Early Development

Question 1

What is fertilisation/conceptual age?

Answer 1

The time from the point of fertilisation

Assumed to be one day from the last ovulation

Question 2

What is gestational age?

Answer 2

The time from the beginning of the last menstrual period
Fertilisation age +14 days
Can be measured by comparing embryo size charts

Question 3

What is carnegie stage?

Answer 3

This is a measure of 23 stages of embryo development by comparing embryo features (i.e. first heart beat)

Question 4

State are the 3 stages in embryo development

Answer 4

Embryogenic
Embryonic
Fetal

Question 5

What occurs during the embryogenic stage (give time)

Answer 5

2 weeks post fertilisation
This is the establishment of the early embryo
There are 2 cell populations: pluripotent embryonic cells and extraembryonic cells

Question 6

What is occurs during the embryonic stage (give time)?

Answer 6

2-8 weeks after fertilisation

The establishment of the 3 germ layers and the body plan

Question 7

What occurs during the fetal stage and when?

Answer 7

Trimesters 2+3
The formation of organ systems
Migration of organs to final destination
Acquisition of fetal viability

Question 8

Outline the cell stages of development (include cell numbers)

Answer 8

Ovulated oocyte - 1 cell
Fertilised zygote - 1 cell
Embryo cleavage - 2-8 cells
Morula - 16 cells
Blastocyst - 200-300 cells

Question 9

What is the maternal to zygotic transition?

Answer 9

A shift away from maternal to zygotic RNA at the 4-8 cell stage

Question 10

What occurs during compaction?

Answer 10

Outer cells become compressed and more wedge shaped as the inner cells grow and compress against the zona pellucida
2 cell populations form - inner and outer

Question 11

Which 2 cell jucntions form during contraction?

Answer 11

Desmosomes and tight junctions

Question 12

Which type of stem cells make up the inner cell mass?

Answer 12

Pluripotent stem cells - contribute to final organism

Question 13

What is contained within the blastoceal?

Answer 13

Na+ ions which drive water to follow in

Question 14

What is the role of the zona pellucida?

Answer 14

Protects early embryo

Prevents polyspermy

Question 15

What occurs during blastocyst hatching?

Answer 15

There are cellular contractions and enzymatic digestions - allow embryo to continue to grow

Question 16

Briefly outline the role and formation of the syncitotrophoblast

Answer 16

It arises from the trophectoderm
The fusion of trophectoderm cells froms the syncitiotrophoblast
It invades the maternal endometrium and breaks capillaries to establish a blood supply

Question 17

What is the role of the cytotrophoblast?

Answer 17

They provide a source of syncitiotrophoblast cells

Question 18

What 2 things does the inner cell mass divide into?

Answer 18

Epiblast - form the fetal tissue

Hypobalst - forms the yolk sac

Question 19

Which hormone is released from the syncitiotrophoblast?

Answer 19

beta hCG

Question 20

Describe bilaminar disc formation?

Answer 20

Amniotic cavity separates the cells of the epiblast

The upper layer forms the amnion and cells of epiblast and hypoblast are separated by amniotic cavity

Question 21

What is gastrulation?

Answer 21

The formation of the 3 germ layers and establishment of the body axis

Question 22

What occurs during gastrulation?

Answer 22

Primitive streak form through the bilaminar disc
Cells from the epiblast move through the streak and it becomes a pit
This forms the definitive endoderm and hypoblast is displaced
Cells in the middle and top become the mesoderm and ectoderm respectively

Question 23

Which organs arise from the endoderm?

Answer 23

Liver
Pancreas
Lung 
GI tract
Thyroid

Question 24

What arises from the ectoderm?

Answer 24

Tooth enamel
Skin epithelia
CNS
Neural crest

Question 25

What arises from the mesoderm?

Answer 25

``` Blood Muscles Gonads Adrenal cortex Kidneys Bone and cartilage ```

Question 26

What is the notochord and where does it develop?

Answer 26

It is a rod like structure made from cartilage like cells. It is the organising centre for neurulation Develops on the ectoderm

Question 27

Outline what occurs during neurulation?

Answer 27

Notochord releases signals which cause the ectoderm to invaginate, forming the neural crest. Neural ridges form and they move over the neural grove and eventually fuse

Question 28

What cells are found in the neural folds?

Answer 28

Neural crest cells which eventually migrate from the folds

Question 29

When do the head and tail ends close?

Answer 29

Head closes at d23 | Tail closes at d 27

Question 30

What 2 conditions arise due to failure of the neural plates to fuse?

Answer 30

Anencephaly - failure to close at the head end (born without brain and most of skull) Spinal bifida - failure to close at the tail end (lower spine)

Question 31

What do neural crest cells migrate into?

Answer 31

``` Cranial NC Cardiac NC Sacral NC Vagral NC Trunk NC ```

Question 32

What defects can arise due to failure to migrate?

Answer 32

Deafness Pigmentation disorders Cardiac and facial defects Gut innervation failure

Question 33

What are somites and what is somitogenesis

Answer 33

Somitogenesis is the production of somites | Somites are paired blocks of mesoderm

Question 34

What is rate of budding in humans?

Answer 34

1/90mins

Question 35

What type of development measuring can somite number be used in?

Answer 35

Carnegie staging

Question 36

What 2 types of tissue do somites form initially?

Answer 36

Sclerotome - vertebrae and rib cartilage | Dermomyotome - dematome is skin, fat and connective tissue; myotome is muscle

Question 37

What 2 types of folding occur in primitve gut formation?

Answer 37

Ventral folding - head and tail end curl together | Lateral folding - the two sides of the embryo roll

Question 38

What is the result of this folding?

Answer 38

Pinches off part of the yolk sac which forms the fore/mid/hindgut

Question 39

What are the derivatives of the primitive gut?

Answer 39

Foregut - upper duodenum, pancreas, liver, gall bladder, stomach, esophagus Midgut - lower duodenum, rest of SI, ascending colon, 2/3 of transverse colon Hindgut - 1/3 of transverse colon, descending colon, rectum and upper anal canal

Question 40

Describe the derivation of the heart

Answer 40

From the mesoderm | Heartbeat detectable from 6 weeks

Question 41

Describe the derivation of the lungs

Answer 41

Arises from the endoderm - lung bud | Lung bud progressively splits into 2 from 4 weeks

Question 42

Describe the derivation of the gonads

Answer 42

Arises from the mesoderm as bipotential - not committed to male or female - forms gonadal ridge XY embryos - SRY gene triggers sertoli cells to develop into testes and leydig cells from XX embryos - absence of SRY gene causes granulosa formation and ovary formation - reinforced by FOXL2 gene