Chapter 1 — Tissue

Definition and idea: A tissue is a group of similar cells arranged together to perform a specific function. While a single cell can carry out simple tasks, many processes in multicellular organisms need large numbers of cells cooperating. Tissues are the second level of organisation after cells and together form organs and organ systems.

Why cells form tissues: Grouping increases efficiency. For instance, a single heart muscle cell cannot pump blood for the whole body, but a large mass of coordinated muscle cells can. Tissues also allow specialisation: some cells become thin and flat to cover surfaces, others become long and contractile for movement.

Levels of organisation and examples: The usual order is cell → tissue → organ → organ system → organism. Examples: many epithelial cells form the skin; groups of xylem and phloem cells form vascular bundles in plants. Recognising tissues helps explain how complex structures work — for example, the stomach wall has muscle tissue for churning, epithelium for secretion and connective tissue for support.

Characteristics of tissues: Tissues show patterns: cell shape (flat, cuboidal, columnar), arrangement (single layer or many), presence or absence of intercellular material, and living or dead cells. Animal tissues often have less extracellular material between cells except connective tissue, while plant tissues commonly have cell walls and may become lignified.

Observation and classroom work: Simple microscope work helps students see tissues: onion epidermis as a sheet of cells resembling epithelium, cheek cells as animal epithelial cells, and thin stem sections showing vascular bundles. Students should practise drawing observations, labelling parts and noting functions. This basic concept of tissue links simple cell studies to organs and systems studied later in biology.

Main groups: Animal bodies are built from four principal tissue types: epithelial, connective, muscular and nervous. Each group contains cells specialised for particular functions and often shows a typical arrangement and appearance.

Epithelial tissue: Forms continuous sheets covering body surfaces and lining internal cavities. It protects, absorbs nutrients, secretes substances and contains sensory cells. Cells are closely packed with little intercellular substance and often rest on a basement membrane.

Connective tissue: Connective tissues support and join other tissues and organs. Cells are scattered within an extracellular matrix that may be liquid (blood), jelly-like (loose connective tissue), or solid (bone). Fibres such as collagen give strength, while elastic fibres allow stretch.

Muscular tissue: Muscle cells are specialised to contract and produce movement. Muscle tissues differ in control and appearance: skeletal muscle is under voluntary control and appears striated; cardiac muscle pumps the heart and is involuntary with special junctions; smooth muscle works involuntarily in organs and blood vessels.

Nervous tissue: Composed of neurons and supporting glial cells. Neurons transmit electrical impulses to sense stimuli, coordinate activities and direct responses. Nervous tissue forms the brain, spinal cord and peripheral nerves.

Integration in organs: Organs are made of combinations of these tissues. For example, lungs have epithelial lining for gas exchange, connective tissue as support, blood (a connective tissue) for transport, and nervous tissue to regulate breathing. Learning each tissue type helps explain organ function, disease and healing.

Structure: Epithelial tissue consists of closely packed cells with little intercellular material, forming sheets that cover surfaces and line cavities. Cells rest on a thin basement membrane that anchors the epithelium to underlying connective tissue. Epithelia are classified by layers and shape: simple (single layer) or stratified (multiple layers); shapes: squamous (flat), cuboidal (cube-like) and columnar (tall).

Specialised features: Many epithelial cells show specialisations on their free surface: cilia are hair-like projections that move fluids and particles (as in the trachea), microvilli increase surface area for absorption (as in the intestine), and secretory cells form glands. Some epithelia have keratin on the surface (skin) to increase toughness and water resistance.

Functions: Protection: stratified squamous epithelium protects against abrasion and infection. Absorption: simple columnar epithelium with microvilli absorbs nutrients in the gut. Secretion: glandular epithelium forms glands that secrete enzymes, hormones or mucus. Sensation: specialised epithelial cells in sensory organs detect stimuli like taste and smell.

Regeneration and repair: Many epithelial tissues divide frequently to replace lost or damaged cells because they face the external environment and sustain wear. Rapid regeneration helps close wounds, but deep damage may require connective tissue repair and lead to scarring.

