INTRODUCTION

In unicellular organisms, all functions such as digestion, respiration, and reproduction are performed by a single cell. In contrast, in the complex body of multicellular organisms, the same basic functions are carried out by different groups of cells organized in a structured manner. For example, the body of a simple organism like Hydra is composed of various types of cells, with counts in the thousands for each type, whereas the human body consists of billions of cells fulfilling diverse functions.

Tissue is defined as a group of similar cells, along with intercellular substances, that perform a specific function. This group of cells provides a fundamental framework in multicellular organisms. The study of tissues is known as Histology. Tissues were first identified in coelenterates. They are organized in specific proportions and patterns to form organs such as the stomach, lung, heart, and kidney. When two or more organs interact physically and/or chemically to perform a common function, they collectively form an organ system, e.g., the digestive system or respiratory system. The division of labor among cells, tissues, organs, and organ systems contributes to the overall survival and efficiency of the organism.

ANIMAL TISSUES

The structure of cells varies according to their specific functions, which leads to different types of tissues in animals. Animal tissues are broadly classified into four main types:

  1. Epithelial Tissue
  2. Connective Tissue
  3. Muscular Tissue
  4. Neural Tissue
Epithelial Tissue

Epithelial tissue can be further classified based on the number of cell layers and shapes. The characteristics of epithelial tissue include:

  • It is derived from ectodermal, mesodermal, and endodermal origins.
  • An epithelium is a tissue consisting of one or more layers of cells, providing a covering or lining for various parts of the body.
  • It features two distinct surfaces:
      - Free surface: This side faces body fluids or the external environment.
      - Basal surface: This side is in contact with the basement membrane.

Epithelial cells are tightly packed with minimal intercellular matrix and are avascular, implying they lack blood supply.

Basement Membrane
  • The basement membrane is a crucial element that is secreted by both the epithelial and connective tissue.
  • It is acellular and consists of two layers, known as laminae:
      - Basal Lamina: Composed of mucopolysaccharide and glycoprotein.
      - It provides support and separates the epithelial layer from the connective tissues beneath it.
Classification of Epithelial Tissues (By Number of Layers)
  1. Simple Epithelium
       - Composed of a single layer of cells and functions mainly as a lining for body cavities, ducts, and tubes.
  2. Compound Epithelium
       - Consists of two or more cell layers and typically serves protective functions (as seen in skin).
Shapes of Epithelial Tissues
  • Squamous Epithelium: Thin, flat cells.
  • Cuboidal Epithelium: Cube-shaped cells.
  • Columnar Epithelium: Tall, cylindrical cells.
Types of Simple Epithelial Tissue
(i) Simple Squamous Epithelium
  • This epithelium is essential for forming diffusion barriers.
  • Locations include: walls of blood vessels, lymph vessels, heart, peritoneum, pleura, Bowman's capsule, thin segment of the loop of Henle, and air sacs of lungs.
  • Characterized by a single thin layer of wavy flattened cells with irregular boundaries, often termed tesselated epithelium or pavement epithelium due to its tile-like appearance.
(ii) Simple Cuboidal Epithelium
  • It consists of a single layer of cube-shaped cells resting on a basement membrane, with centrally located nuclei.
  • Found in glands (e.g., salivary and pancreatic ducts) and tubular sections of nephrons in the kidneys.
  • Functions include secretion and absorption. The proximal convoluted tubule (PCT) of nephrons features microvilli that enhance reabsorption, increasing the surface area of the epithelial cells.
Modifications of Simple Cuboidal Epithelium
  1. Brush Bordered Cuboidal Epithelium
       - Features a microvillus-covered surface found in the PCT of the nephron, enhancing cellular surface area.
  2. Ciliated Cuboidal Epithelium
       - Contains cilia on the apical surface that aid in movement; located in the respiratory bronchioles.
(iii) Simple Columnar Epithelium
  • Comprising a single layer of tall and slender cells, with nuclei positioned at the base.
  • Free surfaces may possess microvilli, enhancing absorption and secretion.
  • Typical locations include the lining of the stomach, intestines, and gall bladder.
Modifications of Simple Columnar Epithelium
  1. Brush Bordered Columnar Epithelium
       - This variant has microvilli on its free surface aiding in absorption, primarily found in the small intestine.
  2. Ciliated Columnar Epithelium
       - Characterized by hair-like cilia, aiding in the movement and clearing of substances; found in the fallopian tubes and the ependymal epithelium of brain ventricles.

STRUCTURAL ORGANISATION IN ANIMALS

  • The organization of tissues into functional units is critical for the maintenance of body functions and the interaction of different systems to ensure overall homeostasis and efficiency in multicellular organisms. The arrangement of tissues leads to the formation of organs, which then come together to create organ systems, facilitating the complex processes that sustain life.

TRAIN YOUR BRAIN

  • Fill in the following points based on the above excerpts:
      1. Quamous epithelium is made of a single thin layer of flattened cells with irregular boundaries.
      2. The epithelium of PCT of nephron in the kidney has microvilli.
      3. The human body is composed of billions of cells to perform various functions.