Animal Tissues

In multicellular animals, a group of similar cells along with intercellular material perform a specific function. Such an organisation is called tissue.

The animal body is composed of four basic types of tissues:

Epithelial tissue: Covers the surface of the body and lines internal cavities and organs.
Connective tissue: Supports and connects other tissues and organs.
Muscular tissue: Responsible for movement.
Neural tissue: Conducts nerve impulses.

These tissues are organised in specific proportions and patterns to form organs (e.g., stomach, lung, kidney). Organs then combine to form organ systems (e.g., digestive system, respiratory system).

Epithelial Tissue

Epithelial tissue (also called epithelium) forms the covering or lining of external and internal surfaces of the body. Cells in this tissue are compactly packed with little intercellular matrix.

Types of Epithelial Tissue (Based on Number of Layers)

Simple Epithelium: Composed of a single layer of cells. It functions as a lining for body cavities, ducts, and tubes.
Compound Epithelium: Consists of two or more cell layers. It has a protective function, like in the skin.

Types of Simple Epithelium (Based on Shape of Cells)

The simple epithelium is further classified based on the structural modification of the cells:

Squamous Epithelium: Made of a single layer of flattened cells with irregular boundaries. The nuclei are flattened and usually centrally located.
Function: Forms a delicate lining, involved in processes like diffusion and filtration.

Location: Found in the walls of blood vessels, air sacs of lungs (alveoli), lining of Bowman's capsule in kidney. This is also called pavement epithelium.

*(Image shows a single layer of flattened, irregular-shaped cells)*

Cuboidal Epithelium: Composed of a single layer of cube-like cells. The nuclei are round and centrally located.
Function: Involved in secretion and absorption.

Location: Found in the lining of kidney tubules, ducts of glands (e.g., salivary glands, pancreas), and parts of nephrons. Epithelium of proximal convoluted tubule (PCT) in the kidney has microvilli for increased absorption (brush border cuboidal epithelium).

*(Image shows a single layer of cube-shaped cells)*

Columnar Epithelium: Composed of a single layer of tall and slender cells. The nuclei are usually located at the base.
Function: Involved in secretion and absorption.

Location: Lines the stomach and intestine.
- If these cells bear microvilli on their free surface, it is called brush border columnar epithelium, which increases the surface area for absorption (found in the intestine).
- Some cells may get modified into glandular cells that secrete mucus (goblet cells).
*(Image shows a single layer of tall, rectangular cells)*

Ciliated Epithelium: Either cuboidal or columnar cells that bear cilia on their free surface.
Function: To move particles or mucus in a specific direction over the epithelium.

Location: Found in the inner surface of hollow organs like bronchioles and fallopian tubes.

Pseudostratified Epithelium: Although appearing multilayered, it is actually a single layer of cells where some cells are shorter than others, but all cells rest on the basement membrane. The nuclei are found at different levels, giving a stratified appearance. Often ciliated and may contain goblet cells.
Function: Mucus secretion and transport (ciliated type).

Location: Lining of the respiratory tract (trachea, large bronchi).

Glandular Epithelium

Some epithelial cells become specialised for secretion and are called glandular epithelium. They can be:

Unicellular glands: Consisting of single isolated glandular cells (e.g., goblet cells in the lining of the alimentary canal).
Multicellular glands: Consisting of a cluster of cells (e.g., salivary gland).

Based on the mode of pouring secretion, glands are:

Exocrine glands: Secrete mucus, saliva, earwax, oil, milk, digestive enzymes, and other cell products through ducts or tubes (e.g., salivary gland, sweat gland).
Endocrine glands: Secrete hormones directly into the fluid surrounding the gland (usually blood), without ducts (ductless glands) (e.g., thyroid gland, pituitary gland).

Compound Epithelium

Made up of more than one layer of cells.
Function: Primarily provides protection against mechanical, chemical, or thermal stress.
Location: Found in the dry surface of the skin, the moist surface of the buccal cavity, pharynx, inner lining of ducts of salivary glands and pancreatic ducts.
Example: Stratified Squamous Epithelium (e.g., skin epidermis - keratinised; lining of mouth - non-keratinised).

Cell Junctions

Cells of the epithelium are held together by specialised structures called cell junctions. These junctions provide structural and functional links between adjacent cells.

Tight junctions: Help to stop substances from leaking across a tissue.
Adhering junctions: Provide cementing to keep neighbouring cells together.
Gap junctions: Facilitate the cells to communicate with each other by connecting the cytoplasm of adjoining cells, for rapid transfer of ions, small molecules and sometimes larger molecules.

