embryology-01-fertilization

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2 reasons to study embryology: understand normal anatomy & dev. of birth defects
what is the leading cause of neonatal death: birth defects
% of newborns with birth defects: 3%
% of birth defects recognized in early childhood: another 3%
somatic cell karyotype is what normally?: 22 prs autosomes, 1 pr. sex chrom. (46 chromosomes)
failure of chromosomes to distribute normally during meiosis leads to ?: gametes with abnormal numbers of chromosomes because of non-disjunction
abnormal number of chromosomes called?: aneuploidy
% of non-disjunction w/in sperm cells?: 2 Â– 3%
% of non disjunction in oocytes in women approaching menopause?: 20%
what happens with most aneuploid embryos?: donÂ’t come to term Â– spontaneously abort
most common trisomy?: 21: down syndrome
effect of monosomy?: usually lethal. In sex chromosome usually results in early death & range of problems
% of sperm which may be abnormal?: 10-15% (above 20% = infertility)
% of abnormal forms above which may cause infertility?: 20%
define gametogenesis: production of a population of cells specialized for fertilization
2 events of gametogenesis?: 1: gametes acquire structural and functional characteristics nec. for successful union 2: meiosis occurs (only occurs in gametes) = half the number of chromosomes
# of chromosomes in somatic cells: 46N
number of chromosomes in gametes: 23N
non-disjunction: failure of chromosomes to distribute normally during meiosis = embryo with abnormal number of chromosomes (aneuploid).
spermatogenesis begins when?: puberty
spermatogenesis occurs where?: seminiferous tubules of the testis
phases in spermatogenesis, & their results?: two phases: phase 1 spermatogonia undergo meiosis 1 & 2 to create spermatids. phase 2: spermatids mature, undergoing morphogenesis/spermiogenesis creating spermatozoa
how many sperm are created every day?: between 50 and 150 million
central part of head of sperm and its purpose: nucleus, contains chromosomes
outer covering of head of sperm called? purpose?: cell membrane Â– contains binding sites for sperm receptors on zona pellucida and on oocyte membrane
portion of head of sperm covering nucleus called what, purpose: acrosome Â– contains hydrolytic emzymes to digest pathway to egg
how long does spermatogenesis take?: about 9 weeks
three parts of the head of the sperm(diagram p 1-3 in notes): cell membrane, nucleus, acrosome
three parts of the tail of the sperm: middle piece (nearest sperm head), principle piece, end piece
what structures create seminal fluid and in what percentages? (How many structures?): 2 seminal vesicles (60%),1 prostate gland (30%), 2 bulbourethral glands (10%)=5 structures total
When is seminal fluid added to spermatozoa?: at time of ejaculation, to constitute semen
three purposes of seminal fluid?: nutrients (simple sugars) for spermatozoa, neutralize vaginal acidity, promote sperm motility (need slightly alkaline environment to swim well.
what is considered a normal concentration of spermatozoa per ml of semen?: 40 Â– 200 million
spermatozoa live how long in female reproductive tract?: 5 days or more
How long do oocytes live for?: 24 hours
normal site of fertilization?: ampulla of uterine tube
what carries some spermatozoa to ampulla of uterine tube within minutes of ejaculation?: contractions of uterus and tubal musculature
where are spermatozoa that are not carried to ampulla?: colonize crypts of the cervix Â– majority of sperm
why so many sperm?: 50% chance of going to the wrong ampulla, as ovaries take turns ovulating
why donÂ’t fertilization and ovulation have to coincide?: sperm can live within the cervix and then swim to the ampulla by tail beat
what occurs to create fully functional spermatozoa?: capacitation (after 4 Â– 6hrs in fem. rep. tract) increases motility & metabolism, nec. for sperm-egg attachment & acrosome reaction
acrosome reaction: acrosome perforates, releasing enzymes, which allow sperm to digest a path through the zona pellucida
internal os: constricted opening of cervical canal: communicates with uterus. (external os communicates w/ vagina)
probability of conception near day of ovulation: within 5 days of ovulation, probability still exists, with highest probability of conception on day of ovulation.
24 Â– 36 hrs before ovulation Â– what occurs?: hormonal changes: spike in leutinizing hormone:- triggers follicular cells to disassemble gap junction- releases oocyte from meiotic inhibition, causing oocyte to undergo meiosis I- oocyte floats freely in antrum
when does oogenesis begin/end?: begins: during fetal life in fetal ovary, ends: menopause
what occurs in the ovary during the first phase of oogenesis?: Meiosis I begins, but does not complete until just before ovulation à this creates 2ndary oocyte from primary oo.
what occurs just before ovulation?: meiosis II begins, but does not complete unless egg is fertilized. beginning of Meiosis II creates an Â“ovumÂ” from a secondary oocyte.
what is the name given to the cellular units in the ovary that contain the developing egg?: follicles
an oocyte develops from what? 2 main characteristics: primordial follicle: primary oocyte arrested in prophase of Meiosis I, enclosed by a layer of cells.
