L18 Derivatives of Germ Layers

Question 1

which structures are formed by the end of week 2 and what were they formed from?

Answer 1

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.

Question 2

when does the primitive streak form and what does it establish?

Answer 2

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.

Question 3

what is formed at the cranial and caudal ends of the embryo whilst the primitive streak is being formed (day 15)?

Answer 3

At the cranial end the oropharyngeal membrane is formed

At the caudal end the cloacal membrane begins to form

Question 4

how does situs invertus occur and what is it’s prevalance?

Answer 4

occurs when abnormal gastrulation leads to right and left asymmetry of the body being reversed. occurs in approx 1 in 10,000 of people

Question 5

what is Kartagener’s syndrome and what is it’s prevalance in those with situs invertus?

Answer 5

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.

Question 6

what is partial situs inversus and what major complications can occur?

Answer 6

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

Question 7

how is the first germ layer formed?

Answer 7

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

Question 8

how is the second germ layer formed?

Answer 8

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

Question 9

how is the third germ layer formed?

Answer 9

the remaining epiblast cells on top become the ectoderm. therefore gastrulation is complete

Question 10

what is the trilaminar embryonic disk formed from?

Answer 10

ectoderm
mesoderm
endoderm

Question 11

what structures form from the ectoderm?

Answer 11

CNS, eyes, peripheral nervous system, epidermis of skin

Question 12

what structures form from the mesoderm?

Answer 12

musculoskeletal system, heart, vessels, kidneys, ureters, dermis of skin

Question 13

what structures form from the endoderm?

Answer 13

epithelial lining of respiratory tract, gall bladder, pancreas, GI tract, urethra, trachea, tonsils, lungs

Question 14

looking at the trilaminar disk through a transverse section, what structures make up the mesoderm?

Answer 14

paracial mesoderm is closest to primitive streak. intermediate mesoderm is next and the lateral plate mesoderm is most lateral

(DAY 17)

Question 15

what structures are made up from the paraxial mesoderm?

Answer 15

majority of skeleton, skeletal muscles, dermis of skin

Question 16

what structures are made from the intermediate mesoderm?

Answer 16

gonads, internal reproductive tracts, kidneys

Question 17

what structures are formed from the lateral plate mesoderm?

Answer 17

lining of body cavities

Question 18

what happens on day 20 post fertilisation to the paraxial mesoderm?

Answer 18

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.

Question 19

what is sirenomelia?

Answer 19

“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.

Question 20

what is sacrococcygeal teratoma?

Answer 20

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.

Question 21

what happens as the primitive streak regresses?

Answer 21

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)

Question 22

how is the notochord formed?

Answer 22

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)

Question 23

what does the notochord initially signal?

Answer 23

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.

Question 24

what does the notochord secondarily signal?

Answer 24

initiates and organises formation of vertebrae from somites.

the paraxial membrane divides into segments/somites.

Question 25

what 3 parts does the notochord induce the somites to differentiate into?

Answer 25

the notochord induces differentiation of the somites into 3 parts: sclerotome (bone and cartilage), myotom (skeletal muscle) and dermatome (dermis)

Question 26

how are vertebrae formed?

Answer 26

the notochord organises the sclerotome. sclerotome cells surround the notochord to form the vertebral body and surround the neural tube to form the vertebral arch

Question 27

what happens to the notochord after formation of the vertebrae?

Answer 27

Majority of notochord degenerates as bodies of vertebrae form

Remnants possibly persist as nucleus pulposus of intervertebral discs? (we believe)

Question 28

what is spina bifida?

Answer 28

Failure of induction of vertebral arch around neural tube

Question 29

what is menigocele?

Answer 29

protrusion of membrane through gap which was formed by failure of induction of vertebral arch around neural tube

Question 30

what is myelomeningocele?

Answer 30

neural tissue (nerves) also protrude through the gap formed by improper induction of vertebral arch around neural tube.

Can be spotted on ultrasound but can also operate in utero - baby at 20 weeks, surgeons can go into the uterus and begin to mend the gap

Question 31

what is meninges?

Answer 31

tissue around the nervous system

inflammation of this tissue is meningitis

Question 32

rapid proliferation of cytotrophoblast cells form the syncytiotrophoblast cells. what do these cells do?

Answer 32

are immune protection, form the placenta and secrete hCG