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BodyLog * > S4 + S6 Embryology > Flashcards

S4 + S6 Embryology Flashcards

1
Q

How many weeks is the pre-embryonic period? And what happens during this period?

A

2 weeks long

Cleavage (formation of morula) and implantation begins

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2
Q

What is the process when the ovum is travelling from the ovary to the uterus?

A
  1. Oocyte released from ovary
  2. Oocyte travels along Fallopian tube
  3. Oocyte is fertilised by sperm in the ampulla (of the Fallopian tube)
  4. Zygote travels to the uterus
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3
Q

What is the name of a fertilised oocyte?

A

Zygote

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4
Q

How long is an oocyte viable for?

A

1 day

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5
Q

How long is a sperm viable for?

A

Up to 3 days

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6
Q

What are the 6 steps that happen in week 1?

A
  1. Cleavage
  2. Zona pellucida
  3. Morula
  4. Compaction
  5. Hatching
  6. Implantation begins
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7
Q

What happens during cleavage?

A

This is the first mitotic division which begins 30 hours after fertilisation. It results in 2 blastomeres of equal size

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8
Q

What is a blastomere?

A

Each cell of the embryo formed after the first division to produce two cells until the embryo becomes a blastocyst

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9
Q

What is the zona pellucida?

A

A glycoproteins shell that keeps sperm away from the blastocytes.

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10
Q

What is the morula?

A

The stage when all cells are totipotent (capacity to become any cell type). Made up of 16 cells (blastomeres)

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11
Q

What is the compaction stage?

A

The formation of the 1st cavity and the blastocyst.

After compaction the cells are pluripotent (capacity to become many cell types)

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12
Q

What is a blastocyst?

A

A hollow sphere of cells formed from the morula through compaction - fluid is pumped in to create the blastocoele which results in inner and outer cell masses.

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13
Q

What is the hatching stage?

A

The blastocyst hatches from the zona pellucida so it is now free to enlarge and interact with the uterine surface for implantation

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14
Q

What happens as implantation begins?

A

There is now about 100 cells - 8 of these cells make the embryo, the rest (about 92) will contribute to development of feral membranes

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15
Q

What two layers is the outer cell mass made out of?

A
  1. Cytotrophoblast (single cell sheet)

2. Syncytiotrophoblast (multi-cell sheet)

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16
Q

What two layers make up the initial inner cell mass (bilaminar disk)?

A
  1. Epiblast

2. Hypoblast

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17
Q

What happens during week 2?

A
  1. Differentiation
  2. Conceptus implanted
  3. Cavity formation
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18
Q

What are the two cavities in week 2?

A
  1. Amniotic sac

2. Yolk sac

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19
Q

What suspends the cavities?

A

The connecting stalk (in the chorionic cavity)

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20
Q

What happens on day 11?

A

The primitive yolk sac membrane is pushed away from the cytotrophoblast layer by an acellular extraembryonic reticulum

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21
Q

What is the acellular extraembryonic reticulum converted in to? And how does this happen?

A

Extraembryonic mesoderm

By cell migration

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22
Q

What happens on day 9?

A
  • at the embryonic pole, there’s rapid development of the synctiotrophoblast
  • at the abembryonic pole, the primitive yolk sac is formed (the yolk sac membrane is in contact with the cytotrophoblast layer)
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23
Q

What happens on day 12?

A
  • maternal sinusoids invaded by syncytiotrophoblast - lacunae become continuous with sinusoids - this is the start of uteroplacental circulation
  • the uterine stroma prepares for support of the embryo
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24
Q

What is the uterine stroma?

A

Layer of connective tissue

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25
Q

What does the syncytiotrophoblast contain?

A

Lacunae

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26
Q

What happens on day 13?

A

Formation of the secondary yolk sac (the definitive yolk sac) - it pinches off from the primitive yolk sac

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27
Q

What happens on day 14?

A

Spaces in the extraembryonic mesoderm merge to form the chorionic cavity

28
Q

What is the role of the outer cell mass?

A

Designed to produce the tissues supporting the embryo/foetus during development

29
Q

What is the role of the inner cell mass?

A

Cells inside the blastocyst go on to form the embryo proper

30
Q

What is another name for the inner cell mass?

A

Embryoblast

31
Q

What are the 5 stages in early development?

A
  1. Fertilisation and implantation
  2. Gastrulation
  3. Neurulation
  4. Segmentation
  5. Folding
32
Q

What 4 things does the mesoderm differentiate into?