Connective Tissue

Connective tissue is the most abundant and widely distributed tissue in the body of complex animals. It connects or supports other tissues/organs. They are diverse in structure and function.

Components of Connective Tissue

Connective tissues generally consist of three basic components:

Cells: Various types of cells depending on the specific connective tissue (e.g., fibroblasts, macrophages, mast cells, adipocytes, lymphocytes).
Fibres: Protein fibres secreted by cells. They provide strength, elasticity, and flexibility. Main types are:
- Collagen fibres: White fibres, provide strength.
- Elastic fibres: Yellow fibres, provide elasticity.
- Reticular fibres: Fine branching fibres, form a delicate network.
Ground substance (Matrix): The intercellular material, a transparent, jelly-like substance made of modified polysaccharides and proteins. It fills the space between cells and fibres and acts as a medium for transport. The nature of the matrix determines the specific function of the connective tissue (e.g., fluid in blood, rigid in bone, flexible in cartilage).

Unlike epithelial tissue, connective tissue cells are loosely spaced, and there is abundant intercellular matrix.

Types of Connective Tissue

Connective tissues are classified into three main types:

Loose Connective Tissue
Dense Connective Tissue
Specialised Connective Tissue

Loose Connective Tissue

Fibres and cells are loosely arranged in a semi-fluid ground substance.

Areolar Tissue: Contains fibroblasts (secrete fibres), macrophages, and mast cells. Fibres (collagen and elastic) are loosely scattered.
Function: Serves as a supporting framework for epithelium. Provides support and acts as packing material around organs, nerves, and blood vessels.

Location: Present beneath the skin.

*(Image shows fibroblasts, macrophages, mast cells, collagen fibres, elastic fibres in ground substance)*

Adipose Tissue: Specialised to store fats. Contains cells called adipocytes, which are filled with fat globules.
Function: Stores fat, acts as an insulator (prevents heat loss), provides cushioning to organs.

Location: Located mainly beneath the skin (subcutaneous fat), around internal organs.

*(Image shows round or oval cells filled with large fat droplets, pushing the nucleus and cytoplasm to the periphery)*

Dense Connective Tissue

Fibres and fibroblasts are compactly packed. Orientation of fibres can be regular or irregular.

Dense Regular Connective Tissue: Collagen fibres are arranged in parallel rows between parallel bundles of fibres.
Function: Provides strength and flexibility in one direction.

Example:
- Tendons: Connect muscle to bone. Made of tough collagen fibres.
- Ligaments: Connect bone to bone. Contain collagen and more elastic fibres than tendons.
*(Image shows closely packed, parallel collagen fibres with fibroblasts between them)*

Dense Irregular Connective Tissue: Fibres (mostly collagen) and fibroblasts are oriented in different directions.
Function: Provides strength in multiple directions.

Location: Found in the skin (dermis) and surrounding the capsules of organs.

*(Image shows randomly arranged collagen fibres with fibroblasts)*

Specialised Connective Tissue

Includes cartilage, bone, and blood, which have specialised structures and functions.

Cartilage:
- The intercellular material (matrix) is solid, pliable, and resists compression.
- Cells (chondrocytes) are enclosed in small cavities (lacunae) within the matrix.
- Blood vessels are generally absent (avascular).
- Function: Provides support and flexibility, found in joints, nose tip, outer ear joints, trachea.
- Example: Hyaline cartilage (most common), Elastic cartilage (external ear), Fibrocartilage (intervertebral discs).
*(Image shows chondrocytes within lacunae embedded in a matrix)*

Bone:
- The intercellular material (matrix) is hard and non-pliable, rich in calcium salts (calcium phosphate) and collagen fibres, which gives bone its strength.
- Cells (osteocytes) are present in lacunae. Lacunae are arranged in concentric rings (lamellae) around a central canal (Haversian canal) in compact bone. Canals connecting lacunae are called canaliculi.
- Bone marrow in the cavity of bones is the site of blood cell production.
- Function: Provides structural framework and support to the body, protects organs, helps in movement, stores calcium and phosphate.
*(Image shows a cross-section of compact bone highlighting lamellae, lacunae with osteocytes, canaliculi, and a central Haversian canal)*

Blood:
- Considered a fluid connective tissue.
- The matrix is fluid, called plasma.
- Contains blood cells (corpuscles): Red blood cells (RBCs), White blood cells (WBCs), and platelets.
- Fibres are absent (except for clotting factors in plasma).
- Function: Transport of gases ($O_2, CO_2$), nutrients, hormones, waste products. Involved in defence (WBCs) and clotting (platelets).
*(Image shows red blood cells, white blood cells (various types), and platelets in a liquid background (plasma))*

Muscular Tissue

Muscular tissue is composed of elongated cells called muscle fibres. Muscle fibres are capable of contraction and relaxation, enabling movement and locomotion.

