Connective Tissue
Most abundant and widely distributed of the tissues
Its major functions include : binding and support, protection, insulation, and transport of substances
Common characteristics of connective tissue
- Common origin
: all types of connective tissue have a common origin – mesenchyme
- Different degrees of vascularity
: some connective tissues have no blood supply, others are very well supplied
- Extracellular matrix
: connective tissue cell are sparsely strewn throughout a matrix of fibers and/or minerals.
Structural components of connective tissue
Ground substance : amorphous material that contains fibers and fills the space between the cells. Composed of cadherins, proteoglycans, and interstitial fluid. This area functions as a sort of filter and is part of the extracellular matrix.
Fibers : provide support. Collagen fibers are thick, cross-linked ropes of collagen. Reticular fibers are also made of collagen, but are much thinner. Elastic fibers are made of elastin, which is stretchy. The fibers are also part of the extracellular matrix.
Cells : obviously cells must be part of a tissue. Each class of connective tissue has a particular type of immature, mitotically active blast cell that forms the ground substance, fibers and cells of that type of tissue.
- Fibroblast
: connective tissue proper
- Osteoblast
: bone
- Chondroblast
: cartilage
- Hematopoietic stem cell
: blood
- Other types of cells found in the matrix : fat cells (nutrient storing), white blood cells (mast cells and macrophages).
Types of connective tissue
Mesenchyme : embryonic connective tissue. Its function is to produce all other types of connective tissues.
Connective tissue proper
: a gel-like matrix with all three fiber types. Include fibroblasts, macrophages, mast cells, and other white blood cells. It’s found directly under the epithelia. It functions as a cushion for organs and plays a role in inflammation. