8.14:
Classification of Connective Tissues
Mesenchyme, an embryonic connective tissue, gives rise to all mature connective tissues in humans.
Mature connective tissues are classified into proper, supporting, and fluid based on their physical forms and functions.
The loose connective tissue proper is composed of different cells with loosely arranged protein fibers in the extracellular matrix or ECM.
In contrast, the dense connective tissue proper has a relatively acellular ECM with tightly packed thick bundles of collagen fibers.
Loose connective tissue is further divided into areolar, adipose, and reticular, while the dense connective tissue is categorized into regular, irregular, and elastic types.
The supporting connective tissues—bones and cartilage—comprise the human skeletal system.
Bones have calcified ECM, and based on the organization of cells in the ECM, they are compact or spongy.
Cartilage has properties intermediate to dense connective tissues and bones and can withstand stress without deformation. It has a gel-like ECM enriched with fibrous proteins and is subdivided into three types — hyaline, fibrocartilage, or elastic.
Lastly, blood and lymph are fluid connective tissues with cells floating in a fluid ECM.
8.14:
Classification of Connective Tissues
The connective tissues have different properties and functions in the human body. They are broadly categorized into proper, supporting, or fluid connective tissues.
Connective Tissue Proper
Connective tissue proper is the most abundant class of connective tissues. As its name implies, it predominantly connects different tissues in the body. Depending on the cell types, ground substance, viscosity, and fiber types in the ECM, connective tissue proper is further categorized into loose and dense. Loose connective tissue provides strength, elasticity, and support, and is the 'packaging material' of the body. The areolar connective tissue has the most diverse cell types of the three loose connective tissues. In contrast, the adipose and reticular tissues primarily comprise adipocytes and reticular cells, respectively. Dense connective tissues have more fibers than cells to withstand tension. They are classified as regular or irregular according to the arrangement of fibers. The elastic tissue is a type of dense connective tissue that provides elasticity to organs, such as the lungs and the heart.
Supporting Connective Tissue
Bone and cartilage are categorized as supporting connective tissues. The bone matrix is rigid and calcified because of the deposited calcium salts. Soft or trabecular bone has a spongy, porous appearance, while compact bone is solid and has greater structural strength. In contrast, the cartilage matrix has abundant collagen and elastic fibers and is rich in proteoglycans. It houses cells called chondrocytes in empty spaces or pockets called lacunae. Cartilage tissue has three types: hyaline cartilage, fibrocartilage, and elastic cartilage.
Fluid Connective Tissue
The blood and lymph— types of fluid connective tissue— have a liquid extracellular matrix with various cells floating in them. These cells originate from the hematopoietic stem cells in the bone marrow. Both blood and lymph participate in immune response, wound healing, and transportation of wastes and nutrients in the body. Additionally, the lymphatic system balances the composition and volume of the tissue fluids and blood in the body.