Embryology I

Question 1

define the pre-embryonic, embryonic and fetal periods of human development

Answer 1

Pre-embryonic is 1-2 weeks following fertilisation.
Embryonic is the 3-8 week.
Foetal is 9-38 weeks.

Question 2

Pre-embryonic I

Answer 2

Cleavage: forming 2 identical blastomeres
More cleavage to form morula with totipotent cells
Compaction: Forms blastocyst cavity so blastocyst formed
- 1st differentiation to form trophoblast & embryoblast
Blastocyst hatches from zona pellucida forms conceptus with pluripotent cells
2nd differentiation forms bilaminar disc
- embryoblast = epiblast + hypoblast
- trophoblast = syncytiotrophoblast + cytotrophoblast

Question 3

Pre-embryonic I acronym

Answer 3

Because Men Bewitch Chicks Badly
Blastomeres Morula Blastocyst Conceptus Bilaminar disc

Question 4

Pre-embryonic II

Answer 4

Implantation complete: fibrin plug seals entrance
Embryonic pole: syncytiotrophoblast develops
Abembryonic pole: 1* yolk sac forms
Lacunae + sinusoids connect allowing uteroplacental circulation
2* yolk sac forms by pinching off 1*
Spaces in extra-embryonic mesoderm merge = chorionic cavity
- embryo + cavity suspended by connecting stalk

Question 5

Pre-embryonic II acronym

Answer 5

Ignore Yelling Unless Secretly Confident

Implantation Yolk sac Uteroplacental circulation Secondary yolk sac Chorionic cavity

Question 6

Placenta previa

Answer 6

Implantation in inferior uterine segment
- placenta develops there
- covers cervix
- haemorrhage
- c section

Question 7

Ectopic pregnancy

Answer 7

Implantation elsewhere in the body
e.g. ampulla (common)
- invade pelvic blood vessels
- life threatening haemorrhage

Question 8

Gastrulation

Answer 8

1. primitive streak on epiblast’s dorsal surface at caudal end
   - body midline
   - marks beginning of gastrulation
2. epiblast cells multiply + migrate to primitive streak
3. epiblast cells invaginate through primitive streak + replace hypoblast with trilaminar disc

Question 9

Left vs Right

Answer 9

Ciliated cells at primitive node beat LHS-signalling molecules that determine LHS+RHS

immotile cilia = RHS flow signal molecules

Question 10

Situs inversus

Answer 10

complete mirror image viscera

Question 11

Ectoderm derivatives

Answer 11

organs & structures that maintain contact with outside world
e.g. nervous system, epidermis

Question 12

Mesoderm derivatives

Answer 12

supporting tissues
e.g. muscle, cartilage, bone, vascular system

Question 13

Endoderm derivatives

Answer 13

internal structures
e.g. epithelial lining of GI tract, respiratory tract, parenchyma of glands

Question 14

Neuralation

Answer 14

1. notochord forms
2. notochord signals ectoderm -> neuroectoderm
3. neuroectoderm (neural plate) thickens
4. neuroectoderm edges curl inward -> neural tube
   - cranial ends close first

Question 15

What is segmentation

Answer 15

when mesoderm forms subdivisions

Question 16

Paraxial mesoderm

Answer 16

31 somite pairs that guide innervation

Question 17

Somite pairs

Answer 17

sclerotome -> hard tissues/bone

myotome -> muscle cell precursor

dermatome -> dermis

Question 18

What does folding achieve

Answer 18

• puts heart + primordium of diaphragm in right place
• creates gut primordium
• pulls amniotic membrane around trilaminar disk
  - embryo suspended in sac
• creates ventral body wall
• creates new cavity