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Development & Ageing > Early Fetal Development > Flashcards

Early Fetal Development Flashcards

1
Q

How do you measure time in embryo-fetal development?

A
  • Fertilisation age
  • Gestational age
  • Carnegie stage
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2
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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3
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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4
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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5
Q

What are the stages of pregnancy?

A

Embryogenic stage - 1st trimester
Embryonic stage - 1st trimester
Fetal stage - 2nd & 3rd trimester

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6
Q

What is the embryogenic stage?

A

establishing the early embryo from the fertilized oocyte

Determining two populations of cells:

  • pluripotent embryonic cells (contribute to fetus)
  • extraembryonic cells (contribute to the support structures eg placenta)
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7
Q

What is the embryonic stage?

A
  • Establishment of the germ layers and differentiation of tissue types
  • Establishment of the body plan
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8
Q

What is 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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9
Q

What does the morula form?

A
  • Inner cell mass

- Trophoblast

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10
Q

What does the inner call mass form?

A
  • Epiblast

- Hypoblast

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11
Q

What does the epiblast form?

A
  • Epiblast

- Amnion

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12
Q

What does the trophoblast form?

A
  • Cytotrophoblast

- Synctitrophoblast

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13
Q

What occurs in the first few days of life?

A
  • Ovulated oocyte 1 cell undergoes fertilisation forming a zygote
  • The zygote undergoes mitotic cleavage divisions finally forming an 8 cell embryo, these are known as cleavage stage embryos
  • 8 cell embryo undergoes more mitotic divisions forming the morula (16+ cells)
  • Morula forms blastocyst (200-300 cells)
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14
Q

Where do the first few days of life occur?

A

Occurs during migration through fallopian tube into the uterus

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15
Q

When does the maternal to zygotic transition occur?

A

4-8 cell stage

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16
Q

What is the embryo like until the maternal to zygotic transition?

A
  • Embryo is dependent on maternal mRNAs and proteins to get through the first divisions
  • These mRNA and proteins are synthesized and stored during oocyte development (i.e. pre-ovulation)
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17
Q

What could happen if the proteins fail to synthesise?

A

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

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18
Q

What happens duing the maternal to zygotic transition?

A
  • Transcription of embryonic genes (zygotic genome activation)
  • Increased protein synthesis
  • Organelle (mitochondria, Golgi) maturation
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19
Q

When does compaction occur?

A

8 cell stage or later

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20
Q

What occurs during compaction?

A

Formation of 2 cell populations

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21
Q

How does compaction occur?

A
  • Outer cells become pressed against zona
    • Change 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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22
Q

What are the layers of the blastocyst?

A
  • Zona pellucida
  • Trophoectoderm
  • Inner cell mass
  • Blastocoel
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23
Q

What is the zona pellucida?

A

Hard protein shell

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24
Q

What is the role of zona pellucida?

A

inhibiting polyspermy and protects early

embryo

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25
Q

What is the inner cell mass?

A

Pluripotent embryonic cells that will contribute to the final organism

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26
Q

What is the trophoectoderm?

A

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

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27
Q

What is the blastocoel?

A

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

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28
Q

What is hatching?

A

The eggs escape from the zona pellucida so it can implant

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29
Q

How does hatching occur?

A
  • Enzymatic digestion

- Cellular contractions

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30
Q

What occurs during peri-implantation?

A

Once the blastocyte has implanted itself into the endometrium:

  • *Trophoectoderm lineage** separates further:
  • 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
  • *Inner cell mass** separates further into:
  • epiblast: from which the fetal tissues will be derived.
  • hypoblast: which will form the yolk sac (extraembryonic structure)
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31
Q

When does bi-laminar disc formation occur?

A

Day 12+

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32
Q

What occurs during bi-laminar disc formation?

A

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

33
Q

What secretes hCG?

A

Syncitiotrophoblast would now secrete hCG - this would be detected in a pregnancy test

34
Q

What is gastrulation?

A

Process whereby the bilaminar embryonic disc undergoes reorganisation to form a trilaminar disc

35
Q

When does gastrulation occur?

A

Around the end of the second week

36
Q

What makes up the bilaminar embryonic disc?

A
  • Hypoblast

- Epiblast

37
Q

What happens around day 15?

A

Thickened structure forms around midline mear near the caudal end of the bilaminar embryonic disc, this is the primitive streak

38
Q

What does the primitive streak do?

A

Defines the major body axes

  • Cranial end towards the head
  • Caudal ends towards the tail
  • Left & right sides
39
Q

What is the primitive node?

A

Expansion of the primitive streak at the cranial end

40
Q

What is the primitive pit?

A

A depression along the midline of the epiblast that continues towards the caudal end of the streak

41
Q

What happens once the primitive groove is formed?

A

Cells migrate inwards towards the streak detaching from the epiblast and slip into the embryo (Invagination)

42
Q

What do the first cells that invaginate do?

A

Invade the hypoblast and displace cells

43
Q

What is the definitive endoderm?

A

The cells that completely replace hypoblast forming a new proximal cell layer

44
Q

What happens by day 16?

