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5BBA2300 Essentials of Embryology > Cleavage to Gastrulation > Flashcards

Cleavage to Gastrulation Flashcards

1
Q

what is cleavage in embryology?

A

series of rapid cell divisions (of zygote), without growth, that divides embryo into many small cells (blastomeres)
- outcome = formation of blastula

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2
Q

what are the two types of cleavage?

A
  1. holoblastic cleavage - complete (mammals, flatworms)
  2. meroblastic cleavage - incomplete (fish, reptiles)
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3
Q

describe the polarity in embryogenesis

A
  • embryo divided into 2 hemispheres within blastula :
  • ANIMAL POLE:
    consists of small cells that divide rapidly
  • VEGETAL POLE:
    contains large yolky cells that divide very slowly
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4
Q

where does holoblastic cleavage occur? and on what axis does the first and second holoblastic cleavage occur?

A
  • frogs and mouse
  • in cells with little/no yolk
  • first cleavage always occurs along vegetal-animal axis of egg
  • second cleavage is perpendicular to the first
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5
Q

what are the two types of holoblastic cleavage, describe them

A

radial cleavage: division planes are perpendicular/parallel to main axis of embryo - resulting in cells stacked on top of each other (pattern typically seen in sea urchins)
spiral cleavage: oblique/diagonal divisions, leading to cells arranged in spiral patterns (common in snails & some worms)

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6
Q

describe holoblastic cleavage in humans

A
  • rotational cleavage
  • cells divide along planes that aren’t fixed - instead shift slightly
  • not strictly parallel or perpendicular to any specific axis
  • rotational movement results in cells being positioned at angle to one another, giving rise to loosely arranged cellular structure
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7
Q

what are the two types of meroblastic cleavage?

A
  • discoidal
  • superficial
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8
Q

what is discoidal meroblastic cleavage?

A
  • cleavage that occurs in organisms with large, yolk-rich eggs (e.g. birds, reptiles, some fish)
  • cleavage partial and limited to small disc-shaped area above yolk (yolk remains undivided)
  • division occurs in cytoplasm, forming layered structure of cells on yolk surface
  • so embryo develops on top of yolk (forms blastodisc= disc of cells) at animal pole , utilising its nutrients for growth
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9
Q

what is superficial meroblastic cleavage?

A
  • occurs in insects with large eggs (e.g. drosophila)
  • division incomplete and limited to periphery of egg (central region, containing majority of yolk, is undivided)
  • nuclei in periphery divide rapidly, forming multinucleated blastoderm (=syncytium), w/o complete cell formation
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10
Q

what is the end product of cleavage?

A
  • blastula [term used for non-mammalian organisms] (blastoderm/blastocyst) form with fluid-filled cavity in centre called the blastocoel
  • in mammals, blastula organises into 2 layers = inner cell mass [embryo] + trophoblast [placenta] - the blastocyst (in mammals)
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11
Q

what is embryonic induction?

A
  • where one group of cells influences the fate and differentiation of nearby cells
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12
Q

Describe the “Spemann and Mangold organizer graft experiment”

A
  • removed dorsal lip region from one embryo and transplanted it onto a different embryo
  • transplanted dorsal lip had the ability to influence the nearby cells in the host embryo
  • transplanted dorsal lip had the ability to influence the nearby cells in the host embryo
  • result was the formation of a secondary embryonic axis and the notochord (which becomes the backbone) and the neural tube (which becomes the nervous system)
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13
Q

the “Spemann and Mangold organizer graft experiment” provides evidence for the existence of what?

A
  • an organizing center/”organizer” that emits signals controlling the development of surrounding tissues
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14
Q

what did the “Spemann and Mangold Organiser graft experiment” demonstrate?

A
  • that there are specific signals/instructions that cells in embryo receive from the dorsal lip, which guide their development and differentiation
  • these signals are responsible for determining body plan and fate of cells during embryonic development
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15
Q

what is required of cells in host tissues for embryonic induction to take place?

A
  • cells in host tissues need to have particular receptors/ transcription factors
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16
Q

what is cell fate?

A

the characteristic cell types that an early embryonic cell will normally give rise to

i.e. describes what a cell will become

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17
Q

what is specification?

A

there is plasticity to the cells development & it can be influences by environment

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18
Q

what is determination?

A

once it is a certain level of specificity and cannot be changed

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19
Q

what is a morula?

A

early-stage embryo consisting of small ball of blastomere cells
- 16-cell stage
- day 3
- precursor of zygote

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20
Q

what is a blastocyst?

