Animal Tissue


In multicellular animals, various specialized cells work together to perform vital functions, exhibiting a division of labor that ensures the survival of the entire organism. This organizational hierarchy consists of cells, tissues, organs, and organ systems.

1. Cells:

  • Unicellular organisms perform all essential functions, such as digestion, respiration, and reproduction, within a single cell.
  •  In complex organisms, like humans, these functions are distributed among different cell types, each with a unique role.

2. Tissues:

  • Tissue is an organization of similar cells, along with intercellular substances, that work collectively to perform specific functions.
  • Complex animals primarily consist of four basic types of tissues: epithelial, connective, muscular, and nervous.

 3. Organs:

  • Organs are formed when various tissues are organized in specific proportions and patterns to serve a specialized purpose.
  • Examples of organs in the human body include the stomach, lungs, heart, and kidneys.

 4. Organ Systems:

  • Multiple organs can collaborate to carry out a common function through physical and/or chemical interactions.
  • These groups of organs collectively form organ systems, such as the digestive system and respiratory system.

 5. Division of Labor:

  • Within multicellular organisms, the division of labor is essential for efficient functioning and overall survival.
  • Cells, tissues, organs, and organ systems work in harmony to carry out distinct tasks, contributing to the well-being of the entire organism.

 In multicellular animals, tissues play a crucial role in maintaining the structure and function of various organs and systems. These tissues are broadly classified into four main types, each characterized by specific cell structures and functions.

(i) Epithelial,

(ii) Connective,

(iii) Muscular and

(iv) Neural

 Epithelial tissue

  • Epithelial tissue, often referred to as epithelium, is a critical component of animal tissues.
  • It covers body surfaces and linings, providing protection and facilitating various functions. 
  • Epithelial tissues can be broadly categorized into simple and compound epithelium, with further differentiation based on structural modifications of the cells.

1. Simple Epithelium:

  • Simple epithelium consists of a single layer of cells.
  • Functions: It serves as a lining for body cavities, ducts, and tubes.

 Types of Simple Epithelium:

a. Squamous Epithelium:

  • Composed of thin, flattened cells with irregular boundaries.
  • Found in the walls of blood vessels and air sacs of the lungs, facilitating diffusion. 

b. Cuboidal Epithelium:

  • Composed of cube-like cells.
  •  Commonly found in gland ducts and parts of nephrons in the kidneys, involved in secretion and absorption. 

c. Columnar Epithelium:

  • Composed of tall, slender cells with nuclei located at the base.
  • Found in the lining of the stomach and intestines, aiding in secretion and absorption. 

d. Ciliated Epithelium:

  • Contains cilia on the free surface.
  • Function is to move particles or mucus in a specific direction.
  • Found in organs like bronchioles and fallopian tubes.



e. Glandular Epithelium:

  • Glandular epithelium is a specialized type of columnar or cuboidal epithelial tissue responsible for secretion.
  • These glands can be categorized into two main types: unicellular and multicellular.
  • Additionally, glands are further divided based on the mode of secretion into exocrine and endocrine glands.

 Types of Glandular Epithelium:

  •  Glandular epithelium refers to cells specialized for secretion and can be found in columnar or cuboidal forms.
  • Glandular epithelium can be classified into two main types based on the structure and organization of the gland cells. 

a. Unicellular Glandular Epithelium:

- Unicellular glands consist of isolated glandular cells.

- Example: Goblet cells found in the alimentary canal, which secrete mucus to protect and lubricate the digestive tract.

 b. Multicellular Glandular Epithelium:

- Multicellular glands are formed by clusters of glandular cells working together.

- Example: Salivary glands, which secrete saliva to aid in digestion.


Classification Based on Secretion Mode:

 Glands can be further categorized based on how they release their secretions. 

a. Exocrine Glands:

- Exocrine glands secrete their products into ducts or tubes that lead to specific target locations.

- Common exocrine gland products include mucus, saliva, earwax, oil, milk, and digestive enzymes.

- The secretions are released at the body's surfaces or into cavities. 

b. Endocrine Glands:

- Endocrine glands differ from exocrine glands in that they do not have ducts.

- Instead, they secrete hormones directly into the fluid bathing the gland.

- These hormones are then transported through the bloodstream to distant target organs and tissues, regulating various physiological processes. 

2. Compound Epithelium:

  • Consists of more than one layer of cells.
  • It has a limited role in secretion and absorption.
  • Mainly provides protection against chemical and mechanical stresses.
  •  Locations include the skin's dry surface, the moist surfaces of the buccal cavity, pharynx, inner linings of salivary gland ducts, and pancreatic ducts.



Cell junction in Epithelial Tissue

  • Cell junctions are specialized structures that play a crucial role in maintaining the integrity and functionality of epithelial tissue, as well as other animal tissues.
  • These junctions provide both structural support and enable cellular communication.
  • There are three main types of cell junctions: tight junctions, adhering junctions, and gap junctions.

