2nd/3rd week of development

Question 1

Overview of the events that occur in the 2nd week of development (6)

Answer 1

1. Finish implantation
2. modifies trophoblast
3. 2nd germ layer formed
4. amniotic cavity formed
5. yolk sac formed
6. extraembryonic mesoderm formed

Question 2

End of implantation process during the 2nd week

Answer 2

1. blastocyst becomes completely embedded into the EM
2. formation of a coagulation plug that seals the open space
3. Formation of the trophoblastic cavities (by the lytic enzyme degeneration) that form a continuum with blood vessels to the flow of maternal blood –> intake of nutrients and glandular secretions for the development of the embryo
4. Decidual reaction to end the implantation

Question 3

process of forming the STB

Answer 3

-agregation of cells of the cytotrophoblasts
-form microvilli that embedd into EM
-multinucleated

Question 4

Decidual reaction def and process

Answer 4

def: process to ensure that the implantation is stopped despite the continuous release of the lytic enzymes in the EM

PROCESS:
1. fibroblasts of uterine CT become enlarged by the uptake of lipids and glycogen

2. their morphology changed from elongated cell to rounded cell of epithelial shape –> become the decidual cells
3. this modification of the phenotype causes resistance to the lytic enzymes secreted by STB

Question 5

what do the terms cranial and caudal mean

Answer 5

cranial means in the direction of the embryo head and caudal is in the direction of the anus

Question 6

Differentiation of the embryoblast cells

Answer 6

1. epiblast: high columnar cells adjacent to the amnotic cavity
2. hypoblast: small cuboidal cells adjacent to blastocyst cavity (later the YS)

!! this forms the bilayer disc

Question 7

what is the only region which the mesoderm does not split (day 12)

Answer 7

separation occurs everywhere apart from the tip between amniotic cavity and STB bcos this is where the umbilical cord will be formed (connection to maternal tissue)

Question 8

what are the components of the chorion and what is the chorion used to form

Answer 8

CHORION:
1. E Somatic M
2. CTB
3. STB

!! all layers are ajdacent

USE: forms the embryonic part of the placenta (along with the decidua basalis)

Question 9

Events that occur in the 3rd week of embryonic development (6)

Answer 9

1. Gastrulation
2. Notochord formation
3. Neural plate induced
4. chorionic villi development
5. gonocyte formation
6. development of allantois

Question 10

3 components of the primitive streak

Answer 10

1. node
2. groove
3. pit

!! originally forms as solid tissue but is then canalised by the dying of the inner tissue (forms canal for cell migration)

Question 11

what chemical secretions triggers the migration of cells for the gastrulation + why

Answer 11

FGF 8 (fibroblastic GF)
TGF-beta (transforming GF)

released by the cells of the primitive streak

they inhibit the expression of e-cadherin that holds the cells of the epiblast together and hence allows EMT (migration)

Question 12

process of cell migration in gastrulation and products formed

Answer 12

1st migration: epiblast cells migrate and replace the hypoblast cells to form the endoderm (innermost layer)

2nd migration of epiblasts that forms the intraembryonic mesoderm

the epiblasts remaining in their original position make up the ectoderm

Question 13

what would occur if the neurochord failed to form

Answer 13

events leading to the formation of the neural crest wouldnt be triggered

Question 14

tissues derived from the ectoderm

Answer 14

epidermis
nervous system cells
adrenal medulla

Question 15

tissues deriving from the endoderm

Answer 15

lungs
pancreas
thyroid

Question 16

3 subsections of the mesoderm and the tissues derived from each

Answer 16

1. PARAXIAL:
   -vertebrates, ribs, muscle, dermis
2. INTERMEDIATE:
   -kidneys and gonads
3. LATERAL (2 laminae):
   -splanchnic (blood vessels, heart, adrenal cortex)
   -somatic (smooth muscle, dermis, cartilage, bone)

!! 1 to 3 are closest to further from the central notochord

Question 17

Left-Right Axis Assymtry definition and 2 types

Answer 17

def: process that regulates the morphological symmetry/ assymetry of the body tissues / organs as needed

1. NODAL FLOW MODEL: occurs in the cells of the P.node –> some of them have primary cilia that generate a nodal flux in a precise direction via rotation (by the release of SHH, TGF-beta)
2. MECHANORECEPTOR MODEL: immotile primary cillia thatdrive fluid which activates calcium influx and differently stimualtes cells

Question 18

pathology stemming from faulty L/R axis assymetry mechanism

Answer 18

SITUS INVERSUS: subjects have complete or partial reversed distribution of organs on the left or right side

Question 19

Pathology stemming from insufficient gastrulation

Answer 19

SIRENOMELIA
premature regression of the primitive streak and hence incorrect distribution of info of the mesoderm
leads to defects in limb morphology (leg fusion)

Question 20

Neurulation definition

Answer 20

Process leading to the formation of the neural tube and crest which differentiates into the tissues of the nervous system

Question 21

What is the chemical that induces neurulation and what happens if its not released

Answer 21

SHH which is released by the notochord

!! the SHH inhibits the Wnt pathway, and if it was NOT inhibited the neural plate and crest would become the epidermis

Question 22

Cells that are derived from the neural tube

Answer 22

NEUROECTODERM

components of the CNS, neurones and CNS gial cells

!! not microglial cells bcos they are macrophages and are derivateives of monocytes

Question 23

Cells that are derived from the neural crest

Answer 23

4TH GERM LAYER

-PNS components, Scwann cells, dorsal root ganglia, autonomic NS ganglia
-pia and dura mater (of meninges)
-dermis and cartilage of head and neck
-melanocytes in basal layer of the epidermis
-cells of medulla (modified neurones)

Question 24

What types of folding occur in the 4th week of embryonic development

Answer 24

1. cranial folding
2. caudal folding
3. lateral folding : to squeeze the yolk sac and create the primitive intestine