Chapter 2 Tissues and body cavities

KEY POINTS

• The cells of the body are arranged into four basic tissue types: epithelial, connective, muscle and nervous tissue.

• Epithelial tissue covers the outside of the body and lines all the body cavities and the structures within them. Its primary function is to protect but in some areas it may also be absorbent or secretory. Secretory epithelial tissue forms glands.

• Connective tissue is found in varying forms such as blood, fibrous connective tissue, cartilage and bone. Its main function is to connect and support the parts of the body, but it also carries nutrients to the tissues and conducts waste material away.

• Muscle tissue brings about the movement of the body. It is found as striated muscle attached to the skeleton, smooth muscle within the internal organs of the body and cardiac muscle found only in the myocardium of the heart wall. Control of striated muscle is voluntary, while that of smooth and cardiac muscle is involuntary and brought about by branches of the autonomic nervous system.

• Nervous tissue is found all over the body and its function is to conduct nerve impulses to and from parts of the body and the central nervous system.

• The body is divided into three body cavities, which contain the visceral systems. The thoracic and abdominal cavities are lined with a single layer of serous epithelial tissue, which is named according to its location within the cavity.

Within the body individual cells are grouped together to form tissues and organs. Thus:

• A tissue is a collection of cells and their products in which one type of cell predominates, e.g. epithelial tissue or muscle tissue

• An organ is a collection of tissues forming a structure within an animal that is adapted to perform a specific purpose, e.g. liver, larynx, kidney

• A system is a collection of organs and tissues that are related by function, e.g. the respiratory system.

Body tissues

Each tissue type consists of three main components:

• Cells – one type forms the majority of the cells and gives the tissue type its name, e.g. muscle tissue consists mainly of muscle cells

• Intercellular products – these are produced by the cells and lie in the spaces between them

• Fluid – interstitial fluid flows through specialised channels running through the tissue.

There are four main types of tissue:

• Epithelial – protects the body; may also be secretory and absorbent

• Connective – binds the tissues together

• Muscle – brings about movement

• Nervous – conveys nerve impulses from one area to another and coordinates the response.

Epithelial tissue

Epithelial tissue or epithelium covers the surface of the body and the organs, cavities and tubes within it – it covers the internal and external surfaces of the body. Its main function is to protect delicate structures lying beneath it but in some areas the epithelium may be secretory, e.g. glands, or absorbent, e.g. in the small intestine. The epithelium lining structures such as the inside of the heart, blood vessels and lymph vessels is referred to as endothelium.

Epithelium may be described according to the number of layers of cells, i.e. its thickness:

• If an epithelium is one cell thick it is said to be simple (Fig. 2.1)

• If there is more than one layer it is said to be stratified or compound (Fig. 2.1).

Fig. 2.1 The different types of epithelium found in the body.

The thickness of the epithelium reflects its ability to protect: the more layers of cells, the more protection is provided. The epithelium on the footpads consists of many layers of cells providing protection when walking on rough surfaces, while the epithelium over the abdominal wall is only a few cells thick and additional protection is provided by fur. Further protection may be provided by the presence of the protein keratin. The epithelium is described as being a keratinised stratified epithelium and this type can be seen in claws and nails.

Epithelium may also be described according to the shape of the cells within it (Fig. 2.1). There are three basic shapes of epithelial cell:

• Squamous cells – flattened in shape

• Cuboidal cells – square or cube-shaped

• Columnar cells – column-shaped (the height is greater than the width) (Fig. 2.2).

Fig. 2.2 Light micrograph of the mucosa of the small intestine showing a simple columnar epithelium with a border of microvilli that increase the surface area for absorption.

(Taken from D. Samuelson. Textbook of Veterinary Histology. Saunders. 2007, p 42.)