Muscles are typically composed of bundles of muscle fibres. Muscle fibres contain numerous fine fibrils called myofibrils. Myofibrils contain contractile proteins (actin and myosin).

Types of Muscular Tissue

Muscular tissue is of three types:

Skeletal Muscle:
- Appearance: Striated (show light and dark bands or striations).
- Control: Voluntary (under conscious control).
- Structure: Long, cylindrical, unbranched fibres. Many nuclei per cell (multinucleate), located at the periphery.
- Location: Attached to bones by tendons.
- Function: Movement of body parts, locomotion, posture.
*(Image shows elongated, cylindrical, striated fibres with multiple peripheral nuclei)*

Smooth Muscle:
- Appearance: Non-striated (lack striations).
- Control: Involuntary (not under conscious control).
- Structure: Spindle-shaped (tapering at both ends) fibres. Single nucleus per cell, located centrally.
- Location: Found in the walls of internal organs (viscera) such as the stomach, intestine, blood vessels, uterus, iris of the eye.
- Function: Involuntary movements like peristalsis in the digestive tract, regulation of blood flow, changing pupil size.
*(Image shows spindle-shaped, non-striated cells with a single central nucleus)*

Cardiac Muscle:
- Appearance: Striated.
- Control: Involuntary.
- Structure: Branched fibres. Cells are joined together by specialised junctions called intercalated discs, which allow rapid communication and coordinated contraction. Usually single nucleus per cell (sometimes two), located centrally.
- Location: Found only in the wall of the heart.
- Function: Pumping of blood through the heart. Contraction is rhythmic and self-generated (myogenic).
*(Image shows branched, striated fibres with central nuclei and intercalated discs between cells)*

Feature	Skeletal Muscle	Smooth Muscle	Cardiac Muscle
Striations	Present	Absent	Present
Control	Voluntary	Involuntary	Involuntary
Shape	Cylindrical	Spindle-shaped	Branched
Nuclei per cell	Many (multinucleate)	One	One (or two)
Nuclei location	Peripheral	Central	Central
Intercalated Discs	Absent	Absent	Present
Location	Attached to bones	Walls of internal organs	Heart wall

Neural Tissue

Neural tissue (also called nervous tissue) is specialised for conducting electrical impulses. It forms the nervous system (brain, spinal cord, nerves).

Components of Neural Tissue

Neural tissue consists of two main types of cells:

Neurons (Nerve cells): The functional units of the nervous system, responsible for transmitting nerve impulses.
Neuroglia (Glial cells): Supporting cells that protect and support neurons. They make up more than half the volume of neural tissue.

Neuron (Nerve Cell)

Each neuron is composed of three main parts:

Cell body (Soma): Contains the nucleus and cytoplasm. The cytoplasm contains characteristic granular bodies called Nissl's granules (made of ribosomes and RER).
Dendrites: Short, branched processes extending from the cell body. They receive signals (nerve impulses) from other neurons and transmit them towards the cell body.
Axon: A single, long projection extending from the cell body. It transmits nerve impulses away from the cell body towards other neurons, muscles, or glands.
- The axon may be covered by a myelin sheath (formed by Schwann cells or oligodendrocytes) with gaps called Nodes of Ranvier. Myelinated axons conduct impulses faster (saltatory conduction).
- The terminal end of the axon branches into bulb-like structures called synaptic knobs, which contain neurotransmitters.

Diagram showing the structure of a neuron (nerve cell)

*(Image shows a neuron with cell body, nucleus, dendrites, axon, myelin sheath, Nodes of Ranvier, and synaptic terminals)*

Neuroglia (Glial Cells)

These cells do not conduct nerve impulses.
They provide support, protection, and nourishment to neurons.
They also clear cellular debris and participate in the formation of the blood-brain barrier.
Example: Astrocytes, Oligodendrocytes (form myelin sheath in CNS), Schwann cells (form myelin sheath in PNS), Microglia.

Function of Neural Tissue

Neural tissue is highly excitable. When stimulated, neurons generate and transmit electrical signals (nerve impulses) rapidly from one part of the body to another.

This allows the nervous system to coordinate body activities, sense stimuli, process information, and initiate responses.

Diagram showing a synapse where a neuron transmits signal to another cell

*(Image shows the junction (synapse) between the axon terminal of one neuron and the dendrite/cell body of another neuron, illustrating signal transmission)*