Embryology Flashcards
what is embryology
the study of development of an embryo from the stage of fertilisation until the foetal stage
stages of life before birth
preimplantation stage (week 1)
embryonic stage/ organogenesis (weeks 2-8)
foetal stage (weeks 9-38)
preimplantation stage DAYS 1-3
oocyte fertilised -> zygote and moves along fallopian tube
- triggers cleavage of zygote
- overall size remains the same to allow passage down isthmus
zona pellucida prevents premature implantation
- pronuclei
preimplantation stage DAY 4
morula formation (held together by tight junctions)
enters the uterus
until now all cells are the same
preimplantation stage DAY 5
blastocyst formation
- differentiation begins
- inner cell mas (ICM) will form embryo and the extraembryonic tissues
- trophoblasts/ trophectoderm will contribute to the placenta
- fluid enters via zona pellucida into spaces of ICM
- blastostyle forms
why does implantation occur
after week 1 the blastocyst begins to run out of nutrients
ICM cells proliferate + fluid builds up in the cavity - ‘hatching’
week 2: implantation
decidualisation in stromal cells (blastocyst contacts endometrium)
- cells contacted form maternal part of placenta
- molecules produces
- promotes trophoblast cells -> invasive
implanting trophoblast cells differentiate
- cytotrophoblast
- syncytiotrophoblast
ICM - epiblast + hypoblast (bilaminar disc)
towards end of wk 2 - implanting syncytiotrophoblast communicates with maternal placenta and establishes O2 connection, waste and nutrient diffusion via blood
end of wk 2 - syncytiotrophoblast produces human chronic gonadotrophin (hCG)
ectoderm
outer epithelium, nervous system, teeth, adrenal medulla
mesoderm
notochord, skeletal system, muscular system, excretory system, circulatory and lymphatic systems, reproductive system, dermis, adrenal cortex
endoderm
lining of intestinal tract/ respiratory system/ urethra/ urinary bladder/ reproductive system; liver; pancreas; thymus; thyroid/ parathyroid
week 3 - gastrulation
formation of gut
formation of trilaminar embryo from bilaminar epiblast
3 important structures
- primitive streak - starts gastrulation
- notochord
- neutral tube
primitive streak
thickened area of cells that starts at the future tail
bulbous area = primitive node
cells move toward the primitive streak, down and then out again -invagination
- fibroblast profunctor 8
extraembryonic membranes
derived from blastocyst
- amnion
- chorion
-yolk sac
- allantois
amnion
continuous with the epiblast of the bilaminar disc
lines amniotic cavity (filled with fluid to protect embryo)
present till birth
chorion
double layered membrane formed from trophoblast + extraembryonic membranes
lines chorionic cavity (seen in early pregnancy but disappears due to amnionic cavity expansion)
forms foetal component of placenta
yolk sac
continuous with hypoblast of bilaminar disc
important in nutrient transfer in weeks 2=3 but gone by week 20
important in blood cell + gut formation
allantois
outgrowth of yolk sac
umbilical arteries and connects to foetal bladder
non-human mammals, acts as reservoir for foetal urine
ectopic pregnancy
implantation occurs in the wrong place
- most commonly due to slow movement through uterine tube or premature hatching of blastocyst
- more common in IVF
- cannot develop to full term (cant form a functional placenta)
- may spontaneously miscarry
teratoma
type of tumour with tissue or organ components resembling derivatives of the germ layers
ectoderm is the most commonly represented layer
can be malignant or benign
- normally good outcome once removes
can be derived from germ cells, oocyte/sperm or embryonal cells
germ cell teratomas
usually found in testes/ ovary
germ cells that failed to differentiate into oocyte/ sperm
rare so not entirely understood
inherited defects associated with increased risk (CNS + genitourinary tact malformations/ major malformations of lower spine)
cryptorchidism = increased risk
embryonal teratomas
normally congenital (midline)
cartilage found is mesodermal in origin
endoderm origins can be found at histological level