Locations and examples: Epithelia cover the skin, line the mouth, trachea, gut, blood vessels, body cavities and form glands (salivary glands, sweat glands). In lab slides, students can identify thin flat cells of simple squamous epithelium, or tall columnar cells with nuclei aligned near one end.

General description: Connective tissue links, supports and protects body parts. It differs from epithelial tissue by having cells scattered in a prominent extracellular matrix. The matrix contains fibres (collagen for strength, elastic for elasticity) and ground substance that can be watery, gel-like or hardened by minerals. This composition allows a wide variety of connective tissues with different functions.

Loose connective tissue: Also called areolar tissue, it contains many cells such as fibroblasts, macrophages and mast cells embedded in a soft matrix with collagen and elastic fibres. It wraps organs, holds them in place, and provides a reservoir of salts and water.

Dense connective tissue: Rich in collagen fibres aligned in the direction of stress. Tendons (muscle to bone) and ligaments (bone to bone) are examples. These tissues resist pulling forces and provide strong mechanical links.

Specialised connective tissues: Cartilage is flexible and smooth, found in joints, ear and nose; cells called chondrocytes lie in lacunae within a firm matrix. Bone is a hard, mineralised tissue with osteocytes in lacunae and a matrix of calcium salts and collagen for support and protection. Blood is considered a fluid connective tissue: plasma is the matrix and red and white blood cells and platelets are suspended in it; blood transports gases, nutrients and immune cells. Adipose tissue stores fat in adipocytes and cushions organs and insulates the body.

Cells and repair: Fibroblasts make matrix components, osteoblasts build bone, chondroblasts build cartilage. Connective tissues often heal by forming scar tissue: fibroblasts make collagen to close gaps, which restores strength though sometimes with reduced function compared to original tissue.

Purpose and basic action: Muscular tissue is specialised to contract and generate force, enabling movement of body parts, maintenance of posture and production of heat. Muscle cells contain contractile proteins (actin and myosin) arranged in a way that allows shortening when stimulated, converting chemical energy into mechanical work.

Types of muscle tissue: There are three major types: skeletal, smooth and cardiac. Each differs in structure, control and function.

Skeletal muscle: Attached to bones by tendons, skeletal muscle produces voluntary movements like walking and lifting. Muscle fibres are long, cylindrical and multinucleated, showing alternating light and dark bands (striations) due to organised sarcomeres. Contractions are under conscious control through motor neurons and occur rapidly.

Smooth muscle: Found in walls of internal organs such as intestine, bladder, ureter and blood vessels, smooth muscle cells are spindle-shaped and lack striations. Contractions are slower and involuntary, controlled by the autonomic nervous system and local hormones. Smooth muscle performs actions like peristalsis in the gut and constriction of arteries to regulate blood flow.

Cardiac muscle: Present only in the heart, cardiac muscle has striations like skeletal muscle but works involuntarily. Cells are branched and connected by intercalated discs that contain gap junctions and desmosomes, enabling rapid, coordinated contraction across the heart wall for effective pumping. Cardiac muscle has abundant mitochondria to meet high energy demands.

Energy, fatigue and training: Muscles require oxygen and nutrients; during intense work anaerobic respiration may produce lactic acid causing fatigue. Regular exercise increases muscle strength by hypertrophy (increase in fibre size) and more mitochondria, while endurance training improves oxygen delivery and fatigue resistance.

Overview and role: Nervous tissue forms the communication network of animals. It senses internal and external changes, transmits signals, integrates information and coordinates responses. Nervous tissue forms the brain and spinal cord (central nervous system) and peripheral nerves that connect the rest of the body.

Neurons — the functional units: Neurons are specialised cells that transmit electrical impulses. A typical neuron has a cell body (soma) containing the nucleus and synthetic machinery; dendrites are short branched projections that receive incoming signals; an axon is a long extension that conducts impulses away from the cell body toward other neurons or effectors. Axons may be covered by a myelin sheath (produced by glial cells) that insulates and speeds conduction.

