Tissues and Supporting Systems in Animals – SS2 Biology Lesson Note (First Term)

A skeleton is the framework of the body that supports animals, gives them shape and protects soft tissues and organs. It is made of chitin, cartilage or bone, and it may be a hydrostatic skeleton, an exoskeleton or an endoskeleton. In vertebrates it has two parts: the axial skeleton and the appendicular skeleton.

Behavioural objectives

By the end of the lesson, students should be able to:

• name the three forms of skeletal materials and state where each is found;
• state the differences between bone and cartilage;
• describe the hydrostatic skeleton, exoskeleton and endoskeleton;
• list the functions of the skeleton;
• name the bones of the axial and appendicular skeletons;
• describe the features and functions of the different types of vertebrae.

What are tissues and the skeleton?

To carry out life processes, all organisms (plants and animals) need tissues. Tissues are groups of similar cells that carry out specific functions.

The skeleton is the framework of the body which provides support, shape and protection to the soft tissues and organs in animals. It forms the central core of the human body and it is covered by muscles, blood vessels and skin.

What are the forms of skeletal materials?

There are three forms of skeletal materials found in animals. These are:

1. Chitin
2. Cartilage
3. Bone

Chitin

Chitin is a tough, non-living material present in arthropods (invertebrates). It acts as a hard outer covering for the animal and is made up of a series of plates covering or surrounding the organism. Chitin is very tough, light and flexible.

However, it can be strengthened by impregnation with “tanned” proteins and, particularly in aquatic crustaceans like crabs, by calcium carbonate.

Cartilage

Cartilage is a tissue present in the skeleton of complex vertebrates. It consists of a hard matrix penetrated by numerous connective tissue fibres. The matrix is secreted by living cells called chondroblasts.

These later become enclosed in spaces (lacunae) scattered throughout the matrix. In this condition, the cells are termed chondrocytes. Cartilage acts as a shock absorber between bones during movement because it is tough and flexible with great tensile strength. It is found predominantly in mammals and cartilaginous fishes, e.g. sharks.

Types of cartilage

Cartilage is of three main types in mammals:

1. Hyaline cartilage: This dense meshwork is the most common type. It is found on the surface of moveable joints, in the trachea and bronchi (for ease of respiration) and in the protruding parts of the nose.
2. White fibrous cartilage: It is tougher than hyaline cartilage and is found in the intervertebral discs of the vertebral column.
3. Yellow elastic cartilage: It is found in the external ear (pinna) and the epiglottis (a cartilaginous flap that covers the trachea during swallowing).

Bone

Bone is the major component of the skeletal system. It consists of living cells (osteocytes), protein fibres (collagen), and minerals such as calcium carbonate and calcium phosphate. These minerals (the non-living constituent) make up two-thirds of the mass of bone. Hence, bone is strong and very rigid, unlike cartilage. Bones are highly vascularised.

The skeleton of a young vertebrate embryo is made up of cartilage. As the embryo grows, bone cells (osteocytes) replace cartilage cells. Hence, the cartilage tissue becomes hardened into bone through the addition of minerals in a process called ossification.

What are the differences between bone and cartilage?

What are the types of skeleton?

The three main types of skeleton in animals are:

1. Hydrostatic skeleton: This is the type present in soft-bodied animals, e.g. earthworms and sea anemones. Such animals use pressure to support themselves. They have a muscular body wall which is filled with fluid. The muscles contract and exert force against the fluid, which presses back against the muscular wall.
2. Exoskeleton: This is the outer skeleton present in arthropods. It is secreted by the cells covering the body of the animal and its main component is chitin (a non-living substance). The exoskeleton also supports animals against gravity and enables them to move about. Animals with this type of skeleton periodically shed the old skeleton and grow rapidly in size while the new exoskeleton is still soft and extensible. The shedding process is called moulting or ecdysis.
3. Endoskeleton: This is an internal skeleton present in all vertebrates. It is composed mainly of bones, and the bones grow steadily as the animal grows (hence there is no need for moulting). Bones of many sizes and shapes make up the endoskeleton. The bones are attached at moveable joints by tough, flexible fibres called ligaments, so the skeleton is flexible. Muscles are also attached to the bones, usually by tendons, to provide posture and bring about body movement.

