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Early Fetal Development Flashcards

1
Q

What is fertilisation age?

A

measured from the time of fertilization (assumed to be +1 day from last ovulation)
difficult to know time of fertilization exactly (unless IVF)

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2
Q

What is another name for fertilisation age?

A

Conceptual

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3
Q

Why is fertilisation age not very practical?

A

Difficult to measure in practice

Variaiblity in time between intercourse and fertilisation in natural conception
Can be inferred if we know time of ovulation

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4
Q

What is gestational age?

A

calculated from the time of the beginning of the last menstrual period (LMP)

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

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5
Q

What is carnegie stage?

A

23 stages of embryo development based on embryo features not time
Allows comparison of developmental rates between species
Covers the window of 0-60 days fertilization age in humans

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6
Q

What are the division of pregnancy?

A

Embryogenic stage
Embryonic stage
Fetal stage

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7
Q

When is the embryogenic stage?

A

14-16 days post-fertilization

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8
Q

What is the purpose of the embryogenic phase?

A

establishing the early embryo from the fertilized oocyt
e
Determining two populations of cells:

pluripotent embryonic cells (contribute to fetus)

Extraembryonic cells (contribute to the support structures eg placenta)

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9
Q

When is the embryonic stage?

A

16-50 days post fertilisation

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10
Q

What happens during the embryonic stage?

A

Establishment of the germ layers and differentiation of tissue types
Establishment of the body plan

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11
Q

When is the fetal stage?

A

50-270 days post-fertilisation

8-38 weeks

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12
Q

What happens during the fetal stage?

A

Major organ systems now present
Migration of some organ systems to final location
Extensive growth and acquisition of fetal viability (survival outside the womb)

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13
Q

What is the first trimester comprised of?

A

Embryogenic stage

Embryonic stage

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14
Q

When does the change from embryo to fetus occur?

A

Roughly at the end of the first trimester

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15
Q

What is the developmental trajectory for the first few days of life?

A 
Ovulated oocyte
Zygote
Cleavage state embryos (2-8 cells)
Morula (16+cells)
Blastocyst (200-300 cells)

All occurs whilst travelling along the fallopian tube towards uterus

Zone pellucida present for all stage

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16
Q

What is the first developmental event?

A

Maternal-toZygotic transition

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17
Q

What is the Maternal-toZygotic transition?

A

Occurs at the 4-8 cell stage

Embryotic genes take over - begin transcription

Embryo increases protein production

Maturation of organelles (mitochondria and golgi)

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18
Q

How does the cell divide prior to Maternal-toZygotic transition?

A

No genes are transcribes

Divisions dependent on maternal mRNA and proteins

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19
Q

What is the second major event?

A

Compaction

Occurs at 8 cell stage

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20
Q

What happens in compation?

A

Cells start pressing up against the zona pellucida

Go from spherical to wedge-shaped

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

Forms barrier to diffusion between inner and outer embryo

Outer cells become polarised

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21
Q

What does compaction result in?

A

2 distinct cell types in the early embryo

  1. Inner
  2. Outer
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22
Q

What is the zona pellucida?

A

Hard protein shell inhibiting polyspermy and protects early embryo

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23
Q

What is the inner cell mass?

A

Pluripotent embryonic cells that will contribute to the final organism

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24
Q

What is the trophectoderm?

