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TOB > Early Embryology > Flashcards

Early Embryology Flashcards

0
Q

What are the number of gestational weeks?

A
  • 38
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1
Q

What are the stages of fertilisation?

A
  • Oocyte is released from the ovary
  • Travels down Fallopian (uterine) tube
  • Fertilised by sperm in the ampulla
  • Fertilised ooctyce is called the zygote
  • Ideal site for implantation is the posterior uterine wall
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2
Q

What are the number of pregnancy weeks?

A
  • 40

- Different to gestational as is calculated from the last menstrual period

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3
Q

What weeks is the pre-embryonic period?

A
  • 1-2
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4
Q

What weeks is the embryonic period?

A
  • 3-8
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5
Q

What weeks is the fetal period?

A
  • 9-38
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6
Q

Define zygote

A
  • A fertilised ovum/ooctye
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7
Q

Define cleavage

A
  • The dividing of cells into 2 masses known as blastomeres (are identical to each other and to zygote)
  • First cleavage happens ~30 hours after fertilisation (resting period)
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8
Q

Define zona pellucida

A
  • The glycoprotein shell to prevent polyspermy
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9
Q

Define morula

A
  • The result of further cleavage divisions of the fertilised oocyte
  • Each cell is totipotent (has the capacity to become any cell)
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10
Q

Define ovary

A
  • The female reproductive organ producing oocytes/eggs for fertilisation
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11
Q

Define Fallopian tube

A
  • Tube that the ooctye travels along to reach the uterus

- The egg is fertilised at the ampulla of the Fallopian tube

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12
Q

Define uterus

A
  • Female reproductive organ where implantation of the zygote and the growth of the embryo occurs
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13
Q

Define blastocyst

A
  • Formed from the compaction of cells that make up the morula
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14
Q

Define trophoblast

A
  • Outer cell mass that will later firm support structures for the embryo ie placenta
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15
Q

Define embryo last

A
  • Inner cell mass that will later become the bilaminar disc
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16
Q

Define implantation

A
  • The attachment of the blastocyst to the wall of the uterus at day 6-7
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17
Q

Define cytotrophoblast

A
  • Is a derivative of the trophoblast

- Forms the placental membrane around the yolk sac

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18
Q

Define syncytiotrophoblast

A
  • Derivative of the trophoblast

- Cells that invade maternal sinusoids (irregular blood vessels) resulting in uteroplacental circulation

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19
Q

What stages occur during week 1?

A
  • Compaction (day 4)
  • Hatching (day 5)
  • Implantation begins (day 6-7)
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20
Q

What happens during compaction?

A
  • Blastomeres making up the morula compact to form the blastocyst
  • First differentiation has occurred (cells are now pluripotent-can become many cells
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21
Q

What does the blastocyst consist of?

A
  • Embroblast (inner cell mass)
  • Trophoblast (outer cell mass)
  • Blastocyst cavity
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22
Q

What happens during hatching?

A
  • Blastocyst hatches from the zona pellucida
  • Is no longer constrained and is now free to enlarge
  • Can now interact with the uterine surface to implant
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23
Q

What is the structure of the conceptus at the beginning of implantation?

A
  • Conceptus now has 107 cells
    ~ 8 will make the embryo
    ~ 99 will begin the development of the foetal membrane
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24
Q

What is the second week know as and what begins?

A
  • ‘Week of twos’
  • Both embryo and placenta start to form
  • Placenta has priority at earliest
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25
Q

What happens on week 2, day 8?

A
  • Embryoblast differentiates into the epiblast and hypoblast, which both make up then bilaminar disc
  • Trophoblast differentiates in the cytotrophoblast and the syncytiotrophoblast
  • The amniotic cavity forms above the bilaminar disc
  • The blastocyst cavity is still present below the bilaminar disc
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26
Q

What has happened by day 9-10?

A
  • Implantation is complete
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27
Q

Describe implantation

A
  • Is interstitial ie is invasive, surrounded and embedded
  • The uterine epithelium is breached and the conceptus implants within the uterine stroma
  • Establishes maternal blood flow within the placental ie support of the embryo changes from histotrophic (dependent on simple diffusion across cell membrane) to haemotrophic (has access to maternal blood flow)
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28
Q

What are implantation defects? Give examples

A
  • Implantation at inappropriate sites can cause problems
  • Ectopic pregnancy: implantation at site other than the uterine body (most commonly the Fallopian tube); can quickly become a life-threatening emergency
  • Placenta Praevia: implantation in lower uterine segment; placenta grows across cervical opening; requires c-section delivery
  • Pre-eclampsia
  • IUGR (inter-uterine growth restriction)
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29
Q

What happens on week 2, day 9?

A
  • At the embryonic pole:
    ~ rapid development of the syncytiotrophoblast
  • At the abembryonic pole:
    ~ primitive yolk sac formed by Heuser’s membrane (hypoblast cells and extracellular matrix) spreading across the blastocyst cavity
    ~ yolk sac membrane is in contact with the cytotrophoblast layer
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30
Q

What happens on week 2, day 11?

A
  • Primitive yolk sac is pushed away from the cytotrophoblast layer by an acellular extraembryonic reticulum
  • Reticulum is later converted to extraembryonic mesoderm by cell migration
31
Q

What happens on week 2, day 12?

A
  • Maternal sinusoids are invaded by syncytiotrophoblast
  • Lacunae becomes continuous with the sinusoids
  • Ureroplacental circulation begins
  • Uterine storm prepares for support of the embryo
32
Q

What happens on week 2, day 13?

A
  • Secondary yolk sac (definitive yolk sac) is formed

- Is pinched off the primary yolk sac (primitive yolk sac)

33
Q

What happens on week 2, day 14?

A
  • Spaces within the extraembryonic mesoderm merge to form the chorionic cavity
  • The connecting stalk suspends the embryo and it’s cavities in the chorionic cavity
  • The connecting stalk is a column of mesoderm and is the future umbilical cord
  • Bleeding around now can be confused with menstrual bleeding
34
Q

What has happened by the end of the second week?

A
  • Conceptus has implanted
  • The embryo and its two cavities (amniotic and yolk) are suspended by a connecting stalk in a supporting sac (chorionic cavity)
35
Q

Define bilaminar disc

A
  • The epiblast and the hypoblast
  • Derived from the embryoblast
  • Later will become the trilaminar disc with the formation of the primitive streak
36
Q

Define germ layer

A
  • Layer from which other developed tissues originated from
37
Q

Define ectoderm

A
  • The outermost layer of cells giving rise to the epidermis and nerve tissue
38
Q

Define endoderm

A
  • The innermost layer of cells giving rise to the gut lining
39
Q

Define doral surface

A
  • Outermost surface during early development
40
Q

Define ventral surface

A
  • Innermost layer during early development
41
Q

Define mesoderm

A
  • Middle germ layer giving rise to muscle, bone and more
42
Q

Define primitive streak

A
  • A narrow groove that develops of the dorsal surface of the epiblast
  • Has an important role in the orientation of the embryo, determining the front and the back
43
Q

Define gastrulation

A
  • The reorganisation of the germ layers into the ectoderm, mesoderm and the endoderm, and hence the establishing the origin of all tissues
44
Q

Define trilaminar disc

A
  • The three germ layers: ectoderm, mesoderm and endoderm
45
Q

What are the most important ectodermal derivatives?

A
  • Skin
  • Skin derivatives (hair, nails, lens and cornea of eye, mouth and anus lining)
  • Brain and spinal cord
  • Peripheral nerves
  • Retina and iris of eye
46
Q

What are the important mesodermal derivatives?

A
  • Smooth, skeletal and cardiac muscle
  • Connective tissue
  • Bone
  • Cartilage
  • Blood and blood vessels
  • Urinary system
47
Q

What are the important endodermal derivatives?

A
  • Lining of digestive tract
  • Glands associated with digestion (eg liver, pancreas)
  • Other gut derivatives (eg lungs)
48
Q

Describe the embryonic period

A
  • Weeks 3 to 8
  • Period of greatest change
  • All the major structures and systems are formed
  • Most perilous period for the developing child
49
Q

What is week 3 known as and why?

A
  • ‘Week of threes’
  • By the beginning of the week there are 3 cavities: amniotic cavity, yolk sac and chorionic cavity
  • Three germ layers are established during week 3: ectoderm, mesoderm and endoderm
50
Q

What is gastrulation?

A
  • Process of establishing 3 germ layers and hence the origin of all tissues
  • Trilaminar disc is formed
  • Axes observed in an adult are set:
    ~ anterior/posterior
    ~ dorsal/ventral
    ~ right/left
  • Starts with the appearance of the primitive streak
  • Cellular rearrangement occurs through migration and invagination
51
Q

What does the primitive streak consist of?

A
  • Primitive node

- Primitive pit

52
Q

Describe the appearance of the primitive streak and what happens to it?

A
  • The primitive streak appears on the dorsal surface of the epiblast in the third week
  • The streak is a narrow groove with bulging edges
  • Primitive node is located at the cranial end of the streak
  • Primitive pit is located at the centre of the node
  • As the three germ layers are established as gastrulation proceeds, the primitive streak regresses
  • Development proceeds in a cranial/rostral to caudal direction
53
Q

Describe the migration and invagination of the epiblast cells

A
  • Cells on the epiblast migrate towards the primitive streak
  • They then invaginate into the epiblast and displace the hypoblast creating a third layer the mesoderm
  • Forms a trilaminar disc: ectoderm; mesoderm; endoderm
  • Cells spread laterally and cephalad (towards the head) in the mesoderm
  • Mesoderm spreads out leaving 2 gaps
54
Q

What does the fate of the invaginating epiblast cells depend on?

A
  • Where in the streak they invaginate
55
Q

What is the notochord?

A
  • Notochord defines the phylum Chordata
  • Basis for the axial skeleton
  • Drives the formation of the nervous system (neurolation)
  • Prenotochordal cells of epiblast migrate through the cranial part of the primitive pit
  • Forms a solid rod of cells running in the midline with an important signalling role
  • Defines the midline
  • Axial skeleton forms around it
  • Regresses - vestigial remnant in the adult is in the nucleus pulposus of the invertebral discs
56
Q

How are axes set?

A
  • Primitive streak appearing at one end of the bilaminar disc defines the front and back (anterior/posterior)
  • Molecular signals from the primitive streak ensures correct left right development
57
Q

What has happened by the end of week 3?

A
  • Embryo has been gastrulated and the bilaminar disc is converted to a trilaminar disc (3 germ layers): ectoderm; mesoderm; endoderm
  • Axes have been set
58
Q

What is the notochord?

A
  • Solid rod of cells running in the midline

- Has an important signalling role

59
Q

How is the notochord involved with neurolation?

A
  • Notochord directs the conversion of overlying ectoderm to neuroectoderm
  • Notochord signals overlying ectoderm to thicken (forms a keyhole-shaped neural plate)
  • Edges elevate out of the plane of the disc and curl towards each other, creating the neural tube
60
Q

What are somites and when do they develop?

A
  • Segments of the paraxial mesoderm
  • 1st pair appears at day 20 in the occipital region
  • More appear in the craniocaudal sequence at 3 pairs per day until there are 42-44 pairs by the end of week 5
  • Some pairs disappear leaving 31 in total
61
Q

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

A
  • Repeating structures eg vertebrae; ribs; intercostal muscles
  • Guides innervation
62
Q

How is the mesoderm structured in the 4th week after the neural tube has been formed?

A
  • Immediately on both sides of the neural tube is the paraxial mesoderm (which develops into somites)
  • Intermediate mesoderm outside the paraxial mesoderm
  • Somatic mesoderm on the outside at the top
  • Splanchnic mesoderm on the outside at the bottom
  • Intraembryonic coelom is the space between the somatic and splanchnic mesoderm
63
Q

How is the intraembryonic mesoderm formed?

A
  • Formed when mesoderm differentiates

- During lateral folding the lateral branches of the mesoderm connect causing the intraembryonic mesoderm to join within

64
Q

What structures are derived from the paraxial mesoderm?

A
  • Axial skeleton (vertebral column and ribs)
  • Dermis
  • Muscles of the axial body wall
  • Some limb muscles
65
Q

What structures are derived from the somatic mesoderm?

A
  • Connective tissue of limbs

- Contributes to the axial body wall

66
Q

What structures are derived from the splanchnic mesoderm?

A
  • Smooth musculature

- Connective tissue and vasculature of gut

67
Q

What structures are derived from the intermediate mesoderm?

A
  • Urogenital system eg kidneys, ureters, gonads
68
Q

How do somites differentiate into dermatome, myotome and sclerotome?

A
  • Somites appear as regular blocks of mesoderm cells arranged around as small cavity
  • The ventral wall of the somite breaks down in ‘organised degeneration’, which leads to the formation of the sclerotome
  • Further organisation of the dorsal portion forms the combined dermomyotome
  • Myotome proliferates and migrates, and the dermatome disperses
69
Q

What are the fates of the dermatome, myotome and sclerotome and what will they make up together?

A
  • Dermatome: dermis
  • Myotome: muscles
  • Sclerotome: bones
  • Together are the fundamental precursors of the musculoskeletal system
70
Q

What do the terms dermatome and myotome mean developmentally and clinically?

A
  • Dermatome:
    ~ developmentally: part of the somite that gives rise to the dermis
    ~ clinically: a strip of skin supplied by a single spinal nerve
  • Myotome:
    ~ developmentally: gives rise to muscles
    ~ clinically: a muscle/group of muscles supplied by a singalong spinal nerve
71
Q

What do the nerves produced by the neural tube innovate?

A
  • Dermomyotome
72
Q

What happens to the heart during embryonic folding?

A
  • Heart lies at the cranial end

- Heart moves inwards until it lies in the future chest

73
Q

What does folding of the embryo achieve?

A
  • Achieves 3D structure
  • Creates ventral body wall
  • Pulls the amniotic membrane around the disc so that the embryo becomes suspended in the amniotic sac
  • Pulls the connective stalk centrally
  • Creation of a new cavity within the embryo (coelom)
  • Creates primordium of the gut from the yolk sac
  • Puts the heart and primordium of the diaphragm in the right place
74
Q

What has happened by the end of week 4?

A
  • Neurulation: formation of the primordium of the nervous system
  • Organisation of the mesoderm: derivatives of the somites; segmentation

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