The full classification of the type of epithelium is based upon the shape of the cell and the number of layers present. There are a number of different types of epithelial tissue in the body, these include:

• Simple cuboidal epithelium – this is the least specialised type of epithelium. It is one cell thick and the cells are cube-shaped. Cuboidal epithelium lines many of the glands and theirducts. This type of epithelium has an absorptive or secretory function depending on its location in the body, e.g. lining the renal tubules.

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• Simple squamous epithelium – this type has flattened cells and is one layer thick. Simple squamous epithelium is thin and delicate and is found in areas of the body where the covering surface needs to be easily permeable to molecules such as oxygen, e.g. lining the blood vessels and the alveoli of the lungs.

• Simple columnar epithelium – this has tall narrow cells and is one layer thick. Generally, simple columnar epithelium lines organs that have an absorptive function, e.g. the small and large intestines, or a secretory function, e.g. digestive glands (Fig. 2.2).

• Ciliated epithelium – this is a more specialised epithelium consisting of a single layer of column-shaped cells (Figs 2.1, 2.3). The free surface of the cells has tiny hair-like projections called cilia whose function is to ‘waft’ foreign particles along the epithelial surface and out of the body. Ciliated epithelium lines the upper respiratory tract, where it helps to trap solid particles that have been inhaled, preventing them from entering the more distal parts of the respiratory system. The uterine tubes are also lined with ciliated epithelium, which helps to move the fertilised egg along the reproductive tract.

• Stratified epithelium – this is composed of a number of layers of cells and is thicker and tougher than the other types of epithelium. It is found in areas that are subjected to wear and to friction and shearing forces, e.g. the epidermis of the skin (see p. 44). Pseudostratified epithelium (Fig. 2.3) appears to be multilayered because of the irregular positioning of the nuclei but is actually a single layer of cells. This may be found in areas such as the trachea.

• Transitional epithelium – a type of specialised stratified epithelium found lining parts of the urinary system, i.e. structures and tubes that are capable of considerable distension and variations in internal pressure and capacity, such as the bladder and ureters. The cells are able to change their shape according to circumstances and thus their appearance varies with the degree of distension of the structure.

Fig. 2.3 Photomicrograph of the lining of the trachea showing a pseudostratified ciliated columnar epithelium

(Taken from D. Samuelson. Textbook of Veterinary Histology. Saunders. 2007, p 43.)

Glands

Glandular tissue is a modification of epithelial tissue. The epithelium, in addition to its protective function, may also be a secretory membrane. Glands are either:

• Unicellular glands – these have individual secretory cells that are interspersed throughout the tissue. The most common type is the goblet cell (Fig. 2.4), which secretes clear sticky mucus directly on to the membrane surface. The epithelium is known as a mucous membrane. Mucus traps particles, providing extra protection, and also lubricates the epithelial surface. Mucous membranes are found covering the oral cavity, lining the vagina and the trachea and in many other parts of the body.

• Multicellular glands – these consist of many secretory cells folded to form more complex glands. They vary in shape and intricacy relating to their position and function in the body. Examples of some of the types of gland found in the body are shown in Figure 2.5.

Fig. 2.4 Photomicrograph of goblet cells within a pseudostratified ciliated columnar epithelium

(Taken from D. Samuelson. Textbook of Veterinary Histology. Saunders, p 64.)

Fig. 2.5 The different types of gland found in the body.

Glands may be categorised as either:

• Exocrine glands – these have a system of ducts through which their secretory products are transported directly to the site where they will be used

• Endocrine glands – do not have a duct system (ductless glands) and their secretions, known as hormones, are carried by the blood to their target organ, which may be some distance away (see Ch. 6).

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Connective tissue

Connective tissue is responsible for supporting and holding all the organs and tissues of the body in place. It also provides the transport system within the body, carrying nutrients to the tissues and waste products away. Connective tissue consists of cells embedded in an extracellular matrix or ground substance. The properties of this ground substance depend on the type of connective tissue. There are many types of connective tissue which, in order of increasing density are:

1 Blood

2 Haemopoietic tissue

3 Areolar tissue or loose connective tissue

4 Adipose or fatty tissue

5 Fibrous connective tissue or dense connective tissue

6 Cartilage

7 Bone.

Blood

Blood is a specialised connective tissue that circulates through the blood vessels to carry nutrients and oxygen to the cells, and waste products to the organs of excretion. It consists of a number of different types of blood cell within a fluid ground substance – the plasma. (This is covered in more detail in Ch. 7).

