Gastrulation Flashcards
Briefly describe Gastrulation
It begins with the formation of the primitive streak on the surface of the epiblast and is essentially the creation of three layers. Cells from the epiblast migrate towards the primitive streak and when they arrive, the break off, and dive under it, ending up between the epiblast and the hypoblast.
What controls the cells from the epiblast migrating towards the primitive streak?
FGF8 (fibroblast growth factor)—it downregulates E-cadherin
What are the specific names of the three layers formed?
The surface will be ectoderm, the middle layer will become mesoderm, and the hypoblast is replaced by the invaginating cells to become endoderm. The embryo then becomes a trilaminar disc
What is the source of all 3 layers?
the epiblast
What is the ectoderm and what does it differentiate into?
Connects to the “outside”. Produces the nervous system (peripheral and central), the epidermis (skin, nails, hair), sensory epithelium of the ear/nose/eye. Also produces subcutaneous glands, mammary glands, pituitary gland, and enamel of the teeth
What does the mesoderm differentiate into?
It’s the “middle”. Supporting tissue (cartilage, bone), muscle, blood and lymph cells, walls of the blood vessels and heart, genitourinary system, cortical portion of the adrenal, the spleen.
What will the connecting stalk eventually become?
the umbilical cord
What does the endoderm differentiate into?
It’s the “inside”. Epithelial lining of GI tract/respiratory tract/bladder/urethra, thyroid, parathyroid, liver, pancreas, lining of tympanic cavity and auditory tube
Gastrulation begins with the formation of the ___________.
Primitive streak
True or false: once the primitive streak is formed on the surface of the epiblast there is now a head (cranial) and tail (caudal) end.
True; the cephalic end of the streak = primitive node
How are three layers created during gastrulation?
Cells from the epiblast migrate towards the primitive streak and when they arrive, they break off, and dive under it, ending up between the epiblast and the hypoblast. The invaginated cells become the endoderm and mesoderm while the cells remaining in the epiblast become the ectoderm.
What growth factor is key in gastrulation? What effect does it have on gene expression?
FGF8 is produced by streak cells and downregulates E-cadherin (binds epiblast cells together—thus the cells will become loose, break off and move) and it regulates Brachyury (T) which controls cell differentiation into mesoderm
Describe the migration of mesoderm.
Cells move into the primitive streak and then dive out to the edges in a predictable pattern depending on where they dove into the streak. For example, the cells that enter at the primitive node head straight up the middle (cranially).
Cells that migrate through the midline are also called what?
prenotochordal cells—they will form the prechordal plate and the notochordal process
Which of the three germ layers will form the spinal cord?
ectoderm
Discuss the formation of the definitive notochord
The prenotochordal cells initially become enmeshed in the hypoblast/endoderm forming the notochordal plate (like an “in between” stage). As the hypoblast is replaced by endodermal cells (moving in from the primitive streak), the notorchord plate gets pushed back up to form the definitive notochord
True or false: the notochord becomes the spinal cord
False; it more or less serves as a marker or “scaffolding” and then disappears; it’s been hypothesized that the nucleus pulposus is of notochordal origin
The notochord originates from which of the three germ layers?
mesoderm
Appearance of the notochord induces thickening of ectoderm leading to what structure?
Neural plate
Where is there a temporary connection between the ectoderm and endoderm? What happens if there’s a persistence of this connection?
the notochordal plate; if there’s a persistence of this connection (in other words, the notochordal process didn’t “bounch” back up) we get a neuroenteric canal
How does the neural plate form?
the appearance of the notochord induces thickening of the ectoderm which forms the neural plate. In other words, the notochord tells the overlying cells of the ectoderm to become the neural plate. The induction of the neuroetoderm is called neurulation
Describe the origin of the neural folds and their role in creation of the neural tube
The edges of the neural plate elevate and fold inward creating the depression known as the neural groove. The edges of the neural folds move towards the midline to fuse, creating the neural tube. NOTE: the cells at the crest of the neuroectoderm begin to dissociate from their neighbors and migrate throughout the body.
Where does the fusion of the neural folds first take place?
Folding starts at the 5th somite and then progresses both caudally and cranially
After the neural tube has formed, describe the spatial relationship of the ectoderm, neural crest, and neural tube
The ectoderm is on top of both the neural crest and neural tube (think of the skin over your spinal cord). The neural crest drapes over the neural tube.
Which neuropore will close first, the cranial or caudal?
the cranial neuropore will close first at day 25, while the caudal neuropore closes at day 27. Note: the closure of the neural tube is the final step in neurulation.
What pathology is associated with the failure to close the caudal neuropore
spina bifida and spina bifida occulta
What pathology is associated with the failure to close the cranial neuropore?
encephalocele and anencephaly
What is significant about the cells that sit at the crest of the neural fold?
They are designated neural crest cells and migrate away from the neuroectoderm to different parts of the body. For example, they can migrate to form melanocytes in the skin, sensory ganglion, or the cartilage/bone in the head.
