Principles of cell biology

Chapter 1 Principles of cell biology

• All living organisms can be classified into different orders, classes and families linked by certain common characteristics. These groups can be further divided into a genus and species which describes an individual type of organism.

• The body is made up of a number of systems, each of which has a specific function. These systems form the structural framework of the body or lie within one of three body cavities.

• Each system consists of a collection of tissues and organs which are comprised of the smallest units of the body – the cells.

• Cells can only be seen under the microscope and all have a basic structure with certain anatomical differences which adapt them to their specific function.

• Each structure within the cell plays a vital part in the normal function of the cell and therefore in the normal function of the body system.

• Cells grow and divide by means of mitosis. Each mitotic division results in the production of two identical daughter cells containing the diploid (or normal) number of chromosomes.

• The healthy body contains 60–70% water, distributed into two principal fluid compartments – the extracellular fluid (surrounding the cells) and the intracellular fluid (within the cells).

• Body fluids move between these compartments and this movement is controlled by the chemical constituents of the fluid and the physical processes of diffusion and osmosis.

• Body fluids contain inorganic and organic compounds. The structure and percentage of all of these is fundamental to the balance and normal function of the body. Within the body there are many systems involved in maintaining a state of equilibrium – this is known as homeostasis.

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:

• Amphibia – approximately 3080 species

• Reptilia – approximately 6600 species

• Aves or birds – approximately 8500 species

• Fish – approximately 30 000 species.

• Mammalia – approximately 4070 species.

These classes are then further divided into orders, and so on, until a species is identified, as in Table 1.1.

Most of this section of the book concerns the mammals, because the majority of animals seen in veterinary practice will be from this class. The distinctive features of mammals are the production of milk by the mammary glands and the possession of hair as a body covering. Examples of mammalian orders include:

• Insectivores, e.g. shrews, moles

• Rodents, e.g. mice, rats

• Lagomorphs, e.g. rabbits, hares

• Carnivores, e.g. cats, dogs, bears, seals

• Ungulates, e.g. cows, sheep, horses

• Cetaceans, e.g. whales, dolphins

• Primates, e.g. monkeys, apes.

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:

• Median plane – divides the body longitudinally into symmetrical right and left halves, i.e. it can be described as ‘the line down the middle of the animal’ from nose to tail

• Superficial – near to the surface of the body

• Deep – closer to the centre of the body

• Cranial/anterior – towards the front of the animal, i.e. towards the head

• Caudal/posterior – towards the rear end or tail of the animal, i.e. away from the head

• Medial – structures that lie towards or near the median plane, i.e. closer to the middle of the animal

• Lateral – structures that lie towards the side of the animal, i.e. away from the median plane

• Dorsal – towards or near the back or vertebral column of the animal and the corresponding surfaces of the head, neck and tail

• Ventral – towards or near the belly or lowermost surface of the body and the corresponding surfaces of the head, neck and tail

• Rostral – towards the nose; this is used to describe the position of structures on the head

• Proximal – structures or part of the structure that lie close to the main mass of the body, e.g. the ‘top’ of the limb that attaches to the body; this is also used to describe parts that lie near the origin of a structure

• Distal – structures or part of the structure that lie away from the main mass of the body or origin, e.g. the free end of the limb

• Palmar – the rear surface of the fore paw that bears the footpads; the opposite surface (i.e. the front surface of the paw) is the dorsal surface

• Plantar – the rear surface of the hind paw that bears the footpads; the opposite surface (as above) is the dorsal surface.

Fig. 1.1 Anatomical planes and directional terms used to describe the relative position of structures in the body.

(With permission from Colville T, Bassett JM 2001 Clinical anatomy and physiology for veterinary technicians. Mosby, St Louis, MO, p 3.)

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 – provide the basic ‘framework’ and transport system for the body

• Coordinating systems – the control mechanisms of the body

• Visceral systems – includes all the basic functional systems that do the general duties for the body; found within one of the three body cavities: thoracic, abdominal and pelvic.

Coordinating systems

• Nervous system – carries information to and from the central ‘control station’ of the body, i.e. the brain; it controls and monitors the internal and external environment of the body

• Endocrine system – controls the body’s functions via a communication system of chemical messengers or hormones.

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:

• Cell membrane

• Organelles:

Rough endoplasmic reticulum

Smooth endoplasmic reticulum

Golgi apparatus

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:

• Pores in the cell membrane – small molecules can pass through these pores

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Tags: Introduction to Vetrinary Anatomy and Phisiology

Jul 18, 2016 | Posted by admin in PHARMACOLOGY, TOXICOLOGY & THERAPEUTICS | Comments Off

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