Connective tissue

Definition and Origin
- Connective tissue is a fundamental type of tissue with mesodermal origin.
- Provides structural and metabolic support to other tissues and organs in the body.
- Typically contains blood vessels that mediate nutrient, metabolite, and waste exchange between tissues and the circulatory system.
- Comprises isolated cells surrounded by an extracellular matrix (matrix), synthesized by these cells.
- The viscosity and tensile strength of connective tissues vary based on the arrangement of their fibrous and amorphous components (e.g., fibrinogen, collagen, proteoglycans).
- Serves various functions, indicating its versatility as a supporting tissue (e.g., bones).

Connective tissues are ubiquitous in the body, with every organ containing or enveloped by connective tissue.
Functions and Features
- Forms robust capsules around organs (e.g., liver and spleen).
- Surrounds the central nervous system with protective sheaths (meninges).
- Binds epithelial tissues to underlying structures, forming organs (e.g., lungs, liver).
- Connects skeletal muscles to bones through tendons and binds bones to each other through ligaments.
- Includes specialized connective tissues such as bone, cartilage, and peripheral blood.

List of Significant Cells and Functions:
- Basophil
- Red blood cell
- Eosinophil
- Plasma cell
- Megakaryocyte
- B lymphocyte
- T lymphocyte
- Monocyte
- Neutrophil
- Hematopoietic stem cells
- Mast cell
- Mesothelial cells
- Osteoclast
- Endothelial cells
- Smooth muscle cells
- Macrophage
- Langerhans cell
- Microglia
- Undifferentiated mesenchymal cells
- Fibroblast
- Osteocyte
- Osteoblasts
- Chondrocytes
- Chondroblast
- Adipocyte (fat cell)

Characteristics:
- Mesenchyme cells are unspecialized and capable of differentiating into all connective tissue cell types.
- Morphology:
- Irregular, stellate or fusiform shape with delicate branching cytoplasmic extensions.
- Oval nuclei with visible nucleoli.
- Some mesenchymal cells persist in mature tissues, serving as a pluripotential cell source.

Active Fibroblasts:
- Stellate shape with extensive cytoplasmic processes.
- Large nucleus with prominent nucleolus; exhibits numerous organelles indicative of active protein synthesis.
Mature Fibroblasts (Fibrocytes):
- Smaller than active fibroblasts, elongated with reduced cytoplasm.
- Nucleus is elongated, maintaining tissue integrity through slow turnover of extracellular materials.

Characteristics:
- Darkly stained due to cytoplasmic granules.
- Participate in defense mechanisms (e.g., involving allergies) and promote increased vascular permeability.

Main Types:
- Collagen Fibers: The predominant type in most connective tissues, it is the most abundant protein in the body.
- Flexible with high tensile strength.
- Secreted into the extracellular matrix as tropocollagen, consisting of three polypeptide α chains (two α1 and one α2) that form a helical structure.
- Fibrils are formed by polymerization of tropocollagen in the extracellular matrix, aggregating to form thicker fibers.
- Different types of collagen are identified based on function and location (e.g., Type I in skin, tendons; Type II in cartilage).
- Reticular Fibers:
- Composed of type III collagen and may also include type IV collagen.
- Form a mesh-like network in tissues (e.g., around glands and in lymphoid organs).
- Elastic Fibers:
- Composed of oxytalan, elaunin, and elastin.
- Located in skin, lungs, blood vessels, providing properties of elasticity and stretchability.
- Thinner than collagen fibers and interwoven to prevent tearing during stretching.

Description:
- An amorphous, transparent material occupying spaces between cells and fibers, exhibiting semi-fluid gel properties.
Composition:
- Composed of glycosaminoglycans (e.g., hyaluronic acid) and proteoglycans.
- Forms a flexible gel that allows metabolite diffusion.
Permeability and Properties:
- Determined by the size of spaces between glycosaminoglycan molecules.
- Mechanical properties reinforced by fibrous proteins in the extracellular matrix.

General Connective Tissue Proper:
- Loose (areolar) connective tissue
- Dense connective tissue (regular, irregular)
Special Connective Tissue:
- Adipose tissue
- Reticular tissue
- Elastic tissue
- Mucous tissue
Supporting (Specialized) Connective Tissue:
- Cartilage
- Bone
- Peripheral blood

Loose (Areolar) Connective Tissue:
- Characterized by loosely arranged fibers, abundant cells, and significant ground substance.
- Thin and sparse fibers
- Important for diffusion of oxygen and nutrients and site of immune reactions.
- Locations include mesenteries, lamina propria, and adventitial layers of blood vessels.
- Beneath epithelia that covers organs & glands, blood vessels
Dense Connective Tissue:
- More fibers per unit volume, less ground substance.
- Subdivided into regular (e.g., tendons, ligaments) and irregular types.
- Regular type features orderly and densely packed fibers.
Reticular Tissue:
- Supports highly cellular organs (e.g., lymph nodes, spleen) and consists of reticular fibers.
Elastic Tissue:
- Found in yellow ligaments of the vertebral column, contains abundant elastic fibers providing elasticity.
Mucous Tissue:
- Main component of the umbilical cord (Wharton's jelly) and also found in the pulp of young teeth.
Adipose Tissue:
- Contains adipocytes for fat storage and serves as an energy source with rich blood supply.
Types of Adipose Tissue:
- White Adipose Tissue:
  - Unilocular, found throughout the body, acts as energy store and thermal insulator.
  - Composed of large polyhedral cells (~50-150 μm) surrounded by reticular fibers.
- Brown Adipose Tissue:
  - Multilocular, present in newborns and hibernating animals for heat generation.
  - Smaller cells with a prominent role in temperature regulation.

Has a rich blood supply and significant mitochondrial content contributing to its role in thermogenesis, important for maintaining body temperature in newborns due to their high surface area to body mass ratio.