Connective Tissue: Types and Functions

Connective Tissue Overview

• Connective tissue characteristics include supporting epithelia, vascularization, lack of cell polarity, and less cohesiveness
• Connective tissue consists of loosely embedded cells in an extracellular matrix of fibers and ground substance

Functions of Connective Tissue

• Connective tissue connects, joins, and supports tissues
• It provides form and subdivides organs
• Connective tissue gives physical support to the body, such as bone tissue
• It regulates temperature through adipose and vascular effects
• Connective tissue enables nutrition and storage via adipose tissue
• It facilitates defense and repair through inflammatory and immune cells

Relevance of Understanding Connective Tissue

• Knowledge of connective tissue is essential for understanding tissue organization, development, and functioning
• It is crucial in trauma, healing, inflammation, and edema
• Connective tissue knowledge helps in understanding tumor terminology and development
• Malignant tumors of connective tissue are called sarcomas

Components of Connective Tissue

• Cells: Includes resident and transient cells
• Fibers: Consists of collagen, elastic, and reticular fibers
• Ground substance: Includes macromolecules and interstitial (tissue) fluid

Resident Cells: Fibroblasts

• Fibroblasts are the most numerous cell population in connective tissue
• They synthesize collagenous, elastic, and reticular fibers
• Active fibroblasts are different from inactive fibroblasts (fibrocytes)

Resident Cells: Reticular Cells

• Reticular cells are specialized fibroblasts
• These synthesize a fine type of collagen fiber, and offers scaffolding to the parenchyma
• Parenchyma includes specialized epithelial cells of various organs like lymph nodes, liver, and spleen
• Parenchymatous organs require support for internal organization but are not subjected to high mechanical stress

Resident Cells: Adipocytes

• Adipocytes are cells specialized in depositing lipids
• Multilocular adipocytes contain multiple lipid droplets ("brown fat")
• Unilocular adipocytes contain a single large lipid droplet ("white fat")

Resident Cells: Macrophages

• Macrophages are resident phagocytic cells
• Kupffer cells in the liver and osteoclasts in bone are examples of macrophages named by location

Resident Cells: Mast Cells

• Mast cells contain basophilic granules (stained blue with H&E)
• They participate in inflammatory and allergic reactions
• Degranulation (activation) of mast cells releases histamine and chemical mediators, triggering allergy symptoms

Transient Cells

• Transient connective tissue cells are white blood cells

Fibers of Connective Tissue: Collagen Fibers

• Collagen fibers are the most numerous fibers in connective tissue
• They appear white; when numerous, they give a white color to tissues like tendons and sclera
• Collagen fibers have very high tensile strength, but can only stretch 5% of their initial length
• Collagen fibers stain pink with H&E, while black arrows point to fibrocyte nuclei

Collagen (as a substance)

• Collagen is the body's most abundant protein, representing 30% of human's dry weight
• Many cells synthesize collagen including fibroblasts, chondroblasts, osteoblasts, odontoblasts, etc.
• Procollagen is formed in the fibroblast, transported outside, and assembled into collagen fibrils
• Vitamin C is essential for collagen synthesis
• Type 1 collagen makes up 90% of body's collagen, and is found in skin, bone, dentin, tendons, and fibrocartilage
• Type 1 resists tensile forces
• Type 2 collagen is found in cartilage and resists compression
• Type 3 collagen is reticular fibers and found in the stroma; it is located in expandable organs
• Type 4 collagen is found in the basal lamina of epithelia
• Deficiency of Collagen Type III can result in fragile skin syndromes

Elastic Fibers

• Elastic fibers are individual, branching, and anastomosing fibers
• Many cell types, including fibroblasts, produce elastic fibers
• They appear pale yellow
• Elastic fibers can stretch 2 ½ times their original length and recoil
• Location include the aorta, elastic arteries, lungs, vocal cords, dermis, pinna of the ear, and epiglottis

Reticular Fibers

• Reticular fibers form a fine network that can be stained by silver
• They are also known as argentaffin fibers
• Reticular cells, liver cells, smooth muscle cells, and skeletal muscle cells synthesize these fibers
• Reticular fibers act as a scaffolding for cells or cell groups in various organs

Ground Substance

• Amorphous, gel-like, and bathed in interstitial fluid
• Ground substance is produced by resident cells
• Glycosaminoglycans (GAGs) are included; for example hyaluronic acid, chondroitin sulfate, keratan sulfate, dermatan sulfate, and heparan sulfate
• GAGs link to core proteins to make proteoglycans
• Properties include the ability to bind water, resist compressive forces, enable cell adhesion, support cell migration, and provide a barrier to pathogen penetration

Interstitial (Tissue) Fluid

• It is a solution of nutrients and dissolved gases derived from plasma
• It bathes the cells, fibers, and ground substance, and removes waste
• Fluid dynamics maintain proper balance in connective tissues
• Tissue fluid determines skin turgor, and a decrease in skin turgor is a late sign of dehydration
• Lymphatics play an important role in fluid movement and preventing edema
• Interstitial fluid is not visible on slides

Edema

• Edema is the abnormal accumulation of tissue fluid

Adipose Connective Tissue

• Composed of adipocytes (fatty cells)
• Energy storage
• Endocrine function
  • Adipocytes produce leptin, which regulates body fat
  • Leptin levels are proportional to body fat and signal the brain when the body has had enough to eat
• Protection
  • Adipose tissue absorbs forces in the digital fat pad
• Insulation
• Adipose tissue is formed when adipocytes outnumber other cell types

White Adipose Tissue

• Unilocular fat cells, and possess one lipid droplet Functions:
  • Energy storage
  • Insulation
  • Endocrine ("Leptin" secretion)

Brown Adipose Tissue

• Multilocular fat cells with many lipid droplets and mitochondria
• Found in hibernating and newborn animals
• Function: Heat generation Unilocular adipocytes can generate very common benign tumors called lipomas

Whole Blood as a Specialized Connective Tissue

• Functions include transport of water, solutes, and cells; temperature regulation; and water balance (blood volume)
• It connects and supports all tissues of the body
• Blood cells exist in a complex extracellular matrix of plasma
• Cells lack polarity (no apical or basolateral surface)

Whole Blood Composition

• Composition is 5-10% of total body weight (TBW)
• 60% plasma (water, solutes)
• 40% cells (Red Blood Cells, White Blood Cells, Platelets)
• Changes in cells can be measured and inform of disease processes (e.g. anaemia and inflammation)

Plasma/Serum

• Made up of ~90% water, ~8% proteins, and ~2% other solutes
• Measurement identifies proteins and solutes
• Variations in concentrations reflect disease processes