Chapter 1 Principles of cell biology
Anatomy and physiology are scientific terms used to describe the study of the structure of the body (anatomy) and how the body actually ‘works’ (physiology). In this section, we will study the anatomy and physiology of the dog and cat. In Section 2, the anatomy and physiology of some of the most commonly kept exotic species and of the horse are covered. We start by looking at the basic unit of the body – the cell – and then work our way through the tissues, organs and systems until the picture is complete.
Animal classification
When studying any aspect of biology it is important to have a basic understanding of the classification system used to group animals. How the species that one may meet in a veterinary practice fit into this classification system should also be understood. Classification is the way in which we ‘sort’ species into orderly groups, depending on how closely they are related in terms of their evolution, structure and behaviour. The science of classification is known as taxonomy.
If organisms have certain basic features in common they are grouped together into a kingdom. For example, if an organism is composed of more than one cell, i.e. it is multicellular, and obtains its food by ingestion, it is placed in the animal kingdom. Other kingdoms include plants and fungi. The animal kingdom is then further subdivided, based upon similarities of organisms, into a hierarchical system (Table 1.1). This narrows the classification down until we eventually reach a particular genus and species. Most living organisms are identified by a genus and species – a method known as the binomial system and invented by the Swedish scientist Carl Linnaeus.
Table 1.1 Classification of the domestic dog and cat
| Taxonomic group | Dog | Cat |
|---|---|---|
| Kingdom | Animal | Animal |
| Phylum | Chordata (Vertebrate) | Chordata (Vertebrate) |
| Class | Mammalia (Mammal) | Mammalia (Mammal) |
| Order | Carnivora | Carnivora |
| Family | Canidae | Felidae |
| Genus | Canis | Felis |
| Species | familiaris | catus |
| Common name | Domestic dog | Domestic cat |
All the species within the animal kingdom are divided into those with backbones – the vertebrates – and those that do not have backbones – the invertebrates, e.g. insects, worms, etc. The vertebrates are divided into eight classes. The classes that are of the most veterinary importance are:
These classes are then further divided into orders, and so on, until a species is identified, as in Table 1.1.
Generally speaking, all mammals have a similar basic structural plan in terms of anatomy and physiology, but each species has been modified to suit its specific lifestyle. In other words, mammals have become specialised for activities such as running, digging, gnawing, jumping and eating specific foods.
Anatomical definitions
When studying anatomy and physiology it is important to understand the terms that are used to describe where structures lie in relation to one another. These are illustrated in Figure 1.1 and named as follows:
The basic plan of the body
The body is made up of a number of systems and each of them has a specific job, enabling the body to function effectively. These systems can be placed in one of three groups depending on their function:
Structural systems
Coordinating systems
Visceral systems
Each system of the body is made up of a collection of specific types of tissue arranged as organs. Each tissue is composed of a specialised type of cell (the smallest unit of the body).
The mammalian cell
Cells are the minute units of a tissue that can only be seen under a microscope. Cells can be considered to be the basic structural and functional unit of an organism. In fact they are like ‘little bodies’ themselves because they carry out a number of basic functions such as taking in nutrients and excreting waste, respiring or ‘breathing’, and reproducing. These and other functions are carried out by various structures that make up the cell – mainly by the organelles, or ‘little organs’, that float within the cytoplasm of the cell.
Cell structure and function
The components of a cell are shown in Figures 1.2 and 1.3 and are as follows:
Fig. 1.2 Components of the mammalian cell.
(With permission from Colville T, Bassett JM 2001 Clinical anatomy and physiology for veterinary technicians. Mosby, St Louis, MO, p 11.)
Fig 1.3 Transmission electron micrograph of a plasma cell showing extensive rough endoplasmic reticulum and scattered mitochondria (m).
(With permission from Samuelson DA 2007 Textbook of veterinary histology. Saunders-Elsevier, St Louis, MO, p 86.)
Cell membrane
The cell membrane covers the surface of the cell and may also be called the plasma membrane. It is responsible for separating the cell from its environment and controls the passage of substances in and out of the cell. Carbohydrates are found on the surface of the cell membrane and it is believed that these help cell recognition, e.g. they enable a cell to recognise whether or not it is in contact with another cell of the same type. The cell membrane of a mammalian cell is composed of a phospholipid bilayer (Fig. 1.4). This is a double layer of phospholipid molecules and has protein molecules embedded within it.
Fig. 1.4 Structure of the cell membrane showing the phospholipid bilayer. This structure is also known as the ‘fluid mosaic model’.
The nature of its structure means that the cell membrane is selectively permeable, allowing some substances to pass through it while others may either be excluded or must travel across the membrane by means of specialised transport systems. These include:
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