Section 1: Cells

Ronald A. Bergman, Ph.D., Adel K. Afifi, M.D., Paul M. Heidger, Jr., Ph.D.
Peer Review Status: Externally Peer Reviewed

Plates

Through the process of cell division, differentiation, and specialization, four basic tissues arise from a single cell, the fertilized ovum. These tissues, the epithelial, connective, muscular, and nervous tissues, carry out all the diverse functions essential for life. The cells that constitute these basic tissues share certain common characteristics but also differ strikingly in their size, shape, organelle content, and function. General and special techniques have been developed by the biologist to visualize cellular structure and to establish functional correlates. The photomicrographs in this section were selected to reveal specific organelles and inclusions common to most cell types as demonstrated to advantage by a variety of preparative and staining methods. Cytological structural/functional correlations of the four basic tissues will be emphasized in subsequent sections that deal with the specialized cells that form the organs. Understanding cellular function depends upon the recognition of the role played by each component part of the cell. Examples follow.

In general, all cells possess:

1. a cell membrane or plasmalemma;
2. one or more nuclei with nucleoli containing primarily deoxyribonucleic acid (DNA) and ribonucleic acid (RNA), respectively;
3. cytoplasmic RNA;
4. a Golgi apparatus;
5. membranes in the form of vacuoles or saccules;
6. mitochondria; and
7. energy stored in the form of glycogen and lipid.

The plasmalemma, demonstrated by electron microscopy to be about 100 Angstroms (10 nm) in thickness, cannot be resolved, per se, by light microscopy, because the use of visible light as the illuminating source limits resolution to about 2750A (0.275 µm*). However, the plasmalemma together with associated connective tissue and surface polysaccharide coat may be stained and resolved as the cell boundary under certain conditions. Such images are present in Plates 15, 17, and 64 and elsewhere in this book.

The nucleus is of special importance in understanding cell function. Because it is large enough for detailed examination by the light microscope when stained even by routine methods (such as hematoxylin and eosin, H. & E.), its varying functional states can be assessed. It has been demonstrated that active DNA does not stain with nuclear stains; the nucleus may thus appear empty except for a nucleolus, which will be stained. Inactive DNA is readily stained with hematoxylin, toluidine blue, and other similar basic dyes. Most nuclei contain varying amounts of functional (active) and nonfunctional (inactive) DNA. The stainable DNA may appear in clumps or may be in a reticulated pattern. The functional DNA is termed euchromatin, whereas the nonfunctional, or inactive, DNA is called heterochromatin. The nerve cell nucleus seen in Plate 1 contains no stainable DNA, which indicates its active involvement in the metabolism of the cell. By contrast, the densely stained heterochromatin seen in the nucleus of the maturing red blood cell (or erythrocyte), Plate 1, signals the termination of nuclear involvement in the cytoplasmic synthesis of hemoglobin. Such nuclei are called pyknotic. In the case of the red blood cell, the useless heterochromatin is eventually ejected from the cell and phagocytized by macrophages (Plate 54). During cell division, the stainable, inactive DNA appears in the form of threads or rods called chromosomes (Plates 3 and 4).

The nucleus also contains one or more nucleoli, which stain routinely with one of the nuclear stains cited previously. The nucleolus consists principally of RNA and is the source of cytoplasmic RNA (Plates 1 and 26).

The cytoplasm of most cells contains some RNA that may not be detectable by routine methods. In these instances, it is likely that the protein synthesis related to this RNA is mainly associated with the maintenance and repair of cellular structures or organelles. In certain instances, however, the cytoplasm contains a significant amount of RNA that is readily stained and can be directly related to some specific function, such as the elaboration of digestive enzymes (Plate 5). In certain nerve cells, cytoplasmic RNA appears as specific blue-staining (so-called basophilic) patches called Nissl bodies (Plate 1). In these two examples, the staining pattern is a permanent and recognizable feature of the normal cell. It has been detected by electron microscope that, in these cases, the RNA is bound to cytoplasmic membranes. In the developing red blood cell and muscle fiber, however, the RNA is not membrane-bound and gradually disappears when these cells become structurally and functionally mature (Plate 1).

The Golgi apparatus seen in Plate 7 is well developed in cells actively engaged in protein synthesis and secretion, and its role is well understood in the enzyme-producing pancreatic acinar cell. Proteins synthesized through the interaction of nuclear, nucleolar, and cytoplasmic nucleic acids are first concentrated in the sacs of the Golgi apparatus in the form of granules or droplets. Except for protein glycosylation and conversions of proproteins, it is unlikely that the Golgi apparatus is directly involved enzymatically in synthetic activity of the cell and appears to be "packaging" the secretory product for transport to the extracellular space. Although this organelle was first convincingly demonstrated by Golgi in nerve cells, its precise role in these cells is not completely understood.

The cytoplasm of many mature cells contains little RNA, and, when these cells are stained with hematoxyln and eosin, the most widely used combination of stains, the cytoplasm binds the eosin and appears red. In these cells, functions other than protein synthesis predominate. The parietal cell of the stomach, which elaborates hydrochloric acid, is an example of such an eosinophilic (or acidophilic) cell (Plate 5). The cytoplasm of this cell contains numerous mitochondria and membranes, but little cytoplasmic RNA.

Mitochondria are found in all cells except the mature red blood cell. They vary in number, size, shape, and distribution, depending upon cell type and its specific energy requirements. Mitochondria are membranous sacs with membranous partitions to which enzymes may be tightly or loosely bound and which are themselves integral component parts of the organelle. This organelle produces the energy- rich and ubiquitous adenosine triphosphate necessary for synthetic and other cellular functions such as muscular contraction and active transport. Additional details will be found in the legends to Plates.

The substrates utilized by the mitochondrial enzymes in the elaboration of energy-yielding compounds include stored glycogen and lipid droplets. These cellular inclusions are seen in Plates 8, 9, and 12. Other inclusions found in certain cells are pigment granules (Plates 11 and 13), protein granules (Plates 52 and 136), and phagocytized dust (Plate 30).

Other specialized organelles are associated with specific cell types; these will be discussed in subsequent sections of this atlas.

*1 µm = 1/25,400 inch = 1/1000 mm = 10,000 Å= 1000nm.

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