Animal Tissues

Bone vs cartilage

Similarities (2 marks)

1. Both are specialised connective tissues derived from embryonic mesenchyme.
2. Both consist of cells, fibres (collagen) and a matrix, and the cells lie in spaces called lacunae.
3. Both give support and shape to the body, and both grow by apposition (new matrix added at the surface).
4. Both are covered by a fibrous membrane (periosteum in bone, perichondrium in cartilage).

Differences (2 marks)

Conclusion: bone is hard, vascular and calcified, suited for rigid support and protection, whereas cartilage is flexible and avascular, suited for cushioning and reducing friction at joints.

Both are connective tissues, with cells scattered in a matrix, but they differ as follows.

(b) They contain more collagen fibres

Tendons are made mainly of dense, parallel collagen fibres, which give high tensile strength but very little stretch, so they are less elastic. Elastin (option a) is what would make a tissue more elastic.

(d) Mast cells

Mast cells are the connective-tissue cells that store histamine and heparin in their granules and release them during allergic and inflammatory responses.

Squamous epithelium is called pavement epithelium:

Squamous epithelial cells are thin, flat and arranged in a single continuous layer like tiles fitted closely together. Because this tile-like (pavement-like) arrangement resembles the paving stones of a road or floor, squamous epithelium is also called pavement epithelium. It forms the lining of the mouth, oesophagus, alveoli of lungs, and the inner surface of blood vessels (endothelium).

Three differences between squamous epithelium and cuboidal epithelium:

(a) Mast cell

Areolar tissue is a loose connective tissue whose cells include mast cells, fibroblasts, macrophages and plasma cells. Chondrocytes occur in cartilage, peptic (chief) cells in the gastric glands, and microglial cells in nervous tissue.

Voluntary (skeletal/striped) muscles

• Attached to the skeleton (bones) and under the control of our own will.
• Made of long, cylindrical, unbranched fibres that are multinucleate and show clear cross-striations.
• Contract quickly but tire easily (fatigue).
• Controlled by the somatic nervous system.
• Examples: muscles of the arms, legs, tongue, and face.

Involuntary (smooth/unstriped) muscles

• Found in the walls of internal organs such as the stomach, intestine, blood vessels, and urinary bladder; not under our will.
• Made of spindle-shaped, uninucleate fibres that are non-striated.
• Contract slowly but work continuously without fatigue.
• Controlled by the autonomic nervous system.
• Examples: muscles of the alimentary canal and blood vessels.

Contrast (2 marks)

Similarity (2 marks)

• Both are contractile tissues made of muscle fibres containing actin and myosin.
• Both contract by the sliding-filament mechanism using ATP and calcium ions.
• Both are supplied with blood vessels and nerves, and both help in movement of body parts.

Myelinated nerve fibres

1. The axon is covered by a fatty myelin sheath secreted by Schwann cells (in the peripheral nervous system).
2. The sheath is interrupted at regular gaps called nodes of Ranvier.
3. The myelin acts as an insulator, so depolarisation occurs only at the nodes.
4. The nerve impulse travels by saltatory conduction, jumping from node to node, so conduction is fast.
5. Fibres are thicker (larger diameter) and appear white.
6. Found in the somatic nervous system — e.g. motor fibres to skeletal muscles.

Non-myelinated nerve fibres

1. The axon is not covered by a myelin sheath; the Schwann cell simply encloses the axon without wrapping it in myelin.
2. No nodes of Ranvier are present.
3. The whole membrane is excitable, so depolarisation occurs along the entire length.
4. The impulse travels by continuous conduction, so conduction is slow.
5. Fibres are thinner and appear grey.
6. Found in the autonomic nervous system and in visceral (smooth muscle and gland) nerves — e.g. postganglionic sympathetic fibres.

Contrast (summary)

Similarity: Both are axons of neurons, both carry nerve impulses (action potentials) by changes in membrane permeability to Na⁺ and K⁺, and both obey the all-or-none law.

(B) Adipose tissue

Adipose tissue is a specialised connective tissue whose cells (adipocytes) store fat globules in their cytoplasm, serving as a fat reserve, insulator and shock absorber.

Simple epithelial tissues are composed of a single layer of cells resting on a basement membrane. They are classified on the basis of the shape of the cells and their function.

