Third Week of Development

Question 1

Characteristics of the third week

Answer 1

Formation of germ ayers, early tissue, orga differentiation

• coincides with the first missed menstrual period
• Includes the formation of the primitive streak, notochord, neural tube, somites
• Beginning of the embryonic period

Question 2

When does the embryonic period begin

Answer 2

Week 3

Question 3

Gastrulation

Answer 3

Process by which the bilaminar embryonic disc is converted to a trilaminar embryonic disc

• Occurs via. the formation of the primitive streak
• Cells of epiblast origin migrate down the prmitive groove to establish the intraembryonic mesoderm and endoderm
• Epiblast is renamed ectoderm

Question 4

What are the trilaminar layers

Answer 4

Intraembryonic mesoderm, intraembryonic endoderm, hypoblast

Question 5

Notochord

Answer 5

• axis for the development of the axial skeleton
• induces the formation of the bodies of the vertebrae
• Part persists as the nucleus pulposus of the intervertebrae discs
• induces the overlying ectoderm to form the neural plate

Question 6

When does neural tube formation occur

Answer 6

Begins in the 3rd week, completes by the end of the 4th

Question 7

Neurulation

Answer 7

• Neural tube formation
• ectoderm thickens to form the neural plate
• Day 18: neural plate invaginates to form the neural groove and neural folds on either side
• End of 3rd week: neural folds fuse, converting the neural groove into the neural tube
• Anterior neuropore closes day 25
• Posterior neuropore closes day 27
• Neural c5rest cells form from margins of folds

Question 8

When does the anterior neuropore close

Answer 8

Day 25

Question 9

When does the posterior neuropore close

Answer 9

Day 27

Question 10

What occurs when neural tubes are open

Answer 10

Amniotic fluid flows through

Question 11

Development of the Somites

Answer 11

• Paraxial mesoderm forms paired, cube-shaped blocks (somites)
• Occurs days 20-38

Question 12

What do somites give rise to

Answer 12

• Vertebrae (sclerotomes)
• Muscles of neck, body wall, limbs (myotomes)
• Dermis of scalp, neck, trunk (dermatomes)

Question 13

Development of the intraembryonic coelom

Answer 13

• Spaces appear in the lateral mesoderm and coaslesce to form the intraembryonic coelom

Question 14

Intraembryonic coelom

Answer 14

Divides into intraembryonic splanchnic mesoderm and intraembryonic somatic mesoderm
- gives rise to the pericardial, pleural, and peritoneal cavities

Question 15

Intraembryonic splanchnic mesoderm

Answer 15

forms the gut wall with the endoderm, derived from the intraembryonic coelom

Question 16

Intraembryonic somatic mesoderm

Answer 16

Forms the body wall with the ectoderm, derived from the intraembryonic coelom

Question 17

What forms the somatopleure

Answer 17

(gut wall)

- Intraembryonic splanchnic mesoderm and the endoderm

Question 18

What forms the splanchnopleure

Answer 18

(body wall)

- Intraembryonic somatic mesoderm and the ectoderm

Question 19

Pericardial cavity

Answer 19

Derived from the intraembryonic coelom, gives rise to the heart

Question 20

Pleural cavity

Answer 20

Derived from the intraembryonic coelom, gives rise to the lungs

Question 21

Peritoneal cavity

Answer 21

Derived from the intraembryonic coelom, gives rise to the gut

Question 22

Vasculogenesis

Answer 22

Formation of new vascular channels by assembly of individual cell precursors (angioblasts)

Question 23

Angiogenesis

Answer 23

Formation of new vessels by budding and branching from preexisting vessels

Question 24

What is the first functional system

Answer 24

cardiovascular system

Question 25

How are secondary villi formed

Answer 25

in week 3, mesoderm grows into the primary villi, converting them to secondary villi

Question 26

How are tertiary villi formed

Answer 26

mesoderm differentiates to form blood vessels, converting secondary villi to tertiary villi