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Body logistics > Early embryology > Flashcards

Early embryology Flashcards

1
Q

how does one cell become a multicellular body

A
  • growth
  • morphogenesis development of form and structure
  • differentation specialisation for function
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2
Q

when is the pre-embryonic period

A

first two weeks of development

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3
Q

when is the embryonic period

A

weeks 3-8

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4
Q

when is the fetal period

A

weeks 9-38

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5
Q

what stages occur in pre-embryonic period

A

week 1
- cleavage
- compaction
- hatching
- implantation begins

week 2
- differentiation
- implantation complete

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6
Q

how does fertilisation occur (day 1)

A
  • oocyte released from ovary
  • travels along fallopian (uterine) tube
  • fertilised by sperm in the ampulla
  • fertilised oocyte is called zygote
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7
Q

when does cleavage occur

A
  • days 2-4
  • when zygote is transported to the uterus from ampulla end of fallopian tube to isthmus (junction with uterus) by specialised cilia in the oviduct
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8
Q

what happens in cleavage

A
  • zygote undergoes series of mitotic divisions
  • results in blastomeres which get smaller with each division
  • divisions occur without an increase in cell size due to the glycoprotein shell called the zona pellucida
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9
Q

what is a morula

A
  • blastomeres form a compact ball of cells held together by tight junctions
  • each cell is totipotent - can become any cell type
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10
Q

assisted reproductive techniques

A
  • oocytes are fertilised in vitro and alowed to divide to 4 or 8 cell stage
  • morula transferred to uterus
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11
Q

what is PGD (pre-implantation genetic diagnosis)

A

a cell can be safely removed from the morula and tested for serious heritable disorders prior to the transfer of the embryo into the mother

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12
Q

what happens in compaction (day 4)

A

spaces between cells of the morula merge to form one large central cavity called the blastocoele converting the morula into a hollow sphere of cells called the blastocyst

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13
Q

what is a blastocyst

A
  • result of compaction
  • cells are pluripotent
  • inner cell mass - embryoblast becomes embryo
  • outer cell mass - trophoblast becomes placenta and fetal membranes to support and sustain developing fetus
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14
Q

what happens in hatching (day 5)

A
  • blastocyst hatches from zona pellucida
  • no longer constrained so free to enlarge
  • can interact with uterine surface to implant
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15
Q

what happens in days 6-7

A

implantation begins
- conceptus has around 100 cells
- 8 will make the embryo and remainder begin development of fetal membranes

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16
Q

what occurs during week 2

A
  • formation of bilaminar disc (epiblast and hypoblast) from embryoblast
  • formation of syncytiotrophoblast and cytotrophoblast layers from the trophoblast
  • formation of amniotic cavity, yolk sacs, chorionic cavity
  • conceptus has implanted
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17
Q

how does the bilaminar embryonic disc form (day 8)

A
  • inner cell mass (embryoblast) shows signs of cell differentiation and forms two layers
  • epiblast and hypoblast
  • the bilaminar disc has dorsal (ectodermal) and ventral (endodermal) surfaces but no head to tail orientation
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18
Q

how does implantation occur (days 9-10)

A
  • trophoblast becomes cytotrophoblast and syncytiotrophoblast which is a special layer that adheres to and invades the endometrium
  • uterine epithelium is breached and conceptus implants within uterine stroma
  • establishes maternal blood flow within placenta to support embryo from maternal circulation
  • establishes basic structural unit of materno-fetal exchange called the chorionic villus
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19
Q

two layers of trophoblast

A
  • cytotrophoblast inner layer of trophoblast that continually differentiates into syncytiotrophoblast
  • syncytiotrophoblast placental barrier between maternal and fetal blood that allows exchanges in nutrients and gases
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20
Q

what happens in each day of week 2

A

day 8
- bilaminar disc - epiblast and hypoblast
- cytotrophoblast and syncytiotrophoblast

day 9
- rapid development of syncytiotrophoblast
- primitive yolk sac formed
- yolk sac membrane in contact with cytotrophoblast

day 9-10
- implantation

day 11
- primitive yolk sac membrane pushed away from cytotrophoblast layer by reticulum
- reticulum converted to extraembryonic mesoderm by cell migration

day 12
- maternal sinusoids invaded by syncytiotrophoblast
- uteroplacental circulation begins
- uterine stroma prepares for support of embryo

day 13
- formation of secondary yolk sac which pinches off from primitive yolk sac

day 14
- spaces within extraembryonic mesoderm merge to form chorionic cavity
- embryo and cavities suspended by connecting stalk (future umbilical cord)
- implantation bleeding can occur

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21
Q

how does the amniotic cavity form

A

formed from spaces within the epiblast

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22
Q

what is the blastocoele

A

first cavity, formed as a result of compaction

23
Q

how does the primitive yolk sac form

A

(aka exocoloemic cavity) formed by hypoblast lining blastocoele

24
Q

how does the secondary yolk sac form

A

(aka definitive yolk sac) formed within primitive yolk sac

25
Q

how does the chorionic cavity form

A

(aka extraembryonic coelom) formed from spaces within extraembryonic reticulum and mesoderm