A

Most of the cells have been replaced and the remaining cells of the epiblast are referred to as the ectoderm

45
Q

What forms the mesoderm?

A

The invaginated epiblast cells that remain in the space between the endoderm & ectoderm

46
Q

What does the endoderm give rise to?

A
  • GI tract
  • Liver
  • Pancreas
  • Thyroid
    Lung
47
Q

What does the ectoderm give rise to?

A
  • CNS
  • Skin epithelia
  • Tooth enamel
  • Neural crest
48
Q

What does the mesoderm give rise to?

A
  • Blood (endothelial cells, RBCs & WBCs)
  • Muscle
  • Gonads, kidneys & adrenal cortex
  • Bones & cartilage
49
Q

What happens after gastrulation?

A

Notochord formation

50
Q

What is the notochord?

A

A rod-like tube structure formed of cartilage-like cells

51
Q

Where does notochord form?

A

Forms along the embryo midline, under the ectoderm

52
Q

What is notochords role?

A

Acts as a key organizing centre for neurulation and mesoderm development

53
Q

What is the neural plate?

A

Thickened ectoderm that sits on top of the embryo

54
Q

What is neuralation?

A

Formation of the neural tube & CNS

55
Q

How is the CNS formed?

A
  • Notochord signals direct the neural plate ectoderm to invaginate forming neural groove
  • Creates two ridges (neural folds) running along the cranio-caudal axis
  • Neural crest cells specified in neural folds
56
Q

How does the neural tube form?

A

Neural folds move together over neural groove

Ultimately neural folds fuse, forming a hollow tube

Neural tube overlaid with epidermis (ectoderm)

Migration of the neural crest cells from folds

57
Q

What is required for neural development to proceed?

A

Closure at head end precedes formation of brain structures

head end - day 23

tail end day - day 27

58
Q

What does the failure of neural tube closure lead to?

A
  • Anencephaly (absence of most of the skull and brain) arises from failure to close at the head end (1/10,000 births)
  • Spina bifida (open neural tube at birth, usually lower spine due to failure to close tail end– varying severity – (0.4-5/1000 births)
59
Q

What are neural cells?

A

Endoderm derived, plastic cells that migrate during development

60
Q

How can neural cells be classified?

A
  • Cranial NC
    cranial neurones, glia, lower jaw, middle ear bones (ossicles), facial cartilage
  • Cardiac NC
    aortic arch/pulmonary artery septum, large arteries wall musculoconnective tissue
  • Trunk NC
    dorsal root ganglia, sympathetic ganglia, adrenal medulla, aortic nerve clusters, melanocytes
  • Vagral & Sacral NC
    parasympathetic ganglia and enteric nervous system ganglia
61
Q

What do neural crest defects lead to?

A

Defects of neural crest migration/specification lead to diverse birth defects including pigmentation disorders, deafness, cardiac and facial defects and failure to innervate the gut.

62
Q

What occurs after neuralation?

A

Somatogenesis

63
Q

What is somatogenesis?

A

Segmentation of the body axis

64
Q

What happens during somatogenesis?

A

Blocks of paraxial mesoderm condense and bud off in somite pairs

One of each pair either side of the neural tube.

Somitogenesis commences at the head end and progresses down the long axis of the embryo

The rate of ‘budding’ or appearance of somite pairs is species-specific, as is the number of pairs.
Humans 1 pair/90 min, 44 pairs

65
Q

What do somites form?

A

2 types of embryonic tissue:

  • Scierotome - vertebrae & rib cartilage
  • Dermomyotome
    • Dermatome - dermis of skin, some fat & connective tissues of neck & trunk
    • Myotome - muscles of the embryo
66
Q

What occurs after somatogenesis?

A

Formation of the gut tube

67
Q

Where is the yolk sac derived from?

A

Hypoblast

68
Q

Where does the primitive gut arise from? and how?

A

Yolk sac
Ventral folding: where the head and tail ends curl together

Lateral folding: where the two sides of the embryo roll

69
Q

What makes up the foregut?

A

esophagus, stomach, upper duodenum, liver, gallbladder, pancreas

70
Q

What makes up the midgut?

A

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

71
Q

What makes up the hindgut?

A

last third of the transverse colon, descending colon, rectum and upper anal canal

72
Q

Where is the heart derived from?

A

Tube of mesoderm ~day 19

73
Q

What commences around day 22?

A

Beating & pumping

74
Q

When can you detect a fetal heartbeat?

A

~6 weeks gestational age

75
Q

Where do the lungs arise from?

A

lung bud, an endodermal structure adjacent to the foregut, in the 4th week of development

76
Q

When does the lung bud split?

A

End of the 4th week

77
Q

Where do the gonads form from?

A

mesoderm as bipotential (i.e. not committed to testis or ovary) structures known as gonadal/genital ridges

78
Q

How do XY embryos develop?

A

presence of SRY gene on Y chromosome directs gonadal cells to become Sertoli cells, triggering testis development, Leydig cell formation and testosterone production

79
Q

How do XX embryos develop?

A

absence of SRY leads to gonadal cells adopting a granulosa cell fate and ovary development, requires reinforcement by FOXL2

Development & Ageing (4 decks)

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