Topic: disturbances of growth and cancer,
Question: Enumerate four disturbances of growth. Describe the mechanism of atrophy.
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four main disturbances of growth:
1. Atrophy – Decrease in cell size due to decrease in cell contents and organelles. Can occur due to disuse, malnutrition, denervation, or aging. Cells shrink in size.
2. Hypertrophy – Increase in cell size due to increased intracellular contents and organelles. Seen in cells like cardiac and skeletal muscle in response to increased work demands.
3. Hyperplasia – Increase in number of cells. Occurs in tissue repair and in endocrine hyperstimulation. Total mass of tissue increases.
4. Metaplasia – Change of one mature cell type into another mature cell type. For example, squamous metaplasia of bronchial epithelium in smokers.
ATROPHY
Is reduced size of an organ or tissue resulting from a decrease in cell size and number.Atrophy can be physiologic or pathologic.Physiologic atrophy is common during normal development. Some embryonic structures, such as the notochord and thyroglossal duct, undergo atrophy during fetal development. The uterus decreases in size shortly after parturition, and this is a form of physiologic atrophy.
Pathologic atrophy depends on the underlying cause and can be local or generalized. The common causes of atrophy are the following:- Decreased workload (atrophy of disuse). – Loss of innervation (denervation atrophy). – Diminished blood supply. – Inadequate nutrition. – Loss of endocrine stimulation. – Pressure.
The fundamental cellular changes associated with atrophy are identical in all of these settings. The initial response is a decrease in cell size and organelles, which may reduce the metabolic needs of the cell sufficiently to permit its survival. In atrophic muscle, the cells contain fewer mitochondria and myofilaments and a reduced amount of rough ER.
Mechanisms of Atrophy
Atrophy results from decreased protein synthesis because of reduced metabolic activity and increased protein degradation in cells, occurs mainly by the ubiquitin-proteasome pathway. Nutrient deficiency and disuse may activate ubiquitin ligases, which attach the small peptide ubiquitin to cellular proteins and target these proteins for degradation in proteasomes
In many situations, atrophy is also accompanied by increased autophagy, with resulting increases in the number of autophagic vacuoles.(Autophagic vacuoles are membrane-bound vacuoles that contain fragments of cell components. The vacuoles ultimately fuse with lysosomes, and their contents are digested by lysosomal enzymes.)
Some of the cell debris within the autophagic vacuoles may resist digestion and persist as membrane-bound residual bodies that may remain as a sarcophagus in the cytoplasm. When present in sufficient amounts, they impart a brown discoloration to the tissue (brown atrophy). Autophagy is associated with various types of cell injury, and we will discuss it in more detail later.
