Connective Tissue Histology: Types, Cells, ECM, and Functions

Parenchymal cells

Provide the functional activity within a tissue.

Support cells

Connective tissue cells that provide structural support for a tissue and mechanical stability.

Extracellular matrix (ECM)

Helps determine the physical/mechanical properties of a tissue.

Connective tissue (CT)

Characterized by support cells and ECM; the most abundant tissue in the body.

Embryonic CT

Originates from mesenchyme during week 3 of embryonic development when germ layers differentiate.

Mesoderm

The middle germ layer from which undifferentiated cells called mesenchyme are derived.

Mature connective tissue

Has few cells and much extracellular matrix.

Matrix

Consists of fibers and amorphous ground substance that provides mechanical support for parenchymal cells.

Fibroblasts

The most common type of cell found in connective tissue.

Ground substance

The matrix component of connective tissue that can be fluid, gel-like, or solid.

Classification of CT

Based on the relative amount and type of matrix and the abundance/kinds of cells.

Characteristics of connective tissue

Very vascular, can replicate to repair organs, and has more extracellular matrix than cells.

Mechanical support

Provided by the matrix which binds and anchors parenchymal cells.

Intercellular spaces

Abundant in connective tissue and filled with matrix.

Connective tissue vs Epithelial tissue

Connective tissue is characterized by a greater abundance of intercellular spaces filled with matrix.

Mesenchyme

Undifferentiated cells derived from the mesoderm.

Types of connective tissue

Varies widely in elements, making precise definitions difficult in certain areas.

Embryonic connective tissue

Connective tissue present during the embryonic period (6 weeks, 3-8th week) characterized by high tissue differentiation and organ formation.

Areolar (Loose) CT

Most abundant type of connective tissue with a loose arrangement of cell sheets, holding unbound thin protein fibers and many cells, primarily located beneath epithelia.

Subcutaneous tissue

A type of areolar connective tissue that binds skin to muscles and surrounds blood vessels and nerves.

Dense (White) CT

Connective tissue characterized by fewer cells and ground substance, more collagen fibers, and tightly packed bundles of collagen fibers running in one direction.

Dense irregular CT

Type of dense connective tissue with fibers irregularly woven into a felt-like fabric, allowing it to withstand stress from multiple directions.

Dense regular CT

Type of dense connective tissue with bundles oriented into uniform parallel patterns, exemplified by tendons and ligaments.

Elastic CT

Connective tissue with irregularly arranged elastic fibers that allow for stretching and recoiling, found in the walls of large arteries and the larynx.

Reticular CT

Connective tissue characterized by irregularly arranged reticular fibers.

Macrophages

Cells that perform nonspecific phagocytosis in connective tissue.

Plasma cells

Cells that secrete antibodies to mark enemy cells.

Mast cells

Cells that secrete histamine (to dilate capillaries) and heparin (an anticoagulant).

White blood cells

Cells that attack and destroy bacteria, viruses, and parasites.

Adipose cells

Cells that store fat in connective tissue.

Scarce ground substance

A characteristic of dense connective tissue where ground substance is present in minimal amounts.

Tendons

Dense regular connective tissue that connects muscle to bone.

Ligaments

Dense regular connective tissue that connects bone to bone.

Aponeuroses

Flat tendons made of multiple layers of collagen fibers oriented at 90 degrees to each other.

Strain

Excessive stretch causing no serious damage.

Sprain

Tearing of a ligament causing little to very serious damage.

Permanent residents

Types of support cells that include mostly fibroblasts, macrophages, and reticular cells.

Visitors (wandering cells)

Types of support cells that include mostly plasma cells, mast cells, white blood cells (leukocytes), and pigment cells.

Functions of Fibroblasts

Support vessels and nerves, provide physical compartmentalization using septa, and are essential for tissue repair.

Myofibroblasts

Cells with features similar to both fibroblasts and smooth muscle cells, containing actin and myosin fibers, and function to contract, especially after tissue damage.

Mesenchymal cells

Embryonic connective tissue cells that may persist in adults and can differentiate into a variety of connective tissue cells.

Reticular cells

Embryonic cell type that builds reticular fibers, providing mechanical support and forming a cellular reticulum outside cells.

Functions of Reticular Cells

Support for cells within bone marrow, lymph nodes, and spleen, and may be phagocytic if needed.

Adipoblasts

Immature adipose cells with few small vacuoles of neutral fat that increase in size and coalesce into one large globule in the fetus.

Adipocytes

Mature adipose cells where lipid fills the cell.

Types of adipose tissue

Includes unilocular (white) and multilocular (brown) adipose tissue.

Unilocular adipose tissue

The predominant type in adults characterized by cells with a single large droplet of lipid, which is not membrane-bound and is very metabolically active.

Functions of Unilocular Adipose Tissue

Energy storage, insulation, and cushioning of vital organs.

Hypodermis

Also known as 'panniculus adiposus', it is the layer under the skin containing unilocular adipose tissue.

Internal storage of adipose tissue

Connective tissue mesenteries holding or surrounding organs, found in palms, soles, eye orbits, and between muscles.

Influencing factors of adipose tissue amount

Genetics, stress, exercise, caloric intake, and toxic load within the body.

Multilocular (brown fat)

Present in newborn; Body uses this type of lipid to create heat; Brown color = many mitochondria & sER.

Leukocytes

All white blood cells; Abundant during inflammation or infection; Creep through capillary walls into tissue (ameboid movement).

Monocytes

Avid phagocytes found in Loose CT.

Neutrophils

Common during bacterial infections.

Eosinophils

Often found in breast, lung & GI tract; Increase during parasitic infections & asthma.

Melanocytes

Cytoplasmic organelles that construct melanin granules; Function = absorb & disperse UV light to protect DNA.

Fibers

Composed of fibrillar proteins; Surrounded by large areas of gel-like GAG's (glycosaminoglycans) & proteoglycans.

GAG's (glycosaminoglycans)

Large unbranched polysaccharide chains; Many sulfur atoms induce negative charge to molecule, attracting Na+ and water.

Proteoglycans

GAG molecules attached to proteins; Huge molecules that have large spaces to hold water.

Collagens

MOST numerous CT fibers; Provide the greatest tensile (stretch) strength; Produced by fibroblasts.

Collagen structure

Assembles into a right hand triple helix called procollagen; Cleaved ends form Tropocollagen molecule.

Tropocollagen

Molecules that polymerize to form a microfibril; Strength due to covalent bonds between adjacent tropocollagen molecules.

Principle amino acids in collagen

Glycine (34%), Proline (12%), Hydroxyproline (10%) are required for procollagen helix to form.

Nutrition needs for collagen

Vitamin C is needed for proline conversion to hydroxyproline; Cannot build collagen without vitamin C.

Reticular fibers

Produced by fibroblasts or reticular cells; Made up of collagen fibers that form a network (reticulum).

Elastic fibers

Produced by fibroblasts & smooth muscle cells; Made up of elastin & fibrillin; More elasticity than collagen.

Location of elastic fibers

Found in ligaments between vertebrae, larger arteries & cartilages (ear & epiglottis).