1. Structural Features making endospores highly resistant:
Endospores are dormant structures formed by certain bacteria, allowing them to survive harsh environmental conditions. The key structural features that contribute to their resistance include:
- Spore coat: A multi-layered protein coat that provides physical protection against chemicals and enzymes.
- Cortex: A layer located beneath the spore coat, consisting of peptidoglycan, which provides structural rigidity.
- Core: The innermost part of the endospore containing the bacterium's DNA and essential components.
- Small acid-soluble proteins (SASPs): Proteins that bind to DNA, protecting it from damage during dormancy.
- Dipicolinic acid (DPA): A molecule present in high concentrations in the endospore core, which provides resistance against heat and desiccation. 

2. Triggers and Environmental Signals initiating transition from spore to vegetative cell:
When environmental conditions become favorable, endospores can transition back to their vegetative, metabolically active form. This process, known as germination, can be initiated by several triggers and environmental signals, including:
- Nutrient availability: The presence of specific nutrients can signal the endospore to germinate.
- Heat: Elevated temperatures can induce germination in some endospore-forming bacteria.
- pH changes: Alterations in pH levels can stimulate germination.
- Certain enzymes: Enzymes produced by other bacteria can trigger germination by breaking down the spore coat. 

3. Contribution of bacterial endospores to pathogenesis:
Bacterial endospores play a significant role in the pathogenesis of certain diseases. These dormant structures allow bacteria to survive in adverse conditions, persist in the environment, and initiate infections when favorable conditions arise. Some examples of pathogenic bacteria producing endospores include: 

- Clostridium tetani: The bacterium responsible for tetanus forms endospores in the presence of oxygen, allowing it to survive in soil and contaminated objects. When introduced into deep tissue wounds, these endospores can germinate and release toxins, causing tetanus. 

- Bacillus anthracis: The causative agent of anthrax, Bacillus anthracis, produces highly resistant endospores. These spores can survive in the soil for long periods and can cause disease when inhaled, ingested, or exposed through cuts or abrasions. 

- Clostridium botulinum: This bacterium produces endospores that can survive in various environments and is responsible for botulism. In an anaerobic environment, such as improperly preserved canned food, the endospores can germinate and produce potent neurotoxins. 

These examples highlight how bacterial endospores contribute to the pathogenicity of certain diseases.