Gastrulation and Neurulation Flashcards
Describe the 1st week of embryological development
- Ovulation
- Fertilisation- conception in uterine tube
- Migration - to uterus
- Cleavage - cell division to morula then blastocyst
- Implantation - in uterine mucosa, complete by day 9
Describe the state by the end of the 2nd week of embryological development
Bilaminar disc with connecting stalk
Within the chorionic sac
Amniotic sac on the dorsal side
Yolk sac on the ventral side
Epiblast and Hypoblast are in contact
Day 14 primitive streak appears and the cells begin to move.
Gastrulation has begun
In the next 3-4 days a lot happens
When does the primitive streak form ?
Around day 14-15
Gastrulation
Bilaminar disc becomes the trilaminar disc
2 layers ————-> 3 layers
Describe early week 3
Formation of the primitive groove.
A groove with raised edges and a pit, formed on the epiblast along ahead-tail axis.
Groove – Primitive Streak
Pit - Primitive pit, associated with the primitive node which is a key signalling region in gastrulation.
Where does the primitive streak develop ?
Develops at the caudal end of the embryo
Primitive node function
A key signalling region in gastrulation
Sends out signalling factors
Describe what happens in cell migration 1
The epiblast cells divide and migrate through the primitive streak.
The epiblast cells displace and replace the hypoblast cells.
The epiblast cells start becoming endoderm.
Describe what happens in cell migration 2
Second wave of epiblast migration “fills in” between 2 layers.
This establishes a trilaminar disc, except at 2 locations.
The epiblast is now ectoderm.
Describe the location of the notochord
From the primitive pit rostrally, into the mesoderm, grows a tube-shaped structure.
Describe development of the notochord
Changes form a tube to a disc to a solid rod (17-20 days)
Describe the dorsal/ventral axis
The primitive node/notochord organise dorsal-ventral axis.
Skin/ ventral (belly) development uses signals from bone morphogenic proteins (BMP)
BMP
Bone Morphogenic Protein
Function of the notochord
Notochord is essential for setting up dorsal and ventral axes and inducing neurulation.
Node and notochord release factors (chords, noggin, follistatin) that block BMP.
Function of BMP
BMP would drive formation of skin on the back, but notochord drives back/neural tissue development instead.
Neurulation
The neural plate converts to a groove then a tube.
Forming of neural tissue, induced by the bar shaped tissue-notochord (deep to the neural epithelium)
epithelial cells
Describe neurulation
Epithelial cells become columnar in an area called the neural plate.
The neural plate converts to a groove then a tube (neurulation)
Middle neural groove becomes apparent after day 19
Neurulation - fold and groove
Cells on the edge of the plate thicken, forming a grove and then a fold.
Occurs between day 20-21
Neurulation - tube
The edges of the fold roll over to make a tube.
Occurs from day 22 onwards
Neurulation - closure
The folds close like a zip in cephalic and caudal directions, the open ends being the anterior and posterior neuropores.
They close to form a complete tube.
Anterior neuropore - 25 days
Posterior neuropore - 28 days
What forms the brain ?
Three dilations within the rostral tube - primary brain vesicles - go onto form the brain.
Describe secondary neurulation
The neural tube is lengthened caudally by a process of secondary neurulation
The medullary cord forms and develops a lumen before fusing with the caudal end of the neural tube.
Explain the formation of neural tube during neurulation
Fold and groove
Tube
Closure
Neural tube defects causes
Failure of the neuropores to close, leads to various levels of :
- Brain/skull defects : anterior neuropore
- Spina bifida : posterior neuropore
Neural tube defects linkage
Closely linked to low levels of folate which is essential for cell division.
Origin of neural crests
Neural crests derive from the edges of the neural tube.
They undergo an epithelial to mesenchymal transformation, become motile and migrate away from the neural tube to widespread destinations.
Neural crests migrate all over the body, but in particular contribute to the sensory and autonomic nervous systems.
