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Development and Ageing > Early fetal development > Flashcards

Early fetal development Flashcards

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1
Q

What is ‘Fertilisation age’?

A

Also known as conceptual age

Measured from the time of fertilisation

However it is difficult to know time of fertilisation exactly unless its IVF

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2
Q

How is time of fertilisation estimated?

A

+1 from last ovulation

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3
Q

What is gestational age?

A

Calculated from time of the beginning of the last menstrual period - LMP

Determined by fertilisation date (+14) if known, or early obstetric ultrasound and comparison to embryo size charts

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4
Q

What is Carnegie staging system?

A

23 stages of embryo development based on embryo FEATURES rather than time

Covers 0-60 days post fertilization

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5
Q

Why is the Carnegie system good

A

Allows comparison of developmental rates between specifie

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6
Q

How do we measure time in embryo-fetal development?

A

Embrogenic stage
Embryonic stage
Fetal stage

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7
Q

What is the embryogenic stage?

A

14-16 days post fertilisation

establish early embryo from the fertilized oocyte

determine two populations of cells as pluripotent

extraembryonic cells

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8
Q

What is the embryonic stage?

A

16-50 days

Establishment of germ layers and differentiation of tissue types

Establishement of the body plan

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9
Q

What is the fetal stage?

A

50-270 days post fertilization or 8-38 weeks

major organ systems now present

Migration of some organs to final location

Extensive growth andacquistion of fetal viability

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10
Q

What stages make up the first trimester?

A

Embryogenic and embryonic stage

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11
Q

What stages make up the second and third trimester?

A

Fetal stage

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12
Q

What changes occur in the first few days of life e.g. from oocyte to blastocyte

A

Ovulated oocyte 1 cells

Fertilisation

Zygote 1 cell

Cleavage stage embryos - 2-8 cells

Morula 16+ cells

Blastocyst 200-300

  • during this is migrates from fallopian tube to uterus
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13
Q

What is the maternal to zygotic transition?

A

The embryo is dependent on material mRNA and proteins to get through first divisions

These are synthesised and stored during oocyte development which is pre-ovulation

Failure to synthesise, store or interpret these mRNAs and proteins during oogenesis can impair embryonic development

So maternal to zygotic transition including:

  • transcription of embryonic genes known as ZYGOTIC GENOME ACTIVATION
  • Increased protein synthesis
  • organelle maturation
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14
Q

What does compaction start?

A

The formation of the first two cell types around the 8 cell stage or later

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15
Q

WHat does Compaction do?

A

Outer cells become pressed against zona
Change from spherical to wedge shapes

Outer cells connect to each other through tight gap junctions and desmosomes

Forms barrier to diffuson between inner and outer embryo

Outer cell becomes polarised.

Forms copacted morula and then blastocyst

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16
Q

How does a blastocyst make two cell types?

A

Inner cell mass : pluripotent embryonic cells

Trophoectoderm : extra embryonic cells that contribute to extraembryonic structures that support development

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17
Q

What does the Zona pellucida do?

A

Hard protein shell inhibiting polyspermy and protects early embryo

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18
Q

What is the blastocoel?

A

Fluid filled cavity formed osmotically by trophoblast pumping Na+ ions into cavity

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19
Q

Why is hatching necessary?

A

Escape of blastocyst from the zona pellucida to allow further growth and implantation

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20
Q

How does hatching occur?

A

Enzymatic digestion
Cellular contraction from blastocyte

Creates a weak spot and comes out the zone pellucida leaving it empty

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21
Q

How do trophoectoderm lineage separate further?

A

Trophoblast cells fuse to form syncitiotrophoblast and cytotrophoblast

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22
Q

What do syncitiotrophoblast?

A

These cells invade and destroy local maternal cells in the endometrium

this creates interface between embro and maternal blood supply

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23
Q

What do the cytotrophoblasts do?

A

Remain sources to make syncitiotrophoblasts

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24
Q

What does the inner cell mass divide into?