Supporting glial cells: Neuroglia or glial cells are non-neuronal cells that support and protect neurons. They provide nutrients, remove debris, maintain ion balance, and form myelin (oligodendrocytes in the CNS, Schwann cells in the PNS). Glial cells also help in repair and form the blood–brain barrier components.

How impulses travel (simple view): A stimulus causes a change in the neuron's membrane potential; this electrical change travels as an action potential along the axon. At the axon terminal, neurotransmitters are released into the synaptic cleft, crossing to the next neuron or muscle cell and starting a new impulse. This allows fast, directed communication from sensory organs to the brain and then to muscles or glands.

Functions and clinical notes: Nervous tissue controls voluntary actions (walking), involuntary actions (heart rate), reflexes and higher functions such as thinking and memory. Some nervous tissue, especially in the peripheral nervous system, can regenerate to an extent, while many central nervous system neurons have limited ability to regrow, making injuries to spinal cord or brain often permanent.

Broad categories: Plant tissues are mainly classified into meristematic and permanent tissues. Meristematic tissues contain actively dividing cells responsible for growth. Permanent tissues are differentiated cells that perform specialised functions such as support, storage and transport.

Meristematic tissues and growth: Apical meristems at shoot and root tips produce new cells for lengthwise growth (primary growth). Lateral meristems, like vascular cambium and cork cambium, add thickness in woody plants (secondary growth). Intercalary meristems occur at internodes or leaf bases in grasses and permit regrowth after cutting.

Simple permanent tissues: Parenchyma, collenchyma and sclerenchyma are simple tissues made of similar cells. Parenchyma cells are generally living, thin-walled and involved in storage and photosynthesis. Collenchyma has unevenly thickened walls and gives flexible support in growing regions. Sclerenchyma cells have thick, lignified walls and provide rigid support and protection.

Complex permanent tissues: Xylem and phloem form the vascular system. Xylem conducts water and minerals and also provides structural support; it consists of vessel elements, tracheids, xylem parenchyma and fibres. Phloem transports sugars and includes sieve tube elements and companion cells, as well as phloem fibres and parenchyma.

Epidermis and adaptations: The epidermis is the outer protective layer and may bear stomata for gas exchange, hairs for protection, and a waxy cuticle to reduce water loss. Many plant tissues are key to agricultural practices: cambium activity explains tree rings and grafting; meristematic tissue is used in tissue culture to propagate plants.

Definition and cell features: Meristematic tissue is made up of actively dividing, undifferentiated cells that allow plants to grow. Meristematic cells are small, with thin primary cell walls, dense cytoplasm, a prominent nucleus, and little or no large central vacuole. Their ability to divide by mitosis produces new cells that either remain meristematic or differentiate into permanent tissues.

Types by position and function: Apical meristems are found at the tips of shoots and roots and are responsible for primary growth — increasing length. Lateral meristems, such as the vascular cambium and cork cambium (phellogen), are responsible for secondary growth — increasing girth in woody plants. Intercalary meristems are located at internodes or leaf bases (common in grasses) and help regrowth after grazing or cutting.

Activity and differentiation: Cells produced by apical meristems pass through zones: a meristematic zone where cells divide, a zone of elongation where cells lengthen, and a zone of differentiation where cells specialise into tissues such as xylem, phloem, epidermis and ground tissue. Lateral meristems like the vascular cambium form secondary xylem (wood) towards the inside and secondary phloem towards the outside, creating rings in temperate trees which are records of annual growth.

Practical importance: Meristems are used in plant propagation and biotechnology. Cuttings root because cells near the cut can form new meristematic tissue; grafting requires cambium alignment for successful union. Plant tissue culture uses explants rich in meristematic cells grown in sterile nutrient medium with hormones to regenerate whole plants, enabling mass propagation of disease-free plants and conservation of rare species.

Observation and safety: Under the microscope shoot and root tips show small closely packed cells; students should learn to identify the meristematic zone and the adjacent elongation and differentiation zones. Handling meristems in labs requires careful sterile technique for tissue culture work.

