Embryonic Disc Formation

Question 1

By what week post-fertilisation have many of the major organ systems formed?

Answer 1

8 weeks

Question 2

Describe the formation of a Morula

Answer 2

Day 1 - 4

Morula forms by repeated mitotic divisions

Question 3

Describe the formation of a Blastocyst

Answer 3

Day 5

Blastocyst forms: Trophoblast and Embryoblast (Inner cell mass)

Question 4

What happens on day 6?

Answer 4

Implantation

Question 5

What needs to disintegrate before implantation?

Answer 5

Zona Pellucida

Question 6

What happens on day 8?

Answer 6

Amniotic Cavity opens up
Bilaminar disc: Epiblast (Top)
Hypoblast (Bottom)

Question 7

What happens on day 9?

Answer 7

Primary Yolk Sac forms below hypoblast

Question 8

What is a syncytium?

Answer 8

A single cytoplasmic mass containing multiple nuclei

Question 9

What cell layers will eventually form the placenta?

Answer 9

Extraembryonic mesoderm, Syncytium and Cytotrophoblasts

Question 10

What happens on day 12?

Answer 10

Extraembryonic mesoderm develops and forms cavities within (Will eventually form Chorionic Cavity)

Question 11

What happens on day 13?

Answer 11

Connecting stalk forms and chorionic cavity fills with fluid.

Question 12

How is the trilaminar disc formed?

Answer 12

Week 3 - Gastrulation
Rapid epiblast cell division at the primitive groove and downwards migration forms 3 layers: Ectoderm, Mesoderm and Endoderm.

Question 13

What is the Primitive node and pit?

Answer 13

Week 3 - Neural Tube Formation
Raised area of epiblast. Cells migrate from the node to form the Notochord.

Tip: Node = ‘Nodochord’

Question 14

What are the derivatives of the Endoderm?

Answer 14

Lining of gut tube, respiratory system and urinary system

Question 15

What are the derivatives of the Ectoderm?

Answer 15

Skin & neural tissue

Question 16

What are the derivatives of the Mesoderm?

Answer 16

Organs and muscles

Question 17

Define Somites

Answer 17

Somites form parts of the axial skeleton (skull, vertebrae, ribs), associated musculature and the dermis of the skin

Question 18

Describe the 2 directions of embryonic folding

Answer 18

Longitudinal - Moves heart, brain, mouth & anus into an adult position (process called Reversal)
Lateral - draws amniotic cavity around the embryonic disc

Question 19

Briefly describe gut tube folding

Answer 19

The gut tube grows rapidly and herniates out of the umbilical region. The gut tube rotates 270º anticlockwise along the axis of the SMA

Question 20

Name 2 pathologies of gut tube development

Answer 20

Omphalocele - The gut tube may not fully return to the abdomen
Gastroschisis - Gut tube may herniate through the abdominal wall

Question 21

What is a volvulus

Answer 21

Twisting of a loop of intestine and supporting mesentery resulting in bowel obstruction.

Question 22

Name the remnants of the vitellointestinal duct and their clinical significance.

Answer 22

Cyst, band, fistula, sinus or diverticulum
Clinical:
Cyst - protrusion from umbilicus
Fistula - faeces leaking from umbilicus
Meckel’s Diverticulum - can inflame and cause tenderness

Question 23

What is the cloaca?

Answer 23

The cloaca is separated by the urorectal septum into the urogenital sinus (developing bladder) and rectum

Question 24

What problem(s) can occur if the urorectal septum does not form properly?

Answer 24

Imperforate Anus

Question 25

What are pharyngeal arches and what to they form?

Answer 25

4 Interconnecting loops of ectoderm, mesoderm and endoderm (lower face and neck)

Question 26

What layer does the notochord form within?

Answer 26

Mesoderm - creates a longitudinal axis in the embryo

Question 27

What does the 1st pharyngeal arch form?

Answer 27

Nerve supply: CNV (Trigeminal nerve)
Muscles: e.g. Mastication and tensor tympani
Bones: e.g. malleus, incus, maxilla, zygoma & mandible