Rucete ✏ Campbell Biology In a Nutshell
Unit 7 ANIMAL FORM AND FUNCTION — Concept 47.3 Cytoplasmic Determinants and Inductive Signals Regulate Cell Fate
The development of an animal body plan depends on coordinated gene expression that determines cell fate, establishes body axes, and organizes tissues into organs. Mechanisms like cytoplasmic determinants, induction, and signaling pathways control these events across species.
1. Cell Fate and Cytoplasmic Determinants
- Determination commits a cell to a fate; differentiation expresses that fate via structure and function
- All cells share the same genome but express different gene sets
- Cytoplasmic determinants and inductive signals guide gene expression
- In C. elegans, P granules determine germ cell fate and are asymmetrically distributed in early divisions
2. Fate Mapping and Cell Lineage
- Fate maps trace embryonic regions to their future tissues
- In C. elegans, cell lineage has been fully mapped from zygote to adult
- Adults have 959 somatic cells, with 131 undergoing apoptosis
- This revealed conserved cell death pathways in all animals
3. Axis Formation
- Most animals show bilateral symmetry with defined body axes
- In frogs, the anterior-posterior axis is set during oogenesis
- The dorsal-ventral axis forms from sperm entry and cortical rotation, creating the gray crescent
- The left-right axis forms later, often influenced by cilia-driven fluid flow
4. Restriction of Developmental Potential
- Early cells are totipotent, capable of forming all tissue types
- Potential narrows as development progresses
- In mammals, totipotency extends through the 8-cell stage
- Spemann’s experiments confirmed early cytoplasmic determinants guide axis formation
5. Induction and Pattern Formation
- Induction: cells influence nearby cells using signaling molecules
- Spemann and Mangold showed the dorsal lip of the blastopore acts as an organizer
- The organizer suppresses BMP-4, promoting neural and notochord development
6. Vertebrate Limb Development
- Three axes define limb patterning: proximal-distal, anterior-posterior, dorsal-ventral
- The AER (apical ectodermal ridge) promotes outgrowth via FGF
- The ZPA (zone of polarizing activity) sets A–P pattern using Sonic hedgehog (Shh)
- Adding ZPA or Shh creates mirror-image limbs
- Hox genes control limb identity and segmentation
7. Cilia in Development and Asymmetry
- Monocilia act as sensory antennae for developmental signals like Shh
- Motile cilia create leftward flow to establish L–R asymmetry
- Defects lead to situs inversus (reversed organs), seen in Kartagener’s syndrome
In a Nutshell
Animal development relies on coordinated gene expression and signaling pathways that guide cell fate, tissue organization, and body plan establishment. From cytoplasmic determinants and induction to axis specification and morphogen gradients, these conserved mechanisms transform a zygote into a structured, functional organism.