DA2 Flashcards
Fate of embryonic epiblast cells
- 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
Roles of primitive streak
- 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
Neurulation
Establishment of nervous system
- Notochord induces formation of neural plate
- Folds of neural plate fuse to establish neural tube
Roles of notochord
- Inductor of CNS
- Supporting structure for developing embryo
- Formation of vertebral column (persists in adults as nucleus pulposus)
Neurulation: Neuropores
- Temporary communication of neural tube with amnion
- Neural tube differentiates to form brain vesicles, spinal cord
Neurulation - Neural Crest Cells
Break free from the crest of neural plate
Derivatives of pluripotent crest cells
- Sensory and autonomic ganglia
- Adrenal medulla
- Melanocytes
- Connective tissue structures of the head and face
Paraxial Mesoderm
- Becomes segmented into paired somites
- Formed successivley, unidirectionally
- Number of pairs is species-specific
- Used to estimate age of embryo
Somites
the foundation of metameric organization of body
Intermediate mesoderm differentiates into_______.
Urogenital structures
Lateral plate mesoderm divides into two layers:
- Dorsal layer associates with ectoderm to form parietal pleurae/peritoneum
- Ventral layer associates with endodern to form visceral pleurae/peritoneum
Space between the dorsal layer and ventral layer of the divded lateral plate mesoderm
Intraembryonic body cavity (coelom)
Establishment of cylindrical body form
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
Body form result:
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
- 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
Placodes
Ectodermal thickenings in head region; form sensory organs
Pharyngeal arches
agregations of mesoderm in head region
Basic Processes of Development: Growth
- 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)
Basic Processes of Development: Differentiation
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)
Stages in cell differntiation
- Uncommited (totipotent) cell: to 8-cell morula stage
- Commited (pluripotent): cell with restricted developmental potential
Phases of commitment
-
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
Basic Processes of development: Morphogenesis
- A mass of cells becomes a structure morphogenic field - a subset of cells forming an organ (each field - controlled by tissue-master genes)
Basic Morphogenic Processes
- Cell migration: controlled and influenced by interaction with their environment
- Cell aggregation: cells with like tissue-receptors adhere to form compact masses
- Splitting - cells delaminate and reaggregate to form new layers
- Fusion - growth together of two localized areas
- Cell Death - apoptosis, essential for shaping complex organs
- Folding - outward (evagination, outpocketing); inward (invagination, inpocketing); linear (groove)
