Tissue Level of Organization and Embryology

How are Tissues Formed?

• After fertilization occurs, the cells divide by mitosis.

• Within one week, a blastocyst forms:

  • Blastocyst Structure:

  • Outer cell layer: Trophoblast

    • Function: Will form the placenta.

  • Inner cell mass:

    • Function: Will form the embryo.

Three Primary Germ Layers

• Formation Timeline:

  • After fertilization and the formation of the blastocyst (about one week).

• The three primary germ layers are:

  1. Ectoderm:

  • Development: Forms the neural tissues and epidermis.

  1. Mesoderm:

  • Development: Forms muscles, skeleton, and cardiovascular system.

  1. Endoderm:

  • Development: Forms the internal epithelial tissues.

Characteristics of Epithelial Tissues

1. Very little matrix; tightly packed cells.

2. Contains both an exposed apical surface and a basal surface.

3. Sits on a basal lamina.

4. Avascular: Lacks blood vessels.

5. Replaced constantly from stem cells.

6. Can be one or more layers thick.

Functions of Epithelial Cells

1. Protect the underlying tissues from physical and environmental forces.

2. Act as a selectively permeable barrier.

3. Produce secretions.

4. Provide sensation via sensory cells.

Why do Epithelia Lack Blood Vessels?

• A) Current understanding is limited; the reason remains unclear.

• B) To protect red blood cells from solar damage.

• C) To prevent materials from directly entering the bloodstream.

Basal Lamina

• Composition: Made by both epithelial and connective tissues.

• Function:

  • Anchors the epithelium in place.

  • Prevents the passage of large molecules deeper into the tissues.

Cell Types in Epithelial Tissue

Cell Shape:

1. Squamous: Flat and thin cells.

2. Cuboidal: Cube-shaped cells.

3. Columnar: Tall and column-like cells.

Cell Layers:

1. Simple Epithelium: One layer of cells.

2. Stratified Epithelium: Multiple layers of cells.

3. Pseudostratified Epithelium: Appears layered but is not.

Types of Epithelial Tissues

1. Simple Squamous Epithelium

2. Simple Cuboidal Epithelium

3. Simple Columnar Epithelium

4. Pseudostratified Columnar Epithelium

5. Transitional Epithelium

6. Stratified Squamous Epithelium:

   • Two Types:

     • A. Keratinized

     • B. Non-Keratinized

Exocrine Glands

• Definition: Invaginations of epithelial tissues.

• Functions: Produce secretions through a duct onto the skin or into a passageway.

• Examples of secretions:

  • Sweat, milk, and digestive enzymes.

Types of Exocrine Glands

1. Single Cell Exocrine Gland:

   • Goblet Cell:

     • Function: Produces mucin, which turns to mucus in the presence of water.

2. Multiple Cell Exocrine Gland:

   • Structure: Consists of duct and secretory portion.

   • Structural Categories of Secretory Portion:

     • A. Tubular

     • B. Acinar

3. Gland Categories by Structure:

   • Simple Glands:

     • Examples:

     • Simple Tubular: Intestinal glands

     • Simple Coiled Tubular: Merocrine sweat glands

     • Simple Branched Tubular: Gastric glands, mucous glands of esophagus, tongue, duodenum

   • Compound Glands:

     • Examples:

     • Compound Tubular: Mucous glands in the mouth, bulbo-urethral glands (male)

     • Compound Alveolar: Mammary glands

     • Compound Tubuloalveolar: Salivary glands, glands of respiratory passages, pancreas

Exocrine Glands by Secretion Categories

1. Eccrine/Merocrine:

   • Mechanism: Use exocytosis to release secretions.

   • Example: Salivary glands.

2. Apocrine:

   • Mechanism: Pinch off the apical surface of the cell.

   • Example: Mammary glands.

3. Holocrine:

   • Mechanism: Release products by disintegration (cell death).

   • Example: Sebaceous glands.

Characteristics of Connective Tissue

1. High matrix content with few cells.

2. Lacks an apical surface.

3. Positioned deep to epithelial tissue.

Components of Connective Tissue

1. Cells:

   • Types include: Fixed cells (fibroblasts, adipocytes, chondrocytes, osteocytes) and wandering cells (white blood cells, mast cells).