A
  • form after morula phase
  • is a hollow ball of cells with a cluster of cells inside known as the inner cell mass
  • 32-cell stage
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21
Q

how do the terms blastocyst and blastula differ?

A

blastula refers to hollow sphere of cells
similar to blastocyst but blastocyst has an inner cell mass (ICM)

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22
Q

how do the terms blastocyst and blastula differ?

A

blastula refers to hollow sphere of cells
similar to blastocyst but blastocyst has an inner cell mass (ICM)

23
Q

what are the difference between the morula and blastocyst?

A

they are both two distinct stages in early development of embryo
differences:
- morula occurs around 3 days after fertilisation, blastocyst around 5/6 days after fertilisation
- morula cells are totipotent, blastocyst cells differentiated into embryoblast (gives rise to fetus) and trophoblast (gives rise to extra-embryonic tissues)
- morula is stage that preceded implantation, blastocyst is stage that implants in uterus
- blastocyst has cavity inside zona pellucida and an inner cell mass (ICM), morula is a solid ball of cells
- blastocyst larger than morula due to presence of fluid-filled blastocoel

24
Q

what is gastrulation?

A
  • invagination of blastocyst to form gastrula
  • results in formation of 3 germ cells: ectoderm, mesoderm and endoderm
25
Q

ectoderm gives rise to which structures?

A
  • epidermis (outer skin layer)
  • nervous system
  • sensory organs
  • other structures like hair, nails, teeth
26
Q

the middle germ layer, mesoderm gives rise to which structures?

A
  • muscle
  • connective tissue
  • cardiovascular system
  • kidneys
  • reproductive organs
  • and portions of respiratory system
27
Q

endoderm gives rise to which structures?

A
  • epithelial lining of digestive, respiratory tract, liver, pancreas and other internal organs
28
Q

what is invagination in gastrulation?

A
  • gastrulation begins with formation of primitive streak
  • cells migrate from surface of blastula towards primitive streak, they ingress into embryo and start rearranging themselves (invagination)
29
Q

difference between blastula and blastocyst

A
  • blastula is an early developmental stage of the embryo
  • blastomere is a cell that results from division of a fertilized egg
30
Q

at the end of cleavage stage the blastula organises into what?

A

inner cell mass (will become the embryo) and trophoblast (placenta) – the blastocyst
- some cell specification has occurred during cleavage - you get the ectoderm, mesoderm and endoderm
(gastrulation is getting germ layers into order)

31
Q

what are maternal determinants?

A

mRNAs and proteins synthesised by mother, required for early patterning

32
Q

give examples of maternal determinants

A

xenopus:
- Coco expressed in vegetal pole
- required for endoderm & mesoderm expression
drosophila:
- nanos expressed in posterior part of embryo
- determines posterior patterning of fly [protein diffuses + forms conc grad whilst embryo is in syncitium]

33
Q

what is the VegT gene?

A
  • maternal effect gene (i.e. expressed nd deposited in egg by mother during oogenesis)
  • expressed at vegetal pole
  • required for endoderm and mesoderm specification
34
Q

where is the mRNA of the VegT gene localised?

A
  • localised to vegetal pole of xenopus egg, which is future site of endoderm & mesoderm formation
35
Q

state the hierachy of gene expression in AP (Anterior-Posterior) patterning - Drosophila

A
  1. Bicoid (expressed in anterior part) is required to switch on gap genes
    - Gap genes expressed in broad domains of embryo
  2. These switch on pair rule genes and segmentation genes
  3. Homeotic selector genes are expressed – give positional identity
36
Q

what is patterning in embryology? (this flashcard is more for understanding than memorisation)

A
  • process where body plan and spatial organisation of embryo is established during its development
  • involves precise arrangement and differentiation of cells/tissues in specific spatial patterns to form structures and organs
  • essential for establishing: body axes, symmetry and regional identities
37
Q

what key aspects contribute to patterning in embryology?

A
  1. axis formation: establishing anterior-posterior (AP), dorsal-ventral (DV) and left-right(LR) axes
  2. cell signalling and gradients: signal molecules create concentration gradients and provide positional info to cells incl. growth factors and morphogens
  3. germ layer specification: sets foundation for subsequent regional and organ-specific patterning
  4. segmentation and regionalisation: patterning involves division of embryo into segments along AP and DV axes
  5. cell fate determination: leads to differentiation of diverse cell types and formation complex tissues and organs
  6. morphogenetic movements: incl cell migration, tissue folding, invagination and fusion which are coordinated to achieve proper spatial organisation
38
Q

in mammals cell fate depends on what?