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Haemopoietic tissue

This jelly-like connective tissue forms the bone marrow within the long bones and is responsible for the formation of the blood cells (see Ch. 7).

Areolar tissue

Areolar (meaning spaces) or loose connective tissue (Figs 2.6, 2.7) is the most widely distributed type of connective tissue and is found all over the body, e.g. beneath the skin, around blood vessels and nerves, between and connecting organs and between muscle bundles. The ground substance contains a loose weave work of two types of protein fibre: collagen fibres, with a high tensile strength secreted by the main cell type (the fibroblast) and elastic fibres, which enable the tissue to stretch and return to its former shape. Fat cells may be present in varying quantities depending on location and the degree of obesity of the animal. Macrophages, cells which are capable of phagocytosis, are also present.

Fig. 2.6 Composition of areolar tissue. A Collagen fibres: flexible but very strong and resistant to stretching. B Ground substance: this contains the different fibres and cells of the tissue. C Fibroblast: long, flat cell that produces collagen and elastic fibres. D Mast cell: secretes an anticoagulant. E Fat cell: stores fat. F Macrophage: large cell capable of phagocytosis of foreign particles. G Elastic fibres: form a loose and stretchable network.

Fig. 2.7 Light micrograph of loose connective or areolar tissue showing a delicate network of collagen and elastic fibres in a piece of mesentery.

(Taken from D. Samuelson. Textbook of Veterinary Histology. Saunders, p 92.)

Adipose tissue

Adipose tissue (Fig. 2.8) is similar to areolar tissue but its matrix contains mainly fat-filled cells, closely packed together, giving it the name fatty tissue. These fat cells act as an energy reserve and, in the dermis of the skin, the tissue insulates the body to reduce heat loss. In some areas, such as around the kidney, adipose tissue provides a protective layer.

Fig. 2.8 Light micrograph of adipose tissue.

(Taken from Textbook of Veterinary Histology. D. Samuelson. Saunders, p 98.)

Dense connective tissue

Dense or fibrous connective tissue consists of densely packed collagen fibre bundles with relatively few fibroblasts and other cells in between them (Fig. 2.9). The fibres may be arranged in two ways:

• Parallel arrangement – this is known as regular fibrous connective tissue, e.g. tendons, which are strong bands of fibrous tissue linking muscles to bone; and ligaments, which link bone to bone

• Irregularly interwoven fibres – this is seen in the dermis of the skin and in the capsules of joints, as well as in organs such as the testes and lymph nodes. Irregular dense connective tissue is often found in sheets and forms the basis of most fascias and aponeuroses.

Fig. 2.9 Transmission electron micrograph (×25 000) of collagen fibres.

(Taken from Textbook of Veterinary Histology. D. Samuelson. Saunders, p 87.)

Cartilage

Cartilage is a specialised connective tissue which is rigid but flexible and resilient and is able to bear weight (Fig. 2.10). It is composed of cells (known as chondrocytes) and fibres within a gel-like ground substance. Cartilage has no blood supply and its nutrition is supplied by the fibrous sheath or perichondrium that surrounds it.

Fig. 2.10 The structure of hyaline cartilage.

There are three types of cartilage:

• Hyaline cartilage – this has a translucent, bluish-white appearance. The randomly arranged collagen fibres are not easily visible under the microscope as they have the same refractive index as the gel matrix. Hyaline cartilage is the most common type of cartilage in the body, and forms the articular surfaces of joints and provides support in the nose, larynx, trachea and bronchi. It also forms the skeleton of the embryo before it becomes ossified by the process known as endochondral ossification.

