Fertilization & Early Pre-Implantation/Pre-attachment Development Flashcards
what term do we use other than pre-implantation in our common domestic spp?
pre-attachment
What is the structure of the salpinx?
infundibulum (w/ fingers over oviduct) to ampulla to isthmus
What is mating?
- union of female & male gametes (requires mating behaviour; accomplishes delivery of sperm-containing semen to a mature ova or egg)
- in most vertebrates, MATING OCCURS @ PEAK OF FEMALE FERTILITY (estrus (oestrus) in most mammals)
Facts about semen deposition?
- volumes are spp specific
- vaginal semen deposition (humans, rabbits, rodents, cows; in carnivores the cervix is open - Ca & Fe)
- uterine semen deposition (llamas, camelids, Eq, Sw, AI techniques)
Are copulatory plus &/or gels present?
- cervical cap in rodents
- human semen coagulates but then liquefies w/in ~30 mins
- camelid semen also coagulates
- canine penis acts as plug
How fast is sperm transported through the uterus?
- rodents & Sw: sperm reach oviducts w/in 30 mins of mating
- large and small ruminants: 8-10 hr are required after mating for sufficient sperm numbers in oviduct (strong uterine contractile activity during estrus)
- primates: sperm can reach oviducts w/in ~10 mins (uterus has waves of smooth muscle contraction in late follicular phase)
- sperm must undergo capacitation before fertilization
What is capacitation?
- process of physiological alterations of the sperm so they are competent to fertilize the oocyte (REQUIRES the female reproductive tract)
What do sperm undergo during capacitation?
- removal of the membrane cholesterol to improve oocyte binding
- increase in intracellular Ca2+ concentration
- increase in intracellular pH
- protein phosphorylation (esp tyrosine residues)
- hyperactivated motility (ex: asymmetrical beating)
What are the functional affects of capacitation?
- penetrate cumulus cell matrix (un-capacitated sperm adhere to the outer edge of the oocyte)
- adherence to zona pellucida of oocyte
- undergo zona-stimulated acrosome reaction
what is the function of the utero-tubal junction (UTJ)?
- after ejaculation, sperm become maintained in high numbers at the UTJ (beyond the cervix, UTJ is second major selective barrier w/ lots of folds)
- in domestic spp, mechanisms used in selection are largely unknown
- there is selection for live, motile, normal morphology, uncapacitated, & acrosome intact sperm
- removal of the genes that code for adhesion of the sperm head prevent fertilization from occurring
How does the isthmus act as a reservoir?
- oviduct epithelial cells (OECs) are ciliated & non-ciliated
- sperm bind ciliated OECs that have specific glycoproteins
- sperm can be bound to such OECs particularly in the isthmus for 2-4 days in domestic spp (SUCH SPERM SEEM TO BE OF HIGHEST QUALITY - a functional sperm reservoir which can be provided @ time of ovulation)
What are the effects of ovulation time?
- in most mammals, E2 concentrations in circulation & oviductal fluid are elevated during pre-ovulatory period (drop after ovulation) (PROMOTE SHORT TERM SPERM STORAGE)
- post-ovulation, progesterone levels rise in circulation & in oviductal fluid (CHEMOTACTIC SIGNAL FOR SPERM TO MOVE TO AMPULLA & UNDERGO CAPACITATION IN AMPULLA)
What is fertilization?
- sperm contact & penetration (through cumulus cell layer (corona radiata))
- zona pellucida (ZP) binding & acrosome reaction (increased by zona binding)
- zona penetration (sperm penetration of zona)
- SPERM FUSION w/ oocyte mb and ooplasm
- CORTICAL GRANULE RELEASE from oocyte (“zona reaction” & “vitelline block”)
- PRONUCLEAR FUSION & initiation of metabolism
How is the acrosome reaction initiated?
sperm plasma mb covering acrosome has 2 receptor regions
- ZONA BINDING REGION - ZBR that binds zona pellucida-3 glycoprotein on oocyte zona (ZP3) (attaches sperm to zona)
- ACROSOME REACTION PROMOTING REGION (ARPR) (binds ZP3 & starts acrosome reaction ex: fusion of membranes)
What is the acrosome reaction?
- before acrosome reaction, membranes of sperm head are intact
- during reaction, overlying plasma mb fuses w/ outer acrosomal mb (contents released (ex: ACROSIN) & DIGEST ZONA PROTEINS & INCREASE SPERM BINDING TO ZONA)
- inner acrosomal mb & equatorial portion of sperm head increase binding to oocyte mb
what happens post acrosome reaction?
