Development of endoderm and mesoderm derivatives Flashcards
What is the location of the endodermal germ layer?
Covers the ventral surface of the embryo and forms the roof of the yolk sac
What is the main organ system derived from the endoderm?
Epithelial lining of the primitive gut (GIT tract)
What are some other derivatives of the endoderm?
1) Epithelial lining of the respiratory tract
2) Parenchyma of the thyroid, parathyroids, liver and pancreas
3) Reticular stroma of the tonsils and the thymus
4) Epithelial lining of the urinary bladder and the urethra
5) Epithelial lining of the tympanic cavity and auditory tube
What are the two types of folding that occur in the development of the primitive gut?
Craniocaudal/cephalocaudal and lateral folding
What occurs during craniocaudal folding?
Ventral movement of head and tail regions to curve the fetal position due to lengthening of the neural tube
What happens during lateral folding?
Ventral movement of two lateral body wall folds to close the ventral body wall
What are the results of such folding?
1) The folds pull the amnion down such that the embryo lies within the amniotic cavity
2) The ventral body wall closes completely except for the umbilical region where the connecting stalk and yolk sac duct remain attached
3) A continuously larger portion of the endodermal germ layer is incorporated into the body of the embryo to form the gut tube
4) Partial incorporation of the allantois into the body of the embryo where it forms the cloaca
What is the fate map during gastrulation?
The ultimate fates of the epiblast cells that are decided according to the region of the primitive streak that the epiblast cells migrate and ingress through.
Cells that ingress through the cranial region of the primitive node become?
Prechordal plate and notochord
Cells that ingress through the lateral edges of the node and from the cranial end of the streak become?
Paraxial mesoderm
Cells migrating through the midstreak region become?
Intermediate mesoderm and cardiogenic mesoderm
Cells migrating through the more caudal part of the streak form?
Lateral plate mesoderm
Cells migrating through the caudalmost part of the streak become?
Extraembryonic mesoderm
What is the notochord?
Invagination of the prenotochordal cells in the primitive node
How is the notochordal plate formed?
When prenotochordal cells become intercalated in the hypoblast
What connects the amniotic and yolk sac cavities?
Temporary neurenteric canal
How is the definitive notochord formed?
Proliferation of the notochordal plate and detachment from the endoderm
What are the two components of the paraxial mesoderm?
Somitomeres/head mesoderm and somites
Where do somites originate from?
The occipital region caudally
Where do the first pair of somites originate from?
Occpital region of the embyro at approx 20th day of development
What happens after the first pair of somites appear?
New somites appear in the craniocaudal sequence at a rate of three pairs per day until the end of the fifth week where there are 42-44 pair present
How can somites tell the age of an embryo?
Because somites appear with a specified periodicity so we can count somites to tell the age of the embryo
What are the walls that each somite has?
1) Ventral and medial walls (ventro-medial portion)
2) Dorsal and lateral walls (dorso-lateral portion)
What does the ventro-medial portion give rise to?
Sclerotome
What are the three regions of the dorso-lateral portion?
A) Dorsomedial edge: muscle cells
B) Ventrolateral edge: muscle cells
C) Cells between these two edges: dermatome
What is the intermediate mesoderm?
A structure that temporarily connects paraxial mesoderm with the lateral plate
What urogenital structures does the intermediate mesoderm differentiate into?
1) In cervical and upper thoracic regions (forms segmental cell clusters)
2) More caudally (forms an unsegmented mass of tissue)
What is the lateral plate mesoderm?
Appearance and coalescence of intercellular cavities in the lateral plate
What are the two layers of the lateral plate mesoderm?
1) Somatic or parietal mesoderm layer (continuous with mesoderm covering the amnion)
2) Splanchnic or visceral mesoderm layer (continuous with mesoderm covering yolk sac)
What is the intraembryonic cavity?
Formed after dividing lateral plate mesoderm into two layers
Where is the somatic/parietal layer located?
Lines the intraembryonic cavity and together with the overlying ectoderm it forms the lateral body wall folds
What does the somatic or parietal layer form?
1) Dermis of the skin in the body wall and limbs
2) Bones of the limbs
3) Connective tissue of the limbs
4) Sternum
What are the two cells that the somatic or parietal layer receives?
1) Sclerotome cells from costal cartilages
2) Muscle precursor cells from limb muscles and most of the body wall muscles
Where is the visceral or splanchnic layer of the lateral plate mesoderm located?
Covers the yolk sac (primitive gut)
What does the visceral/splanchnic layer form?
Together with embryonic endoderm forms the wall of the gut tube. Thin serous membrane around each organ.
Which germ layer gives rise to blood and blood vessels?
Mesoderm
Through which two ways do blood vessels form?
1) Vasculogenesis - vessels arise from blood islands
2) Angiogenesis - vessels entails sprouting from existing vessels
Where do the first blood islands appear?
In the mesoderm surrounding the wall of the yolk sac at 3 weeks development. Slightly later in lateral plate mesoderm and other regions.
Where are definitive hematopoietic stem cells formed?
1) Aortagonad-mesonephros region (AGM)
2) Liver
3) Bone marrow
