Embryology

Question 1

what is embryology

Answer 1

the study of development of an embryo from the stage of fertilisation until the foetal stage

Question 2

stages of life before birth

Answer 2

preimplantation stage (week 1)
embryonic stage/ organogenesis (weeks 2-8)
foetal stage (weeks 9-38)

Question 3

preimplantation stage DAYS 1-3

Answer 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

Question 4

preimplantation stage DAY 4

Answer 4

morula formation (held together by tight junctions)
enters the uterus
until now all cells are the same

Question 5

preimplantation stage DAY 5

Answer 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

Question 6

why does implantation occur

Answer 6

after week 1 the blastocyst begins to run out of nutrients
ICM cells proliferate + fluid builds up in the cavity - ‘hatching’

Question 7

week 2: implantation

Answer 7

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)

Question 8

ectoderm

Answer 8

outer epithelium, nervous system, teeth, adrenal medulla

Question 9

mesoderm

Answer 9

notochord, skeletal system, muscular system, excretory system, circulatory and lymphatic systems, reproductive system, dermis, adrenal cortex

Question 10

endoderm

Answer 10

lining of intestinal tract/ respiratory system/ urethra/ urinary bladder/ reproductive system; liver; pancreas; thymus; thyroid/ parathyroid

Question 11

week 3 - gastrulation

Answer 11

formation of gut
formation of trilaminar embryo from bilaminar epiblast
3 important structures
- primitive streak - starts gastrulation
- notochord
- neutral tube

Question 12

primitive streak

Answer 12

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

Question 13

extraembryonic membranes

Answer 13

derived from blastocyst
- amnion
- chorion
-yolk sac
- allantois

Question 14

amnion

Answer 14

continuous with the epiblast of the bilaminar disc
lines amniotic cavity (filled with fluid to protect embryo)
present till birth

Question 15

chorion

Answer 15

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

Question 16

yolk sac

Answer 16

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

Question 17

allantois

Answer 17

outgrowth of yolk sac
umbilical arteries and connects to foetal bladder
non-human mammals, acts as reservoir for foetal urine

Question 18

ectopic pregnancy

Answer 18

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

Question 19

teratoma

Answer 19

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

Question 20

germ cell teratomas

Answer 20

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

Question 21

embryonal teratomas

Answer 21

normally congenital (midline)
cartilage found is mesodermal in origin
endoderm origins can be found at histological level