• Elastic cartilage – this has chondrocytes within a matrix and numerous elastic fibres. Elastic cartilage occurs in places where support with flexibility is required, e.g. the external ear and epiglottis.

• Fibrocartilage – this has a similar basic structure but has a higher proportion of collagen fibres giving it great strength, e.g. in the intervertebral discs and in the menisci of the stifle joint. It also attaches the tendons and ligaments to bone.

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Bone

Bone is a living tissue that is capable of remodelling and repairing itself when damaged. It is a specialised type of connective tissue, which provides the rigid supportive framework of the body and forms a system of levers for locomotion.

Bone consists of an extracellular matrix or ground substance that contains the protein osteonectin and collagen fibres. Together, these form osteoid, within which crystals of insoluble calcium phosphate are deposited as the bone tissue becomes calcified. Calcification gives bone its characteristic rigidity and hardness. As the ground substance becomes calcified the bone cells or osteocytes are trapped in spaces called lacunae. Running through the bone matrix are fine channels, called Haversian canals, which carry the blood vessels and nerves of the bone(Figs 2.11, 2.12). Each Haversian canal is surrounded by a series of concentric cylinders of matrix material called lamellae and the osteocytes within their lacunae. Each series of these cylinders, together with the canal, is called a Haversian system (Figs 2.11, 2.12). A fibrous membrane, the periosteum, covers the outer surface of all types of bone.

Fig. 2.11 The structure of compact bone.

Fig. 2.12 Light micrograph (×100) of compact bone showing Haversian canals (CC) surrounded by lamellae of osteocytes (O) within their lacunae.

(Taken from D. Samuelson. Textbook of Veterinary Histology. Saunders, p 122.)

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There are two types of bone tissue:

• Compact bone – this is solid and hard and is found in the outer layer or cortex of all types of bone. The Haversian systems of compact bone are densely packed together.

• Cancellous or spongy bone – this consists of an internal meshwork of bony ‘struts’ or trabeculae with interconnected spaces between filled with red bone marrow. Cancellous bone is found in the ends of long bones, and in the core of short, irregular and flat bones.

Panosteitis is a condition that occurs in young dogs. It is an idiopathic inflammation of all bone tissues and presents as pain in the long bones.

Muscle tissue

Muscle tissue is responsible for organised movement in the body.

Skeletal or striated muscle

This type of muscle is found attached to the skeleton and brings about movement (Fig. 2.13). It is under voluntary or conscious control, i.e. an animal uses its brain to move its limbs.

Fig. 2.13 A Skeletal muscle, as found attached to the bones. It is voluntary and consists of striped (striated) cells, which are long and cylindrical in shape. Each cell has several nuclei. B. Light micrograph (×1000) of skeletal muscle showing characteristic bands and lines and nuclei located at the edges of the fibres.

(Taken from D. Samuelson. Textbook of Veterinary Histology. Saunders, p 165.)

The muscle cells or fibres are long and cylindrical and lie parallel to each other. Each individual muscle fibre is composed of bundles of microfilaments known as myofibrils that are made of two contractile proteins called actin (thin filaments) and myosin (thick filaments). It is their highly regular arrangement that gives the muscle its striated or striped appearance when viewed under a microscope. Each fibre has several nuclei, which lie on the outer surface of the cell as the presence of the myofibrils pushes all the cell structures to the outer margins.

The muscle fibres are grouped together in bundles or fascicles by connective tissue. Groups of fascicles are then held together by connective tissue and form a large muscle. The whole muscle is surrounded by the muscle sheath, which is continuous with the tendons that connect the muscle to a bone.

Smooth muscle

Smooth muscle is also called unstriated, involuntary and visceral muscle. It is found in regions of the body that are under involuntary control, e.g. in the walls of the blood vessels, digestive tract, respiratory tract, bladder and uterus. It is therefore responsible for moving food through the digestive system, controlling the flow of blood through blood vessels, and other unconscious processes. Smooth muscle is controlled by the autonomic nervous system.

