Introduction
Animals are a large and diverse group of living things that make up about three-quarters of all the species on Earth. Animals have certain abilities, like moving, responding to changes in their environment, adapting to different conditions, and defending themselves or finding food. These abilities come from their different body parts. But unlike plants, animals cannot make their own food. This means animals must rely on plants for food in one way or another.
All living things are made up of cells. Some living things are made of only one cell (these are called unicellular organisms), while other living things are made of many cells (these are called multicellular organisms).
A cell is the smallest part of a living thing that can still be alive. It's like the building block of life. The cells that make up animals are called animal cells, while the cells that make up plants are called plant cells.
Most cells are surrounded by a protective layer called a cell wall. This cell wall helps the cell keep its shape and stay strong.
Animal cells are a type of eukaryotic cell, meaning they have a nucleus (where the DNA is stored). However, animal cells do not have a cell wall. Instead, they are surrounded by a plasma membrane (a thin layer that holds everything inside the cell). The plasma membrane also surrounds the cell’s organelles (small parts inside the cell that have different jobs).
Unlike plant cells, animal cells don’t have a rigid cell wall. Plant cells have a cell wall, but animal cells don’t.
Because animal cells don’t have a stiff wall, they can form many different kinds of cells, tissues, and organs. This allows animals to have a wide variety of body parts, like nerves and muscles. The nerves and muscles in animals are made from special cells that plant cells don’t have. These special cells allow animals to move.
Animal Cell Size and Shape:
Animal cells can be many different shapes and sizes. Some are very small (in micrometres) and some can be much larger (in millimetres). The biggest animal cell is an ostrich egg, which is about 5 inches wide and weighs about 1.2 to 1.4 kilograms. The smallest animal cells are neurons, which are about 100 microns wide.
Animal cells are generally smaller than plant cells, and they often have an irregular shape because they do not have a cell wall. This is why animal cells come in many different forms, such as round, oval, flat, rod-shaped, spherical, concave, or rectangular. Their shape depends on the job they do in the body. Note: Because these cells are so small, we can only see them using a microscope.
Even though animal cells are different from plant cells, they do share some similar organelles (the small parts inside the cell) because both animals and plants are made of eukaryotic cells (cells with a nucleus).
As already mentioned, animal cells are eukaryotic, meaning they have a nucleus where the DNA (genetic material) is stored. The nucleus is protected by a membrane. Animal cells also have many other organelles inside, which help the cell do specific jobs, like keeping the cell alive and making sure the body works properly.
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Types of Animal Cells
Animal cells come in different types, each doing specific jobs in the body. They include:
Skin Cell: These cells make up the tissues that protect the body’s surface. They include:
Melanocytes: Produce pigment (colour) in the skin.
Keratinocytes: Make keratin, a protein that protects skin.
Merkel Cells: Help with the sense of touch.
Langerhans Cells: Involved in the immune system (protect against infections).
Muscle Cell: These long tubular cells help the body move. They include:
Skeletal Muscle Cells: Responsible for voluntary movements.
Cardiac Muscle Cells: Found in the heart and helps it pump blood.
Smooth Muscle Cells: Found in organs like the stomach, helping with involuntary actions like digestion.
Nerve Cell: These are found in the nervous system and are also called neurons. They have:
Dendrites and Axons: Extensions that send and receive signals.
Schwann Cells: Support neurons and help nerve signals travel faster.
Glial Cells: Provide support and protection to neurons.
Blood Cell: These cells carry oxygen throughout the body. They include:
Red Blood Cells: Carry oxygen from the lungs to other parts of the body. These cells have no nucleus.
White Blood Cells: Help fight infections.
Fat Cell: Also called adipocytes, these cells store fats and other lipids (fats).
Animal Primary Tissues
In animals, there are four main types of tissues. These tissues are made of groups of similar cells working together to do specific jobs. The four types of tissues are:
Epithelial Tissue: This tissue covers and protects the body and its organs (like skin or the lining of the stomach).
Connective Tissue: Provides support and structure to the body (like bones and blood).
Muscle Tissue: Helps with movement (like muscles in your arms and legs).
