LECTURE 10 (Third week of development) Flashcards
What happens in the third week of development?
- Appearance of primitive streak
- Development of notochord
- Differentiation of three germ layers
What is Gastrulation?
When the three germ layers (precursors of all embryonic tissues) and the axial orientation are established in embryos
During the third week, what is the embryo referred to as?
Gastrula
What are the three germ layers that arise from the Epiblast?
- Ectoderm (outer)
- Mesoderm (middle)
- Endoderm (inner)
What does Embryonic Ectoderm give rise to?
- Epidermis
- CNS and PNS
- Eyes and internal ears
- Neural crest cells
What does the Embryonic Mesoderm give rise to?
- All skeletal muscles
- Blood cells & lining of blood vessels
- Visceral smooth muscle coats
- Kidney
- Reproductive system
- Bone
- Spleen
What does the Embryonic Endoderm give rise to?
- GI tract
- Liver
- Endocrine system
- Urethra
- Bladder
What is the first morphologic sign of gastrulation?
The formation of the PRIMITIVE STREAK on the surface of the epiblast of the bilaminar embryonic disc
What happens during gastrulation?
Epiblast cells move towards the PRIMITIVE STREAK -> enter the primitive streak -> migrate away from primitive streak as individual cells
Describe the formation of the three germ layers
ENDODERM = Epiblast cells move inside primitive pit by INGRESSION -> Populate inside of embryo -> Replace cells in HYPOBLAST
MESODERM = Some epiblast cells diverge into space between EPIBLAST and DEFINITIVE ENDODERM -> forming INTRAEMBRYONIC MESODERM
ECTODERM = Last layer to form
Describe the formation of the Notochordal process
Some MESENCHYMAL CELLS migrate cranially from the PRIMITIVE NODE and PIT forming the NOTOCHORDAL PROCESS -> The process acquires a lumen, the NOTOCHORDAL CANAL -> Notochordal process grows cranially between the ectoderm and endoderm until it reaches the PRECHORDAL PLATE
(Prechordal plate = a small circular area of columnar endodermal cells where the ectoderm and endoderm are fused)
What is caudal to the primitive streak?
A circular area, THE CLOACAL MEMBRANE, which indicates the future site of the anus
Describe the instructive signals that induce notochord precursor cells to form the notochord
- From the primitive streak region
- Molecular mechanism involves Shh signalling from the floor plate of the NEURAL TUBE
What is the function of the Notochord?
- Defines the primordial longitudinal axis of the embryo and gives it some rigidity
- Provides signals for development of MUSCULOSKELETAL STRUCTURES and CNS
- Contributes to the intervertebral discs between the bodies of adjacent vertebrae
What is the Notochordal process?
A cellular tube that extends cranially from the primitive node to the prechordal plate
Describe the formation of the Neural tube
NEURAL ECTODERM folds to form NEURAL FOLD -> fold forms NEURAL GROOVE (with NEURAL FOLDS on each side) which invaginate and closes forming NEURAL TUBE -> NEURAL CREST CELLS are detached from NEURAL ECTODERM and undergo an epithelial to mesenchymal transition
The notochord functions as the _____________________ in the early embryo
Signalling center
What does the developing notochord induce?
Overlying embryonic ectoderm to thicken and form the NEURAL PLATE (primordium of the CNS)
What happens to the Notochordal process to form the Nucleus Pulposus
1) NOTOCHORDAL PROCESS becomes a cellular tube that extends cranially from the PRIMITIVE NODE to the PRECHORDAL PLATE
2) As the openings merge, the floor of the NOTOCHORDAL CANAL disappears and the remains of the NOTOCHORDAL PROCESS for the flattened, grooved NOTOCHORDAL PLATE
3) Beginning at the cranial end of the embryo, the NOTOCHORDAL PLATE cells proliferate and undergo infolding creating the NOTOCHORD -> Notochord detaches from endoderm
4) NOTOCHORD degenerates as the bodies of the vertebrae form -> small portions of it form the NUCLEUS PULPOSUS of each intervertebral disc
What is the Allantois?
- Appears on day 16
- A small expansion of the mesoderm and forms blood vessels that will become the PLACENTA
- Blood vessels of the allantoic stalk become UMBILICAL ARTERIES
When is neuralation complete?
At the end of the fourth week
[when closure of the caudal neuropore occurs]
What are Neural Tube Defects (NTDs)?
NTDs result when neural tube closure fails to occur
What is Anencephaly?
When the neural tube fails to close in the cranial region so most of the brain fails to form
[It’s a lethal defect -> most cases are diagnosed prenatally and the pregnancies are terminated]
What is Spina Bifida?
When the neural tube fails to close from the cervical region caudally
[more susceptible to genetic/environment factors + children lose a degree of neurological function]