A

Extra-embryonic cells that contribute to the extraembryonic structures that support development

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25
What is the balstocoel?
Fluid-filled cavity formed osmotically by trophoblast pumping Na+ ions into cavity
26
What is hatching?
Escape of the blastocyst from the zona pelluida shell | Essential for implantation
27
How does hatching occur?
Enzymatic digestion | Cellular contractions
28
What are the peri-implantation events?
Further differentiation of the embryo 1. Trophectoderm lineage separates 2. Inner cell mass separates
29
What does the trophoblast separate to?
trophoblast cells fuse to form syncitiotrophoblast Syncitiotrophoblast invasion destroys local maternal cells in the endometrium Creates interface between embryo and maternal blood supply cytotrophoblast cells remain individual to provide source of syncitiotrophoblast cells
30
What does the inner cell mass separate to?
epiblast: from which the fetal tissues will be derived. hypoblast: which will form the yolk sac (extraembryonic structure)
31
What is the yolk sac important for?
Gut development | Early haematopoesis
32
What occurs at day 12+?
Bi-laminar embryonic disc formation
33
What is Bi-laminar embryonic disc formation?
Some cells become separated from the epiblast by the formation of a new cavity – the amniotic cavity. These amnion cells will contribute to the extra-embryonic membranes. This leaves a two-layer disc of epiblast and hypoblast, sandwiched between cavities. Embryo is now ready for gastrulation
34
Why is it called bi-laminar?
Looks like two discs stitting on top of eachother
35
What does syncitiotrophoblast secrete?
hCG
36
What forms the basis of pregnancy testing?
Detection of beta hCG subunit in blood/urine is basis of pregnancy testing
37
What is gastrulation?
The process whereby the bilaminar embryonic disc undergoes reorganisation to form a trilaminar disc
38
What happens during the third week of developement?
Differentiatied to establish three primary germ layers
39
What is the primitive streak?
Thickened structure forms along the mid-line at the caudal end Defines major body axis of the embryo
40
What occurs to the primitive streak at the cranial end of the embryonic disc?
Expands to form a primitive node | Containing a circular depression - primitive pit
41
What happens to the primitive pit?
Depression continues along the midline of the epiblast, towards the caudal end Forms the primitive groove
42
What is invagination?
Cells of the epiblast migrate inwards towards to streak Detach from the epiblast Slip beneath it into the interior of the embryo
43
What do the first invaginated cells do?
Invade the hypoblast and displace its cell Forms new proximal layer - the definitive endoderm
44
What happens by day 16?
Majority of hypoblast have been replaced Remaining cells of the epiblast are referred to as the ectoderm
45
What does the ectoderm form?
Most exterior, distal layer
46
What do the other invaginated cells do?
Remain in the space between the ectoderm and definitive endoderm - form a germ layer the mesoderm
47
What are the three germ layers?
Ectoderm (upper) Mesoderm Definitive endoderm
48
What organs are formed by the ectoderm?
CNS and neural tract Skin epithelia Tooth enamel
49
What organs are formed by the endoderm?
GI tract Liver, pancreas Lung Thyroid
50
What organs are formed by the mesoderm?
Blood (endothelial cells, red and white blood cells) Muscle (smooth, skeletal and cardiac) Gonads, kidneys and adrenal cortex Bone, cartilage
51
What is the first major event after gastrulation?
Notochord formation at day 13
52
What is the notochord?
rod-like tube structure formed of cartilage-like cells Forms along the embryo midline, under the ectoderm
53
What is the function of the notochord?
Acts as a key organizing centre for neurulation and mesoderm development Releases growth factor signals
54
What is neurulation?
Process of CNS formatuon
55
How does the notochord organise neurulation?
Through controlling the neural plate
56
What is the neural plate?
Area of ectoderm on top pf embryo Thickened Notochord directs neural plate to form neural tube
57
What does the neural plate do?
1. Part of neural plate to invaginate to form neural groove | 2. Creates two ridges (neural folds) running along the cranio-caudal axis
58
What happens after neural fold formation?
Neural folds move together over neural groove | Ultimately neural folds fuse, forming a hollow tube
59
What happens to the neural tube?
