Neurulation Flashcards
Week 4
Neurulation
Steps of neurulation
1.) Neuroectodermal tissues differentiate from the ectoderm and thicken into the neural plate; the neural plate border separates the ectoderm from the neural plate
2.) The neural plate bends dorsally, with the two ends eventually joining at the neural plate borders, which are now referred to as the neural crest
3.) The closure of the neural tube disconnects the neural crest from the epidermis; neural crest cells differentiate to form most of the peripheral nervous system
4.) The notochord degenerates and only persists as the nucleus pulposus of the intervertebral discs; other mesoderm cells differentiate into the somites, the precursors of the axial skeleton and skeletal muscle
Neurulation highlights
- notochord induces overlying ectoderm to develop into the neural plate
- the neural plate folds into the neural tube and the neural crests are pinched off
- the neural tube derives the CNS
- the neural crests derive the PNS and select other cells
During gastrula (week 3 development), the body axis is formed
- bilateral symmetry
- left and right
- dorsal and ventral
- cranial and caudal
Week 3
- triploblastic/trilaminar disc
What germ layers are formed during gastrulation?
- ectoderm
- mesoderm
- endoderm
Notochord
Sonic Hedge Hog (SHH)
The mesoderm differentiates into how many types?
3
Primitive streak becomes
primitive groove
Primitive node becomes
primitive pit
The notochord secretes SHH, which dictates what?
cellular involvement
Neural plate
- neural groove
- neural folds
During neurulation, the neuroectodermal cells transform into what?
neural tube
Neural tube becomes what?
brain and spinal cord
The neural tube activates what?
- neural crest cells
- further differentiation of ectoderm, mesoderm, and endoderm
Central Nervous System (CNS)
- brain
- brain stem
- spinal cord
Peripheral Nervous System (PNS)
- cranial nerves
- peripheral body nerves
- ganglion
- sensory and motor
Steps at ectoderm: primitive area makes
primitive streak –> groove
primitive node –> pit
Primitive node
“Hensen’s node” or “Spemann-Mangold Organizer”
- notochord and SHH and axial mesoderm
Notochord and SHH and Axial mesoderm stimulates what?
Neural plate (replaces primitive streak at day 25)
Neural plate makes
- neural groove
- neural folds
which makes the neural tube
Steps at ectoderm: Cells at primitive NODE (let’s make a tube)
- they direct organization
- cells that DIVE through groove and travel CRANIALLY become the prechordal plate and notochord (which stimulate neurulation)
Steps at ectoderm: Cells at primitive NODE
Ectoderm: neural plate –> _______ ____
Ectoderm: surrounding plate –> ______
neural tube; epidermis
At primitive NODE, what happens to the cells at the cranial end?
they proliferate
At the primitive NODE, once plate is formed it undergoes __________ by _______ _______
elongation; convert extension
Neural plate
communication
How do germ cells communicate at the neural plate?
induction
At the neural plate, mesoderm causes overlaying ectoderm to undergo molecular and cellular changes
1.) Cells proliferate (increase mitosis)
2.) Cells express new sets of mRNA’s specific for neural development “markers” including CAM’s (cell adhesion molecules)
If cells transplant incorrectly—
two-headed organism
Neural plate – communication – cell movement
cells undergo shape and behavioral changes
- columnarization
- apical constriction
- convergent extension
- cell crawling
Neural plate –> hinge and close –> neural tube
1.) The sheet bends at a median hinge joint, creating neural folds
2.) The sheet bends further at lateral hinge joints, closing at the top to form the neural tube
3.) Neural crest cells detach from the epithelial tube and become wandering mesenchyme cells
Neural crest cells “4th germ layer”
- specialized cells within neural tube
- migrate outward through the fetus
- give rise to NEW tissues
Neural crest cells make up the sensory epithelium of what?
- ears
- nose
- eyes
- mouth
Neural crest cells make up what glands?
- sweat
- mammary
- pituitary
Movement of cells for neurulation in dorsal view
- neural tube is OPEN
- “pores” close –> neural tube is CLOSED
- Neural tube is NOW formed
Day 18-Day 23
- Cranial (anterior) neuropore
- Caudal (posterior) neuropore
Otic placode
- otic vesicles
- cochlea
- inner ear
Surface ectoderm
- nose
- mouth
- epidermis of fetal skin
- hair
- nails
- sweat glands
- mammary glands
Where are the somites found?
