Tissues : Plant and Animal Tissues

1. Introduction to Tissues

• A tissue is defined as a group of similar cells performing a specific function.
• There is a continuous organizational relationship in living bodies: cells group to form tissues, tissues contribute to forming an organ, organs coordinate to create an organ system, and organ systems together constitute the organism.

2. Plant Tissues

Plant tissues are fundamentally categorized into two types: Meristematic and Permanent tissues.

A. Meristematic Tissue (Actively Dividing)

• Found at all growing points of a plant, such as the tips of roots, stems, and branches.
• Characteristics: Cells are small, usually cubical, tightly packed with almost no intercellular spaces, feature thin walls, have large nuclei, and almost lack vacuoles.
• Categories: Includes Apical/Terminal meristem (located near tips for longitudinal growth) and Cambium/Lateral meristem (situated below the bark to increase stem diameter).

B. Permanent Tissue (Non-Dividing)

• Composed of mature cells that have lost their ability to multiply and have taken a permanent shape to perform specific functions. They are classified into three functional types:
• 1. Protective Tissue: Composed of thick-walled cells found on the surfaces of roots, stems, and leaves (e.g., the epidermis and cork).
• 2. Supporting Tissue: Provides support and features three primary varieties:
  • Parenchyma: Contains large, thin-walled cells (usually with a single large vacuole) that store food and offer temporary support.
  • Collenchyma: Made of elongated cells with walls thickened at the corners to help support plant parts.
  • Sclerenchyma: Consists of dead, long, narrow cells with highly thickened lignified walls to provide exceptional strength (found in fibres and "stone cells").
• 3. Conducting (Vascular) Tissue: Facilitates passage for fluids and comprises complex tissues known as xylem and phloem.
  • Xylem: Contains elongated, thick-walled tubular structures (tracheids, vessels, xylem parenchyma) that provide upward movement of water and dissolved minerals from the soil.
  • Phloem: Consists of sieve tubes, companion cells, phloem parenchyma, and fibres to provide passage for the downward and upward movement of manufactured food from the leaves.

3. Animal Tissues

Animal tissues are broadly organized into four primary kinds: Epithelial, Connective, Muscle, and Neural tissues.

A. Epithelial Tissue

• A thin, continuous protective sheet of cells covering body surfaces and lining internal organs.
• Functions: Provides protection, absorption, secretion, and sensory perception.
• Categories: Can be Squamous (flat), Stratified (several layers), Cuboidal (cubical), Columnar (tall, cylindrical), Ciliated (equipped with thread-like projections called cilia), and Glandular (folded inward to form secretory glands).

B. Connective Tissue

• Serves to bind one tissue to another and connects various organs, defined by an abundant intercellular matrix, fibres, and fewer cellular elements.
• 1. Connective Tissue Proper: Includes Areolar tissue (for packing and binding), Adipose tissue (specialized cells for fat storage/insulation), and Fibrous connective tissue (forms tendons linking muscle to bone, and ligaments linking bones).
• 2. Supportive Connective Tissue: Includes Cartilage (a semi-transparent, elastic, non-porous tissue lacking blood vessels) and Bone (a hard, porous tissue with a rich supply of blood vessels and nerves).
• 3. Fluid Connective Tissue: Comprises Blood (liquid plasma with red cells, white cells, and platelets) and Lymph (blood plasma lacking red cells) primarily concerned with transportation and immunity.

C. Muscle Tissue

• Composed of specialized cells that can contract and relax, helping the body in movement.
• Types:
  • Striated (Skeletal/Voluntary): Nucleated, striped muscles under conscious control (e.g., arms and legs).
  • Unstriated (Smooth/Involuntary): Spindle-shaped, uninucleated muscles functioning independently of will (e.g., intestine walls).
  • Cardiac: Involuntary, striated, uniquely branched muscles found exclusively in the walls of the heart.

D. Neural (Nervous) Tissue

• Forms the nervous system and is primarily concerned with the perception of stimuli and formulation of responses.
• Composed of specialized, elongated cells known as neurons.
• A typical neuron structure includes a cell body (perikaryon or cyton) containing the nucleus, multiple hair-like extensions (dendrites), and one long extension (axon) which can be up to three metres long and bundle together to form nerves.