Connective Tissue Cells Quiz

Connective Tissue Overview

• Connective tissue is one of the four basic tissue types
• It provides structural and metabolic support to organs and other tissues
• Functions include support (structural and mechanical), packing (fills spaces), storage (energy, water, electrolytes), transport (nutrients and metabolic wastes), repair (matrix and fibers), and defense (phagocytosis or antibodies)

Connective Tissue Structure

• Composed of cells (resident and wandering) and extracellular matrix
• Resident cells: fibroblasts, adipocytes, macrophages, mesenchymal cells, mast cells
• Wandering cells (transient): lymphocytes, plasma cells, eosinophils, basophils, neutrophils, monocytes
• Extracellular matrix: protein fibers (collagen, elastic, reticular) and ground substance (amorphous)

Connective Tissue Classification

• Embryonic Connective Tissue: mesenchyme, mucous connective tissue (Wharton's jelly)
• Connective Tissue Proper: loose (areolar), dense regular, dense irregular
• Specialized Connective Tissue: cartilage, bone, adipose tissue, blood, hemopoietic tissue, lymphatic tissue

Learning Objectives

• Describe histological characteristics common to connective tissue
• List components (cells, ground substance, fibers) of connective tissue
• Describe structure of each cell type and correlate with functions
• Describe structural organization of loose (areolar), dense regular, dense irregular, embryonic and adipose tissues
• Differentiate between embryonic, loose, dense regular and dense irregular connective tissues based on images
• Describe the synthesis, secretion and assembly of collagen fibers
• Describe the three most common extracellular fibers (collagen, elastic, reticular)
• State locations of collagen, elastic and reticular fibers
• List common sites of collagens I-VI and describe the consequences of deficiencies
• Describe biochemical structure and functions of glycosaminoglycans (GAGs), proteoglycans and glycoproteins
• Identify macrophages, mast cells, eosinophils, neutrophils, lymphocytes, plasma cells, fibroblasts, pericytes, and adipocytes in microscopic sections
• Describe functions of mesenchymal stem cells and myofibroblasts
• Differentiate between permanent and wandering cell types
• Identify residual bodies in macrophages
• Describe functions of B and T lymphocytes, neutrophils, mast cells, basophils, plasma cells, and eosinophils
• Differentiate between mast cells and basophils

Connective Tissue Proper: Loose (Areolar) Tissue

• Predominantly ground substance, few cells (mostly fibroblasts).
• Cells are primarily migratory.
• Key locations: beneath epithelial layers, around glands and smallest blood vessels.
• Supports microvasculature, nerves and immune cells.

Connective Tissue Proper: Dense Regular Tissue

• Primarily type I collagen fibers and fibroblasts aligned in parallel.
• Provides resistance to prolonged or repeated stresses
• Poorly vascularized: Slow repair
• Found in tendons, ligaments, aponeuroses.

Connective Tissue Proper: Dense Irregular Tissue

• Contains mostly collagen fibers, not as much ground substance.
• Bundles of fibers oriented in various directions.
• Provides resistance to tearing when stretched.
• Found in the reticular layer of skin dermis and submucosa of GI tract

Resident Connective Tissue Cells

• Fibroblasts: Primary cells of connective tissue; synthesize collagen, elastin, and reticular fibers; and complex carbohydrates of ground substance (GAGs); show morphological variation based on activity level; fibroblast or fibrocyte.
• Myofibroblasts: Fibroblasts with some characteristics of smooth muscle cells (e.g., contractile filaments); implicated in wound contraction.
• Macrophages (Histiocytes): Phagocytic cells; derived from monocytes; can be seen in clusters or as large phagocytic cells with residual bodies.
• Adipocytes: Store energy in form of triglycerides; unilocular (fat cells): large single droplet in cytoplasm of mature cells; multilocular (brown fat): presence many small lipid droplets; involved in energy metabolism, particularly in infants for heat generation.
• Mast Cells: Granular cells; originate from bone marrow basophil/mast cell precursors; part of innate immune system, respond to various stimuli and release inflammatory mediators (e.g., histamine, tryptase).
• Undifferentiated Mesenchymal Cells/Pericytes: Can differentiate into other cell types—like fibroblasts or myofibroblasts—during injury repair, located around blood vessels, crucial in neovascularization and wound healing