A
  1. Notochord
  2. Somites
  3. Intermediate mesoderm
  4. Lateral plate
33
Q

What is teratogenesis?

A

Major congenital malformation

34
Q

When is teratogenesis the biggest risk?

A

Weeks 3 to 8 (embryonic period) as it’s the period of biggest change

35
Q

When does the primitive streak (and primitive node) form?

A

In the 3rd week

36
Q

What is the primitive streak?

A

A thickened region of the epiblast a the caudal end of the bilaminar disc. It is the site from which gastrulation proceeds

37
Q

What happens during migration and invagination?

A

Epiblast cells migrate to the primitive streak and push through the tight junctions and displace the hypoblast layer, creating 3 layers

38
Q

What is the trilaminar disc made up of?

A
  1. Ectoderm
  2. Mesoderm
  3. Endoderm

All from the epiblast

39
Q

What does the ectoderm form?

A
  • nervous system
  • epidermis

Organs and structures that maintain contact with the outside world

40
Q

What does the mesoderm form?

A
  • muscle
  • cartilage
  • bone
  • vascular system (incl. heart and vessels)

Supporting tissues

41
Q

What does the endoderm form?

A
  • epithelial lining of GI tract and respiratory tract
  • parenchyma of glands

Internal structures

42
Q

What is situs inversus? How does it occur?

A

A congenital condition when the major internal organs are reversed/mirror-image.

Occurs due to immotile cilia that develop during the embryonic period.

Usually no problems unless the person has both normal and mirror-image disposition.

43
Q

What is responsible for establishing left and right asymmetry?

A

The primitive node - actions of ciliated cells at the node result in left-ward flow of signaling molecules which leads to a cascade of side-specific signals.

44
Q

What is the notochord and what is its role?

A

It is a solid rod of cells running in the midline.

Has an important signalling role.

45
Q

When does gastrulation occur?

A

In the 3rd week (start of the embryonic period)

46
Q

What happens in gastrulation?

A
  • bilaminar disc —> trilaminar disc

* formation of the 3 germ layers - ectoderm, mesoderm and endoderm

47
Q

How does gastrulation occur?

A

Due to the formation of the primitive streak in the caudal epiblast which leads to the migration and invagination of epiblast cells

48
Q

Why does gastrulation occur?

A

To ensure the correct placement of precursor tissues (left/right) to allow subsequent morphogeneis to occur

49
Q

What does the notochord direct conversion of?

A

Conversion of the ectoderm to neuroectoderrm

50
Q

What do notochord signals cause?

A

Causes the ectoderm to thicken and the edges to elevate out of the plane of the disc and curl towards each other - this creates the neural tube

51
Q

What areas does the mesoderm differentiate into?

A
  1. Paraxial mesoderm
  2. Intermediate mesoderm
  3. Somatic mesoderm
  4. Splanchnic mesoderm
  5. Intraembryonic coelom
52
Q

What does the intraembryonic coelom (cavity) allow?

A

The cavity in which organs of the body can grow

53
Q

What does the splanchnic mesoderm differentiate into?

A

The viscera

54
Q

What does the somatic mesoderm differentiate into?

A

The body connective tissue

55
Q

What does the paraxial mesoderm form?

A

It forms somites

56
Q

How many somites form? When do they start forming?

A

31 (42-44 initially but some are lost)

Day 20

57
Q

What do somites undergo to give rise to sclerotome, myotome and dermatome?

A

Organised degeneration

58
Q

What does the dermatome form?

A

Dermis

59
Q

What does the myotome form?

A

Muscles

60
Q

What does the sclerotome form?

A

Bones

61
Q

What repeating structures does organisation of the mesoderm into somites give rise to?

A
  1. Vertebrae
  2. Ribs
  3. Intercostal muscles
  4. Spinal cord segments
62
Q

What do somites guide?

A

Innervation

63
Q

What two types of folding does the trilaminar disc undergo?

A

Cephalocaudal folding and lateral folding

64
Q

Why does folding occur?

A
  • It draws together the margins of the disc to pull the amniotic membrane around the disc
  • creates a ventral body wall
  • creates the primordium of the gut
  • puts primordium of organs in correct places
  • it creates a new cavity within the embryo
65
Q

What has happened by the end of the fourth week?

A
  • the nervous system has started to form
  • segments have appeared - assigning specific tasks to specific cells
  • embryo has folded - puts everything in the right place

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