Nervous Tissue: Sends signals throughout the body, allowing you to think, feel, and move.
These tissues combine to form organs, like the skin or kidney, which have specialised functions. For example:
The circulatory system is an organ system made up of the heart and blood vessels.
The digestive system includes organs like the stomach, intestines, liver, and pancreas.
Together, organ systems form an entire organism (like humans or animals).
Epithelial Tissues
Epithelial tissues are like a covering layer. They cover the outside of organs and the inside linings of different parts of the body. They can be just one layer thick, or they can have multiple layers of cells.
Simple epithelia: This type has only one layer of cells. It’s simple and covers areas where not much protection is needed but where materials might pass through by diffusion or absorption.
Stratified epithelia: This type has more than one layer. It’s like having extra protection or padding. It covers areas that need more protection from wear and tear.
Squamous Epithelia
Structure: Squamous epithelial cells are round, flat, and have a small, centrally located nucleus.
Shape: The cell outlines are slightly irregular, and the cells fit tightly together to form a continuous layer.
Function: These cells facilitate diffusion and filtration processes in the body, such as gas exchange in the lungs and nutrient exchange in blood capillaries.
Example: Found in areas like lung alveoli and blood vessels.
Cuboidal Epithelia
Structure: Cuboidal epithelial cells are cube-shaped with a single, central nucleus.
Function: These cells help in secretion and absorption in glandular tissues and the ducts of organs.
Example: They are commonly found in glandular tissues like the ducts of the kidney, liver, and in the walls of tubules.
Columnar Epithelia
Columnar epithelial cells are cells that are taller than they are wide. They look like columns in a layer of epithelial tissue. These cells are mostly found in a single layer arrangement.
In the digestive tract, the nuclei of these cells are lined up at the bottom of the cells, near the base. They help absorb materials from the digestive system and send them into the body through the circulatory and lymphatic systems.
Here’s a diagram of the columnar epithelial cells along with goblet cells, which are special cells that produce mucus to protect the lining of the digestive tract.
Transitional Epithelium
Transitional or urothelial cells are found only in the urinary system, mainly in the bladder and ureter. These cells are arranged in layers and can look like they are stacked on top of each other when the bladder is relaxed and empty.
When the urinary bladder starts to fill up with urine, the epithelial layer spreads out and expands to hold the increasing volume of urine. As this happens, the epithelial layer becomes thinner.
So, in simple terms, when the bladder is empty, the tissue appears thicker. But as the bladder fills with urine, the tissue transitions and becomes thinner.
Connective Tissue
Connective tissue is one of the four basic types of tissues in the human body (the others being epithelial, muscle, and nervous tissues). Its primary function is to support, bind, and protect other tissues and organs in the body. Connective tissue is characterised by the presence of cells embedded in an extracellular matrix, which consists of fibres (such as collagen and elastin) and ground substance (a gel-like material).
The functions of connective tissue include:
Support: Provides structural framework for organs and the body.
Protection: Cushions and insulates organs, and in some cases, protects them from injury.
Binding: Connects different tissues and organs, holding them in place.
Storage: Stores energy in the form of fat.
Transport: Blood, a specialised form of connective tissue, transports oxygen, nutrients, and waste products.
Types of Connective Tissue
Connective tissue can be classified into several types based on the composition of the extracellular matrix and the types of cells they contain. It is broadly divided into connective tissue proper and specialised connective tissues.
1. Connective Tissue Proper
Connective tissue proper is the most abundant form of connective tissue in the body, providing structural support and flexibility.
a. Loose Connective Tissue
Structure: Contains loosely arranged fibres (collagen, elastin) and abundant ground substance.
Function: Provides cushioning, flexibility, and serves as a medium for nutrient and waste exchange.
Location: Found around blood vessels, nerves, and organs. The two common subtypes of loose connective tissue are:
Areolar Tissue: The most common type, found beneath the skin, around organs, and in mucous membranes. It holds organs in place and provides a reservoir of water and salts.
Adipose Tissue: Stores fat, insulates the body, and cushions organs. It is found beneath the skin (subcutaneous fat) and around organs like the heart and kidneys.
b. Dense Connective Tissue
Structure: Contains densely packed fibres, mostly collagen, with little ground substance.
