fertilisation of trilaminar embryo Flashcards
in fertilisation, what is released from sperms head and why?
acrosomal enzymes, to help and allow sperm to penetrate through zona pellicuda
what are cortical granules?
secretory vesicles poised at the cortex of an egg that, upon stimulation by sperm contact at fertilization, secrete their contents. These contents modify the extracellular environment and block additional sperm from reaching the egg. (caused to release by sperm penetration which then hardens zona pellicuda so no polybinding of sperm)
what does sperm bind to in zona pellicuda?
zona pellucida glycoprotein (ZP3) = one of the 4 types of gylcoproteins
where does fertilisation occur?
in ampulla, the widest part of fallopian tube
what is zona pellicuda?
specialised extracellular matrix that surrounds oocyte - means “transparent zone” - and it hardens a little bit into matrix to offer protection
what are blastomeres?
when the fertilised egg had undergone mitotic division to form small identical daughter cells (in cleavage stage)
when does cleavage stage start?
when zygote split into 2 cells, it continues to undergo mitotic division forming blastomeres
what does totipotent mean?
those cells that are able to develop into any of cells in body and any cells related to embryo like placenta etc (different from pluripotent which is where it makes any cell in the body)
what are blastomeres thought to be, up to the 8 cell stage?
totipotent
whats mosaicism?
when someone has 2 genetically different cell lines (different chromosomal complement) (you’re supposed to have all identical)
how does mitotic non-disjunction relate to trisomy 21 and what affect does mosaicism have on it?
mitotic non-disjunction is the mitotic problem whihc results in a daughter cell having an extra chromosme.
-Can occur in early division of the zygote.
→In a zygote already with Trisomy 21.
→Some cells will have a normal complement of cells and some with Trisomy.
→Therefore they will show less prominent features.
basically some cells in body have typical chromosome for trisomy 21 (extra chromosome) but some of them might be normal, depending on where mosaicism has happened so some individuals can have little to no characteristics of trisomy 21 even if genetic problem
when does embryo become a morula?
when 16+ blastomeres - morula is solid mass of blastomeres as a result of lots of cleavages
why do blastomeres in morula have such close contact?
because they’re still contained in zona pellucida so they maximise space and form cell junctions
when is embryo called a blastocyst?
when fluid moves into the cell making obvious blastocyst cavity
whats embryoblast?
inner cell mass that moves to embryonic pole in blastocyst
whats trophoblast?
outer layer (flattened epithelial wall) of blastocyst that provides nutrients and eventually makes foetal part of placenta
what must happen with embryo blastocyst to allow implantation?
zona pellucida must be away as its a hardened ring so prevents growth. embryo bursts through zona pellucida through series of contractions and releases (quite forceful). once out envelope it can now interact with endometrial lining to implant
what envades through endometrial lining?
trophoblast
where does implantation usually occur?
posterior uterine wall (sometimes anterior)
what is corpus luteum?
-endocrine gland
-they form in ovary just as egg is released
-if egg not fertilised after around 2 weeks they degenerate and leave scars on ovary
-if fertilised then feedback loop occurs to stay in place to keep providing progesterone until around 12 weeks when placenta takes over progesterone production
-they’re produced by females once a month and extremely important for pregnancy as produce progesterone which maintains endometrial lining
what day roughly does trophoblast divide and what does it divide into?
roughly day 7.5 is when bilaminar embry formed and trophoblast divides into 1. cytotrophoblast and 2. syncitiotrophoblast
what is syncitiotrophoblast?
single multinucleated cell - one that pushes through endometrial lining to invade uterine wall
what is cytotrophoblast?
single cell layer, made of individual cells
what cells produce hCG and why is that important?
syncitiotrophoblast and cytotrophoblast. around day 9 is when hCG levels can be detected which is when you do pregnancy test - important for corpus luteum to be told to keep producing progesterone
what do cells in embryoblast differentiate into? and when does it differntiate?
