4. Embroolgy and development - Module 4

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Last updated 4:54 AM on 9/21/26

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12 Terms

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Extarembryonic mesoderm

grow on inside of cytotrophoblast cells
become structures that support embryo
- includes connecting stalk

<ul><li><p>grow on inside of cytotrophoblast cells</p></li><li><p>become structures that support embryo</p><ul><li><p>includes connecting stalk</p></li></ul></li></ul><p></p>

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Inner mass cell changes (day 12-13)

makes two chambers
ball of cells with 2 cavities

Top layer:

Epiblast cell layers
- becomes embryo

Bottom layer:

Forms extraembryonic tissue

Both layers combined

bilaminar embryonic disk

Top cavity: amniotic sac

amniotic cavity
- Fills with fluid to help surround embryo

Below Hypoblast layer = yolk sac

not much purpose in humans

<ul><li><p>makes <u>two chambers</u></p></li><li><p>ball of cells with 2 cavities </p></li></ul><p></p><p><u>Top layer: </u></p><ul><li><p>Epiblast cell layers </p><ul><li><p>becomes embryo</p></li></ul></li></ul><p></p><p><u>Bottom layer:</u></p><ul><li><p>Forms extraembryonic tissue</p></li></ul><p></p><p>Both layers combined</p><ul><li><p>bilaminar embryonic disk</p></li></ul><p></p><p>Top cavity: amniotic sac </p><ul><li><p>amniotic cavity</p><ul><li><p>Fills with fluid to help surround embryo</p></li></ul></li></ul><p></p><p>Below Hypoblast layer = yolk sac</p><ul><li><p>not much purpose in humans </p></li></ul><p></p>

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Formation of Germ Layers

Gastrulation (day 13-14)

Taking epiblast, then differentiating
- into different tissue types that will form inside the body
At this point, the epiblast layer elongates
- gets longer in one direction
- due to cell proliferation (cells multiply)
  - Also thickens in a region
    - runs halfway along the length of the epiblast layer
      - forms primitive streak

along this process, cells also move
- some more towards primitive streak and through streak to hypoblast layer
  - some displace hypoblast cells
    - take over space
      - new layer called
        Endoderm (forms digestive system, lungs)

not all cells get through and some sandwich between epiblast/ endoderm layer
- called mesoderm layer (forms all body tissue, blood supply, bone, muscles)
All cells that don’t move are called the ectoderm layer (forms skin + most of nervous system)

<ol><li><p>Gastrulation (day 13-14)</p></li></ol><ul><li><p>Taking epiblast, then differentiating</p><ul><li><p>into different tissue types that will form inside the body</p></li></ul></li><li><p>At this point, the epiblast layer elongates</p><ul><li><p>gets longer in one direction</p></li><li><p>due to cell proliferation (cells multiply)</p><ul><li><p>Also thickens in a region</p><ul><li><p>runs halfway along the length of the epiblast layer</p><ul><li><p>forms primitive streak</p></li></ul></li></ul></li></ul></li></ul></li></ul><p></p><ul><li><p>along this process, cells also move</p><ul><li><p>some more towards primitive streak and through streak to hypoblast layer</p><ul><li><p>some displace hypoblast cells</p><ul><li><p>take over space</p><ul><li><p>new layer called</p><ul><li><p>Endoderm (forms digestive system, lungs)</p></li></ul></li></ul></li></ul></li></ul></li></ul></li></ul><p></p><ul><li><p>not all cells get through and some sandwich between epiblast/ endoderm layer </p><ul><li><p>called mesoderm layer (forms all body tissue, blood supply, bone, muscles)</p></li></ul></li><li><p>All cells that don’t move are called the ectoderm layer (forms skin + most of nervous system) </p></li></ul><p></p>

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Primative streak direction

cephallic region formed away from this steak (head)
Claudal region formed close to this region (tail)

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Endoderm

Inner layer
- forms
  - Lining of the digestive tract
    - derivatives

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Mesoderm

Middle layer
- Forms:
  - Muslce
  - Bone
    - Blood Vessels

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Ectoderm

Outer layer
- Forms
  - skin
  - nervous system

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Formation of Notachord (Day 16-18)