What are the functions of the skeleton?

1. It supports the body of organisms.
2. It acts as the framework of the body.
3. It protects delicate organs, e.g. the heart and brain.
4. It is used for locomotion through the action of the limbs.
5. It is an important component of respiration, e.g. breathing involves active movement of the ribs.
6. It produces blood through the bone marrow.

How are vertebrates supported?

The skeleton of vertebrates such as fish, frogs, lizards, birds and man consists of bones and cartilage. It is classified into two parts:

1. Axial skeleton: consists of the skull, ribs, sternum and the vertebral column.
2. Appendicular skeleton: is made up of the limb girdles (pectoral and pelvic girdles) and the limbs (the fore and hind limbs).

Diagram of the human skeleton

The axial skeleton

The skull

Human skull, lateral view

The skull is made up of flat bones joined together by suture joints. It has three parts: the cranium (brain-box), the facial skeleton and the jaws, which include the maxilla (upper) and the mandible.

Functions of the skull

• It protects the brain.
• It also protects the olfactory organ, eyes, and middle and inner ear.
• It gives shape to the head.
• It bears the teeth.

The vertebral column

The vertebral column forms the backbone and protects the spinal cord. It is made up of 5 groups of bones called vertebrae, each of which is built on a similar basic pattern. The vertebrae are held together with strong ligaments and compressible cartilage pads called intervertebral discs.

Types of vertebrae and their location

A typical vertebra

A typical vertebra has the following structural features:

• Neural canal, which is for the passage of the spinal cord.
• Neural spine, which projects upward and backward for the attachment of muscles.
• Transverse processes, for the attachment of muscles and ligaments.
• Centrum, a solid bony piece below the neural canal.
• Zygapophyses, the surfaces for joining together successive vertebrae. These could be prezygapophyses (facing inward and upwards) or postzygapophyses (facing outward and downwards).

Cervical vertebrae

The first cervical vertebra is called the atlas while the second is called the axis.

The atlas: It has a large neural canal, flat and broad transverse processes, and a short neural spine which could be absent at times. It also has a vertebrarterial canal for the passage of blood vessels. The centrum is absent.

Function of the atlas: It permits the nodding of the head.

The axis: It has a broad and flat centrum, a large and flat neural spine, reduced transverse processes and a vertebrarterial canal. It articulates with the atlas through the odontoid process.

Functions of the axis

• It permits the turning or twisting of the head.
• It forms a pivot joint with the atlas.

Thoracic vertebrae

Thoracic vertebrae have a long and prominent neural spine, a pair of short transverse processes, a large neural canal and neural arch, and a large cylindrical centrum. They also have special surfaces for the attachment of the ribs.

Functions

• They aid the attachment of ribs.
• They assist in breathing.
• They give attachment to muscles at the shoulder and back.

Lumbar vertebrae

Each lumbar vertebra has large and flat transverse processes, a broad and flat neural spine, a large and thick centrum and well-developed zygapophyses. It has extra paired projections, namely:

• Anapophysis
• Metapophysis

Functions of lumbar vertebrae

• They provide attachment for abdominal muscles.
• They bear considerable weight of the body.

Sacral vertebrae

The sacral vertebrae fuse together to form a single bony mass called the sacrum. Each sacral vertebra has a narrow neural canal, a reduced neural spine and a large centrum. The first differs from the remaining four by:

1. having a pair of transverse processes which is large and wing-like, while the others are attached to the muscles of the back;
2. having a small neural canal, which generally becomes narrower in the lower four vertebrae.

Function: The sacrum joins the pelvic girdle to provide support, rigidity and strength.

