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HSF Block 1 > Embryo - Early Development > Flashcards

Embryo - Early Development Flashcards

1
Q

*How to calculate the date for delivery from the last menstrual cycle?

A

Gestation period:

  • 280 days from 1st day of last menstruation
  • 266 days from time of fertilization

Caluclation of Expected Date of Delivery:

  • Nagele Rule
    • 1st day of LMP: 10/20/2017
    • Subtract 3 months: 7/20/2017
    • Add 1 year + 7 days: 7/27/2017
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2
Q

*What are the phases & results of fertilization?

A

Results of fertilization:

  1. Restoration of diploid number (46) of chromosomes
    • 23 from mom/23 from dad
  2. Determination of the chromosomal sex of the embryo
    • XX = female
    • XY = male
  3. Variation of the species
    • new combination of chromosomes
  4. Initiation of cleavage
    • mitotic division of the zygote into blastomeres
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3
Q

*What is the process of the cleavage of the zygote & formation of the blastocyst?

A

Sperm & secondary oocyte —> Pronuclei —> Morula —> Blastocyst

Process

  1. Day 1:
    • Mitotic divsion –> blastomere (2 cell stage)
      • polar body + zona pellucida
      • Until 8 cells = totipotent
  2. Day 3:
    • Morula is formed (12-16 cells) aka Blastula
    • Reaches uterus
  3. Day 5:
    • Morula become Blastocyst
  4. Day 5-6:
    • Zona pellucide disappears
    • implantation begins

Early Blastocyst

  • Inner cell mass (embryoblast)
    • involved w/ embryogenesis
  • Outer cell mass (trophoblast)
    • involved w/ implanation
  • Blastocyst cavity

NOTE: Only Blastocyst can be implanted!!!

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4
Q

*What is the process of implantation of the blastocyst?

A

Events in Week 1:

  1. Ovulation
  2. Fertilization
  3. Cleavage
  4. Morulation
  5. Blastocyst formation
  6. Implanation

Implanation:

  • Inner cell mass = Embryoblast
  • Outer cell mass = Trophoblast
    1. ​​Cytotrophoblast = mitotically active;
      • does not invate maternal tissue
    2. Syncytiotrophoblast = expanding mass;
      • invasive to maternal tissue & will form placental villi

Process:

  • Zona pellucida disappears
  • Blastocyst gets attached to uterine endometrium
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5
Q

What are the two layers formed by the trophoblast?

A
  1. Cytotrophoblast (Inner)
    • mitotically active
    • does not invade maternal tissue
  2. Syncytiotrophoblast (Outer)
    • expanding mass
    • invasive & forms placental villi
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6
Q

What are the two layers formed by the embryoblast?

A
  1. Hypoblast
    • Single layer of cells towards the blastocyst cavity
    • Cuboidal
    • Roof of primary yolk sac (exocoelomic cavity)
  2. Epiblast
    • Layer towards the trophoblast
    • Columnar
    • Floor of amniotic cavity

Formation of Amniotic cavity

  • Amniogenic cells from the epiblast line the trophoblast to enclose the amniotic cavity

Formation of Primary Yolk Sac

  • A layer of cells derived from the hypoblast line the trophoblast to enclose the primary yolk sac
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7
Q

What causes Implanation?

A

Basics:

  • Syncytiotrophoblast secretes:
    • Proteolytic enzymes - erode mom’s endometrium
    • Human corionic gonadotrophin (hCG) - maintains the corpus luteum
      • Base of pregnancy test
  • Penetration defect in the surface of epitheloum = closed by fibrin coagulum

Normal site of implantation:

  • posterior wall of body of uterus
  • close to fundus in the compact layer of endometrium
    • starts 5th-6th day & completed 10-12 day (after fertilization)
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8
Q

Notochord Formation

A

Basics:

  • Where?
    • At primative node
      • cephalic end of primitive streak
  • How?
    • Invaginating epiblast cells
      • prenotochordal cells
  • What?
    • cylindrical rode shaped structure by the primitive node
      • lies between ectodermal & endodermal layers of embryonic disc
    • extends from primitive node —> prechordal plate
      • forms basis for vertebral column
      • induces overlying ectoderm
      • forms neuroectoderm & neural plate
        • ie: brain/spine
    • Disappears leaving some remnants
      • nucleus pulposus of the IV discs
      • abnormal persistence = malignant tumor
        • chordoma

Notochordal process

  • grows foward from the primitive node btw the ectodermal & endodermal layers of the embryonic disc to reach the prechordal plate

Result: CNS

Notes:

  • Cephalic = relatin to head
  • Caudal = at or near tail/posterior
  • Cranial = relating to the skull
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9
Q

What do HCG Values mean?

A
  • Low HCG
    • spontaneous abortion
    • ectopic pregnancy
  • Elevated HCG
    • multiple pregnancies
    • hydatiform mole
      • benign overgrowth of villi
    • choriocarcinoma
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10
Q

What is the Bilaminar Disc?

A

Basics:

  1. Layer of ectoderm - formed via amniotic cavity
    • epiblast
  2. Layer of endoderm - formed via roof of primary yolk sac
    • hypoblast
    • prechordal plate = site of future mouth
  3. Layer of mesoderm - formed via extraembryonic mesoderm
    • space btw trophoblast externally & amnion / yolk sac internally

Bilaminar (Embryonic) Disc:

  • plate between amnion & primary yolk sac
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11
Q

What happens the Second Week of Development?

A

Basics:

  • Completion of implanation
  • Formation of bilaminar disc

“Week of 2s”

  • Trophoblast –> 2 layers
    • cytotrophoblast & syncytiotroophoblast
  • Embryoblast –> 2 layers
    • epi & hypo
  • Extraembryonic mesoderm –> 2 layers & 2 cavities
    • Amniotic & yolk sac
    • Splanchnopleuric & Somatopleuric EEM
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12
Q

How do we form the Chorionic Cavity & Chorion?