Simple permanent tissues: Parenchyma, collenchyma and sclerenchyma are simple plant tissues composed of one cell type each and perform fundamental roles such as storage, support and protection.

Parenchyma: Parenchyma cells are living, with thin primary walls and a large central vacuole. They form the bulk of soft plant parts like cortex and pith and are involved in storage of starch, oils and water. Chlorenchyma is a type of parenchyma containing chloroplasts for photosynthesis. Parenchyma cells can often divide and help in wound repair and callus formation during grafting or tissue culture.

Collenchyma: Collenchyma provides flexible mechanical support in young, growing regions of the plant such as stems, leaf stalks and petioles. Cells are living and show uneven thickening of primary cell walls, often at the corners. This arrangement gives tensile strength while allowing growth and bending without breaking; for example, the strings in celery are bundles of collenchyma.

Sclerenchyma: Sclerenchyma cells have thick, lignified secondary walls and are usually dead at maturity. They give rigid structural support and protection. Two common forms are fibres, which are long and slender and used commercially (jute, hemp), and sclereids, which are shorter and give hardness to seed coats and gritty texture to pear fruit. Sclerenchyma strengthens tissues where growth has finished.

Comparison and roles together: Parenchyma is versatile for metabolic functions and storage; collenchyma is a living support tissue in growing parts providing flexibility; sclerenchyma provides permanent rigidity and protection. Understanding these simple tissues explains how plants balance growth and strength and why different parts feel soft or hard.

Function and importance: Xylem is the vascular tissue that transports water and dissolved minerals from roots to stems and leaves. It also provides mechanical strength to the plant. Xylem forms part of the plant's vascular system along with phloem, and its organisation and properties are key to plant survival and growth.

Components and structure: Xylem is a complex tissue composed of vessel elements (in angiosperms), tracheids (common in gymnosperms and some angiosperms), xylem fibres and xylem parenchyma. Vessel elements are long tubular cells arranged end-to-end with open ends to form continuous vessels. Tracheids are elongated cells with tapered ends and pits that allow water passage. Xylem fibres provide strength and xylem parenchyma store food and help in lateral transport.

Cell condition: Many xylem conducting cells are dead at maturity and lack protoplasm, forming hollow tubes lined by thick, lignified walls that resist collapse under tension and provide support. Lignin also waterproofs walls and contributes to wood strength.

Mechanism of upward movement: Water moves up the plant primarily by transpiration pull: water evaporating from leaf surfaces creates a negative pressure (tension) in leaf xylem, pulling water up from the roots. Cohesion (water molecules sticking to each other) and adhesion (water molecules sticking to xylem walls) maintain a continuous water column. Root pressure, caused by active ion uptake in roots, can push water up short distances, especially at night, and helps in some plants.

Practical significance: Xylem structure explains phenomena like wilting (loss of turgor when xylem supply is insufficient), cavitation (air bubbles blocking vessels), and the formation of annual rings in trees where secondary xylem is deposited seasonally. Wood used in construction is largely secondary xylem, and its strength comes from lignified xylem cells.

Role and overview: Phloem is the living vascular tissue that transports organic nutrients, mainly sugars like sucrose, from the leaves (sources) to growing parts and storage organs (sinks). This process, called translocation, moves substances both upward and downward depending on the plant’s needs and supports growth, storage and metabolic processes.

Components: Phloem is a complex tissue composed of sieve tube elements, companion cells, phloem parenchyma and phloem fibres. Sieve tube elements are elongated, living cells arranged end-to-end with sieve plates between them that have pores to allow flow of sap. At maturity sieve tube elements lack nuclei but remain alive. Companion cells are closely associated with sieve tubes, rich in organelles and with a nucleus; they perform metabolic tasks necessary for sieve tube function including loading and unloading sugars.