Neural tube overlaid with epidermis (ectoderm derived) Initially open at each end Must close for formation of brain structures
60
When does the neural tube close at the head end?
day 23
61
When does the neural tube close at the tail end?
27
62
What conditions are caused by failure of neural tube closure?
Anencephaly | Spina bifida
63
What is anencephaly?
absence of most of the skull and brain | arises from failure to close at the head end (1/10,000 births)
64
What is spina bifida?
open neural tube at birth, usually lower spine due to failure to close tail end– varying severity (0.4-5/1000 births)
65
What are the main features of the neural crest cells?
endoderm derived, plastic and migrate extensively during development give rise to a variety of cell types
66
How are the neural crest cells classified?
By where they end up in the embryo
67
What are the different neural crest cells?
Cranial Cardiac Trunk Vagral and Sacral
68
What do the cranial crest cells form?
cranial neurones, glia, lower jaw, middle ear bones (ossicles), facial cartilage
69
What do the cardiac crest cells form?
aortic arch/pulmonary artery septum, large arteries wall musculoconnective tissue
70
What do the trunk crest cells form?
dorsal root ganglia, sympathetic ganglia, adrenal medulla, aortic nerve clusters, melanocytes
71
What do the vagral and sacral crest cells form?
dorsal root ganglia, sympathetic ganglia, adrenal medulla, aortic nerve clusters, melanocytes
72
What do defects of neural crest migration/specification lead to?
diverse birth defects including: pigmentation disorders deafness cardiac and facial defects failure to innervate the gut
73
What is the stage after neuralation?
Somitogenesis
74
What is somitogenesis?
formation of somites
75
What are somites?
arise from paired blocks of paraxial mesoderm flanking the neural tube and notochord
76
What happens to paraxial mesoderm?
Blocks of paraxial mesoderm condense and bud off in somite pairs One of each pair either side of the neural tube. Pairs of condensed mesdoderm = somites
77
Where does somitogenesis occur?
at the head end and progresses down the long axis of the embryo
78
What is the rate of budding?
species-specific, as is the number of pairs Humans 1 pair/90 min, 44 pairs Week 4: 3-4 pairs Week 5: around 30 pairs
79
What do somite pairs look like on embryo imaging?
Chevron structures | Blocks lining up along the back of the embryo
80
What types of embryonic tissue to the somites give rise to?
Sclerotome | Dermomyotome
81
What will the sclerotome give rise to?
vertebrae and rib cartilage
82
What does the dermomyotome divide into?
Dermatome: gives rise to dermis of the skin, some fat and connective tissues of neck and trunk Myotome: forms the muscles of the embryo
83
From what does the primitive gut arise?
around Day 16 from Ventral folding: where the head and tail ends curl together Lateral folding: where the two sides of the embryo roll This pinches off part of the yolk sac (At top) to form the primitive gut
84
What subsequently happens to the primitive gut?
patterned into foregut, midgut and hindgut
85
What does the foregut give rise to?
esophagus, stomach, upper duodenum, liver, gallbladder, pancreas
86
What does the midgut give rise to?
lower duodenum and remainder of small intestine, ascending colon and first two-thirds of transverse colon
87
What does the hindgut give rise to?
last third of the transverse colon, descending colon, rectum and upper anal canal
88
What are the features of heart development?
Begins as tube of mesoderm around day 19, beating and pumping blood commences around day 22 Fetal heartbeat detectable from ~6 weeks gestational age
89
What are the features of lung development?
Arise from the lung bud, and endodermal structure adjacent to the foregut, in the 4th week of development Lung bud splits into two at the end of the 4th week, and progressively branches through development.
90
What do the gonads form from?
Forms from mesoderm as bipotential (i.e. not committed to testis or ovary) structures known as gonadal/genital ridges
91
What are the features of XY embryo development?
presence of SRY gene on Y chromosome directs gonadal cells to become Sertoli cells, triggering testis development, Leydig cell formation and testosterone production
92
What are the features of XX embryo development?
absence of SRY leads to gonadal cells adopting a granulosa cell fate and ovary development, requires reinforcement by FOXL2
93
What is RIF?
Recurrent implantation failure

Year 2 Development (9 decks)

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