Cellular biology and cytological interpretation: the philosophy behind the system

Shape and observation

Aside from the mere pleasure of observation, an activity that is in its own way rather satisfying, the ability to extract information from the object observed is based on the axiom that different shapes and colours (of the object observed) correspond to different information.

This concept is at the heart of diagnostic cytology. The person who observes the cells on the slide (the cytologist) can use the morphological features of the cell observed (shape and colour) to classify it and interpret its characteristic biological behaviour.

Morphology, identity and behaviour

If properly interpreted, the different shapes and colours of a cell can provide information about its metabolism and differentiation. Indeed, specific chromatic features of the cytoplasm may indicate a particular cell's metabolic condition. Moreover, certain visible structures can tell us that a cell is dividing (e.g. the presence of a mitotic figure), or that it is undergoing phagocytosis (e.g. the presence of material within the cytoplasm). There are also morphologies that suggest no immediate functional interpretation. Such morphologies are 'structural' and connected to a specific type of cell (e.g. the polylobed nucleus of neutrophils).

It is also true that certain cell types, due to their ability to carry out a highly specialized and predetermined function (differentiation), have 'acquired' certain morphological features that make them unique and recognizable from other cells. Examples are plasma cells, which, due to their constant protein synthesis, display an intensely blue cytoplasm, or, macrophages, whose vacuole-containing cytoplasm is a distinctive feature, as well as an expression of phagocytosis.

This goes to show how from a plethora of shapes one can understand both the 'type of cell being observed', and, at times, 'what it is doing'.

Identity and interpretation

The observation of cellular morphology allows classification of cells into different 'cytotypes', i.e. it enables the cytologist to classify them into a specific category. In diagnostic cytology, it is common to classify cells into three morphological families: epithelial, mesenchymal and discrete, otherwise known as round cells.

Traditionally, these three morphological groups are defined as follows.

• Epithelial cells - usually large and round or polygonal in shape, with readily identifiable cellular margins. These cells usually form clusters, which are aggregates of cells that establish contact by means of membrane-to-membrane adhesion.
• Mesenchymal cells - usually of medium size, they appear elongated, spindle-shaped or pleomorphic. These cells may form aggregates of cells through interposition of matrix.
• Discrete cells - usually small and round, they do not establish contact with each other.

With some obvious exceptions, this classification system is of great diagnostic value, hence, the above-mentioned terms will be referred to several times throughout this book. Within these categories, subtle differences in shape, size, presence or absence of certain structures, location of the nucleus and other significant areas often allow classification of cells into specific 'cytotypes', in other words a cell that Homo sapiens has dignified with a name and surname. For example, the ability to selectively identify various cytotypes is crucial to tests such as differential and absolute cell counts, which often provide valuable diagnostic information.

Behaviour and interpretation

The adaptability of cells to outside stimuli or modifications of the environment (hormones, maturative stimuli, etc.) induces the same cytotype to modulate the specific morphological features (shape and identity) it 'normally' displays as it adapts to a new function. The evaluation of these changes, which are compared to the normal morphology of the cytotype (defined by its shapes and colours), allows a higher level of understanding compared to the more basic cellular identification: the cell's metabolic status. This, in turn, has important repercussions on cytological diagnosis, especially in the field of oncology. The variation of these features within a specified range will be considered within the normal limits of the phenotype, but a phenotype that is particularly active or reactive will exhibit morphological features far beyond such limits.

Knowledge and interpretation

Each of the features observed in a cell provides specific information that can help both cytotype identification and functional assessment. These features and their biological significance are discussed here individually, in detail. The combination of more features characterizing different cell types will be dealt with in Chapter 3, Cytotypes. The various morphological features observed have been divided into cellular morphologies, nuclear morphologies, cytoplasmic morphologies and supercellular morphologies (those shapes that are determined by the connections between cells).

Cellular morphologies

'Cellular morphology' refers to the set of morphological features (shape and colour) of a cell as a whole. The features considered are:

• size;
• shape;
• nuclear:cytoplasmic ratio;
• presence of certain specialized structures.

Size of cells

The size of cells can vary greatly. It ranges from an erythrocyte 6 or 7?µm in diameter, up to a rhabdomyocyte of several hundred microns. Except for those cells whose size can vary greatly because of their specific activities (for example, macrophages can increase their size due to the accumulation of phagocytosed material), the size of a cell is often a useful tool to identify the cytotype.

Usually, very large cells in normal conditions are actually syncytia: several cellular bodies merging into one unique cytotype, which is characterized by the presence of multiple nuclei in the cytoplasm.

Cellular shape

The shape of a cell, determined by its margins, can provide valuable information about its classification. In general, a well-defined and repeatable cellular shape is given by its cytoskeleton, which determines the shape when the cell itself is originally located within the tissue and/or organ of origin. On the contrary, so-called pleomorphic cells indicate a more plastic cytotype. They will therefore feature a less rigid morphology, which is not characteristic or indicative of the original tissue. Consequently, when a cell is classified as pleomorphic, it is assumed that, within the cytotype to which it belongs, it will not have a specific and repeatable shape.

There are several cells that, within a given cytotype, maintain a typical morphology. A possible classification by shape would include round, ovoid, columnar, fusiform/spindle, cubic, polygonal, star-shaped and pear-shaped cells (Figure 1).

Figure 1 - Schematic representation of the most important cellular morphologies: round (A), ovoid (B), columnar (C), fusiform/spindle (D), cuboidal (E), polygonal (F), star-shaped (G) and pear-shaped (H).

Nuclear:cytoplasmic (or nucleus to cytoplasm) ratio

The nuclear:cytoplasmic ratio determines how much cellular area is occupied respectively by the nucleus and the cytoplasm. A cell whose nucleus occupies almost the entire cellular area is typically at an immature cellular stage. During maturation, cytoplasmic structures (capable of performing different functions), gain space and tend to equalize such area (1:1 ratio) or exceed it. However, there are cases in which cells considered mature retain a high nuclear:cytoplasmic ratio (for example, mature lymphocytes). This feature has, in physiological terms, a major impact on the identification of the cytotype. The implications of this morphological feature with regard to pathological conditions will be dealt with in Chapter 6, Morphological alterations of cells.

The nuclear:cytoplasmic ratio is typically classified as high (the nucleus takes up most of the cellular area), equal/intermediate (nucleus and cytoplasm occupy approximately the same amount of cellular area) and low (the cytoplasm takes up most of the cellular space) (Figure 2). Classifications using numerical values are less typical.

Figure 2 - Schematic representation of the nuclear:cytoplasmic ratio: high (A), equal/intermediate (B) and low (C).

Specialized cellular structures

Some cells have specialized cellular structures, a sign of their specific functional differentiation. Specialized cellular structures are cilia, flagella, microvilli, basal plates, etc. (for more information, see Chapter 3, Cytotypes). These are proof of cellular differentiation, and are thus crucial features for recognizing a cytotype.