Rucete ✏ Campbell Biology In a Nutshell
Unit 7 ANIMAL FORM AND FUNCTION — Concept 47.2 Morphogenesis in Animals Involves Specific Changes in Cell Shape, Position, and Survival
Morphogenesis, the biological process that gives shape to the animal body, involves dramatic cellular movements, changes in shape, and programmed cell death. It begins with gastrulation and continues into organogenesis, laying the groundwork for tissues, organs, and body plans.
1. Gastrulation: Establishing the Germ Layers
- Gastrulation transforms the blastula into a gastrula with 2 or 3 germ layers
- Involves cell movements like invagination, involution, and migration
- Forms ectoderm, mesoderm, and endoderm—each with distinct developmental fates
- The blastopore becomes the mouth (protostomes) or anus (deuterostomes)
2. Gastrulation in Frogs
- Begins at the dorsal lip of the blastopore via invagination
- Involution moves cells inward to form mesoderm and endoderm
- The archenteron forms as the blastocoel shrinks
- Results in concentric arrangement of the three germ layers
3. Gastrulation in Birds and Mammals
- Birds: epiblast cells migrate through the primitive streak to form germ layers
- Humans:
- Embryo implants around day 7
- Epiblast forms embryo; trophoblast aids implantation and placenta formation
- Gastrulation begins ~day 13 via the primitive streak
- Forms four extraembryonic membranes: amnion, chorion, yolk sac, allantois
4. Organogenesis and Neurulation
- Organogenesis begins post-gastrulation; germ layers differentiate into organs
- Neurulation:
- Notochord induces the neural plate to form
- Neural plate folds into the neural tube → brain and spinal cord
- Neural tube defects like spina bifida can occur if closure fails
- Somites form from mesoderm and establish body segmentation
5. Morphogenetic Mechanisms
- Cell shape changes (e.g., apical constriction) bend tissues inward
- Convergent extension: cells elongate and intercalate to lengthen tissues
- Cell migration: guided by cytoskeleton and ECM (e.g., neural crest cells)
- Apoptosis sculpts tissues (e.g., removes webbing or tails)
6. Evolutionary and Functional Context
- Extraembryonic membranes evolved to support reproduction in dry environments
- Each membrane serves key roles: chorion (gas exchange), allantois (waste), yolk sac (nutrition), amnion (protection)
- Apoptosis and shared developmental genes reflect evolutionary conservation
In a Nutshell
Morphogenesis transforms a simple blastula into a complex, layered embryo through precise changes in cell shape, movement, and death. Gastrulation establishes the germ layers, and neurulation forms the nervous system. These processes are remarkably conserved across animals, showcasing both the unity and diversity of developmental biology.