Tissues --- Plant and Animal

A tissue is a cluster of structurally similar cells that work together to perform a specific, specialized function. In multicellular organisms like plants, division of labor ensures high efficiency. Plant tissues are fundamentally divided into two major groups based on their capacity for cell division: Meristematic Tissues and Permanent Tissues.

Meristematic Tissues consist of actively dividing, immature cells that lead to continuous growth in length and thickness. Structurally, these cells are small, spherical or polygonal, possess thin cellulosic cell walls, contain dense cytoplasm, have conspicuous nuclei, and lack vacuoles because they do not store food. Based on their anatomical location in the plant body, they are classified into three distinct types:

Apical Meristem: Located at the growing tips of roots and shoots. It is responsible for primary growth, which increases the overall height/length of the plant body. Lateral Meristem: Found along the lateral sides of stems and roots (e.g., cambium). It divides radially to cause secondary growth, resulting in an increased girth or thickness of the plant. Intercalary Meristem: Positioned at the base of leaves or internodes (frequently seen in grasses). It helps in the elongation of organs and allows parts damaged by herbivores to regenerate rapidly.

When meristematic cells lose their ability to divide, they attain a permanent shape, size, and function through a process called differentiation. These cells form Permanent Tissues. Permanent tissues are classified as Simple Permanent Tissues when they are composed of only one type of cell structurally and functionally. There are three primary types:

Parenchyma: Consists of living, relatively unspecialized cells with thin cell walls made of cellulose. They have large intercellular spaces. Location: soft parts of the plant like cortex, pith, and leaves. Function: storage of food and mechanical support. Specialized forms include chlorenchyma (contains chlorophyll for photosynthesis) and aerenchyma (contains large air cavities providing buoyancy to aquatic plants). Collenchyma: Consists of living cells characterized by unevenly thickened walls at the corners, made of cellulose and pectin. Intercellular spaces are nearly absent. Location: leaf stalks (petioles) and below the epidermis of stems. Function: provides mechanical flexibility and tensile strength, allowing plants to bend without breaking. Sclerenchyma: Consists of dead, long, narrow cells with extremely thickened secondary walls containing lignin (a chemical cement making walls waterproof and rigid). Lignin deposition makes the cell walls so thick that there is no internal cell lumen or intercellular space. Location: stems, veins of leaves, and the hard coverings of seeds and nuts (e.g., coconut husk). Function: provides structural rigidity and protective mechanical strength.

Complex Permanent Tissues are made of more than one type of cell. All these different cells work together as a single unit to perform a common function. In plants, complex tissues function primarily as conducting or vascular tissues, establishing a continuous transportation network throughout the plant body. They are divided into two main types: Xylem and Phloem.

Xylem: It conducts water and mineral salts unidirectionally upwards from the roots to the leaves. It also provides structural mechanical support. Xylem consists of four distinct cellular components:

1. Tracheids: Elongated, dead tube-like cells with tapering ends and lignified walls with pits.
2. Vessels: Long, cylindrical, tube-like structures formed by rows of dead cells placed end-to-end with perforated transverse walls.
3. Xylem Fibres: Elongated, dead cells with highly thickened walls; they function purely for mechanical support.
4. Xylem Parenchyma: The only living cells in xylem. They have thin cell walls and function in storing food and helping in the lateral conduction of water.

Phloem: It conducts prepared organic food materials (photosynthates) bidirectionally from the leaves to storage organs and growing parts of the plant. This active process is known as translocation. Phloem is composed of four structural elements:

1. Sieve Tubes: Elongated, tubular channels formed of living cells placed end-to-end. Their end walls are perforated like a sieve, forming sieve plates. Mature sieve tube cells lack a nucleus.
2. Companion Cells: Specialized living parenchymatous cells closely associated with sieve tubes. They retain their nucleus and control the metabolic activities of the enucleated sieve tube cells.
3. Phloem Parenchyma: Living cells that store food and other substances like resins or latex.
4. Phloem Fibres (Bast fibres): Dead, elongated sclerenchymatous cells that provide mechanical strength to the tissue.

Animal bodies are complex and dynamic, requiring rapid coordination and movement. Based on the structure of cells and the functions they perform, animal tissues are classified into four main categories: Epithelial, Connective, Muscular, and Nervous tissues.