A

Formation of Chorionic Cavity

  • Cleft appears in extra-embryonic coelom
    • chorionic cavity formed by fusion of clefts
    • 3rd cavity
  • Primary yolk sac shrinks
    • due to enlargement of chorionic cavity
  • Extraembryonic coelon / Chorionic cavity splits EEM
    • Somatic (lines inner cytotrophoblast & outer amnion) & splanchnic (covers yolk sac) layers
    • formation of organs around yolk sac

Formation of Chorion:

  • Syncytiotrophoblast
  • Cytotrophoblast
  • Somatopleuric layer

Connecting Stalk:

  • Suspends the amnion, yolk sac, and embryonic disc in the chorionic cavity
  • primordium of the umbilical cord
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13
Q

How is the Secondary Yolk Sac formed?

A
  • Definitive or secondary yolk sac = formed as the extraembryonic coelom increases in size
  • Primary yolk sac = smaller (remnant)

Cavities so far:

  1. Amniotic Cavity
    • eventually surrounds embryo after folding
  2. Secondary Yolk Sac
    • formerly primary yolk sac
    • formerly exocoelomic cavity
  3. Chorionic Cavity
    • formerly EXTRAembryonic coelom
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14
Q

What is the purpose of an Amniocentesis?

A
  • Used for:
    1. cell culture
    2. karyotyping
    3. DNA analysis
    4. estimation of enzymes
    5. alfafetoprotein levels
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15
Q

What is Gastrulation?

A

Basics:

  • Epiblast & Hypoblast –> Ectoderm/Mesoderm/Endoderm

Note:

  • Arguably the most important part of embryogenesis!

Process:

  1. Formation of “primitive streak” on surface of epiblast
    • becomes a narrow groove
    • induces formation of secondary (intraembryonic) mesoderm
    • causes gastrulation
    • allows us to distinguish:
      • dorsal/ventral
      • head/tail
      • left/right
  2. Cells of epiblast migrate towards primative streak
    • primitive streak + primitive node appear in the ectodermal layer of bilaminar disc
    • primitive node –> formation of notocord
  3. Epiblast cells invaginate beneath the epiblast at primative node
    • spread out in between epiblast & hypoblast
  4. Some cells displace hypoblast, forming endoderm
    • others lie in between the epiblast, forming mesoderm
    • remianing in epiblast, form ectoderm
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16
Q

What is the Primitive streak?

A

Basics:

  • Primitive streak = induces formation of 2ndary (intraembryonic) mesoderm
    • causes gastrulation
  • Primative node = induces formation of the notocord

Both Streak & Node:

  • Appear in the ectodermal layer of the bilaminar disc
17
Q

Early Embryonic Flow Chart of Events

18
Q

What 2 Types of Folding does an Embryo Undergo?

A

Basics:

  1. Lateral Folding
    • Disc –> Cylinder
  2. Cephalocaudal Folding
    • Cylinder –> Cylinder w/ end foldings
      • Head/Tail
  • Folding + Flexion = result in amniotic cavity completely surrounding the developing embryo

Embryonic Folding:

End of 3rd Week:

  • Flat embryonic disc –> cylindrical
    • head fold
      • doral part of yolk sac = fore gut
    • tail fold
      • dorsal part of yolk sac = hind gut
    • 2 lateral folds
      • dorsal part of yolk sac = mid gut
  • Growth of neural tube
    • –> head/tail folds
  • Growth of somites
    • –> lateral folds
  • Margin of the 4 folds = primitive umbilical opening

Notes:

  • Mid gut = continuous w/ ventral part of the yolk sac
    • yolk stalk, vitelline duct or vitellointestinal duct
    • yok stalk disappears
19
Q

What is Gastroschisis?

A

Gastroschisis

  • Failure of lateral folds to fuse
    • loops of intestines herniate into amniotic cavity
20
Q

What is Ectopia Cordis?

A

Ectopia Cordis

  • Failure of lateral folds to fuse
    • Heart = partly or completely exposed
21
Q

What are the 2 types of Spina bifida?

A

Basics:

  • defect in fusion of vertebral arches
    • vertebral canal = open
  • tufts of hair may be present
  • no neurological deficiencies

Occulta

  • No budlge over the bony defect
  • Tuft of hair may be present

W/ Meningocele

  • Budlge over defect
  • Contains meninges & CSF in the subarachnoid space
22
Q

How does Notochord Formation occur?

A

Basics:

  • Where: Primative Node
    • cephalic end of the primative streak
  • How: Invagination of epiblast cells

Process:

  • grows forwards from the primitive node
  • between the ectodermal & endodermal layers of the embryonic disc
    • stops at prechordal plate

Notochord:

  • Shape:
    • Cylindrical rod shaped structure
      • induced by Primitive Node
  • Location:
    • btw ectodermal & endodermal layers of embryonic disc
    • extends from primitive node to prechordal plate
  • Forms basis for the vertebral column
    • Induces overlying ectoderm to form the:
      • Neuroectoderm & Neural Plate
  • Destiny:
    • disappears
    • remnant = nucleus pulposus of IVD
    • abnormal persistance = chordoma
      • malignant tumor
23
Q

How does the Formation of the Neural Fold Occur?

A

Neurulation (formation of neural tube)

  • Notocord + paraxial mesoderm = induce overlying ectoderm on dorsal aspect of embryonic disc
    • lateral margins of neural plate = elevate
      • form neural folds
  • Ectoderm becomes continuous again across the dorsal surface
  • Clusters of neural crest cells - migrate dorsolaterally

HSF Block 1 (19 decks)

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