 1. Tight Junctions:

  • Function: Tight junctions are essential for creating a barrier that prevents the leakage of substances across the tissue.
  • Structure: These junctions consist of proteins that form a continuous, impermeable seal between adjacent cells.
  • Role: Tight junctions maintain the selective permeability of the epithelium, controlling the passage of ions, molecules, and preventing the uncontrolled movement of substances.

 2. Adhering Junctions (Adherens Junctions):

  • Function: Adhering junctions perform a "cementing" function, keeping neighboring cells together.
  • Structure: These junctions are formed by protein complexes that link cells and anchor them to the underlying cytoskeleton.
  • Role: Adhering junctions provide mechanical stability to tissues and allow for coordinated movement, especially in tissues that experience stretching or bending. 

3. Gap Junctions:

  • Function: Gap junctions enable direct communication between neighboring cells by connecting their cytoplasm.
  • Structure: Gap junctions are composed of connexin proteins that form channels called connexons, allowing the passage of ions, small molecules, and even larger molecules.
  • Role: These junctions facilitate rapid transfer of signals, nutrients, and other essential substances between cells, ensuring coordinated functions in the tissue.

 Connective Tissue

  • Connective tissues play a vital role in the complex animal body, where they are highly abundant and widely distributed.
  • Named for their primary function of connecting and supporting various organs and tissues, connective tissues range from softer varieties to specialized types, including cartilage, bone, adipose tissue, and blood.
  • In nearly all connective tissues (except blood), cells secrete fibers composed of structural proteins, such as collagen or elastin.
  • These fibers are crucial for providing strength, elasticity, and flexibility to the tissue. Additionally, connective tissue cells secrete modified polysaccharides, which accumulate between cells and fibers, forming a matrix or ground substance.
  • Connective tissues can be classified into three primary types:

i) Loose connective tissue,

ii) Dense connective tissue and

iii) Specialised connective tissue



1. Loose Connective Tissue

  • Loose connective tissue is a versatile type of connective tissue characterized by the loose arrangement of cells and fibers within a semi-fluid ground substance.
  • This tissue provides support for epithelium and plays a crucial role in maintaining the structural integrity of various body structures.
  • Structure: Loose connective tissue features a semi-fluid matrix in which cells and fibers are loosely arranged.
  • Function: It acts as a supporting framework for epithelial tissues, offering flexibility and support.
  • Components:

a. Fibroblasts: Cells responsible for producing and secreting fibers.

b. Macrophages: Immune cells that help defend the body against pathogens.

c. Mast Cells: Cells involved in the immune response, particularly allergic reactions and inflammation.





  • One specific example of loose connective tissue is the areolar tissue. Areolar tissue is a common type of loose connective tissue found beneath the skin, connecting and supporting various structures in the body.


  •  Additionally, adipose tissue is another type of loose connective tissue primarily located beneath the skin, with cells specially adapted for fat storage.

- Structure: Adipose tissue consists of specialized cells known as adipocytes that are designed for fat storage.

- Function: Its primary function is to store excess nutrients in the form of fats. This stored energy can be used when needed.

- Specialization: Adipocytes have a unique structure adapted for fat storage, with a single large lipid droplet occupying most of the cell's volume.

- Location: Adipose tissue is mainly found beneath the skin (subcutaneous) and around internal organs (visceral), providing insulation and cushioning, as well as serving as an energy reservoir.



2. Dense Connective Tissue

  • Dense connective tissues are a specialized category of connective tissues characterized by their compact arrangement of fibers and fibroblasts.
  • These tissues are classified into two main types based on the orientation of fibers: dense regular and dense irregular.
  • Each type serves specific functions within the body and is found in distinct locations.

 a. Dense Regular Connective Tissue:

  • Structure: In dense regular connective tissues, collagen fibers are neatly arranged in rows, forming a regular pattern.
  • Function: These tissues provide strong and durable connections between structures that require unidirectional tensile strength.
  • Examples:

a. Tendons: Attach skeletal muscles to bones, allowing the transmission of muscular forces to enable movement.

b. Ligaments: Connect one bone to another, providing stability and support to joints.

 b. Dense Irregular Connective Tissue:

  • Structure: Dense irregular connective tissue is characterized by its fibroblasts and the presence of many fibers (primarily collagen) that are oriented in various directions.
  • Function: This tissue offers strength and support while accommodating multidirectional stresses.
  • Location: Dense irregular connective tissue is primarily found in the skin and other areas where tissue integrity and flexibility are required.


3. Specialised Connective Tissue

  • Specialized connective tissues play critical roles in the complex animal body, serving unique functions that contribute to structural support, resilience, and the transportation of vital substances.
  • This category includes cartilage, bones, and blood, each characterized by distinct features and functions. 

a. Cartilage:

  • Structure: Cartilage has solid and pliable intercellular material that resists compression.
  • Cells: Chondrocytes, the cells within cartilage, are enclosed in small cavities within the matrix secreted by them.
  • Functions:

- Cartilage provides structural support and resilience.

- It resists compression, maintains the shape of various body structures, and allows flexibility in joints.

  • Examples of Locations: Cartilage is present in the tip of the nose, outer ear, joints, between adjacent bones in the vertebral column, limbs, and hands in adults.