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The cells of smooth muscle are long and spindle-shaped (Fig. 2.14) and are surrounded by small amounts of connective tissue that bind the cells into sheets, or layers. The nucleus in each cell lies in its centre. Smooth muscle does not appear ‘striped’ when viewed under the microscope – hence its name.

Fig. 2.14 A Smooth muscle, as found in the gastrointestinal tract and other viscera. It is involuntary and consists of small spindle shaped cells without striations (hence ‘smooth’). Each cell has one nucleus in the centre. B. Light micrograph (×1000) of smooth muscle cells showing the long spindle shaped cells with no striations.

(Taken from D. Samuelson. Textbook of Veterinary Histology. Saunders, p 161.)

Cardiac muscle

This type of muscle is found only in the heart and forms the myocardium. It is responsible for the rhythmic and automatic contraction of the heart that continues throughout an animal’s life. This inherent contractibility is increased or slowed down by nerves supplying the heart according to the requirements of the body. Control of cardiac muscle is therefore involuntary or unconscious.

Cardiac muscle cells are striated and cylindrical in shape (Fig. 2.15). Unlike the cells of striated muscle, they branch to create a network of fibres, which are linked by intercalated discs. These enable nerve impulses to be conveyed across the myocardium extremely quickly, producing a rapid response to the changing needs of the body.

Fig. 2.15 A Cardiac muscle, found in the heart. It is involuntary and consists of striated, cylindrical cells. Cells are connected by intercalated discs. B. Light micrograph (×1000) of cardiac muscle cells. Arrow indicates intercalated discs.

(Taken from D. Samuelson. Textbook of Veterinary Histology. Saunders, p 173.)

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Nervous tissue

The main cell of nervous tissue is the neuron (Fig. 2.16), whose function is to transmit nerve impulses from one area to another. Each neuron consists of a cell body, containing the nucleus, several short processes known as dendrons and one long process known as an axon. Dendrons carry information towards the cell body, while the axon carries information away from it and towards its destination. Many axons within the body are covered in fatty material known as myelin. This is secreted by specialised cells wrapped around each axon – the Schwann cells – and it increases the speed of transmission of nerve impulses from one place to another.

Fig. 2.16 A. The structure of a neuron (nerve cell). B Light micrograph of nervous tissue.

(Taken from D. Samuelson. Textbook of Veterinary Histology. Saunders, p 182.)

Dendrons and axons are referred to as nerve fibres and the whitish structures identified by the naked eye as ‘nerves’ within the body are collections of large numbers of nerve fibres.

Nerve impulses are transferred from one neuron to another by means of button-like structures known as synapses. All nerve pathways consist of neurons and synapses. (Nervous tissue is covered in more detail in Ch. 5.)

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The body cavities

The body is divided into separate areas referred to as the body cavities (Fig. 2.17). They are described as ‘potential’ spaces because, although they are completely filled with the visceral organs and fluid, there is only a very small amount of free space. All the body cavities are lined with a serous membrane or endothelium, which is a single continuous layer of epithelium that produces a watery or serous lubricating fluid. This is different from the thicker, more proteinaceous secretion mucus, which has a protective function. Serous fluid acts as a lubricant between the surfaces of the cavity and the organs and structures within it.

Fig. 2.17 Longitudinal section through the body to show the three body cavities.

Each part of the serous membrane is a continuous layer, named according to its position within the cavity:

• Parietal describes the serous membrane that lines the boundaries or sides of the cavity

• Visceral describes the serous membrane that covers all the organs within the cavity.

There are three body cavities:

• Thoracic

• Abdominal

• Pelvic.

