© 1999 Robert A. Freitas Jr. All Rights Reserved.

Robert A. Freitas Jr., Nanomedicine, Volume I: Basic Capabilities, Landes Bioscience, Georgetown, TX, 1999

8.5.3.1 Overall Cellular Structure

As recently as the late 1970s, the cell was often incorrectly described as a water-filled membranous sac enclosing a loosely structured population of discrete organelles. In reality, the interior of a cell is extremely compact, only a few times more open than a hydrated protein crystal.⁹⁴¹ The cell interior is crisscrossed by many tens of thousands of cytoskeletal filaments of various gauges, attaching organelles to the nucleus, the plasma membrane, the ECM, and to each other.⁹⁴²

Cells also have various shapes. Most cells are surrounded by and are anchored to neighboring cells. Such immobilized cells usually assume a polyhedral shape. Specialized cells adopt shapes related to the specific functions they perform. A dramatic example is the nerve cell, which has one or more cylindrical processes extending from the cell like the branches of a tree, which processes allow the cell to receive and to transmit electrical signals.

From the nanomedical perspective, the cell (Fig. 8.36) may be regarded as a large machine constructed of many smaller machines.¹⁸² These smaller machines are the organelles. Organelles are typically 0.53 microns in diameter, roughly the same size as the largest bloodborne medical nanorobots. Table 8.17 is a very approximate quantification of the known classes of cellular organelles and other major cellular components. Cytoskeletal number densities are crude but self-consistent estimates. Actual data may differ widely from the values given in the table depending upon:

1. size, age, and type of cell;

2. global and local respiration, nutrition and energy demand;

3. tissue and organ location;

4. environmental factors such as temperature, pressure, salinity, ECM activity, and extracytosolic toxicology; and

5. cellular secretory and mitotic status.

Organelles represent the principal metabolic and structural machinery of the cell. Each organelle type or cellular component is engineered to carry out specific functions while maintaining a specific structure, as described briefly below.

The small size of most organelles makes intra-organelle locomotion by whole nanorobots difficult or impossible, with the possible exception of the nucleus (Sections 8.5.4 and 9.4.6). This does not preclude insertion of specialized sensory tools or small manipulatory devices into organelle interiors.

Last updated on 20 February 2003