Module 2: Gastrulation and Neurulation Flashcards
3 germ layers
-ectoderm
-mesoderm
-endoderm
Ectoderm
-outermost layer
Mesoderm
-middle layer
Endoderm
-innermost layer
When does gastrulation period start
-week 3
Disc before gastrulation
-bilaminar
-composed of epiblast and hypoblast
Disc after gastrulation
-trilaminar
-composed of endoderm, mesoderm, ectoderm
Primitive streak
-lengthens by addition of cells from cranial to caudal ends
Primitive pit
-indentation at the centre of the primitive node
Endoderm formation days
-day 14-15
Mesoderm formation days
-day 16-17
Ectoderm formation days
-after endoderm and mesoderm
Endoderm formation
-epiblast cells migrate along primitive streak to displace hypoblast and form a single layer of cells called the extraembryonic mesoderm
Mesoderm formation
-epiblast cells continue to migrate along primitive streak to form a layer between the endoderm and epiblast cells
Ectoderm formation
-overlying epiblast tissue that does not migrate through the streak and that remains on the surface of embryo facing the amniotic cavity is then referred to as ectoderm
What are the 2 areas where the embryo remains bilaminar
-oropharyngeal membrane
-cloacal membrane
Oropharyngeal membrane
-cranial end of embryo
-will eventually break down to become opening to the oral cavity
Cloacal membrane
-will eventually reabsorb and become the anal and genitourinary openings
Allantois
-an outpouching formed from cloacal membrane to the connecting stalk
Ectoderm layer fate
-tissues that cover body surfaces (epidermis, hair, nails, tooth enamel)
-nervous system
-eyes
-internal ear
-sweat
-subeceous glands
Mesoderm layer fate
-muscle and connective tissue
-bones
-urogenital systems
-cardiovascular systems
Endoderm layer fate
-gut and gut derivatives (liver, pancreas, lungs)
Abnormal gastrulation fate
-incompatible with life
-usually result in structural abnormalities
Tethered cord syndrome
-rare defect relating to the spinal cord as a result of continued cellular communication at the neurenteric canal
Day 19
-notochord induces overlying ectoderm to differentiate into neuroectoderm
-neuroectodermal cells thicken to form neural plate
Day 19 mesoderm proliferation
-proliferates to form longitudinal column of paraxial mesoderm, intermediate mesoderm, and lateral mesoderm
Day 20
-neural plate lengthens and invaginates in midline to form neural groove with neural folds
-meanwhile small spaces form in lateral mesoderm which are known as coelomic spaces
Day 22
-fusion of lateral neural folds begins in middle of embryo and proceeds cranially and caudally
Day 22 somites
-division of paraxial mesoderm that forms paired blocks
-also expand in both cranial and caudal directions
Day 23
-as neural tube forms, the non-neural ectoderm comes together to seal over the neural folds, allowing tube to separate from the surface
-then mesoderm migrates to space between neural tube and ectoderm
Day 23 neural crest formation
-a special group of cells migrate beside neural tube to form this
What separates the lateral mesoderm into somatic and splanchnic mesoderm
-coelom
Cranial neuropore closure day
-day 25
Caudal neuropore closure day
-day 28
When is neurulation complete
-with the closure of both neuropores
-CNS is now represented by a closed tubular structure that will give rise to brain and spinal cord
Anencephaly
-cranial neuropore failing to close
-defects in CNS and malformation in brain and skull
Spina bifida
-caudal neuropore failing to close
-incomplete covering of nervous tissues & protruding sac that can contain meninges, spinal cord, or both
Epithelial-mesenchymal transition
-epithelial cells lose their cell polarity and cell-cell adhesion, and gain migratory and invasive properties to become mesenchymal stem cells
Mesenchymal stem cells
-multipotent stromal cells that can differentiate into a variety of cell types
Trunk migration
-cells that migrate along dorsolateral pathway
What do trunk migration cells form
-melanocytes in skin and hair follicles
What do cells that migrate along the ventral pathway form
-sympathetic neurons and ganglia
-schwann cells
-adrenal medulla
Cranial migration
-cells that migrate from cranial region
What do cranial migration cells form
-craniofacial skeleton
-cranial neurons and ganglia
What do somites develop into
-vertebrae
-intervertebral discs
-ribs
-skeletal muscles
When does the first pair of somites arise
-day 20
-in occipital region
Rate of growth of somites
-3 pairs per day
Intermediate mesoderm fate
-forms most of the urinary and reproductive system
Splanchnic mesoderm fate
-forms the visceral laters of the serous membranes around each organs
Somatic mesoderm fate
-forms dermis, bones, connective tissue, muscles and parietal layers of serous membranes
When does embryonic folding occur
-days 22-28
Embryonic folding
-flat trilaminar embryonic disk becomes a more cylindrical embryo
Primitive foregut location
-cranial end of the embryo
Primitive foregut fate
-forms parts of the mouth, esophagus, stomach, and duodenum to opening of common bile duct
Primitive midgut location
-middle
Primitive midgut fate
-forms small intestine from opening of the common bile duct to ileocecal junction, and large intestine from caecum to transverse colon
Primitive hindgut location
-caudal end of embryo
Primitive hindgut fate
-forms large intestine from distal transverse colon to anal canal and parts of bladder and urethra
What establishes opening for anus
-rupture of cloacal membrane in week 7
Longitudinal (cephalocaudal) folding
-produce head fold and tail fold which will continue to fold ventrally to form caudal and cranial regions
What is special about the head and tail folds
-they never completely fuse
Transverse (lateral) folding initiation
-lateral folds pull amnion down so embryo is in amniotic cavity
-as lateral folds converge, they pinch midpoint of yolk sac together resulting in formation of tubular embryo with gut tube lined by endoderm
Transverse (lateral) folding at foregut and hindgut
-ventral abdominal wall closes completely at these regions
How is gut suspended from dorsal abdominal wall
-dorsal mesentery
Transverse (lateral) folding at midgut
-lateral folds do not completely fuse at the region of the primitive midgut
Vitelline duct
-channel that connects the yolk sac to the primitive midgut
-incorporated into the umbilical cord
