Four Primary Tissue Types

Introduction to the four primary tissue types to be studied in order:
- Connective
- Epithelial
- Muscle
- Nervous

1. Connective Tissue:
- Functions:
- Connects anatomical structures
- Fills internal spaces
- Stores energy
- Provides support
- Transports materials
2. Epithelial Tissue:
- Functions:
- Covers surfaces
- Lines cavities and tracts
- Produces secretions from glands
3. Muscle Tissue:
- Functions:
- Moves the body and organs
4. Nervous Tissue:
- Functions:
- Conducts electrical impulses
- Carries information from the body to the brain
- Sends commands from the brain to the body

Connective tissue (CT) is a diverse group of tissues classified into three types:
- CT Proper: Includes Loose and Dense types.
- Supporting CT: Consists of Cartilage and Bone.
- Fluid CT: Comprises Blood and Lymph.

All types of connective tissue (CT Proper, Supporting CT, and Fluid CT) contain three general components:
- Cells: Relatively few in number.
- Protein Fibers: May be absent in certain types.
- Ground Substance

Structure:
- The structure of the three types of connective tissue and the arrangement of their components vary significantly.
Function:
- The functions of the three types of connective tissue also differ greatly.

Connective Tissue Proper is found throughout the body and performs various functions, including:
- Connecting anatomical structures
- Filling internal spaces
- Providing insulation
- Storing energy
Example: Connective tissue proper is akin to insulation found under the surface of a puffer jacket.

Connective Tissue Proper typically contains:
- Cells: Either fixed or wandering.
- Fibers: Include collagen, reticular, and elastic fibers.
- Ground Substance: Composed of a mixture of water and proteoglycans.

Fixed Cells: Include the following types:
- Mesenchymal Cells: Stem cells that can differentiate into fibroblasts.
- Fibroblasts: Most common cells in CT proper, responsible for producing fibers and ground substances.
- Fibrocytes: Derived from fibroblasts, less active, maintain fibers and proteoglycans.
- Adipocytes: Fat cells that store lipids for energy.
- Melanocytes: Cells that synthesize melanin, protecting skin from UV radiation.
- Fixed Macrophages: Immune cells that engulf bacteria and pathogens, acting as the first line of defense.

Wandering Cells: Include:
- Free macrophages
- Neutrophils
- Lymphocytes
- Eosinophils
- Mast cells
All these cells function as components of the immune response and are part of the lymphatic system.

1. Collagen Fibers:
- The most common protein fibers in CT proper, made of three protein subunits wound together.
- Characteristics:
- Long and straight.
- Very strong when pulled, yet flexible.
- Key in tendons and ligaments, supporting significant forces.
2. Reticular Fibers:
- Formed from a different type of collagen, producing thin, branched fibers that create nets.
- Functions:
- Provide support to tissue cells in organs like the liver and spleen.
3. Elastic Fibers:
- Composed of elastin and fibrillin proteins.
- Capable of stretching up to 2.5 times their resting length and then recoiling to original form.
- Found in tissues requiring flexibility, such as blood vessels, lungs, and skin.

Ground Substance:
- Consists primarily of 90% water and proteoglycan molecules.
- Forms a gel that suspends CT fibers and cells.
Proteoglycans:
- Secreted by fibroblasts; composed of protein and sugar molecules.
- Water binds to proteoglycans, contributing to the gel's properties that resist compression.
Extracellular Matrix (ECM):
- Composed of protein fibers and ground substance, constituting the bulk of CT proper.

Connective tissue proper further divided into:
- Loose CT:
- Subtypes: Areolar, Adipose, Reticular.
- Dense CT:
- Subtypes: Dense Regular, Elastic, Dense Irregular.

Areolar CT:
- Provides cushioning, support, and fills spaces between tissues and organs.
- Commonly found in skin, respiratory, digestive, and urinary tracts; around blood vessels, nerves, and joints.
Reticular CT:
- Supports the functional tissues in organs such as the liver, kidneys, and lymph nodes with a net of reticular fibers.
Adipose CT:
- Dominated by adipocytes, providing energy storage, cushioning, and insulation; it's critical in areas like under the skin and around organs.
Brown Adipose CT:
- Contains many mitochondria, producing heat; primarily found in infants, it can increase in adults with cold exposure.

Dense Regular CT:
- Composed of tightly packed collagen fibers, providing strength.
- Strongest CT proper.
- Found in tendons and ligaments.
Elastic CT:
- A subtype of Dense Regular CT with abundant elastic fibers, allowing for flexibility.
- Present in blood vessel walls and ligaments of the vertebral column.
Dense Irregular CT:
- Composed of collagen fibers oriented in different directions, providing strength against multi-directional forces.
- Found in skin layers and organ capsules.

Fascia: A layer of connective tissue that forms a framework for the body.
- Composed of areolar CT, adipose CT, dense regular CT, and/or dense irregular CT.
Types of Fascia:
- Superficial Fascia:
- Between the skin and underlying organs; composed of areolar and adipose tissue; provides padding and insulation.
- Deep Fascia:
- Forms a fibrous internal framework; dense irregular connective tissue that binds muscles, tendons, and ligaments.
- Subserous Fascia:
- Found between serous membranes and deep fascia; composed of areolar CT.

From conception through week 8, a developing human is called an embryo. After 8 weeks, the developing human is referred to as a fetus.
Early Development:
- Fertilization: A single cell (zygote) containing 46 chromosomes is formed.
- Cell Division: The zygote divides into many cells, creating a blastocyst over about a week.
Germ Layers Development by Week 2:
- The inner cell mass differentiates into three primary germ layers:
- Ectoderm
- Mesoderm
- Endoderm
Organ Development:
- Organ systems can originate from one or more germ layers. For example, the nervous system arises from the ectoderm, while the skeletal system comes from both ectoderm and mesoderm.
By the end of week 4, all major organs have started to develop.