L18 Derivatives of Germ Layers Flashcards
(32 cards)
which structures are formed by the end of week 2 and what were they formed from?
zygote is formed by fertilisation. it forms the morula through cleavage. a blastocyst is formed from the morula and is a fluid-filled cavity.
the blastocyst differentiates into trophoblast (OCM) and embryoblast (ICM).
trophoblast cells form syncytiotrophoblast cells. the synctiotrophoblast forms maternal blood sinusoids and then lacunae whereas the cytotrophoblast cells form the extraembryonic membrane.
the embryoblast cells form the hypoblast and epiblast. the epiblast cells form the amnion and then the amniotic cavity. the hypoblast forms the primary and secondary yolk sac and also the extraembryonic membrane.
the extraembryonic membrane forms the chorionic cavity.
when does the primitive streak form and what does it establish?
commences in week 3 with the formation of the primitive groove, primitive node and primitive pit.
it establishes the left and right axis of the embryo.
what is formed at the cranial and caudal ends of the embryo whilst the primitive streak is being formed (day 15)?
At the cranial end the oropharyngeal membrane is formed
At the caudal end the cloacal membrane begins to form
how does situs invertus occur and what is it’s prevalance?
occurs when abnormal gastrulation leads to right and left asymmetry of the body being reversed. occurs in approx 1 in 10,000 of people
what is Kartagener’s syndrome and what is it’s prevalance in those with situs invertus?
20% of those with situs invertus suffer from this.
Kartagener’s syndrome is a problem with the cilia cells. it is theorised in embryology that the cilia cells sweep the correct cells to the correct sides to the body eg liver to right side. but in kartagener’s syndrome this is not the case.
what is partial situs inversus and what major complications can occur?
only part of the body becomes inverted. dextrocardia is when only the heart is reversed. is a problem when the rest of the vessels are not reversed, so oxygenated blood goes to the lungs and deoxygenated blood goes to the body
how is the first germ layer formed?
the epiblast cells migrate though the primitive streak, pushing the hypoblast cells away and eventually replacing them
Hypoblast cells are only transient and stay for a little while before they are replaced by epiblast cells
the new layer is called the definitive endoderm
how is the second germ layer formed?
Epiblast cells continue to move down through the primitive streak to form a middle layer in between the epiblast and definitive endoderm – this is the mesoderm
how is the third germ layer formed?
the remaining epiblast cells on top become the ectoderm. therefore gastrulation is complete
what is the trilaminar embryonic disk formed from?
ectoderm
mesoderm
endoderm
what structures form from the ectoderm?
CNS, eyes, peripheral nervous system, epidermis of skin
what structures form from the mesoderm?
musculoskeletal system, heart, vessels, kidneys, ureters, dermis of skin
what structures form from the endoderm?
epithelial lining of respiratory tract, gall bladder, pancreas, GI tract, urethra, trachea, tonsils, lungs
looking at the trilaminar disk through a transverse section, what structures make up the mesoderm?
paracial mesoderm is closest to primitive streak. intermediate mesoderm is next and the lateral plate mesoderm is most lateral
(DAY 17)
what structures are made up from the paraxial mesoderm?
majority of skeleton, skeletal muscles, dermis of skin
what structures are made from the intermediate mesoderm?
gonads, internal reproductive tracts, kidneys
what structures are formed from the lateral plate mesoderm?
lining of body cavities
what happens on day 20 post fertilisation to the paraxial mesoderm?
it starts dividing into blocks called somites. this begins in the cranial region and progresses towards the caudal region at a rate of 3 or 4 pairs per day until 42-44 pairs have formed. very predictable process.
what is sirenomelia?
“mermaid syndrome”
In this condition the primitive streak regresses/closes off too early, so not sufficient migration of epiblast cells. Not enough mesoderm formed in caudal region so abnormalities of urogenital system and lower limbs - legs will often be fused together.
what is sacrococcygeal teratoma?
remnants of primitive streak persist and give rise to this tumour. occurs in 35000 to 40000 live births and is common, with 80% in females. mostly diagnosed in neonates are benign, with good prognosis after resection.
what happens as the primitive streak regresses?
the primitive node forms the notochordal process. this forms during day 17. the node cells migrate cranially and as the primitive streak regresses caudally, it pulls the node structure caudally to form the notochordal process (hollow tube)
how is the notochord formed?
The notochordal process (hollow tube) transiently fuses with the endoderm to form the flattened notochordal plate.
The notochordal plate separates from the endoderm and fuses to form solid notochord (solid structure)
(DAY 18)
what does the notochord initially signal?
it initiates neurulation - the first development in CNS.
initiated by signals from notochord that cause overlying ectoderm to thicken and divide and form neural plate (induction). lateral aspects of the ectoderm begin to raise to form a tube.
what does the notochord secondarily signal?
initiates and organises formation of vertebrae from somites.
the paraxial membrane divides into segments/somites.
