Tissue: The Living Fabric Part D

Nervous Tissue

• General Description: Nervous tissue is composed of branched neurons with long cellular processes and specialized supporting cells.

• Cellular Components:

  • Neurons: These are branching cells. They consist of a cell body that contains the nucleus. Long cell processes extend from the cell body.

  • Supporting Cells: Also known as nonirritable supporting cells, these contribute to the tissue and are represented in photomicrographs by their nuclei. They provide a support framework for the neurons.

• Functional Role: The primary function is to transmit electrical signals from sensory receptors to effectors. Effectors include muscles and glands, and the transmission of these signals allows for the control of their activity.

• Anatomical Locations: Nervous tissue is found in the following areas of the body:

  • Brain.

  • Spinal cord.

  • Peripheral nerves.

• Microscopic Identification:

  • Magnification in typical photomicrographs is noted at $100\times$.

  • Visible structures include the cell body of a neuron, neuron processes, and the nuclei of supporting cells.

Muscle Tissue: Three Primary Types

Skeletal Muscle

• Description: Characterized by long, cylindrical cells that are multinucleate (possessing multiple nuclei). These cells exhibit obvious striations (banded appearance).

• Function: This tissue initiates and controls voluntary movement.

• Location: Found in skeletal muscles that are attached to bones or occasionally to the skin.

Cardiac Muscle

• Description: Consists of branching, striated cells that are typically uninucleate (single nucleus). A defining feature is that these cells interdigitate or interlock at specialized junctions called intercalated discs.

• Function: As it contracts, it propels blood into the circulatory system.

• Location: Found exclusively in the walls of the heart.

Smooth Muscle

• Description: Composed of spindle-shaped cells with central nuclei. Unlike skeletal or cardiac muscle, smooth muscle has no striations. The cells are arranged closely to form sheets.

• Function: It propels substances or objects along internal passageways (a process known as peristalsis). Examples of propelled items include:

  • Foodstuffs.

  • Urine.

  • A baby during childbirth.

• Control: This tissue operates under involuntary control.

• Location: Found mostly in the walls of hollow organs.

• Microscopic Identification: Normal photomicrography magnification is approximately $600\times$.

Tissue Trauma and Inflammation

• Cause: Physical injury or tissue trauma leads to inflammation.

• Characteristics of Inflammation: Inflammation is clinically identified by several physiological changes:

  • Dilation of blood vessels.

  • Increase in vessel permeability.

  • Redness.

  • Heat.

  • Swelling.

  • Pain.

The Process of Tissue Repair

• Phase 1: Organization and Restored Blood Supply:

  • The initial blood clot is replaced with granulation tissue.

  • Blood supply starts to be restored to the injured area.

• Phase 2: Regeneration and Fibrosis:

  • The surface epithelium begins to regenerate.

  • The scab formed over the wound detaches.

• Phase 3: Maturation:

  • Fibrous tissue matures and begins to resemble the adjacent tissue.

• Final Result: The process results in a fully regenerated epithelium with underlying scar tissue.

Developmental Aspects of Tissues

• Primary Germ Layers: During early embryonic development (exemplified by a $16$-day-old embryo), three distinct layers of cells are formed:

  1. Ectoderm.

  2. Mesoderm.

  3. Endoderm.

• Tissue Specialization by Germ Layer:

  • Ectoderm: Gives rise to nervous tissue and certain epithelial tissues.

  • Mesoderm: Gives rise to muscle tissue, connective tissue, endothelium, mesothelium, and certain epithelial tissues.

  • Endoderm: Gives rise to most mucosae and certain epithelial tissues.

• Epithelial Tissue Exception: Unlike the other three primary tissues, epithelial tissues arise from all three germ layers (ectoderm, mesoderm, and endoderm).