Caudal vertebrae

The caudal vertebrae are joined together to form a single bony mass called the coccyx. Each has no neural spine, no neural canal and no transverse process. The coccyx appears as a solid rectangular mass of bone.

Functions

• It supports the tail.
• It provides attachment for tail muscle.

The appendicular skeleton

Pectoral girdle

The pectoral girdle is found around the shoulder in man. It consists of two halves which are held by muscles. Each half is made up of three bones:

1. Scapula
2. Clavicle
3. Coracoid

The scapula and coracoid are fixed together. The scapula is flat and triangular with a hollow called the glenoid cavity at its tip. This cavity articulates with the head of the humerus to form the shoulder joint. The clavicle is a small rod of bone attached to a ligament joining the sternum to the scapula.

Functions

• The pectoral girdle gives attachment to muscles and ligaments.
• It provides firm support to the forelimbs.

Pelvic girdle

The pelvic girdle is found around the waist in man. It consists of two halves which are joined to each other ventrally and to the sacrum dorsally. Each half of the pelvic girdle is made up of three bones. They are:

1. Ilium
2. Ischium
3. Pubis

These three bones form a depression (on their outer surface) called the acetabulum, which articulates with the head of the femur to form the hip joint.

Limbs

The limbs include the fore (upper) and the hind (lower) limbs. In most vertebrates, both limbs have the same basic plan: each limb has a long bone, followed by a pair of long bones, then a set of small bones terminating in five digits.

The fore limbs: The fore limb is made up of an upper arm bone called the humerus, which joins with two other long bones (radius and ulna) at its lower end to form the elbow joint. The radius and ulna (the ulna is longer) are the bones of the forearm. Next are the wrist bones called carpals, which are small bones. These are followed by the digit bones called metacarpals, which terminate in the phalanges (finger bones). In man, each digit has three phalanges except the thumb, which has two.

The hind limbs: The hind limb is made up of the thigh bone called the femur, which is the largest and longest bone in the body. Its lower end terminates in two rounded projections called condyles, which form the knee joint together with the tibia. A small flat bone called the patella is found in front of the knee joint. Next to the femur are the tibia and fibula. The tibia is longer and larger. These are followed by the bones of the ankle, called tarsals. The lower limb terminates in the digit bones called metatarsals, and each digit is made up of three phalanges.

The ribs

The ribs are long, semi-circular rods which connect the thoracic vertebrae to the sternum. They are found in the chest region of the body. In man, there are 12 pairs.

Functions

• They form a cage protecting the lungs and the heart.
• They assist in breathing.

A typical rib

A typical rib has a head, a neck and a body. The first seven ribs are connected directly to the sternum through costal cartilage. They are therefore called true ribs. The next five are called false ribs. The eighth to tenth ribs have a common articulation to the sternum, each one attached by costal cartilage to the one above. The eleventh and twelfth pairs of ribs are called floating ribs because they have no connection to the sternum.

Evaluation

1. Name the three forms of skeletal materials found in animals.
2. State three differences between bone and cartilage.
3. Name the three types of skeleton and give one example of an animal that has each.
4. What is ossification?
5. Name the two parts of the vertebrate skeleton and list the bones that make up each.

Assignment

1. …………. is a non-living skeletal material. A. chondroblasts B. osteocyte C. elastic cartilage D. chitin
2. The articulating surface for joining together successive vertebrae is called A. neural spine B. zygapophyses C. transverse processes D. neural canal
3. The canal for the passage of blood vessels in vertebrae is called A. neural canal B. cervical canal C. vertebrarterial canal D. zygaphosis
4. Endoskeleton is present in the following animals except A. dog B. snake C. shark D. lizard
5. The most abundant supporting tissue in plants is A. sclerenchyma B. parenchyma C. xylem D. phloem
6. (a) What is ecdysis? (b) Mention two animals in which it occurs.
7. Differentiate between (a) bone and cartilage (b) atlas and axis vertebrae.