2. Protein Fibers:

   • Types include:

     • 1. Collagen: Strong and thick (found in tendons and ligaments).

     • 2. Elastic: Flexible and stretchy (medium sized).

     • 3. Reticular: Flexible and thinner forming an overlapping mesh.

3. Ground Substance:

   • A non-fibrous substance that can be liquid, gel-like, or solid.

   • Matrix Definition:

     • Combination of protein fibers and ground substance.

Functions of Connective Tissues

1. Provide protection for organs.

2. Offer structural support.

3. Join different structures together.

4. Store energy and minerals.

5. Aid in the transportation of oxygen, nutrients, and waste products in fluid.

6. Provide immune protection.

Classification of Connective Tissues

CONNECTIVE TISSUE PROPER

• LOOSE Connective Tissue:

  • Characteristics: Fibers create a loose, open framework.

  • Types: Areolar tissue, adipose tissue, reticular tissue.

• DENSE Connective Tissue:

  • Characteristics: Fibers are densely packed.

  • Types: Dense regular, dense irregular, elastic.

FLUID CONNECTIVE TISSUES

• BLOOD:

  • System: Contained within the cardiovascular system.

• LYMPH:

  • System: Contained within the lymphoid system.

SUPPORTING CONNECTIVE TISSUES

• CARTILAGE:

  • Features: Contains a solid, rubbery matrix.

  • Types: Hyaline cartilage, elastic cartilage, fibrous cartilage.

• BONE:

  • Features: Contains a solid, crystalline matrix.

Connective Tissues Listed

1. Areolar Connective Tissue

2. Adipose

3. Dense Regular Connective Tissue

4. Blood

5. Cartilage: hyaline, fibrous, and elastic

6. Bone

Blood: Cellular Components

• Types of cells:

  • Erythrocytes: Red blood cells.

  • Leukocytes: White blood cells.

  • Plasma: Liquid component of blood

  • Platelets: Cell fragments involved in clotting.

  • Types of White Blood Cells:

  • Neutrophil

  • Monocyte

  • Lymphocytes

  • Basophil

  • Eosinophil

Cartilage: Common Features

1. Lacunae are spaces in the extracellular matrix that contain chondrocytes.

2. Mixture of collagen and elastic fibers is present in the structure.

3. Mature cartilage is avascular (lacks blood vessels).

Bone: Structural Features

• Components: Contains osteocytes located in lacunae.

• Types:

  • Compact Bone:

  • Features: Contains concentric lamellae and is dense.

  • Spongy Bone:

  • Features: Less dense, contains trabecular structures.

• Key Components of Bone:

  • Canaliculi, blood vessels, matrix, and periosteum (fibrous and cellular layers).

Membranes in the Body

• Composition: Consist of both epithelial and connective tissues.

  1. Serous Membranes: Line ventral cavities and secrete serous fluid.

  2. Mucous Membranes: Cover layers of body compartments that open to the outside, produce mucus.

  3. Cutaneous Membrane: Refers to the skin.

  4. Synovial Membranes: Secrete synovial fluid within joints.

Characteristics of Muscle Tissue

1. Minimal matrix; consists mainly of cells.

2. Length of cells is greater than their height.

Function of Muscle Tissue

1. Contraction is the primary function.

• Types of Muscle Tissues:

  1. Skeletal

  2. Cardiac

  3. Smooth

Characteristics of Neural Tissue

1. Contains few cells and more matrix.

2. Composed of:

   • Neurons (conduct electrical impulses).

   • Neuroglia (support the neurons).

Functions of Neural Tissue

1. Conducting electrical impulses via neurons.

2. Supporting and protecting neurons through neuroglia.

Location of Neural Tissue

• Found in:

  • Brain

  • Spinal Cord

  • Nerves

How do Tissues Change?

1. Metaplasia: Mature epithelium changes to another mature epithelium (e.g., due to smoking).

2. Atrophy: Decrease in cell size or number.

3. Necrosis: Refers to cell death.

4. Apoptosis: Programmed cell death.

5. Hypertrophy: Increase in cell size.

6. Hyperplasia: Increase in cell number, which may lead to neoplasia.