 2. Bones:

  • Structure: Bones have a hard and non-pliable ground substance rich in calcium salts and collagen fibers, contributing to their strength.
  • Cells: Osteocytes are the bone cells present in small spaces called lacunae.
  • Functions:

- Bones form the primary structural framework of the body.

- They support and protect softer tissues and organs.

- Bones interact with skeletal muscles to facilitate movements.

- Some bones contain bone marrow, where blood cells are produced.

  • Examples of Locations: Limb bones, such as the long bones of the legs, serve weight-bearing functions and enable mobility.

 c. Blood:

  • Composition: Blood is a fluid connective tissue consisting of plasma, red blood cells (RBCs), white blood cells (WBCs), and platelets.
  •  Functions:

- Blood is the primary circulating fluid responsible for transporting essential substances throughout the body.

- Red blood cells transport oxygen, while white blood cells play a crucial role in the immune response.

- Platelets are essential for blood clotting and wound healing.

  • Blood plasma carries various solutes and nutrients, contributing to homeostasis and overall health.


 Muscle Tissue

  • Muscles are essential components of the human body responsible for generating movement and maintaining various bodily functions.
  • These contractile tissues are composed of long, cylindrical fibers arranged in parallel arrays.
  • Within these muscle fibers, numerous fine fibrils, known as myofibrils, facilitate the process of contraction and relaxation.
  • The coordinated actions of muscles play a pivotal role in adapting to environmental changes and maintaining the positions of different body parts.
  • Muscles are of three types, skeletal, smooth, and cardiac.


1. Skeletal Muscle

  • Skeletal muscle tissue is a critical component of the musculoskeletal system.
  • Attachment to Bones: Skeletal muscle tissue is closely attached to skeletal bones, allowing for the transmission of muscular forces to the bones and facilitating controlled movement.
  • Organization: In a typical skeletal muscle, such as the biceps, individual skeletal muscle fibers are bundled together in a parallel arrangement.
  • Striated Appearance: Skeletal muscle fibers are striated, displaying a striped appearance due to the repeating arrangement of actin and myosin protein filaments.
  • Connective Tissue Sheath: A protective sheath of tough connective tissue encloses several bundles of muscle fibers within a muscle.
  • This type of muscle is typically found in the limbs and plays a fundamental role in producing voluntary movements.

 2. Smooth Muscle

  • Fusiform Shape: Smooth muscle fibers have a tapered, fusiform shape, meaning they are narrower at both ends.
  • Lack of Striations: Unlike skeletal muscle, smooth muscle fibers do not display striations, which are the repeating dark and light bands seen in striated muscle.
  • Cell Junctions: Cell junctions, including gap junctions, hold smooth muscle fibers together and allow for coordinated contractions.
  • Connective Tissue Sheath: Bundles of smooth muscle fibers are typically enclosed in a connective tissue sheath, providing support and organization.
  • Location-

- Internal Organs: Smooth muscle is primarily found in the walls of internal organs such as the blood vessels, stomach, intestines, and various other structures.

  •  Involuntary Function: Smooth muscles are considered "involuntary" because their functioning is not directly controlled by conscious thought. Contractions of smooth muscles are typically regulated by the autonomic nervous system and hormonal signals.

 3. Cardiac Muscle

  • Exclusive to the Heart: Cardiac muscle tissue is found exclusively in the heart, where its contractions are essential for pumping blood throughout the circulatory system.
  • Cell Junctions: Cardiac muscle cells are held together by cell junctions, particularly intercalated discs. These junctions fuse the plasma membranes of adjacent cells.
  • Intercalated Discs: Intercalated discs are specialized communication junctions located at specific fusion points between cardiac muscle cells.
  •  Coordinated Contraction: Intercalated discs facilitate communication between cardiac muscle cells. When one cell receives a signal to contract, the intercalated discs ensure that neighboring cells are also stimulated to contract.

 Neural Tissue

  • Neural tissue plays a fundamental role in controlling the body's responses to changing conditions and stimuli.
  • At the core of the neural system are neurons, excitable cells that enable the transmission of electrical signals.
  • Neurons:

- Excitable Cells: Neurons are the basic functional units of the neural system and are excitable, meaning they can generate and transmit electrical signals.

- Transmission of Electrical Disturbances: When appropriately stimulated, neurons generate electrical disturbances that swiftly travel along their plasma membranes.

- Output Zone: These electrical disturbances, upon reaching the neuron's endings or output zone, trigger events that can stimulate or inhibit adjacent neurons and other cells.

  • Supporting the neurons are neuroglial cells, which make up a significant portion of the neural system, providing protection and support to the neurons.
  • Neuroglial Cells:

- Supportive Role: Neuroglial cells constitute the rest of the neural system and serve a vital role in protecting and supporting neurons.

- Abundance: In the human body, neuroglia make up more than half of the total volume of neural tissue, emphasizing their significance in neural function.    

  • These cells collectively govern the body's responsiveness to various stimuli, and the transmission of electrical disturbances within neurons is a key mechanism underlying neural function.