Agenesis
Complete failure of organ to develop (at all)
- Di George syndrome - failure of developement of third and fourth pharyngeal pouches (leads to no thymus and parathyroid glands)
Aplasia
Failure to attain size or function - recognizable tissue that has failed to fully develop
- Biliary atresia - failure to develop a lumen within the biliary tree!
Atrophy
Reduction in normal cell size or number. (Cf hypertrophy - increase in size, hyperplasia - increase in number)
- Physiological
- Childhood: thymus, umbilical vessels
- Adulthood: Menopausal uterus/vagina
- Pathological
- Starvation, ischaemia, disuse (immobilisation)