Health Sciences 4320A/B Lecture Notes - Lecture 5: Primitive Streak, Yolk Sac, Amniotic Sac
11 Feb 2020
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LECTURE 5 & 6 – THIRD WEEK OF DEVELOPMENT & NEURULATION
TIMELINE SUMMARY: THE SECOND WEEK (DAYS 8-14)
CLOACAL MEMBRANE
• Connecting stalk – made of extraembryonic mesoderm
• Umbilical vesicle and amniotic cavity – form the cushion
• Cytotrophoblast – remains undifferentiated
• Syncytiotrophoblast – causes all of the remodelling of endometrium
Makes hCG which is registered on pregnancy tests
These cells make hCG which support the corpus luteum, which supports to endometrium
• Cut away the amniotic cavity and umbilical vesicle – left with the bilaminar disc
• Structures deriving from the primitive streak and oropharyngeal membrane
WHY GASTRULATION?
• Gastrulation – process of forming the three embryonic germ layers
Ectoderm, mesoderm, and endoderm
These germ layers go on to form all of the parts of the body
• Trilaminar embryonic disc
Epiblast and hypoblast from bilaminar disc are replaced with the trilaminar embryonic germ layers
• At this point, as the embryo is going through gastrulation, it is called the gastrula
Can be called an embryo at any stage
• Axial orientation is established
Helping to establish cranial, caudal, left, right, ventral and dorsal
• Extensive cell shape change, migration, and rearrangement
• Beginning of morphogenesis
These do not all happen at the exact same time – some parts happen caudally and cranially
GERM LAYER FORMATION: DAYS 15-16
• Dorsal view of the embryonic disc – slice of the amnion
Morphological change in the epiblast cells
Epiblast – top layer, an epithelial cell type (e.g., skin, lining reproductive tract,
urinary tract)
Needs to undergo a transition to start populating the germ layers
Starts creating a Primitive Streak
• Prechordal Plate
Located at the cranial end
Prechordal Plate directs cell fate
• Primitive Streak
Located at caudal end
Creates a long streak along the embryo into the epiblast
Epiblast cells migrate from caudal end to cranial end and start to press
against each other at the midline
Creates a fold in the epiblast cells as they start to dive down into the streak
itself
As they dive down, they are exposed to different factors that reprogram them
into different cell types
• Epithelial cells are held together fairly tightly
Epithelial-to-mesenchyme transition (EMT) – epithelial cells then become mesenchyme cells
Mesenchyme cells – have less of an identity, stem-cell-like
Pseudopodia – walking, migrating cells that go to populate the germ layers
When measuring how old an embryo is, we
measure pregnancy and gestation from the first
day of the woman’s last menses
PRECHORDAL PLATE
GERM LAYER FORMATION (2)
• Blue – epithelial / epiblast cells
• Red – as cells come down primitive streak, they get
reprogrammed and change shape
Allows them to slip through the streak easily
Becomes mesenchyme cells
• Mesenchyme cells
Migrating cells – heading cranially and caudally
They slip in between the epiblast and hypoblast,
creating another layer
As they slip in, they start creating a middle layer
called the Mesoderm
• Mesoderm – gained identity as mesoderm once
mesenchymal cells stopped migrating
Goes on to form heart, bone, muscle, etc.
• EMT – also occurs in metastasizing cancer cells; they lose identity and become mesenchyme, and migrate everywhere
• Mesenchymal cells that receive more signalling by diving into the primitive streak become more active
These mesenchymal cells travel all the way down to the hypoblast instead of staying in the middle
Yellow – these cells displace the hypoblast cells
These are called endoderm cells
• Hypoblast cells get kicked out of embryo and go on to form the extraembryonic mesoderm
• Endoderm – goes on to form the lungs, GI tract, thyroid, etc.
• Not all cells migrate to the primitive streak
Remaining epiblast cells go on to form ectoderm – goes on to form brain, CNS, PNS, etc.
GERM LAYER FORMATION (3)
• Primitive streak has now extended further along embryonic disc
At the end of streak is the primitive node
In the primitive node is the primitive pit
• Cells that migrate down the node receive different signalling than cells that
migrate through the pit
GERM LAYER FORMATION (4)
• Notochordal process
Made up of mesoderm cells that now have an identity
Located in the midline and from the primitive node up to the prechordal plate
• Trilaminar embryonic disc
Epiblast → Ectoderm
Mesenchyme cells → Mesoderm
Hypoblast → Endoderm
GASTRULATION
• Begins with the primitive streak
Narrow, slightly bulging groove on epiblast
• Cells enter groove
Cells enter streak, and spread laterally and cranially
• Oropharyngeal membrane
At cranial end of embryo
Formed from some of the first cells that migrate to the prechordal plate
Region that is tightly adherent between endoderm and ectoderm cells
Cranially and caudally, there is no mesoderm – still bilaminar ectoderm and endoderm
These cells are adhered so tightly that the mesenchymal cells cannot get through
This surrounds the oropharyngeal membrane and the cloacal membrane
Future site of the mouth
• Cloacal membrane
At caudal end (caudal to the primitive streak)
Future site of the anus, vagina, and ureter
Tight association of endoderm and ectoderm cells
• Formation of body axes
Document Summary
Lecture 5 & 6 third week of development & neurulation. When measuring how old an embryo is, we measure pregnancy and gestation from the first day of the woman"s last menses. Cloacal membrane: connecting stalk made of extraembryonic mesoderm, umbilical vesicle and amniotic cavity form the cushion, cytotrophoblast remains undifferentiated, syncytiotrophoblast causes all of the remodelling of endometrium. Makes hcg which is registered on pregnancy tests. These cells make hcg which support the corpus luteum, which supports to endometrium: cut away the amniotic cavity and umbilical vesicle left with the bilaminar disc, structures deriving from the primitive streak and oropharyngeal membrane. Why gastrulation: gastrulation process of forming the three embryonic germ layers. These germ layers go on to form all of the parts of the body. Epiblast and hypoblast from bilaminar disc are replaced with the trilaminar embryonic germ layers: at this point, as the embryo is going through gastrulation, it is called the gastrula.
