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early fetal development Flashcards

1
Q

what are the 3 ways of measuring embryo fetal development

A

fertilisation age
gestational age
carnegie stage

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2
Q

what is the fertilisation age

A

measured from time of fertilisation - ammued 1 day after last ovulation
difficult to know time unless ivf

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3
Q

what is the gestational age

A

calculated from time of beginning of last menstrual period
determined by fertilisation date + 14 days if known
or can use early obstetric ultrasound and compare to embryo size charts

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4
Q

what is the carnegie age

A

the 23 stages of embryo development based on features than time
allows comparison of development rates between species
covers window of 0-60 days

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5
Q

what are the stages we can divide pregnancy into

A

embryogenic stage

embryonic stage

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6
Q

what is the embryogenic stage

A

14-16 days post fertilisation
establishing early embryo from fertilised oocyte
determining 2 types of cells:
pluripotent embryonic and extraembryonic cells

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7
Q

what is the embryonic stage

A

16 - 50 days post fertilisation
establishment of germ layers and differentiation of tissue types
establishment of body plan

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8
Q

what is the foetal stage

A

50-270 days post fertilisation
major organs present
migration of some organs to final location
extensive growth and acquisition of fetal viability - survival outside womb

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9
Q

what trimester do embryogenic and embryonic stages take place

A

1st trimester

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10
Q

when does fetal stage take place

A

2nd and 3rd trimester

8+ weeks

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11
Q

what is the zygote

A

single cell after fertilisation

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12
Q

what are the cleavage stage embryos

A

2-8 cell embryos

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13
Q

what is the morula

A

16 cells+

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14
Q

what is a blastocyst

A

200-300 cells

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15
Q

how does embryo get through first divisions

A

until 4-8 cell stage genes of embryo arent transcribed

embryo is dependent on maternal mRNAs and proteins which are stored during oocyte development

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16
Q

what happens during maternal to zygotic transition

A

transcription of embryonic genes
increased protein synthesis
maturation of organelles e.g mitochondria and golgi

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17
Q

what is compaction

A

formation of first 2 cell types
around 8 cell stage + outer cells become pressed against zona
change from spherical to wedge shaped

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18
Q

what properties do cells pushing on zona gain

A

outercells connect to each other through tight junctions and desmosomes
forms barrier to diffusion between inner and outer embryo
outer cells become polarised

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19
Q

what two cells does blastocyst formation establish

A

inner cell mass - pluripotent embryonic cells

outer cells - trophectoderm - extraembryonic cells that contribute to extraembryonic that support development

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20
Q

what is the zona pellucide

A

hard protein shell inhibiting polyspermy and protects early embryo

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21
Q

what is the blasctocoel

A

fluid filled cavity formed osmotically by trophoblasts pumping na+ ions into cavity

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22
Q

hatching

A

escape of blastocyst from zona pellicuda done throguh enzymatic digestions and cellular contractions
can then implant into endometrium

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23
Q

when does peri implantation take place

A

7-9 days approx

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24
Q

what happens during peri implantation

A

trophectoderm lineage separates further:

  • trophoblast cells fuse to form syncitiotrophoblasts which invade local maternal cells in endometrium
  • creates interface between embryo and maternal blood supply
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25
Q

what are cytotrophoblasts

A

cells remain individual to provide source of syncitiotrophoblast cells

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26
Q

what does inner cell mass separate into

A

epiblast - from which fetal tissues will be derived

hypoblast - form the yolk sac(imp in haematopoeisis) - extraembryonic structure

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27
Q

when does bi laminar embryonic disc formation take place

A

day 12+

= final stage before gastrulation

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28
Q

what happens during bilaminar embryonic disc stage

A

syncitiotrophoblast continue to expand into endometrium

some cells from epiblast become separated by formation of amniotic cavity

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29
Q

what is the amnion

A

cells that continue to contribute to extra embryonic membranes

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30
Q

what is two layered /bilaminar disc

A

epiblast and hypoblast

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31
Q

what does syncitiotrophoblasts secrete

A

human chorionic gonadotrophin ,hcg used for pregnancy testing

32
Q

what is gastrulation

A

change from single layer of epiblast cells to complex structure containing multiple layers

33
Q

what is the first step in gastrulation

A

around 15 days thickened structure forms along midline in epiblast near caudal end of bilaminar disc = primitive streak

34
Q

what does the primitive streak define

A

major axis of embryo incl cranial and caudal ends and left and right side

35
Q

what takes place at cranial end of embryonic disc

A

primitive streak expands to create primitive node

which contains circular depression - primitive pit

36
Q

what is primitive groove

A

when depression continues along midline of epiblast towards caudal end of streak

37
Q

what is invagination

A

cells of epiblast migrate towards streak, detach from epiblast and slip beneath it into interior of embryo

