Connective Tissue Types: Function & Matrix

• The topic is connective tissue
• It is a tissue that has several variants differing in function and structure
• The extracellular substance predominates quantitatively over the cells, and their cells originate from the mesenchyme or the bone marrow

Function

• Connects, supports and sustains other tissues
• Facilitates the distribution of biologically active substances, oxygen, and nutrients throughout the body
• Responsible for most of the body's defense processes

Main Types

• Embryonic connective tissue (mesenchyme)
• Mucous connective tissue
• Proper connective tissue
• Supportive connective tissue
• Blood and lymph tissue
• Connective tissue are made up of cells and an extracellular matrix

Extracellular Matrix

• Ground substance is unstructured, composed of proteins and protein-polysaccharide compounds such as proteoglycans and glycoproteins
• The ground substance is highly hydrated enabling the diffusion of gasses and nutrients to dispersed cells within it
• There are Ground substance and fibers that consist of collagen, elastin and reticular fibers

Fibroblasts

• Elongated cells that intensely synthesize the components of the extracellular matrix
• These elements are secreted into the extracellular space, where they aggregate into larger structures
• The resting form of fibroblasts are called fibrocytes

Macrophages

• Spherical in fluids or irregularly shaped
• Originate from blood monocytes
• Contain numerous lysosomes and move by amoeboid motion
• Participate in defence processes by phagocytizing and digesting debris of dead cells, microorganisms and foreign bodies
• Produce biologically active substances that influence these processes

Plasma Cells

• Originate from B lymphocytes
• Spherical cells with highly developed rough endoplasmic reticulum and Golgi apparatus
• Participate in immunological processes by producing antibodies

Mast Cells

• Oval-shaped cells containing large basophilic granules in the cytoplasm, which house inflammatory mediators, such as histamine, heparin and proteases
• Upon stimulation, these mediators are released in the surrounding environment through rapid exocytosis (degranulation)
• They collaborate with other cells of connective tissue and blood in defense processes and trigger allergic reactions

Fat Cells

• There are large oval cells
• Brown fat: Nucleus is centrally located and has numerous small fat vacuoles
• Yellow fat: Nucleus is pushed to the periphery of the cell membrane and has one large fat vacuole filling the entire cell, so cells appear empty
• Both types are divided into lobules by connective tissue septa

Collagen

• Fibrillar collagen, types I, II, and III, has polypeptide subunits that join together to form large fibrils
• Type I collagen, the most abundant and widespread, forms large, acidophilic bundles referred to as collagen fibers
• These fibers often densely populate connective tissue, forming structures such as tendons, organ capsules, or dermis
• Collagen is arranged in layered networks or sheaths and is mainly type IV, that has subunits produced by epithelial cells that serve as the main structural protein of the outer layers and all basement membranes of epithelia
• Anchoring collagen consists of short collagen fibers forming larger fibers, with other components of the extracellular matrix
• In basement membranes, type VII collagen binds to type IV collagen and attaches the basal lamina to the underlying reticular layer

Elastin

• Thin, stretchable fibers that form networks or sheets, providing elasticity

Reticular fibers

• Constructed from collagen III, thin fibers that form networks with very fine mesh-like structures, enveloping cells or their groups, providing scaffolding

Extracellular Matrix

• A highly hydrated, transparent complex composed of macromolecules, glycosaminoglycans, proteoglycans, and adhesive glycoproteins
• Fills the space between cells and fibers in connective tissue and allows for the diffusion of small molecules
• Functions both as a gel and as a barrier against invading pathogens, because it is gel like

Loose Connective Tissue

• The most common type, it connects organ structures and introduces blood vessels and nerve fibers into organ territory
• It consists of ground substance and fibers (equal amounts), dispersed connective tissue cells, and white blood cells

Dense Connective Tissue

• Consists mainly fibers (primarily collagen), minimal ground substance, and cells (mostly fibrocytes)
• There are two variations, regular arrangements of fibers (tendons, ligaments) and with an irregular arrangement of fibers (dermis, sclera of the eye)

Reticular Connective Tissue

• Composed of a network of reticular fibers, upon which star-shaped cells are spread, serving as scaffolding for lymphoid tissue and bone marrow

Adipose Tissue

• In this connective tissue variant, cells adhere quite closely to each other and significantly outnumber the extracellular substance
• Yellow adipose tissue: Comprised of unilocular fat cells with a single large lipid droplet filling nearly the entire cell cytoplasm and is located in subcutaneous tissue and organ capsules
• The yellow adipose tissue's function is the storage, metabolism, and release of fats
• Brown adipose tissue: Consists of multilocular fat cells with numerous small lipid droplets and many mitochondria in the cytoplasm, present in very small amounts in humans, mainly in newborns
• The brown adipose tissue's function is heat production

Gelatinous Connective Tissue

• Found in the umbilical cord of the fetus and in the dental pulp of young teeth after birth
• It is produced by fibroblasts and consists of few cells, a large amount of hyaluronic acid, and collagen fibers

Mesenchyme

• A type of embryonic connective tissue present only during the embryonic period
• All types of connective tissues, bone tissue, cartilage tissue, and muscle tissue originate from it
• The star-shaped cells have a totipotent character and the gelatinous ground substance is devoid of fibers