1. Squamous epithelium

• Cells are flat, thin and polygonal, like tiles, with a centrally placed nucleus.
• Forms a smooth, thin lining suited for diffusion, filtration and protection.
• Found in the alveoli of lungs (for exchange of gases), walls of blood capillaries and Bowman's capsule (for filtration), and the lining of the mouth and oesophagus.

2. Cuboidal epithelium

• Cells are roughly cube-shaped, with a round nucleus in the centre.
• Provides mechanical support and takes part in secretion and absorption.
• Found in the lining of kidney tubules, ducts of salivary and pancreatic glands, and the thyroid vesicles.

3. Columnar epithelium

• Cells are tall and pillar-like, with an oval nucleus near the base.
• Mainly concerned with secretion and absorption.
• Found in the lining of the stomach and intestine. When its free surface bears cilia, it is called ciliated columnar epithelium (e.g. lining of the trachea and fallopian tubes), which helps in the movement of mucus, dust and ova.

4. Glandular epithelium

• A modified columnar epithelium in which some cells become glandular and secrete substances such as enzymes, hormones, mucus, sweat and saliva.
• Found in glands like salivary glands, sweat glands and intestinal glands. It may be unicellular (goblet cells) or multicellular.

Diagrams

   Squamous            Cuboidal            Columnar
  ___________         _________           ___|___|___
 /___|___|___\       |___|___|___|         |   |   |
 \   |   |   /       |___|___|___|         |   |   |
  \__|___|__/        |___|___|___|         |___|___|
   (flat cells)       (cube-like)          (tall cells)

        Ciliated columnar          Glandular epithelium
        ~~~ ~~~ ~~~ ~~~            (gland cells with ducts)
        |   |   |   |                   \  |  /
        |   |   |   |                    \ | /
        |___|___|___|                     \|/
        (cilia on free surface)        (secretory cells)

Significance (1 mark) Simple epithelia, being only one cell thick, allow rapid diffusion, filtration, absorption and secretion, and hence are found wherever these processes occur rapidly in the body.

A sarcomere is the functional (contractile) unit of a skeletal muscle fibre — the region of a myofibril between two successive Z-lines. Its structure, as shown in the diagram, is as follows:

1. Z-line (Z-disc): A narrow, dark, proteinaceous band that marks the boundary between two adjacent sarcomeres. The thin (actin) filaments are anchored to it.

2. I-band (Isotropic band): The light band lying on either side of the Z-line. It contains only thin (actin) filaments and no thick filaments, so it appears light under the microscope. The Z-line runs through the middle of the I-band.

3. A-band (Anisotropic band): The dark band lying between two I-bands. It contains the thick (myosin) filaments along its whole length, and also the overlapping portions of the thin filaments at its edges. Hence it appears dark. The central part of the A-band, where only thick filaments are present and there is no overlap with thin filaments, is the H-zone; the thick filaments are joined in the middle of the H-zone by the M-line.

4. Thick filaments (myosin): Made of the protein myosin, they lie in the A-band and are held together at the centre by the M-line. Each myosin molecule has a tail and two heads; the heads project outward and form cross-bridges with actin during contraction.

5. Thin filaments (actin): Made mainly of the protein actin, they are attached to the Z-line and extend into the A-band up to the edge of the H-zone. They carry the regulatory proteins tropomyosin and troponin.

Arrangement: The thin and thick filaments are arranged in a parallel, partly overlapping manner. The I-band has only thin filaments, the H-zone only thick filaments, and the region of overlap (edge of A-band) has both — this overlapping arrangement is what allows the sarcomere to shorten during muscle contraction (sliding-filament theory).

Epithelial tissue covers body surfaces and lines cavities. According to the number of cell layers it is simple or compound.

Similarities

• Both consist of closely packed cells with very little intercellular matrix.
• Both rest on a basement membrane and have no blood vessels (they are avascular).
• Both are able to regenerate quickly.

Differences

(a) Mammals

The Haversian system (osteon), with concentric lamellae around a central canal, is the characteristic structure of compact bone in mammals. Bones of reptiles, birds and amphibians are mostly non-Haversian, with simpler lamellar or fibrous structure.

(d) Cartilage

Cartilage is avascular: its cells (chondrocytes) lie in lacunae and receive nutrients by diffusion from the surrounding perichondrium. Areolar tissue, muscle and bone are all supplied with blood vessels.