How translocation happens (pressure-flow hypothesis): Sugars produced in the leaves are actively loaded into sieve tubes at source cells. This increases the solute concentration inside the sieve tube, causing water to enter from adjacent xylem by osmosis, which raises the turgor pressure. At sink tissues sugars are unloaded and used or stored, lowering solute concentration and turgor there. The resulting pressure difference drives bulk flow of sap along the phloem from source to sink. Companion cells use energy (ATP) to load and unload sugars and maintain the activity of sieve tubes.

Importance and evidence: Phloem is essential for distributing the products of photosynthesis to roots, buds, fruits and seeds. Girdling experiments (removing a ring of bark including phloem) show that interrupting phloem transport causes sugar to accumulate above the cut and starves tissues below, often killing them. Microscopically, students can identify sieve tubes and companion cells in vascular bundles and note their position relative to xylem.

Introduction: When animal tissues are damaged, the body initiates processes to stop loss of blood, remove dead cells and microbes, and restore structure and function. Repair may restore tissue to its original state (regeneration) or replace it with scar tissue (repair), depending on tissue type and extent of injury.

Initial steps — haemostasis and inflammation: Immediately after injury, blood clotting (haemostasis) plugs the wound, preventing excessive blood loss and creating a temporary matrix. Platelets release factors that attract immune cells. Inflammation follows: white blood cells (neutrophils and macrophages) clear microbes and debris. This stage is essential but must be controlled to avoid further damage.

Proliferation and new tissue formation: After cleaning, the proliferation phase begins. New blood vessels grow into the wound (angiogenesis) to supply oxygen and nutrients. Fibroblasts migrate into the area and synthesise collagen and extracellular matrix, forming granulation tissue that fills the wound. Epithelial cells proliferate and migrate over the new tissue to re-establish a barrier.

Remodelling and scar formation: Over weeks to months the collagen matrix is remodelled: collagen fibres are reorganised and excess cells are removed, strengthening the healed area. In tissues with good regenerative ability (skin epithelium, liver), cells may be replaced by the same type and function returns. In tissues with limited regeneration (cardiac muscle, central nervous system), repair by fibrous scar tissue predominates, restoring structural integrity but not original function.

Factors affecting healing and care: Healing is influenced by age, nutrition (protein, vitamins), blood supply, presence of infection and systemic diseases like diabetes. Proper wound care (cleaning, protection, medical attention for deep wounds) and sufficient nutrition support repair. Understanding these stages helps explain medical treatments such as suturing to close wounds, antibiotics to prevent infection and skin grafts to replace lost tissue.

Plant repair principles: Unlike many animal tissues, many plant cells retain the ability to divide and differentiate throughout life. When injured, plants often form a callus — a mass of undifferentiated cells — near the wound. Cells in the wound region, especially parenchyma and adjacent meristematic cells, divide and give rise to new tissues that restore structure and seal the damaged area.

Wound response and protective layers: After cutting or injury, plants form protective barriers: cork cambium (phellogen) can produce cork cells that seal the surface and prevent water loss and infection. Suberin and lignin may be deposited to strengthen the area. Formation of callus tissue covers exposed vascular tissues and prevents pathogen entry while new vascular connections form across the wounded surfaces.

Grafting and cambial alignment: Successful grafting depends on the union of cambium layers from scion and rootstock. After the cut surfaces are placed together, callus tissue forms at the interface; cambial cells proliferate and differentiate into secondary xylem and phloem that reconnect vascular pathways, allowing water and nutrients to pass between the graft partners. Proper alignment, sterile conditions and protection from desiccation increase success rates.

Tissue culture and micropropagation: Plant tissue culture exploits meristematic and parenchymatous cells’ ability to form whole plants. Small explants placed in sterile nutrient media containing plant hormones (auxins and cytokinins) can form callus and be induced to develop roots and shoots, producing clones of the original plant. This method is vital for propagating ornamentals, disease-free planting material and conserving rare species.

Agricultural applications and care: Pruning, cutting and grafting practices use knowledge of plant repair: pruning at the right time and proper wound sealing reduce infection; rooting hormones improve success of cuttings. Understanding plant tissue responses helps farmers and gardeners propagate plants, manage crops and recover damaged trees in orchards and forests.