Wandering Cells of Connective Tissue

• Lymphocytes: Primarily involved in immune responses (B cells produce antibodies; T cells mediate cellular immunity; NK cells destroy virus-infected and tumor cells).
• Plasma Cells: Derived from B lymphocytes; produce antibodies.
• Eosinophils: Granulocytes that play a role in allergic reactions and parasitic infections.
• Basophils: Granulocytes that release histamine, involved in allergic reactions and inflammation.
• Neutrophils: Granulocytes that are the first responders to infection, exhibiting phagocytic activity against pathogens.
• Monocytes: Phagocytic cells; differentiate into macrophages, which function in antigen presentation and immune defense.

Collagen Biosynthesis

• Transcription of mRNA for pro-a1 and pro-a2 chains in nucleus.
• Translation in cytoplasm, producing pre-pro-polypeptide chains.
• Post-translational modifications: Signal peptide removal, hydroxylation of specific amino acids (requires vitamin C), and glycosylation.
• Assembly into procollagen molecules in RER.
• Transport of procollagen to the Golgi apparatus.
• Procollagen cleavage (removal of terminal propeptides) outside the cell.
• Assembly of tropocollagen molecules into fibrils.
• Cross-linking of tropocollagen molecules to form collagen fibrils.

Collagen Breakdown

• Proteolytic degradation: MMPs break down ECM.
• Phagocytic degradation: Macrophages and fibroblasts engulf and degrade collagen.

Collagen Fiber Types

• Type I: Most common; found in bone, tendon, dermis.
• Type II: Important in cartilage.
• Type III: Forms reticular fibers; found in supporting tissues like those surrounding blood vessels and lymphatic tissue.
• Type IV: Major component of basement membranes.

Elastic Fibers

• Thinner than type I collagen fibers.
• Form sparse networks in tissues that undergo regular stretching or bending.
• Synthesized by fibroblasts and smooth muscle cells.
• Consist of elastin molecules arranged in a crosslinked network in an amorphous matrix of the microfibrils
• Key locations: ligaments, lungs, blood vessels.

Reticular Fibers

• Composed of type III collagen.
• Visualized with silver staining (argyrophilic).
• Thin, delicate networks that support structures or tissues such as blood vessels and lymph nodes in spleen and bone marrow.

Ground Substance

• Viscous, amorphous component of connective tissue.
• Mostly composed of proteoglycans, GAGs, and multiadhesive glycoproteins.
• Functions: Supports cells, tissues fluids, medium through which molecules can diffuse, and site for cell adhesion.
• Usually lost during tissue fixation.

Specific Cell Types

• Mast cells: key role in inflammation and allergic reactions.
• Pericytes: undifferentiated mesenchymal cells associated with capillaries and venules, function in neovascularization and wound healing.

Other Important Tissue Components

• Mucous connective tissue (Wharton's jelly): specialized type of embryonic connective tissue rich in ground substance, present in the umbilical cord, helping to support and cushion the developing fetus; also has regenerative potential in regenerative medicine.
• Adipose tissue: stores energy in the form of triglycerides and cushions vital organs; categorized into white and brown adipose tissue.
• White adipose tissue (WAT): predominant in adults, primarily stores energy, cushions organs, regulates endocrine function (hormones like leptin.
• Brown adipose tissue (BAT): predominantly found in fetuses and newborns; primarily for heat generation via metabolism
• Lymphoid tissue: specialized connective tissue that supports lymphocytes and other immune cells; an integral part of the body's immune system.

Test your knowledge on the various types of cells found in connective tissue with this quiz. You'll explore the differences between permanent and wandering cells, the roles of macrophages, and the characteristics of different connective tissue types. Challenge yourself on the histological structures and functions of these important biological components.