Under ectoderm
Lens placode
- lens
- cornea
Steps at mesoderm: notochord
- secretes SHH protein
- tells the cells where they are in 3D space
- SHH permeates embryo
- day 20: differentiation of 3 mesoderm types
3 mesoderm types
1.) Paraxial mesoderm
2.) Intermediate mesoderm
3.) Lateral mesoderm
The paraxial mesoderm becomes _________
Somites
Somites
PAIRED blocks of mesoderm, closest to Notochord
Paraxial mesoderm (somites)
- paired
- formed alongside NOTOCHORD
- cranial to caudal development
- 3 somites per day are formed
- can dictate how old embryo is
How many somites are formed each day?
3
Paraxial mesoderm Somites give rise to what 3 groups of mesodermal tissues?
1.) Dermatome (skin)
2.) Myotome (muscle)
3.) Sclerotome (hard)
Sclerotome
- bones
- cartilage
Intermediate mesoderm
- smaller
- lateral to somites
ex. adrenal cortex, kidneys, testes and ovaries
Lateral mesoderm
- thinnest
- most lateral
Lateral mesoderm splits into which two layers at week 3?
1.) Dorsal portion: Somatic/Parietal
2.) Ventral portion: Splanchnic/Visceral
Somatic/parietal (dorsal) mesoderm
Adheres to ectoderm
- parietal pleura; soft tissue of arms and legs
Splanchnic/visceral (ventral) mesoderm
Adheres to endoderm
- visceral pleura; muscle wall of gut, heart, circulatory system
Space between the 2 mesoderm layers is called what?
Intraembryonic coelom
Intraembryonic coelom is a space that houses what?
Thoracic and abdominal cavities
Paraxial mesoderm (somites) forms
- skin
- muscle
- bony tissue
Intermediate mesoderm forms
- adrenal glands
- gonads
- kidneys
Lateral mesoderm forms
- serous membrane
- limb tissue
- heart
- circulatory system
Steps at Endoderm: Endo and Ecto push together to form 2 bilaminar regions called?
1.) Oropharyngeal membrane or buccopharyngeal (head)
2.) Cloacal membrane (rear)
Week 4
opening mouth
Week 7
opening anus and genitourinary tracts
Steps at Endoderm: week 4 Embryo folds in which 2 directions along LONGITUDINAL plane?
1.) Cranial fold
2.) Caudal fold
Movement of cells at the endoderm take on a _______ curved appearance, which helps to form ____ _____ and yolk area at middle
“shrimp”; gut tube
Steps at Endoderm: Mesoderm follows the endoderm to form ____ layer of that structure
2nd
- ex. blood vessels
Steps at Endoderm: The two sides of the body will come together at ventral surface and eventually ____, but the yolk sac is in the ___
fuse; way
Steps at Endoderm: The gut is divided into which 3 regions?
- foregut
- midgut
- hindgut
Foregut
at week 4: mouth opening connects others
at week 5: trachea and lungs
Midgut
- stays connected to yolk sac through yolk stalk or vitelline duct
- stalk thins over time and collapses
- yolk sac dissipates
What is a remnant of yolk stalk called in the midgut?
Meckel’s Diverticulum
Hindgut
- at week 7: anus opening connects anal canals at pectinate line (large intestine/anus junction)
Hindgut feature that connects anal canals
pectinate line
Endoderm forms the linings of the gastrointestinal tube and respiratory tube…
- tonsils
- thyroid
- parathyroid glands
- thymus
- liver
- gallbladder
- pancreas
- parts of ear
- epithelium of urethra and urinary bladder
- GI tract and pharynx
- respiratory tract
Pharynx
pectinate line (GI tract endoderm)
Primitive streak and node becomes what?
neural tube
Notochord
- Transient contributes to nucleus pulposus
- influences embryo folding
- secretes Sonic Hedge Hog
Steps of neurulation
1.) Neural plate (3 types of mesoderm)
2.) Neural groove
3.) Neural folding
4.) Neural tube
- 3 pairs a day
- by week 5, 42-44 pairs
Intermediate mesoderm
- kidneys
- adrenal glands
- urogenital system
Lateral plate mesoderm
- heart
- blood vessels
- organ muscle
- body wall