Epithelial Tissue is the simplest protective tissue in animals. It forms a continuous sheet over external body surfaces and lines internal organs and cavities. The cells are tightly packed together with almost no intercellular spaces or extracellular matrix. Epithelium is always separated from the underlying vascular connective tissue by an extracellular, non-cellular layer called the basement membrane. Based on cell shape and structural modifications, epithelial tissue is classified into five major types:

Squamous Epithelium: Composed of extremely thin, flat, irregular cells forming a delicate lining. Location: Lung alveoli, blood vessels, and esophagus lining. Function: Protects against mechanical injury and allows substances to pass through via selective filtration and diffusion. Multiple layers of these cells form stratified squamous epithelium (e.g., skin) to prevent wear and tear. Cuboidal Epithelium: Consists of cube-like cells with central, rounded nuclei. Location: Kidney tubules, ducts of salivary glands. Function: Provides mechanical support, secretion, and absorption. Columnar Epithelium: Features tall, pillar-like, elongated cells where nuclei are located near the base. Location: Inner lining of the stomach and intestines. Function: Facilitates efficient secretion of enzymes and absorption of nutrients. Ciliated Epithelium: Columnar or cuboidal cells modified with hair-like projections called cilia on their free surface. Location: Respiratory tract, fallopian tubes. Function: The rhythmic beating of cilia moves mucus or eggs forward in a specific direction. Glandular Epithelium: Epithelial sheets that fold inward to form multicellular glands specialized for secretion. Location: Sweat glands, tear glands, gastric glands. Function: Synthesizes and secretes chemical substances like sweat, oil, and enzymes.

Connective Tissue is the most abundant and widely distributed tissue in the animal body. Its primary functions are binding, supporting, and packing various organs together. Unlike epithelial tissue, its cells are widely separated and embedded in a large amount of extracellular matrix. This matrix can be fluid, gelatinous, or rigid and solid. Connective tissue is classified into three main categories:

Loose Connective Tissue:

1. Areolar Tissue: Found between the skin and muscles, around blood vessels and nerves, and in the bone marrow. It fills spaces inside organs, supports internal organs, and helps in tissue repair.
2. Adipose Tissue: Found beneath the skin and between internal organs. Its cells (adipocytes) are filled with fat globules. It acts as a nutrient reservoir and provides thermal insulation against cold temperatures.

Dense Connective Tissue:

1. Tendons: Strong, fibrous structures with great strength but limited flexibility. They connect skeletal muscles to bones.
2. Ligaments: Highly elastic structures with considerable strength. They contain very little matrix and connect bones to other bones, keeping joints stable.

Specialized Connective Tissue:

1. Blood: A fluid connective tissue containing a liquid matrix called plasma, in which red blood cells (RBCs), white blood cells (WBCs), and platelets are suspended. It transports gases, nutrients, and hormones throughout the body.
2. Cartilage: Features widely spaced cells (chondrocytes) embedded in a solid but flexible matrix composed of proteins and sugars. It smoothens bone surfaces at joints and is found in the nose, ear, and trachea.
3. Bone: A hard, unyielding tissue forming the skeletal framework. Cells (osteocytes) are embedded in a rigid matrix made of calcium and phosphorus compounds.

Muscular Tissue is responsible for all movements inside and outside the animal body. It consists of highly elongated, specialized cells called muscle fibers. These cells contain unique proteins called contractile proteins (actin and myosin), which contract and relax rhythmically to cause movement. Muscular tissue is structurally and functionally classified into three distinct types:

Striated (Skeletal / Voluntary) Muscle: These muscles are attached to bones and control movements we can consciously direct, which is why they are called voluntary muscles. Structurally, the muscle fibers are long, cylindrical, unbranched, and contain multiple nuclei located near the edge (multinucleated). Under a microscope, they show alternating light and dark bands or striations. Location: Muscles of limbs, face, and body wall. Smooth (Unstriated / Involuntary) Muscle: These muscles control movements that happen automatically and cannot be consciously controlled, making them involuntary muscles. Structurally, the cells are spindle-shaped (wide in the middle with tapering ends), unbranched, and have only one central nucleus (uninucleated). They lack any light and dark striations. Location: Walls of the stomach, intestines, bronchi, and iris of the eye. Cardiac Muscle: These are involuntary muscles found exclusively in the walls of the heart. They contract and relax rhythmically throughout life without getting fatigued, pumping blood to the body. Structurally, the cells are cylindrical, branched, and uninucleated. They show faint striations and have dark lines called intercalated discs, which allow electrical signals to pass quickly between cells.

Nervous Tissue specializes in receiving stimuli from the environment and transmitting rapid electrical signals throughout the body. This allows organisms to respond quickly to changes around them. It forms the brain, spinal cord, and peripheral nerves. Nervous tissue is composed of two main types of cells: Neurons (Nerve Cells) and Glial Cells (Neuroglia).

A Neuron is the structural and functional unit of the nervous system. Each neuron consists of three distinct parts:

Cell Body (Cyton or Soma): Contains a central nucleus and cytoplasm filled with unique granules called Nissl's granules. It processes incoming signals. Dendrites: Short, branched projections extending out from the cell body. They act as receivers that catch incoming chemical signals and turn them into electrical impulses. Axon: A single, long, cylindrical projection that carries electrical impulses away from the cell body toward the next cell. Many axons are wrapped in an insulating layer called a myelin sheath. The axon terminates at fine branches called nerve endings.

Glial Cells make up more than half the volume of nervous tissue. Unlike neurons, they do not conduct electrical impulses. Instead, they act as a support system that protects, nourishes, insulates, and holds neurons in place.