The thoracic cavity

The thoracic cavity (Table 2.1) contains the heart, lungs and other associated structures. Its skeletal walls are formed by the bony thoracic cage consisting of the ribs, thoracic vertebrae and sternum. The ‘entrance’ into the cavity is known as the cranial thoracic inlet and is formed by the first thoracic vertebra, the first pair of ribs and the manubrium. The exit or caudal border is filled by the diaphragm.

Table 2.1 Summary of the boundaries of the body cavities

The serous membrane lining the thoracic cavity and covering the organs within it is called the pleura (Fig. 2.18). The parietal pleura lines the inside of the thoracic cavity but is named according to which part of the walls it covers, i.e. the diaphragmatic pleura covers the diaphragm and the costal pleura covers the ribs. The thoracic cavity is divided into two pleural cavities by a continuation of the parietal pleura. Each cavity contains one of the lungs and serous pleural fluid. The lungs themselves are coveredin visceral pleura, called the pulmonary pleura. Between the two pleural cavities, the thorax is divided into right and left sides by a vertical connective tissue septum called the mediastinum, which is covered in the mediastinal pleura. The mediastinum is the potential space formed by the double layer of parietal pleura that separates the two pleural cavities. It contains the pericardial cavity, containing the heart, aorta, trachea, oesophagus and the thymus gland in young animals.

Fig. 2.18 The anatomy of the thoracic cavity, showing the mediastinum and pleura.

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The pericardial cavity lies within the mediastinum in the thoracic cavity and is the space in which the heart sits. The heart is contained within the pericardium, which is a double-layered membranous sheath that completely surrounds it. Between the two layers of membrane is serous fluid that acts as a lubricant, enabling the heart to beat freely.

The abdominal cavity

The abdominal cavity (Table 2.1) lies caudal to the thoracic cavity and contains the abdominal viscera (Fig. 2.17). These include the organs of the digestive system and related glands, the urogenital system and all the associated vessels and nerves that supply these systems. The abdominal cavity is bounded cranially by the diaphragm and caudally by the pelvic inlet (there is no actual physical division between the abdominal and pelvic cavities). Dorsally, the boundary is the lumbar vertebrae and the hypaxial muscles. The muscles of the abdominal walls form the dorsolateral, ventral and lateral limits.

The internal surface of the abdominal cavity is lined with a serous membrane called the peritoneum (Fig. 2.19). This is a continuous sheet that forms a closed cavity, the peritoneal cavity. The peritoneal cavity is the potential space between the parietal peritoneum that lines the abdominal walls and the visceral peritoneum that covers the organs. The peritoneal cavity contains only a small volume of lubricating serous fluid known as peritoneal fluid, which allows friction-free movement of the organs and prevents adhesions forming between the organs and the peritoneum.

Fig. 2.19 Sagittal section through the abdominal cavity to show the reflections (folds) of the peritoneum.

The visceral peritoneum is folded on itself in a way that keeps the organs separate, suspending the organs within the abdominal cavity and carrying the various vessels and nerves that serve the viscera. This area of peritoneum is collectively known as the mesentery. The folds of mesentery have different names depending on their position, e.g. the mesentery suspending the duodenum is called the mesoduodenum, while the one suspending the ovary is the mesovarium. The omentum is a mobile fold of peritoneum that contains a ‘lacy’ network of fine vessels and fat and is divided into the greater omentum, arising from the greater curvature of the stomach, and the lesser omentum, arising from the lesser curvature of the stomach.

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The pelvic cavity

The pelvic cavity (Table 2.1) lies caudal to the abdominal cavity (Fig. 2.17) and contains the urinary bladder, the rectum and the reproductive organs. There is no physical separation between the two as there is between the thoracic and abdominal cavities. The cavity is bounded cranially by the pelvic inlet and caudally by the caudal pelvic aperture. The sacrum and the first few coccygeal vertebrae form the dorsal boundary and the pubis and ischium form the floor of the cavity. The walls are formed by muscles and ligaments. A continuation of the peritoneum extends into the cranial parts of the cavity and covers the organs which lie in both cavities, e.g. bladder and reproductive tract.