epiblast and hypoblast. they initially are dispersed amongst embryoblast but they segregate just before implantation
what is epiblast?
layer of high columnar cells that form majority of dorsal (posterior) surface
what is hypoblast?
layer of small cuboidal cells that form ventral (anterior) surface
how does amniotic sac form? (simple)
layers of embryoblast form a flat disc which and a small cavity appears in epiblast which enlarges to form amniotic sac
how is the defect in endometrial tissue repaired?
by plugging essentially a blood clot called fibrin coagulum to keep embryo implanted in endometrial lining
how is circulation set up in embryo?
gaps start forming in syncitiotrophoblast which start communicating with blood vessels in endometrial tissue
how is primitive yolk sac formed?
flattened cells originating from hypoblast form it
what does primitive yolk sac do?
provides nutrition before circulation set up
what happens on day 9?
-formation of primitive yolk sac
-gaps in syncitiotrophoblast
-fibrin coagulum repairs defect
when is establishment of uteroplacental circulation?
day 12
what are maternal sinusoids?
congested and dilated maternal capillaries
how is uteroplacental circulation formed?
trophoblast (syncitiotrophoblast) erodes sinusoids and as more eroded, maternal blood begins to flow through trophoblastic system
- maternal blood flows to lucanae and diffuses into embryoblast
how is chorionic cavity formed?
by formation of membrane called extra embryonic mesoderm layer (fine,loose, connective tissue) between cytotrophoblast and embryo , which eventually degenerates away leaving cavity
summarise what happens on day 12
-uteroplacental circulation established
-chorionic cavity formed
-endothelial layer almost entirely repaired defect
-entirely surrounded by endometrial stroma
summarise what happens in day 13
-further development of chorionic cavity
-presence of connecting stalk (later develops into umbilical cord)
-2nd wave of hypoblast cells migrate to form definitive (secondary) yolk sac
what is ectopic pregnancy?
implantation outside uterus, dangerous and potentially life threatening
what are some of the abnormal sites of implanation in ectopic pregancy?
-ovary
-uterine tube
-peritineum
what is placenta previa?
condition in which placenta lies very low in uterus and covers all or part of cervical opening that sits at top of vagina
where does primitive streak appear?
in midline at caudal end of epiblast -> at the cranial end = a primitive node and primitive pit
what is gastrulation?
when cells migrate turning 2 germ layers into 3 germ layers
what happens in gastrulation to form new layers?
cells from epiblast migrate towards primitive streak then through and go under - forming ectoderm, mesoderm and endoderm
what are the names of the 3 layers formed in gastrulation?
1.ectoderm
2.mesoderm
3.endoderm
what are the 2 areas that the 3 germ layers DON’T spread to in gastrulation?
- oropharyngeal membrane (eventually forms mouth)
-cloacal membrane (eventually breaks down to reproductive tracts)
what is the fate of ectoderm?
epidermis of skin (hair, nails, associated glands) and nervous system (brain, spinal cord, peripheral nerves)
what does mesoderm divide into?
3 regions:
1. paraxial mesoderm
2. intermediate mesoderm
3. lateral plate mesoderm which splits into = somatic layer and visceral layer
what is the fate of paraxial mesoderm?
around midline structures like axial skeleton, voluntary (skeletal) muscle, parts of dermis (back)
what is fate of intermediate mesoderm?
urogenital systems like kidney and ureter - gonads and associated structures
what is fate of both layers of lateral plate mesoderm?
somatic layer = most of dermis, lining of body wall, parts of limbs
visceral layer = cardiovascular system, mesothelial coverings of organs, smooth muscle
what is fate of endoderm?
lining of gut tube, lining of respiratory tract, lining of bladder & urethra
what is phases of embryogenesis?
- gametogenesis
- fertilisation
- cleavage
- gastrulation
- formation of body plan
- organogenesis
what are the secondary processes of of embryogenesis?
- axial formation/polarity (cranial, caudal)
- folding/rotating (3D)
