Connective Tissue Types and Function

• Connective tissue is a tissue with several variants, differing in structure and function.
• The extracellular substance predominates quantitatively over the cells.
• The cells of this tissue originate from the mesenchyme or the bone marrow.
• Connective tissue connects, supports, and sustains other tissues.
• It facilitates the distribution of oxygen, nutrients, and biologically active substances (e.g., hormones) throughout the body.
• It is responsible for most of the body's defense processes.

Types of Connective Tissue

• Embryonic connective tissue (mesenchyme)
• Mucous connective tissue
• Proper: Dense connective tissue, Adipose tissue, Reticular connective tissue, Loose connective tissue
• Supportive: Cartilage tissue, Hyaline, Fibrous, Elastic, Bone tissue
• Blood and lymph tissue

Extracellular Matrix

• Ground substance: unstructured, composed of proteins and protein-polysaccharide compounds such as proteoglycans and glycoproteins.
• Ground substance is highly hydrated, enabling the diffusion of gases and nutrients to dispersed cells within it.
• Fibers are composed of proteins, of which there are three types: Collagen, Elastin, and Reticular Fibers

Fibroblasts

• Fibroblasts are elongated cells that intensely synthesize the components of the extracellular matrix (proteins, proteoglycans).
• These elements are secreted into the extracellular space, where they aggregate into larger structures.
• These structures include forming fibers or high molecular weight complexes of ground substance.
• Fibrocytes are the resting form of fibroblasts found in mature connective tissue.

Macrophages

• Macrophages are spherical (in fluids) or irregularly shaped cells originating from blood monocytes.
• They contain numerous lysosomes and move by amoeboid motion.
• They participate in defense processes by actively phagocytizing and digesting debris of dead cells, microorganisms, and foreign bodies.
• Macrophages produce biologically active substances that influence these processes.

Plasma Cells

• Plasma cells originate from B lymphocytes.
• They are spherical cells with highly developed rough endoplasmic reticulum and Golgi apparatus.
• They participate in immunological processes by producing antibodies.

Mast Cells

• Mast cells are oval-shaped cells containing large basophilic granules in the cytoplasm, which house inflammatory mediators (such as histamine, heparin, proteases).
• Upon stimulation, these mediators are released into 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

• Brown fat: Large oval cells, Nucleus centrally located, Numerous small fat vacuoles, cells resembling a mesh with spaces, Divided into lobules by connective tissue septa
• Yellow fat: Large oval cells, Nucleus pushed to the periphery of the cell membrane, One large fat vacuole filling almost the entire cell - cell appears empty, Divided into lobules by connective tissue septa

Collagen

• Fibrillar collagen, especially types I, II, and III, has polypeptide subunits that join together to form large fibrils.
• Type I collagen is the most abundant and widespread, forms large, acidophilic bundles typically referred to as collagen fibers.
• These fibers often densely populate connective tissue, forming structures such as tendons, organ capsules, or dermis.
• Collagen arranged in layered networks or sheaths (e.g., type IV collagen) has subunits produced by epithelial cells and serves as the main structural protein of the outer layers and all basement membranes of epithelia.
• Linking (anchoring) collagen consists of short collagen fibers linking fibrillar collagen together (forming larger fibers) and with other components of the ECM.
• 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 (thin layers), providing elasticity to the areas where they are present.

Reticular Fibers

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

Extracellular Matrix Substances

• The basic substance of the extracellular matrix is a highly hydrated, transparent complex composed of a mixture of glycosaminoglycans (GAGs), proteoglycans, and adhesive glycoproteins.
• Filling the space between cells and fibers in connective tissue, the ground substance allows for the diffusion of small molecules.
• Because it is gel-like, it functions both as a gel and as a barrier against invading pathogens. The physical properties of the ground substance also significantly influence various cell functions.

Loose Connective Tissue

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

Dense Connective Tissue

• Its composition is mainly fibers (primarily collagen), minimal ground substance, and cells (mostly fibrocytes).
• Two variations: regular arrangement of fibres (tendons, ligaments) and with an irregular arrangement of fibers (dermis, sclera of the eye).

Reticular Connective Tissue

• Serves as scaffolding for lymphoid tissue and bone marrow.
• Composed of a network of reticular fibers, upon which star-shaped cells are spread (mostly fibroblasts and macrophages).

Adipose Tissue

• In this type of connective tissue variant, cells adhere closely to each other and significantly outnumber the extracellular substance.
  • Yellow adipose tissue: Comprised of unilocular fat cells (adipocytes) containing a single large lipid droplet filling nearly the entire cell cytoplasm; found in subcutaneous tissue, capsules of certain organs, stores, metabolizes, and releases fats.
  • Brown adipose tissue: Consists of multilocular fat cells containing numerous small lipid droplets and many mitochondria in the cytoplasm; present in newborns; produces heat.

Gelatinous Connective Tissue

• This is a type of connective tissue found in the umbilical cord of the fetus and in the dental pulp of young teeth after birth, and is produced by fibroblasts.
• Histologically, it consists of few cells, a large amount of hyaluronic acid, and collagen fibers.

Mesenchyme

• Embryonic connective tissue.
• Present only during the embryonic period.
• All types of connective tissues, bone tissue, cartilage tissue, and muscle tissue originate form here.
• Star-shaped cells have a totipotent character.
• The gelatinous ground substance of embryonic connective tissue is devoid of fibers.