The tracheolar cells form the last branches and tips of the tracheal tubes

Tracheoles are blind ending tubes, that are part of a single cell, the tracheoblast. The tracheoblast is a special cell of the tracheal hypodermis (cell layer that surrounds the tracheal tube). One tracheoblast can form multiple tracheolar endings. During molt and ecdysis the tracheoblasts start to produce branches and tips when they receive a signal from the surrounding tissue that oxygen gets low (Hypoxia Induced Factor: HIF). The newly formed branches penetrate the tissues to provide them with air. Sir Vincent Wigglesworth was one of the first scientist who detected that the tracheoles can be filled with water. The water in the tubes changes the ability of the tracheoles to supply cells with oxygen (conductance). The more oxygen is needed from the cells the less water is in the tracheoles.

 

Tracheoles are so tiny (0.1 - 1 micrometer) that they can penetrate tissues. Sometimes they are adjacent to the mitochondria of a cell, separated only by a thin cell membrane. The image shows a cross-section through a flight muscle of a fruit fly. The small circular tracheoles with the dark walls penetrate the muscle tissue and are in close vicinity to the muscle fibers (large light grea fields) and especially of the mitochondria (dark grey fields). Thus, structures of oxygen supply (tracheoles) are adjacent to structures of oxygen consumption (mitochondria).

 

A nice way to visualize Tracheoles is by using their autofluorescence that can be excited with a confocal laser scan microscope. The tracheoles are evenly distributed throughout the tissues. The image shows the distribution of tracheoles in a leg muscle of the darkling beetle Tenebrio molitor that branch from small tracheae. The scale bar has a size of 50 micrometers.