Physiology of Conception and Placentation Flashcards
Embryologic age defintion
- From fertilization on, gestation = 38wks
Menstrual age definition
- From LMP, gestation = 40wks
Preembryonic period
- Weeks 1-2
Embryonic period
- Weeks 3-8
Fetal period
- Weeks 9-birth
Oogenesis key features
- Mitosis in fetus only
- Meiotic arrests
- Polar bodies
- Large single cell
- Immotile cell
- 23X
Spermatogenesis key features
- Limited mitosis in fetus
- Continuous mitosis at puberty till death
- No meiotic arrests
- 4 mature sperm from 1 spermatocyte
- Small, motile cell
- 23X or 23Y
Oogenesis vs. Spermatogenesis Graph
Egg Maturation
1) embryonic oogonia are naked
2) After meiosis oogonia are surrounded = primordial follicles
3) At birth, primary oocytes have complete layer = primary follicle
4) Early and late secondary follicles (has >1 surrounding)
5) Maturing
6) Mature = Graafian follicles
7) Ovulation
8) Post-ovulation Corpus Luteum
9) Post-ovulation Corpus Albicans
Ovum lifespan
- 12-24hrs
Sperm lifespan
- 48-72hrs
When is fertilization most likely to occur?
- When intercourse occurs within the 2-3 days perior just prior to ovulation
Egg Transport
- Egg arrested in Meiosis II
- Lots of cytoplasm, corona radiata, zona pellucida
- Ciliated cells enlarge, more cilia, beat faster
- Secretory Peg cells more active
- Fimbriae move closer, beat
- Peristalsis propels
- 80% time spent in ampulla
- Chemoattractants
Spermatogenesis
- Several divisions of spermatogonia happening in closs association w/ sertoli cells
*Type A dark spermatogonia—>Type A pale spermatogonia (multiple divisions)—>Type B spermatogonia—>Primary spermatocytes (undergoes 1st meiotic division to become…)—>Secondary spermatocytes (undergoes 2nd meiotic division to become…)—>Spermatids—>Spermatozoa
- Once Spermatozoa is fully developed, it moves to the lumen of seminiferous tubules. Moving thru epididymis, it gains full motility and a glycoprotein coat
Sperm transport
- Of ~200-600 million/ejaculate, about 200-300 reach egg
- Loss by expulsion of semen, by vaginal enzymes, phagocytosis
- Seminal vesicles work to counteract
*vesiculase and fibrinogen from coagulates sperm, to prevent backflow
*prostaglandins in semen may stimulate uterine contractions
*fructose from provides energy for sperm
- May be stored in crypts, gradually released
- Midcycle estrogen peak; high mucus production w/ high water content and space b/w glycoproteins
- Capacitation may be initiated as sperm pass thru cervix
Sperm capacitation
- Changes surface characteristics of sperm, affects motility
- Hyper-activated motility in ampulla, result of capacitation, interaction w/ tubal epithelium
*result: greater speed, better direction, prevention of attachment and entrapment
- Results in decreased stability of plasma membrane and acrosomal membrane so will be able to fuse later in acrosomal reaction
Penetration of egg coverings
- Corona radiata = cellular layer w/ extracellular matrix heavy w/ hyaluronic acid
*chemoattractants to guide sperm
*tubal mucosal cells may aid in dispersion of corona radiata cells
- Zona pellucidae (ZP) = 3 glycoproteins (ZP1-3)
*ZP 3 acts as sperm receptor and stimulates acrosomal reaction
*ZP prevents implantation in fallopian tubes
- Swimming aids in penetration
- Binding to Zona Pellucida-3 stimulates acrosomal reaction
- Acrosomal reaction- aids in pentration of membranes
Binding and fusion of sperm and egg
- Sperm now in perivitelline space
- Molecules on sperm head bind to integrin molecules on egg membrane
- Sperm and egg membranes fuse, contents of sperm sink into egg and sperm plasma membrane incorporated into egg membrane
- Mitochondria from sperm enter, degraded by proteasome. Tail also degraded except for centriole, used for sperm aster later.
Prevention of Polyspermy (Zona Reaction)
Fast block- lasts ~5min
- Rapid depolarization of egg plasma membrane within 2-3sec.
- Prevents other sperm from binding and trigger metabolic activity in egg
Slow block- permanent
- Calcium wave sweeps across egg from site of sperm-egg fusion, released from internal stores in egg
- Cortical granules fuse w/ plasma membrane, exocytosis, release contents into perivitelline space
*polysaccharides attract water, ZP elevates
*ovoperoxidase hydrolyzes the sperm receptors in membranes, kills sperm
*structural proteins of ZP become cross-linked and sperm binding molecules inactivated
- Completion of Meiosis II occurs after
Fusion of pronuclei and loss of pronuclear membrane
- After completion of meiosis II
- Male and female pronuclei come together, chromosomes intermingle, now called zygote
- Egg activation occurs after
Blastocyst composition
- Outside layer of trophoblast cells
*give rise to extraembryonic structures like placenta
- Inner cell mass
*give rise to embryo itself
Klinefelter Syndrome
- Extra X chromosomes; the more X-chromosomes the more likely to have cognitive impairment
*e.g. XXXY, XXYY, XXXYY
Turner’s Syndrome
- 45, X karyotype, only monosomy compatible w/ life. 2% of fetuses survive
*e.g. XO
Super-female
- Triple X (XXX) Syndrome
- Often undiagnosed
- Some speech impediment
Super-male
- XYY
- Hypogonadal
- Infertile
- Cognitive impairment
Early Implantation
- Blastocyst reaches uterus b/w 3-4 days, remains in uterine avity 1-3 days before implantation
- Days 6-9, embryo implants into midportion of posterior wall of uterus
- Inner cell mass proliferates into the two-germ-layer stage of embryo epiblast and hypoblast
Late implantation
- Embryo will sink into stroma of endometrium, original site will heal over
- Requires complex preparation by hormones stimulating uterus
- Epiblasts cells become primitive amnion
- Hypoblast cells becoming yolk sac
Source of nutrition graph
Amnion germ cell layers
- Epiblast/ectodermal cells
*later lined by mesoderm
Yok sac germ cell layers
- Hypoblast/endodermal cells
Allantois germ cell layer
- Hypoblast/endodermal cells
Chorion germ cell layer
- Cytotrophoblast/mesodermal
Decidua basalis
- B/w chorionic vesicle and uterine wall, w/ growth it is incorporated into maternal component of placenta
Decidua capsularis
- Overlies the embryo
Decidua parietalis
- Endometrium on sides of uterus not occupied by embryo
Amniocentesis
- Tests for genetic defects, neural tube defects by measuring alpha feto protein directly
- Done at 15-16wks, can be done earlier but have higher fetal losses
Chorionic villus sampling
- Can detect chromosomal abnormalities, X-linked disorders, and metabolic problems
- Slightly more risky than amniocentesis
- Advantage is it can be performed weeks earlier
Formation of early placenta
- Chorionic villi become more highly branched and the layers become thinner w/ growth of placenta
*primary (small projections), wk 2
*secondary (CTB+mesenchyme)
*tertiary (+fetal blood vessels), wk 3
Formation of late/mature placenta
- Chorionic villi become more highly branched and the layers become thinner w/ growth of placenta
*anchoring (direct contact w/ maternal tissues wk5)
*mature (by term reaches >1billion microvilli/cm2 = high surface area)