Forms from mesoderm cells
Solid cylinder of cells under ectoderm
Closer to the cephalic end

Involved in induction
- Formation of the neural plate
- Signals the ectoderm to form neural plate
  - which is a thickened layer of ectoderm surrounding notochord
    - Will become spinal cord + brain

By day 18 neural plate formed

<ul><li><p>Forms from mesoderm cells</p></li><li><p>Solid cylinder of cells under ectoderm</p></li><li><p>Closer to the cephalic end</p></li></ul><p></p><ul><li><p>Involved in induction </p><ul><li><p>Formation of the neural plate</p></li><li><p>Signals the ectoderm to form neural plate</p><ul><li><p>which is a thickened layer of ectoderm surrounding notochord</p><ul><li><p>Will become spinal cord + brain</p></li></ul></li></ul></li></ul></li></ul><p></p><ul><li><p>By day 18 neural plate formed</p></li></ul><p></p>

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Oropharyngeal Region

Cephallic
thin membrane region of ecto derm
becomes mouth

<ul><li><p>Cephallic</p></li><li><p>thin membrane region of ecto derm</p></li><li><p>becomes mouth</p></li></ul><p></p>

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Cloacal Membrane

become other end of digestive system (anus)
Caudal end

<ul><li><p>become other end of digestive system (anus)</p></li><li><p>Caudal end</p></li></ul><p></p>

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Formation of Neural TUBE (day 18-26)

Neural plate formed
- Then it starts to move
  - Lateral edges of the neural plate rise
    - forms neural fold
    - In the center of the folds, we get a neural groove

The neural groove gets deeper because the folds get higher

Crests of neural folds form

Crests get closer together

Notochord doesn’t move
Some cells in the crest region break off
- Neural crest cells

creats merge to form neural tube

outher layer = skin

Neural crest cells migrate off to form parts of the nervous system in this step:

Ex forms:

sensory + postganglionic autonomic portions of PNS
General connective tissue of head

Note:

Folding begins in center of ectoderm layer
- then moves to both cephallic + caudal ends
outside of neural tube “somites” form
- formed from mesoderm

<ul><li><p>Neural plate formed</p><ul><li><p>Then it starts to move</p><ul><li><p>Lateral edges of the neural plate rise</p><ul><li><p>forms neural fold</p></li><li><p>In the center of the folds, we get a neural groove</p></li></ul></li></ul></li></ul></li></ul><p></p><ol start="2"><li><p>The neural groove gets deeper because the folds get higher</p></li></ol><ul><li><p>Crests of neural folds form</p></li></ul><p></p><ol start="3"><li><p>Crests get closer together</p></li></ol><ul><li><p>Notochord doesn’t move</p></li><li><p>Some <u>cells</u> in the crest region break off</p><ul><li><p><u>Neural crest cells</u></p></li></ul></li></ul><p></p><ol start="4"><li><p>creats merge to form neural tube</p></li></ol><ul><li><p>outher layer = skin</p></li></ul><p></p><p>Neural crest cells migrate off to form parts of the nervous system in this step:<br><br>Ex forms:</p><ul><li><p>sensory + postganglionic autonomic portions of PNS</p></li><li><p>General connective tissue of head</p></li></ul><p></p><p>Note:</p><ul><li><p>Folding begins in center of ectoderm layer</p><ul><li><p>then moves to both cephallic + caudal ends</p></li></ul></li><li><p>outside of neural tube “somites” form</p><ul><li><p>formed from mesoderm</p></li></ul></li></ul><p></p>

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Formation of Somites

Form alongside the neural tube (bilateral)
Formed from mesoderm
Become:
- Vertebral column
- ribs
- Some skeletal muscle

<ul><li><p>Form alongside the neural tube (bilateral)</p></li><li><p>Formed from mesoderm</p></li><li><p>Become:</p><ul><li><p>Vertebral column</p></li><li><p>ribs</p></li><li><p>Some skeletal muscle</p></li></ul></li></ul><p></p>