A

-zygotic transcription not maternal factors
- cell fate depends on relative position (move a cell that would become trophoblast to inside - becomes trophoblast only half the time)
- fate not initially specified

39
Q

are maternal signals conserved?

A

Yes
- maternal determinants are conserved through evolution
-

40
Q

in vertebrates, cell fate is determined by what?

A
  • by reception of signals (not by cleavage)
    (many of signals conserved across species, whether maternally or zygotically derived)
41
Q

describe holoblastic cleavage in humans

A
  • cleavage starts when fertilised egg passes from ovary into oviduct (cleavage occurs in oviduct)
  • resulting in early blastocyst that embeds in uterus (at day 5)

similar to mice

42
Q

what is midblastula/midblastula transition?

A
  • before gastrulation
  • stage during the blastula stage of embryonic development in which zygotic gene transcription is activated
43
Q

what is “nodal”?

A
  • signalling protein, acts as morphogen
  • expressed in primitive streak
  • establishes left-right axis and formation of mesoderm germ layer during gastrulation
44
Q

nodal is expressed due to what?

A
  • nodal expressed due to Veg T and 1, where nodal formed you get mesoderm formed
45
Q

high level of nodal lead to what

A
  • you get organiser (Anterior-Posterior Patterning and Germ layer movement)
  • in gastrulation germ layer forming, due to signal expression
46
Q

in fish and mouse divisions are determined by what?

A
  • all divisions before blastula are determined by maternal factors, after that it is by zygotic transcription
47
Q

signals from endoderm induce what?

A

signals from endoderm induce mesoderm in animal cap (presumptive ectoderm of the blastula embryo)

48
Q

what is involution in gastrulation?

A
  • movement of a sheet of cells towards the inside of the embryo, by rolling in under itself
49
Q

describe the gastrulation movements in fish

A

(extra info: involves series of movements and rearrangements of cells that transform single-layered blastula into more complex, multi-layered embryo)

  1. INVOLUTION: blastoderm (group of cells on surface of blastula) moves inwards, folding into embryo forming blastopore (a depression)
  2. EPIBOLY: as involution occurs, remaining cells of blastoderm spread and cover embryo; expansion and thinning of blastoderm layer encloses internal structures of embryo
    3: CONVERGENCE & EXTENSION: movement of cells towards dorsal side of gastrula, cells narrow and elongate in converging pattern towards midline of embryo, which helps lengthen and reshape body axis
    4 germ layers formation: during gastrulation, movement mentioned leads to ectoderm forming outermost layer, endoderm forming innermost layer and mesoderm forming between the two
50
Q

describe the gastrulation movements in chickens

A

Ingression: Movement of INDIVIDUAL cells from the epiblast towards the inside of the embryo. The primitive streak becomes the PRIMITIVE GROOVE formed by the involuting and ingressing mesendodermal cells.
Humans gastrulate like chicks

  1. primitive streak (PS) formation: forms on blastoderm and establishes midline of embryo
  2. invagination and cell migration: cells at posterior end of PS invaginate forming primitive groove (PG) - creates opening called blastopore. some cells migrate through PG and move inward, others spread out and cover yolk surface
  3. epiboly: as cell migration continues, remaining blastoderm cells spread and cover yolk. process involves expansion and thinning of blastoderm layer to enclose yolk and internal structures
  4. formation of germ layers: movements described lead to formation of germ layers
  5. migration and differentiation: cels that have migrated to their respective germ layers continue to proliferate and differentiate into various cell types and tissues (contributing to development of organs and structures in embryo)
51
Q

humans gastrulate like which animals?

A

chickens

52
Q

how does mouse gastrulation differ to other animals?

A
  • forms cup-like blastula (rare in mammals)
  • mouse embryo initially develops inside-out: internally-located ectoderm surrounded by mesoderm and endoderm
  • but gastrulation movements are similar to the chick
53
Q

summarise gastrulation in xenopus & zebrafish and the chick and mouse

A

XENOPUS + ZEBRAFISH: formation of blastopore, involution or internalization of mesoderm + endoderm, convergent extension of mesoderm along antero-posterior axis, ectoderm extension by epiboly
CHICK AND MOUSE: cells ingress through primitive streak and becomes mesoderm + endoderm, remaining epiblast becomes ectoderm

54
Q

The blastopore is also known as what?

A

the organiser

5BBA2300 Essentials of Embryology (12 decks)

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