26
Q

early pregnancy loss

A
  • approximately 50% of zygotes are lost in the first 2-3 weeks
  • 15% diagnosed pregnancies miscarry
  • 1% suffer from recurrent miscarriages
27
Q

what are two implantation defects

A
  • ectopic pregnancy
  • placenta praevia
28
Q

what is ectopic pregnancy

A
  • implantation at a site other than the uterine body
  • most commonly in the ampulla of the fallopian tube
  • can be peritoneal or ovarian
  • can very quickly become life-threatening emergency due to rupture
29
Q

what is placenta praevia

A
  • implantation in the lower uterine segment
  • can cause haemmorhage in pregnancy
  • requires C-section delivery
30
Q

where does implantation usually occur

A

upper posterior uterine wall

31
Q

what happens during the embryonic period

A
  • weeks 3-8
  • period of greatest change
  • all major structures and systems are formed
  • greatest risk of major congenital malformations due to environmental exposure or drug therapy
32
Q

what stages occur in weeks 3 + 4

A
  • gastrulation
  • neurulation
  • segmentation
  • folding
33
Q

when does gastrulation occur

A

3rd week, marking the start of the embryonic period

34
Q

what is a germ layer

A

a group of cells in an embryo that interact with each other as the embryo develops and contribute to the formation of all organs and tissues

35
Q

what is gastrulation

A

when the bilaminar disc is converted to a trilaminar disc with three germ layers: ectoderm, mesoderm and endoderm

36
Q

why does gastrulation occur

A

to ensure the correct placement of precursor tissues to allow subsequent morphogenesis to take place

37
Q

how does gastrulation occur

A
  • primitive streak appears on dorsal surface of epiblast
  • causes migration and invagination of epiblast cells
  • hypoblast displaced and epiblast cells differentiate to create 3 layers
  • ectoderm, mesoderm and endoderm (trilaminar disc)
38
Q

what does the primitive streak and node do

A

make sure cells of right type get to right place

39
Q

derivatives of each germ layer of the trilaminar disc

A

ectoderm
- organs and structures that maintain contact with outside world
- nervous system, epidermis

mesoderm
- supporting tissues
- muscles, cartilage, bone, vascular system

endoderm
- internal structures
- epithelial lining of GI tract and respiratory tract, parenchyma of glands

40
Q

what is the notochord

A
  • solid rod of cells running in the midline with an important signalling role
  • it drives neurulation
  • mesoderm cells differentiate into notochord
41
Q

how does neurulation occur

A
  • notochord signals overlying ectoderm to thicken and differentiate into neuroectoderm
  • forms the neural plate (origin of nervous system)
  • edges of the neural plate elevate out of the plane and curl towards each other, creating the neural tube
42
Q

differentiation of mesoderm

A
  • notochord
  • paraxial mesoderm is either side of axis (notochord and neural tube) becomes organised into somites
  • intermediate mesoderm is lateral to the somites
  • lateral plate mesoderm is a sheet that splits into two layers: somatic mesoderm and splanchnic mesoderm with the intraembryonic coelem in between them
42
Q

differentiation of mesoderm

A
  • notochord
  • paraxial mesoderm is either side of axis (notochord and neural tube) becomes organised into somites
  • intermediate mesoderm is lateral to the somites
  • lateral plate mesoderm is a sheet that splits into two layers: somatic mesoderm and splanchnic mesoderm with the intraembryonic coelem in between them
43
Q

derivatives of the 6 regions of mesoderm

A
  • notochord forms the vertebral column
  • somites form vertebral column, ribs and extensor muscles of spine and muscle of body wall
  • intermediate mesoderm forms the kidneys
  • somatic mesoderm forms diaphragm and limb muscles
  • splanchnic mesoderm forms muscle of heart and viscera
  • intraembryonic coelem becomes the body cavities
44
Q

how does segmentation occur

A

organisation of the paraxial mesoderm into segments called somites in a regular, predictable sequence to give rise to repeating structures such as vertebrae, ribs, intercostal muscles and to guide innervation

45
Q

when do somites form (segmentation)

A
  • 1st pair appear at day 20 in occipital region
  • more appear at 3 pairs/day until 42-44 pairs are present by end of week 5
  • some disappear, leaving 31 somites in total
46
Q

what are somites

A
  • appear as regular block of mesoderm cells arranged around a small cavity
  • followed by further differentiation into subgroups of cells
  • underlie pattern of outgrowth of spinal nerve roots from spinal cord, formation of vertebral column and innervation of muscles and skin
47
Q

how do somites differentiate

A

each somite separates into a dermatome which gives rise to an area of skin, a myotome which gives rise to a block of skeletal muscle and a sclerotome which gives rise to a vertebra and rib

48
Q

how does differentiation of somites lead to innervation of skin and muscles

A
  • spinal nerve roots growing from neural tube enter and innervate dermatomes (skin) and myotomes (muscle)
  • sclerotomes rearranged to lie between nerve roots and become vertebrae
49
Q

why does folding occur

A

draws together the margins of the disk to present ectoderm to outside and endoderm inside
- creates a ventral body wall
- pulls amniotic membrane around the disk so embryo suspended within amniotic sac
- pulls connecting stalk ventrally

puts the heart and gut in right place
creates a new cavity within the embryo

50
Q

types of folding that occurs

A

cephalocaudal folding (head and tail)
lateral folding

51
Q

cephalocaudal (head+tail) folding

A

driven by growth of neural tube
- amniotic sac becomes wrapped around embryo
- little bit of yolk sac inside embryonic body and rest becomes umbilical cord

52
Q

lateral folding

A

driven by pressure from growth of somites
- lateral plate mesoderm opens up to form somatic and splanchnic layers and intraembryonic coelem
- yolk sac creates tube inside embryonic body

Body logistics (20 decks)

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