38
Q

what do first cells to invaginate primitive groove do

A

invade the hypoblast and displace its cells

39
Q

what is the definitive endoderm

A

hypoblasts are eventually replaced by new proximal layers

40
Q

what is the ectoderm

A

remaining cells of epiblast

forms most exterior distal layer

41
Q

what is the mesoderm

A

layer of cells invaginated between endoderm and ectoderm

42
Q

once mesoderm and endoderm form

A

epiblast cells no longer migrate towards primitive streak

43
Q

what does ectoderm do throughout gastrulation

A

continues to form from cranial to caudal end of embryo

44
Q

what organs/tissues does endoderm give rise to

A

gi tract
liver, pancreas
lung
thyroid

45
Q

what organs/tissues does ectoderm give rise to

A

cns and neural crest
skin epithelia
tooth enamel

46
Q

what tissues does mesoderm give rise to

A

blood, endothelial cell,rbcs,wbcs
muscle
gonads, kidneys and adrenal cortex
bone,cartiliage

47
Q

what is the first major event after gastrulation

A

formation of notochord

around day 13

48
Q

what is the notochord

A

rod like tube structure formed of cartilage like cells

forms along embryo midline, under ectoderm

49
Q

what is role of notochord

A

acts as key organising centre for neuralation and mesoderm development

50
Q

what is the neural plate

A

thickened ectoderm

sits on top of embryo

51
Q

what is neuralation

A

forming neural tube and cns

52
Q

how does cns form

A

notochord signals to neural plate ectoderm to invaginate forming neural groove
creates 2 ridges running along craniocaudal axis

53
Q

what does formation of neural fold create

A

neural crest cells which migrate away and populate variety of diff tissues

54
Q

what happens when neural folds move together

A

neural folds fuse forming hollow tube
neural tube is overlaid with epidermis
migration of neural crest cells

55
Q

when does closure of head and tail end of neural tube form

A

head end - around 23 days
tail end - around 27 days
closure at head end precedes formation of brain structure

56
Q

what is anencephaly

A

failure of closure at head end = absence of most of skull and brain

57
Q

what is spina bifida

A

open neural tube at birth

usually lower soine defect

58
Q

what do neural crest cells do

A 
become:
cranial nc - cranial neurones,glia,lower jaw, middle ear bones,facial cartiliage
cardiac nc
trunk nc
vagral and sacral nc
59
Q

what can defects in nc migration cause

A

birth defects e.g pigmentation disorders, deafness, cardiac and facial defects and failure to innervate gut

60
Q

what is somitogenesis

A

formation of somites which arise from paired blocks of paraxial mesoderm flanking the neural tube and notochord

61
Q

what occurs during somitogenesis

A

blocks of paraxial mesoderm condense and bud off in somite pairs
one of each pair either side of neural tube

62
Q

where does somitogenesis commence

A

at head end and progresses down the long axis of embryo

rate of budding or appearance of somite pairs is species specific as is number of pairs

63
Q

what do somites initially form

A

2 types of embryonic videos:
sclerotome
dermomyotome

64
Q

what is the sclerotome

A

vertebrae and rib cartilage

65
Q

what is the dermomyotome

A

in turn subdivides to form dermatome and myotome

66
Q

what is the dermatome

A

gives rise to dermis of skin, some fat and connective tissues of neck and trunk

67
Q

what is the myotome

A

forms muscles of the embryo

68
Q

where does primitive gut arise

A

2 types of folding:
ventral folding - where head and tail ends curl together
lateral folding - where two sides of embryo roll
pinches off part of yolk sac

69
Q

what is primitive gut is split into

A

foregut
midgut
hindgut

70
Q

what does foregut develop into

A

eosophagus, stomach, upper duodenum, liver, gallbladder and pancreas

71
Q

what does midgut develop into

A

lower duodenum and remainder of small intestine, ascending colon and first two thirds of transverse colon

72
Q

what does hindgut develop into

A

last third of tranverse colon, descending colon, rectum and upper anal canal

73
Q

where does heart form from and when

A

begin as tube of mesoderm around day 19, beating and pumping blood commences around day 22
fetal heartbeat detectable from approx 6 weeks gestational age

74
Q

when does lung form and where from

A

arise from lung bud and endodermal structure adjacent to foregut in 4th week of development
lung bud splits into 2 at end of 4th week and progressively branches

75
Q

when does gonads form and where from

A

forms from mesoderm as bipotential ( not committed to testis/ovary) structures known as gonadalgenital ridges
xy embryos - presence of sry gene on y chromosome directs gonadal cells to become sertoli cells, triggering testis development, leydig cell formation and testosterone production
xx embryos - absence of sry leads to gonadal cells adopting granulosa cell fate and ovary development, requires reinforcement of foxl2

brs2 (65 decks)

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