🐾 Structural Organisation in Animals (Animal Tissues)

1. Introduction to Animal Tissues

In multicellular organisms, cells do not work in isolation. A group of similar cells along with intercellular substances performing a specific function is called a tissue. All complex animals consist of only four basic types of tissues.

2. Epithelial Tissue (Epithelium)

This tissue has a free surface which faces either a body fluid or the outside environment. The cells are compactly packed with little intercellular matrix.

2.1 Simple Epithelium

Composed of a single layer of cells. It mainly functions as a lining for body cavities, ducts, and tubes.

• Squamous Epithelium: Made of a single thin layer of flattened cells with irregular boundaries.
  Location: Walls of blood vessels and air sacs (alveoli) of lungs.
  Function: Forms a diffusion boundary.
• Cuboidal Epithelium: Composed of cube-like cells.
  Location: Ducts of glands and tubular parts of nephrons in kidneys.
  Speciality: In PCT of nephron, it has microvilli (Brush border) for absorption.
• Columnar Epithelium: Composed of tall and slender cells. Nuclei are located at the base.
  Location: Lining of stomach and intestine.
  Function: Secretion and absorption.

2.2 Ciliated Epithelium

If the columnar or cuboidal cells bear cilia on their free surface, they are called ciliated epithelium. Their function is to move particles or mucus in a specific direction over the epithelium.
Location: Inner surface of hollow organs like bronchioles and fallopian tubes.

2.3 Glandular Epithelium

Some columnar or cuboidal cells get specialized for secretion. They are of two types:

1. Unicellular: Consisting of isolated glandular cells (e.g., Goblet cells of the alimentary canal).
2. Multicellular: Consisting of cluster of cells (e.g., Salivary gland).

2.4 Compound Epithelium

Made of more than one layer of cells. Their main function is to provide protection against chemical and mechanical stresses. They have a limited role in secretion and absorption.
Location: Dry surface of the skin, moist surface of buccal cavity, pharynx, inner lining of ducts of salivary glands and pancreatic ducts.

3. Cell Junctions

In nearly all animal tissues, specialized junctions provide both structural and functional links between its individual cells.

Junction Type	Function
Tight Junctions	Help to stop substances from leaking across a tissue.
Adhering Junctions	Perform cementing to keep neighbouring cells together.
Gap Junctions	Facilitate the cells to communicate with each other by connecting the cytoplasm for rapid transfer of ions and molecules.

4. Connective Tissue

They are the most abundant and widely distributed in the body of complex animals. They are named connective tissues because of their special function of linking and supporting other tissues/organs of the body.

4.1 Loose Connective Tissue

• Areolar Tissue: Present beneath the skin. It serves as a support framework for epithelium. It contains fibroblasts, macrophages, and mast cells.
• Adipose Tissue: Located mainly beneath the skin. The cells are specialized to store fats.

4.2 Specialized Connective Tissue

• Cartilage: The intercellular material is solid and pliable and resists compression. Cells (chondrocytes) are enclosed in small cavities.
  Location: Tip of nose, outer ear joints, between adjacent bones of the vertebral column.
• Bones: Have a hard and non-pliable ground substance rich in calcium salts and collagen fibers which give bone its strength. Cells (osteocytes) are present in spaces called lacunae.
• Blood: A fluid connective tissue containing plasma, RBC, WBC, and platelets. It is the main circulating fluid that helps in the transport of various substances.

5. Muscular Tissue

Each muscle is made of many long, cylindrical fibers arranged in parallel arrays. These fibers are composed of numerous fine fibrils, called myofibrils.

• Skeletal Muscle: Closely attached to skeletal bones. They are striated and voluntary.
• Smooth Muscle: The fibers taper at both ends (fusiform) and do not show striations. They are involuntary. Found in walls of internal organs like blood vessels, stomach, and intestine.
• Cardiac Muscle: Contractile tissue present only in the heart. Communication junctions (intercalated discs) allow the cells to contract as a unit.

6. Neural Tissue

Exerts the greatest control over the body’s responsiveness to changing conditions. Neurons, the unit of neural system are excitable cells. The neuroglial cells which constitute the rest of the neural system protect and support neurons. Neuroglia make up more than one-half the volume of neural tissue in our body.

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🪳 Cockroach (Periplaneta americana)

Cockroaches are brown or black bodied animals that are included in Class Insecta of Phylum Arthropoda.

Morphology

• The body is divided into three regions: Head, Thorax, and Abdomen.
• The entire body is covered by a hard chitinous exoskeleton (brown in colour). In each segment, exoskeleton has hardened plates called sclerites (tergites dorsally and sternites ventrally).
• Head: Triangular in shape. It bears a pair of compound eyes and a pair of thread-like antennae.
• Thorax: Consists of prothorax, mesothorax, and metathorax. It has three pairs of legs and two pairs of wings.
  • Forewings (Mesothoracic): Called tegmina, dark, opaque and leathery.
  • Hindwings (Metathoracic): Transparent, membranous and used in flight.
• Abdomen: In both males and females, the abdomen consists of 10 segments.
  Male vs Female: In males, a pair of short, thread-like anal styles are present (absent in females). Anal cerci are present in both.

Anatomy (Digestive System)

The alimentary canal is divided into foregut, midgut, and hindgut.

• Gizzard (Proventriculus): Has an outer layer of thick circular muscles and thick inner cuticle forming six highly chitinous plate called teeth. It helps in grinding the food particles.
• Hepatic caecae: A ring of 6-8 blind tubules present at the junction of foregut and midgut. They secrete digestive juice.
• Malpighian tubules: At the junction of midgut and hindgut, 100-150 yellow coloured thin filamentous Malpighian tubules are present. They help in excretion.

Nervous & Reproductive System

The nervous system consists of a series of fused, segmentally arranged ganglia joined by paired longitudinal connectives on the ventral side. Three ganglia lie in the thorax, and six in the abdomen.

Cockroaches are dioecious. Male reproductive system consists of a pair of testes (4th-6th segments). Female reproductive system consists of two large ovaries (2nd-6th segments).