DA2

Question 1

Fate of embryonic epiblast cells

Answer 1

• Those on midline form the primitive streak
• Some displace hypolast to form endoderm
• Some migrate between epiblast and endoderm to form mesoderm
• The rest become ectoderm

Question 2

Roles of primitive streak

Answer 2

• Establishes the longitudinal (cranial-caudal) axis of embryo
• Is the master organizer of mesoderm as segregation of epiblast cells occus according to area of streak they migrate
  
  • Through primitive node - form notochord
  • Caudal to node - form paraxial mesoderm
  • Through midstreak - form intermediate mesoderm
  • Caudal to streak - form lateral plate mesoderm

Question 3

Neurulation

Answer 3

Establishment of nervous system

• Notochord induces formation of neural plate
• Folds of neural plate fuse to establish neural tube

Question 4

Roles of notochord

Answer 4

• Inductor of CNS
• Supporting structure for developing embryo
• Formation of vertebral column (persists in adults as nucleus pulposus)

Question 5

Neurulation: Neuropores

Answer 5

• Temporary communication of neural tube with amnion
• Neural tube differentiates to form brain vesicles, spinal cord

Question 6

Neurulation - Neural Crest Cells

Answer 6

Break free from the crest of neural plate

Question 7

Derivatives of pluripotent crest cells

Answer 7

• Sensory and autonomic ganglia
• Adrenal medulla
• Melanocytes
• Connective tissue structures of the head and face

Question 8

Paraxial Mesoderm

Answer 8

• Becomes segmented into paired somites
• Formed successivley, unidirectionally
• Number of pairs is species-specific
• Used to estimate age of embryo

Question 9

Somites

Answer 9

the foundation of metameric organization of body

Question 10

Intermediate mesoderm differentiates into_______.

Answer 10

Urogenital structures

Question 11

Lateral plate mesoderm divides into two layers:

Answer 11

1. Dorsal layer associates with ectoderm to form parietal pleurae/peritoneum
2. Ventral layer associates with endodern to form visceral pleurae/peritoneum

Question 12

Space between the dorsal layer and ventral layer of the divded lateral plate mesoderm

Answer 12

Intraembryonic body cavity (coelom)

Question 13

Establishment of cylindrical body form

Answer 13

Initiated by differential growth and body foldings

• Head fold: brain extends into amniotic cavity and folds ventrally
• Tail fold: proceeds cranially
• Lateral folding: proceeds bidirectionally
• Result: incorporation of coelom and yolk sac into the body

Question 14

Body form result:

Answer 14

Tube-within-a-tube cylindircal body

• Outer tube represents the body wall
• Inner tube represents the primitive gut
• Space between the two represents the body cavity
• Middle part of the tube remains temporarily
• In fetal stage olk sac is replaced by umbilical cord

Question 15

Answer 15

• A - communication of intraembryonic and extraembryonic cavities
• B - narrowing of the wide connection between the two cavities
• C - completion of cylindrical body
• Figure C:
  
  • 1 - Body cavity
  • 2 - Primitive gut
  • 3 - Body wall

Question 16

Placodes

Answer 16

Ectodermal thickenings in head region; form sensory organs

Question 17

Pharyngeal arches

Answer 17

agregations of mesoderm in head region

Question 18

Basic Processes of Development: Growth

Answer 18

• Most outstanding event in early development
• Differential growth - accounts for diversity of form
• Growth rate of different parts are relatviely constant within same species
• Growth is controlled and made possible by a number of factors (constitutional, nutritional, hormones, vitamins, growth-arresting)

Question 19

Basic Processes of Development: Differentiation

Answer 19

Development of specialized cell types

• Is antagnoistic to growth
• Is determined by cell’s genome and its position in embryo
• Is determined by interactions and effective communication between cells
• Induction: a group of cells directs differentiation of other cells (essential part of cell differentiation)

Question 20

Stages in cell differntiation

Answer 20

• Uncommited (totipotent) cell: to 8-cell morula stage
• Commited (pluripotent): cell with restricted developmental potential

Question 21

Phases of commitment

Answer 21

• Specification: cell can differentiate autonomously
  
  • Transitional commitment –> reversible commitment (cloning)
  • A cell can be re-programmed to give rise to any cell type
• Determination: stage of irreversible commitment
  
  • Conditional commitment –> regulative development
  • The halved number of cells alter their fat to produce a complete organisms
  • Critical in development of identical twins

Question 22

Basic Processes of development: Morphogenesis

Answer 22

• A mass of cells becomes a structure morphogenic field - a subset of cells forming an organ (each field - controlled by tissue-master genes)

Question 23

Basic Morphogenic Processes

Answer 23

1. Cell migration: controlled and influenced by interaction with their environment
2. Cell aggregation: cells with like tissue-receptors adhere to form compact masses
3. Splitting - cells delaminate and reaggregate to form new layers
4. Fusion - growth together of two localized areas
5. Cell Death - apoptosis, essential for shaping complex organs
6. Folding - outward (evagination, outpocketing); inward (invagination, inpocketing); linear (groove)