Function: Provides strong support and resistance to stretching.
Location: Found in areas requiring strength and flexibility. There are two main types:
Dense Regular Connective Tissue: Collagen fibres are arranged in parallel bundles, providing strength in one direction. Examples include tendons (which connect muscles to bones) and ligaments (which connect bones to each other).
Dense Irregular Connective Tissue: Collagen fibers are arranged in an irregular pattern, providing strength in multiple directions. Found in the dermis of the skin, joint capsules, and around organs like the kidneys.
2. Specialized Connective Tissue
This category includes tissues that perform specific functions and have unique properties.
a. Cartilage
Structure: Contains a dense network of collagen and elastic fibres embedded in a firm but flexible ground substance. It is avascular (lacks blood vessels), which makes it slow to heal.
Function: Provides flexible support, reduces friction between bones, and helps absorb shock in joints.
Location: Found in joints, the rib cage, ear, nose, and respiratory passages. There are three types of cartilage:
Hyaline Cartilage: The most common type, providing smooth surfaces for joints and support for structures like the nose, trachea, and larynx.
Elastic Cartilage: Contains more elastic fibers, making it more flexible. Found in the external ear and epiglottis.
Fibrocartilage: Contains thick collagen fibers, making it very strong and capable of withstanding compression. Found in intervertebral discs and the menisci of the knee.
b. Bone (Osseous Tissue)
Structure: Composed of cells called osteocytes embedded in a matrix of collagen fibers and mineral salts (mainly calcium phosphate), giving it rigidity.
Function: Provides structural support, protection for organs, and a site for muscle attachment. It also serves as a reservoir for minerals like calcium and phosphorus.
Location: Found in the skeleton, which supports and protects the body and allows for movement.
c. Blood
Structure: A fluid connective tissue with a liquid matrix (plasma) that contains cells such as red blood cells, white blood cells, and platelets.
Function: Transports oxygen, nutrients, hormones, and waste products throughout the body. It also plays a role in immune defense and clotting.
Location: Circulates throughout the body within blood vessels (arteries, veins, and capillaries).
d. Lymph
Structure: A clear fluid derived from blood plasma, containing white blood cells.
Function: Part of the lymphatic system, it helps in transporting immune cells and removing excess fluid and waste from tissues.
Location: Found in lymphatic vessels and nodes throughout the body.
Muscle Tissues
There are three types of muscles in animal bodies: smooth, skeletal, and cardiac. They differ in several ways, including whether they have striations (bands), the number and location of their nuclei, whether they are controlled voluntarily or involuntarily, and where they are found in the body. Table 3 below summarises these differences.
Smooth Muscle:
Striations: No visible stripes.
Nuclei: One nucleus located in the center of each cell.
Control: Involuntary (works without conscious effort).
Location: Found in the walls of internal organs like the stomach, intestines, and blood vessels.
Skeletal Muscle:
Striations: Yes, it has visible bands.
Nuclei: Multiple nuclei found at the edges of the cells.
Control: Voluntary (you can control these muscles).
Location: Attached to bones and used for movement, like your arms and legs.
Cardiac Muscle:
Striations: Yes, has stripes similar to skeletal muscles.
Nuclei: One nucleus located in the center of each cell.
Control: Involuntary (works automatically, like the heart beating).
Location: Found only in the heart.
Nervous Tissue
Nervous tissues are made of cells specialised to receive and transmit electrical impulses from specific areas of the body and to send them to specific locations in the body. The main cell of the nervous system is the neuron, illustrated in the figure.
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The large structure with a Central nucleus is the body of the neuron. Projections from the cell body are either dendrites specialised in receiving input or a single axon speciality in transmitting impulses. Some glial cells are also shown. Astrocytes regulate the chemical environment of the nerve cell and oligo-dendrites insulate the axon so the electrical nerve impulse is transferred more efficiently. Other glial cells that are not shown the nutritional and waste requirements of the neuron. Some of the glial cells are phagocytic and remove debris or damaged cells from the tissue. A nerve consists of neurons and glial cells.