP1 x-links)
Hyperactivated motility provides motile force
Sperm penetration depends on…
Acrosin-digested pathway
Hyperactivated motility to propel sperm through pathway
Sperm-egg binding
Entire sperm enters perivitelline space
Motility diminishes or ceases
Sperm binds to oocyte cell membrane
-Binding domain restricted to post-acrosomal region of sperm
Sperm ligand (fertilin, PH-30) binds to oocyte surface receptor (integrin?)
-Binding event triggers surface depolarization event
-Momentary opening of calcium channels causes massive increase in oocyte [Ca++]
-Cortical granule exocytosis (Block to polyspermy)
-Sperm-egg fusion (PLC) and endocytosis
-Sperm nucleus decondensation
-Resumption of oocyte meiosis
How the oocyte blocks polyspermy
Cortical reaction
Cortical granule exocytosis once sperm enters into the oocyte Perivitelline space and binds to oocyte surface receptor
Cortical granule exocytosis release a variety of proteases (e.g., plasminogen activator)
Protease diffuse across PVS
Proteases alter zona structure (zona reaction), rendering zona resistant to acrosin digestion
Results in block to polyspermy
Sperm-egg binding: the seminal fertilization event
- Sperm-egg binding
- Cytoplasmic [Ca++]
a. Cortical Reaction (exocytosis)
b. Fast Block to polyspermy
c. Sperm endocytosis
d. Sperm nucleus decondensation
e. Resumption/completion of oocyte meiosis
The Cortical reaction leads to a release of a variety of…
proteases (e.g., plasminogen activator)
Protease diffuse across Perivitelline space
(PVS)
Proteases alter zona structure (zona reaction), rendering zona resistant to acrosin digestion
Results in block to polyspermy
Sperm endocytosis into the oocyte
Increase in cytoplasmic [Ca++]
Sperm and oocyte membranes fuse (endocytosis)
Decondensing sperm nucleus (1N)
Completion of oocyte Meiosis II
(extrusion of PBII)
Resumption/completion of oocyte meiosis happens when….
Sperm endocytosis into the oocyte with increased cytoplasmic calcium
(Egg has already at metaphase and has extruded polar body and is ready to accept more DNA)
Sperm nucleus then decondenses and completion of oocyte meiosis II is complete and 2nd polar body is extruded- HAPLOID male and female pronuclei
First zygotic cell cycle
After Meiosis II is complete there is formation of haploid male pronucleus (1N) and female pronucleus (1N)
Sperm head decondensation
When the sperm head first enters the ooplasm, it immediately begins to swell
The dense chromatin “unpacks”, involving reduction of protamine disulfide bonds
Protamines are replaced by histones
Haploid genome becomes organized into chromosomes
The enlarging sperm head is called the…
Male pronucleus
As it enlarges, it becomes surrounded by a pronuclear envelope/membrane and microtubules
It moves towards the center of the ovum.
It is also busy duplicating its chromosomes so that by the time it reaches the center, it is ready for the metaphase of mitosis.
As the male pronucleus enlarges it becomes surrounded by…
a pronuclear envelope/membrane and microtubules
It moves towards the center of the ovum.
It is also busy duplicating its chromosomes so that by the time it reaches the center, it is ready for the metaphase of mitosis.
In the oocyte, immediately upon extrusion of the second polar body, the remaining haploid genome forms…
a female pronucleus
It also forms a pronuclear envelope, duplicates its chromosomes, and moves towards the center of the fertilized ovum.
In some species the female pronucleus is typically _______ than the male pronucleus.
Smaller
Syngamy process
When the 2 pronuclei reach the center, the pronuclear envelopes disappear and the two genomes become one.
The chromosomes with duplicated chromatids line up on the metaphase plate for the first mitosis (first cleavage).
First zygotic cell cycle
Interphase:haploid male and female pronuclei replicate chromosomes
Prophase: chromosomes condense; nuclear membrane breaks down
Metaphase: syngamy occurs as
maternal and paternal chromosomes align together on metaphase plate
Cell stages of developing embryo
2, 4, 6, 12
At some point between 8 – 16 cells (depending on species), the cells produce a cell adhesion molecule on their surfaces, compaction process to create the MORULA
Morula
At some point between 8 – 16 cells (depending on species), the cells produce a cell adhesion molecule on their surfaces
Uvomorulin, an E-cadhedrin
Forms tight junctions between adjacent cells, so that individual cell boundaries become difficult to see (morula becomes compacted; process is called “COMPACTION”.)
Some cells are now located in the interior of the ball of cells
Early morula- some cell boundaries still visible
Late morula- boundaries between cells not visible
Becomes blastocyst once fluid accumulates in cavities to form blastocoel
Blastocyst description
multiple small fluid-filled cavities are formed. These coalesce into a single cavity – the blastocoel, marks the transition from morula to blastocyst
The cells on the inside form the Inner Cell Mass (ICM) which will eventually give rise to the embryo. The cells forming the periphery will eventually form the embryonic side of placental tissues (trophoblast).
Blastocyst is still surrounded by zona pellucida
Blastocyst growth
ICM and trophoblast cells continue to multiply
Largest increase in blastocyst volume comes from accumulation of blastocoel fluid
This causes outward pressure on zona pellucida which stretches and thins
This pressure-caused thinning, together with proteases in the endometrium, eventually causes a zona rupture, through which the blastocyst escapes (hatches)