A

Epiblast : from which fetal tissues will be derived

Hypoblast : which will form the yolk sac

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25
What is the formation of the bi-laminar embryonic disk?
Some epiblast cells get seperated from the epiblast due to the amniotic cavity forming and then become the amnion membrane The epiblast left below the cavity gives rise to the fetal structures. and the layer below is renamed the hypoblast which becomes the yolk sac Hence why it is called a bi-laminar disk
26
What secretes HCG for pregnancy testing?
Syncitiotrophoblasts
27
What is gastrulation?
The process by which the bilaminar embryonic disk forms a trilaminar disk which is the basis for all organs and body tissues
28
What happens to the epiblast around day 15?
Along the midline near the caudal end a primitive streak is formed This defines the body axis e.g. cranial vs caudal + right vs left
29
How does invagination occur
The primitive streak deepens into a primitive node with a primitive pit which is a depression in the streak This depression goes across to the caudal end and the epiblast cells then detach and slip down this depression. These cells replace the hypoblast and becomes a new layer = endoderm
30
What are the remaining cells of the epiblast post invagination referred to?
The ectoderm forming the most exterior distal layer
31
Where are the mesoderm cells?
The cells left in between the new endoderm and the epiblast turned ectoderm
32
Which organs are made form endoderm?
``` GI Liver pancreas lung thyroid ```
33
Which organs are made frome ecotderm?
CNS neural cest SKin epithelium tooth enamel
34
Which organs are made from mesoderm
``` Blood muscle Gonads kidneys adrenal cortex bone cartilage ```
35
what event occurs on day 13 after grastulation?
Notochord formation
36
What is a notochord?
rod like tube structure of cartilage like cells forms along embryo midline, under ectoderm
37
What does the notochord act as?
key organizing centre for neurulation and mesoderm development, it does this by controlling the neural plate
38
Describe the formation of the neural tube and CNS?
the notochord signals direct the neural plate ectoderm to invaginate forming neural groove creates two ridges called neural folds running along the cranio -caudal axis neural crest cells specified in neural folds
39
Once the neural folds develop what happens?
The folds move together over neural groove folds fuse forming a hollow tube = neural tube the neural tube overlays with epidermis migration of the neural crest cells from folds
40
The neural tube starts as a hollow open tube, what must happen for CNS development to proceed?
Closure at head end around day 23 Closure at tail end at day 27 * closure at head end precedes formation of brain structures
41
What does the failure of neural tube closure cause?
Developmental defects: - anencephaly - absense of most skull and brain due to failure of closure at HEAD END of neural tube - Spina bifida - lower spine gap due to failure to close TAIL END of neural tube
42
What do neural crest cells make in the cranial area?
- cranial neurones - glia - lower jaw - middle ear bones - facial cartilage
43
What do neural crest cells make in the cardiac area?
- aortic arch - pulmonary artery septum - large arteries wall musculoconnective tissue
44
What do neural crest cells make in the trunk area
- dorsal root ganglia - sympathetic ganglia - adrenal medulla - aortic nerve cluster - melanocytes
45
What do neural crest cells make in the vagral and sacral area?
- parasympathetic ganglia | - enteric nervous system
46
What defect causes pigmentation disorders?
Neural crest defect
47
What defect causes deafness?
neural crest - ossicle bones
48
What defect can cause cardiac and facial defects?
neural crest defect
49
What defect can cause failure to innervate the gut?
Neural crest defect
50
What are the properties of neural crest cells?
endoderm derives plastic migrate extensively during development to give rise to other cell types
51
What is somitogenesis?
Formation of somites
52
What are somites?
arise from paired blocks of paraxial mesoderm lflanking the neural tube and notochord
53
How are the pairs of praxial mesoderm crates around the neural tube?
Parts of the paraxial mesoderm CONDENSE and BUD OFF creating somite pairs either side
54
How are 44 pairs of somites created?
Somitogenesis commences at the head end and progresses down the long axis of the embryo rate of 'budding' or appearance of somite pairs if species-specific, as is the number of pairs * check slide for images
55
What two tissues do somites initially form?
- Sclerotome = vertebrae and rib cartilage - dermomyotome = which subdivides to form 1. dermatome (skin, connective and fat tissues of neck and truck 2.myotome
56
What two foldings in the embryo leads to the primitive gut?
ventral folding - where head and tail ends curl together lateral folding - where the two sides of the embryo roll As these folds are folding it pinches off part of the yolk sac to form the primitive gut
57
What 3 parts does the primitive gut form?
Foregut Midgut Hindgut
58
What organs does the foregut give rise do?
``` esophagus stomach upper duodenum liver gall bladder pancreas ```
59
What organs does the midgut give rise do?
lower duodenum remainder of s.intestine acsending colon 2/3 of transverse colon
60
What organs does the hindgut give rise do?
1/3 transverse colon descending colon rectum upper anal canal
61
When is fetal heartbeat detectable?
6 weeks gestational age
62
How does the heart develop?
tube of mesoderm beating and pumping blood around day 22
63
How do the lungs develop?
Arise from lung bud and endodermal structure adjacent to foregut lung bud splits into two at the end and progressively branches through development
64
How do the gonads develop?
forms the mesoderm as bipotential structures known as gonadal/genital ridges
65
What does it mean by bipotential structures?
not committed to testis or ovaries
66
How does a XY embryo develop?
presence of SRY gene on Y chromosome directs gonadal cells to become Sertoli cells triggers testis development, Leydig cell formation and testosterone development
67
How does an XX embryo develop?
absence of SRY leads to gonadal cells adopting Granulosa cell fate and ovary development requires reinforcement by FOXL2

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