- sperm penetrates the zona & into perivitelline space
- Oocyte produced cortical granules accumulate at periphery
- oocyte & sperm fuse @ equatorial region
- CORTICAL GRANULES RELEASE CONTENTS (mucopolysaccharides, proteases, plasminogen activator, acid phosphatases, & peroxidases; CHANGE ZONA TO PREVENT POLYSPERMY (ZONA BLOCK OR ZONA REACTION); CHANGE OOCYTE MB TO PREVENT SPERM ATTACHMENT (VITELLINE BLOCK))
- sperm nuclear contents decondense
what is pronuclear fusion?
- nucleus of the sperm now forms a male pronucleus (both female & male pronuclei are haploid)
- pronuclei fuse & a zygote is created that is diploid
- DNA replication takes place, chromosomes condense, first mitotic division occurs
- male mitochondria including mtDNA are degraded leaving only maternal mitochondria
What are cleavage divisions?
- mitotic cell divisions in early embryos. cells of the divisions are called blastomeres
What deviations from normal cell division do cleavage divisions during preimplantation development have?
- rapid multi-cellularity
- no growth ex: nuclear to cytoplasmic ratio increases & cell size decreases
- shape is maintained
- asynchronous in mammals
What is holoblastic?
division completely cuts through the embryo. this occurs in embryos w/ moderate (ex: amphibians) TO LITTLE/NO YOLK (EX: MAMMALS) & can be unequal or equal (rotational: cleavage will produce blastomeres at right angles to 1 another)
What is rotational cleavage of the mammalian embryo?
- cleavage divisions are asynchronous in mammals (divisions of 2-cell to 4-cell can also appear as 3-cell to 5-cell under the microscope)
What is pre-attachment development?
- in mammals, the period of embryonic development from fertilization to just prior to implantation or uterine attachment
What is the activation of the embryonic genome?
- maternal transcripts & proteins stored in the oocyte play a major role in initiating early development. following embryonic genome activation, cellular machinery becomes that of embryo & maternal transcripts & proteins are degraded
- embryonic genome becomes activated during early cleavage stages
What is the morphogenetic event I?
compaction
- unique event that begins after 3rd cleavage
- cells huddle closely together & form compact ball at 8-cell stage
- just prior to & during compaction, blastomeres become polarized
- outside cells form a tight permeability seal w/ tight junctions, desmosomes, & adherens junctions
- cells on the inside become connected by gap junctions
- 16-cell stage referred to as a morula
- most of the outer cells of the morulae are destined to become trophectoderm & then trophoblast (Ex: extraembryonic lineage)
- the inner cells will derive the embryo by first forming the inner cell mass (ICM)
- the 2 regions are distinct & synthesize different proteins
What is the morphogenetic event II?
Cavitation
- fluid begins to accumulate in the compacted morulae forming small vesicles
- the vesicles coalesce into a blastocoel & the embryo becomes a mammalian blastula known as a blastocyst
- the enzyme Na, K - ATPase appears to be a primary driver of the process (pumping 3 Na+ into developing cavity per 2 K+ into the cell creating an osmotic gradient for water to flow into the blastocoel)
What is the morphogenetic event III?
Blastocyst expansion
- the enzyme Na,K-ATPase again appears to be a primary driver of the process. ex: ouabain inhibits the enzyme & expansion
- requires a very efficient permeability seal that can be disrupted by drugs that breakdown the actin cytoskeleton or Ca2+ requiring adhesion molecules
- process is essential b/c it culminates in the 1st differentiation event: the differentiation of the ICM & the trophectoderm (1st epithelium) (ICM = embryo proper; trophectoderm = extra-embryonic lineages)
What happens during preparation for implantation or attachment: hatching?
- w/ blastocyst expansion, hatching follows
- requires release of proteases (Ex: strypsin) to break down the matrices comprising the zona pellucida
- @ this time, ICM differentiates into: hypoblast & epiblast
- w/ formation of hypoblast & epiblast, the former ICM is referred to as a bilaminar disk
What 3 forces is hatching governed by ?
- growth & fluid accumulation w/in the blastocyst
- production of enzymes by the trophectoderm cells to break down the matrices comprising the zona pellucida (ex: strypsin)
- contraction of the blastocyst
Describe the blastocyst once hatched?
- blastocyst is now free-floating embryo w/in lumen of the uterus (totally dependent on the uterine environment for survival
Why is the Eq zona pellucida & capsule the exception?
- in Eq, the embryo does not hatch from the zona pellucida b/c the ZP disintegrates around d7-8 post-ovulation
- @ ~ d6.5, the trophectoderm of the blastocyst begins to secrete a glycoprotein containing capsule beneath the ZP & the capsule remains until ~d22 of pregnancy