State some things that the neural crest forms
Peripheral nervous system
Melanocytes
Schwann cells
Head mesenchyme
Describe some neural crest linked problems
Hearing problems
Abnormal pattern of melanocytes
Neural crest cell link syndromes
Waardenburg’s syndrome (1/50,000)
Treacher Collins Syndrome (autosomal dominant 1/50,000)
Treacher Collins Syndrome
Defective protein called treacle (TCOF1 gene)
Failure of formation/apoptosis of neural crest cells
Parts of the mesoderm
Paraxial mesoderm
Intermediate mesoderm
Lateral plate mesoderm
How is mesoderm formed ?
Cells from the epiblast flow into the primitive streak/groove (invagination)
These cells replace the hypoblast cells to form endoderm
Cells lie between the hypoblast and epiblast form mesoderm.
Cells that remain in the epiblast become ectoderm.
How are different regions of mesoderm formed ?
Epiblast cells migrate through the streak to form different regions of mesoderm.
What does the lateral plate mesoderm split into ?
Visceral and Parietal layers
What does paraxial mesoderm form ?
Paraxial mesoderm becomes segmented to form somites
Paraxial mesoderm
Paired condensations of paraxial mesoderm in the future trunk - NOT HEAD
Day 20-30, 3/4ths form a day from cranial to caudal.
42-44 pairs develop, but regression in the tail end leaves approx. 37 pairs.
Name the 37 pairs of spinal cord segments and nerves
4 Occipital
8 Cervical
12 Thoracic
5 Lumbar
5 Sacral
3 (ish) Coccygeal
Describe somite formation
Cells the paraxial mesoderm have an internal timer
They go through cycles every 90 minutes defined by a notch signalling clock.
A wave of FGF signal that passes along the embryo in a rostral caudal direction.
When the FGF wave passes cells, they are programmed to change into part of a somite according to where they are in the clock cycle.
Crucial part of somite formation
If the wave passes cells early in the clock cycle, they become the front end of that segment
If the wave passes cells late in the cycle, they become the tail end of the segment
This process is repeated over and over forming a number of somites
Notch clock
A genetic process that has a clock
Homeobox genes (Hox genes)
Expressed by somites and tells structures what to become and when to become it.
The direct formation of body structures AT THE RIGHT PLACE along the body e.g., form CORRECT body segments.
Derivatives of the paraxial mesoderm
Head and Somites.
Somites are differentiated into :
- Sclerotome —> Axial skeleton
- Myotome —> Skeletal muscle
- Dermatome —> Dermis
Derivatives of the intermediate mesoderm
Kidney
Gonads
Urogential structures
Derivatives of the lateral plate mesoderm
Splanchnic and Somatic
Splanchnic :
- Visceral coverings
- Heart
- Blood vessels
- Blood
- Spleen
Somatic :
- Parietal coverings
- Limb cartilage
- Limb bone
- Limb, lateral, ventral, trunk dermis
Derivatives of the midline mesoderm
Prechordal plate :
- future site of the mouth
Notochord :
- nucleus pulposus
Endoderm process
Craniocaudal folding and lateral folding pinches of the endoderm lined yolk sac.
This becomes the primitive gut lining.
Note the former epiblast – now amnion encapsulating the folded embryo.
Small connection to the yolk sac.
The vitelline duct, closed by 5/6 weeks
Note the arrangement of LPM lining the cavities and surrounding the gut tube
Endoderm derivatives
Lining of gut tube
Distal urogenital system
What is Meckel’s diverticulum ?
Remnant of the vitelline duct (yolk sac/ gut connection)
Meckel’s diverticulum statistics
Meckel diverticulum occurs in 2% of the population,
2% are symptomatic,
mostly in children < 2 years
affects males twice as often as females,
is located 2 feet proximal to the ileocecal valve,
is ≤ 2 inches long, and can
has 2 types of mucosal liningE
Ectoderm derivatives
Epidermis and associated glands
Nails and enamel of teeth
Lens and cornea in the eye
Olfactory epithelium
Oral Cavity and anal canal epithelium
Sensory organs of ear
Glands: Salivary glands and Mammary Gland
Adenohypophysis
Terminal male urethra, labia majora and outer surface of labia minora
