Cells have long been known to proliferate and yet the manner in which tissues grow is still poorly understood. Some earlier workers were correct in assuming that the mitotic figures present in some adult tissues were either the expression of general growth in size or local growth of some structures (as in hair follicles). [...]

The histology of the seminiferous epithelium of the rat has been repeatedly investigated since the early work of Von Ebner (1871). These studies have led to the formulation of widely accepted concepts and have produced an extensive terminology. [...]

Classical investigations conducted on the development and morphology of the rat dentition have clarified the main phases of tooth growth (Addison and Appleton, '15; Andrews, '19; Noyes et al, '38; Schour and Massler, '42; Orban, '44; Maximow and Bloom, '50; Ham, '53). The advent of radio-isotopes provided a new tool, which when used in tracer amounts, is more physiologically compatible to the living body than any of those commonly used in the past. In addition to this advantage, the use of a radio-active element offered increased precision of the experimental results. [...]

The effect of mechanical forces on the structure of bone has intrigued biologists for many years. As early as 1838 Ward recognized the resemblance of the structure of the femur to a crane, and the appropriateness of the form of the femur to withstand the possible forces to which it was subjected. Since it has been universally assumed that bone must withstand mechanical forces, many investigators have analysed and tried to relate the structure of bone to the manner in which these forces are distributed throughout the bone.

In animal life, growth manifests itself in many ways, and accordingly, has been defined from many points of view. Richard and Kavanagh (1945) define growth as "an attribute of living organisms manifested by a change in size of the individual or in the number of organisms in a unit of environment". Sholl (1950) and Medawar (1950) agree with this definition while some authors have considered certain important chemical processes to be characteristic of growth, for instance protein synthesis (Boell, 1955) and DNA synthesis (Davidson, 1950). Weiss (1949), bearing in mind these varied attempts to grasp the concept of growth, wrote the following comment.

The science of histochemistry is of primary importance in understanding natural phenomena, for it is a science which permits the visualization of the chemistry of a cell component in relation to the cell itself, to the organ of which the cell is a part, and often to the body as a whole. The histochemist is therefore often able to understand the intimate relationships of biological systems better than the chemist or biochemist who is involved with the study of isolated systems.

With the advent of radioautography, a technique was founded by which to correlate structure and function. The use of radioiodine, by means of radioautography, has helped to elucidate the anatomical sites, wherein take place the successive metabolic steps in the formation and secretion of thyroid hormone. Thus, the accumulation of inorganic iodide and its organification were shown to occur within the follicle lumen or colloid (Nadler and Leblond, 1955). [...]

The adult mammalian thyroid (122) is a bilobed, highly vascular, solid organ related to the pretracheal fascia which binds it to the larynx and trachea. It is invested by a true fibrous capsule, which is continuous with the stroma of the gland. The thyroid consists of a pair of lobes joined across the anterior aspect of the trachea by the isthmus. Each lobe extends proximally along the lateral aspect of the thyroid cartilage, and distally along the upper part of the trachea. The lobes are related medially, to the thyroid and cricoids cartilages, to the trachea and oesophagus, and to the external and recurrent laryngeal nerves.

Progress in the investigation of nucleic acids has been unexpectedly rapid during the past decade. They, like proteins, carbohydrates and fats, have been recognized as indispensable constituents of all living organisms and, indeed, of all tissues. Two, and only two, kinds of nucleic acids, deoxyribonucleic acid (DNA) and ribonucleic acid (RNA), have been identified in living tissues. The morphological evidences of nuclear conjugation at fertilization, and the succeeding nuclear divisions led to the belief that the main genetic material is in the nucleus and consists of the nuclear chromatin.

This thesis reports an attempt to measure the duration of the period during which cells duplicate their nuclear complement of deoxyribonucleic acid (DNA) in preparation for mitosis, in the epithelia of the tongue, oesophagus, and intestine of the rat. Information was also sought on the duration of other stages in the cell cycle, and on the renewal time of the entire cell population of these epithelia. The nucleoside thymidine, labelled with tritium (H3), was used as a DNA precursor, and labelled nuclei were detected by radioautography.