# primary follicles (approx) do females have at birth?: abt. 2 million.
primordial follicle develops into?: primary follicle
Summarize 4 stages primordial Â– primary follicle: growth, follicular cells à granulosa, tissue around granulosa à thica (thickens: theca=thicker!), zona pellucida forms
connections between granulosa and oocyte: gap junctions Â– transfer of nutrients, etc
theca produces what/why?: androgens- substrate for granulosa to produce estrogen
zona pellucida is?: extracellular matrix between primary oocyte and innermost follicular layer
name of thick vascularized outer coating of granulosa: theca
follicular cells do what for primary oocyte: metabolic support, and suspension in meiosis. Are connected to prim.ooc. via gap junctions, so transfers nutrients, plus chemicals which keep oocyte suspended.
follicular cells secrete what: OMI: Oocyte maturation inhibitor- follicle:Meiosis I
after primary follicles, what forms?: secondary follicles
differences between primary and secondary follicles?: - increased size + appearance of antrum (fluid-filled spaces eventually enlarge - coalesce into large space)
follicular cells forming walls of antrum are called?: mural follicular cells (like a mural on a wall)
secondary follicle matures to become what/how?: graffian follicle Â– cumulus oophorus forms around oocyte, projects into antrum
mural follicular cells do what to form what?: stay in ovary after ovulation to form the granulosa lutein cells of the corpus luteum
structure projects into antrum? formed by?: cumulus oophorus, formed by oocyte & follicles around
what is a graffian follicle?: very large secondary follicle Â– these grow to about 2.5cm in size, and can be seen as a bulge on the side of the ovary.
graffian follicles grow large Â– what happens to ovary: white avascular spot seen there: stigma
avascularity why, causes what on ovary surface: Proteolytic enzymes secreted by follicular cell: break down tissue for oocyte escape. Ovary tissue becomes pale because avascular: called stigma.
length of time for a primary follicle to develop into a mature, pre-ovulatory secondary follicle?: about 3 mos.
OMI is what and what does it do?: OOcyte maturation inhibitor, secreted from follicular cells to keep follicle in Meiosis I. Removal of follicular cells removes OMI, and causes continuation of meiosis.
what is released when from the ovary during ovulation?: secondary oocyte, first polar body, zonal pellucida, corona radiata
what happens to the primary oocyte before ovulation?: a few hours before, it completes meiosis I.
characteristic of completion of primary oocyte meiosis I?: both daughter cells still within zona pellucida, but cytoplasm division unequal during cytokinesis, so one large secondary oocyte, one small polar body.
After oocyte completes meiosis I: oocyte begins meiosis II, but only completes if fertilized
what happens to the secondary oocyte etc. at ovulation: released through surface of ovary, into peritoneal cavity, to enter the ostium (mouth) of the uterine tube at Â“ovum pick-upÂ”.
cumulus oophorus becomes?: corona radiata
cells covering the secondary oocyte at ovum pick-up?: corona radiata Â– no longer connected firmly to oocyte by gap junctions Â– fall away easily
3 purposes of corona radiata?: Assist 2ndary oocyte into uterine tube: easier for the cilia of the fimbria of ampulla to Â“grab ontoÂ” the ova; larger target size for sperm; helps Â“trapÂ” sperm cells and direct them to surface of egg
What happens (at this point) if the egg is fertilized?: completes meiosis II, but division is again unequal, so a second polar body is created, which then degenerates.
What does the corpus luteum become after ovulation?: a secretory body that secretes hormones to prepare uterus for implantation
corpus luteum become if no implantation?: corpus albicans Â– a scar
how does the sperm bind with the egg?: species-specific interaction. zona binding sites on sperm cell membrane bind with sperm receptor on zona pellucida (ZP3)
sperm-zona binding trigger?: acrosome reaction Â– sperm cell membrane fuses with acrosome, disappears- enzymes released: zona pellucida breaks down: pathway Â“eatenÂ” through for sperm.
main 2 acrosomal enzymes facilitate zona penetration?: hyaluronidase and acrosin
which is species specific, zona Â–sperm interaction, or sperm-egg binding?: zona-sperm interaction, NOT sperm-oocyte interaction.
sperm crosses what to reach oocyte membrane: perivitelline space betw. zona pellucida & oocyte memb.
after the sperm has crossed perivitelline space - ?: sperm head crosses the space to bind with sperm receptors on the oocyte membrane. (sperm-egg binding)
sperm-oocyte binders?: integrin receptor on oocyte membrane +fertilin on sperm cell membrane = fusion of membranes
3 events triggered following binding?: meiosis completes, other polar body cast off, cortical granules released = zona reaction.
zona reaction is?: chemistry change to zona pellucida because of cortical granule release
zona reaction occurs why?: block to polyspermy
name for cortical granule release: exocytosis
What are formed in the oocyte following fertilization?: male and female pronuclei in the oocyte. (All other parts of the sperm body/tail are taken up into the egg cytoplasm and incorporated into the egg membrane, meaning that only the paternal chromosomes survive.)
mechanisms in place to prevent polyspermy?: zona reaction Â– caused by cortical granule exocytosis changes Â– modify receptors on zona Â– no more sperm can bind with
why prevent polyspermy?: maintain equal genetic contributions, restore diploid chromosome value.
what events occur in the first 12 hours of fertilization to form the zygote?: nucleus of sperm decondenses, forms male pronucleus; secondary oocyte completes Meiosis II, casts off 2nd polar body Â– remaining oocyte nucleus now female pronucleus; DNA replication occurs in both male and female pronuclei; pronuclear membranes break down, chromosomes co-mingle, joining on a mitotic spindle for the first cleavage division of the zygote --> this restores the chromosomes to 46 or 2N = zygote

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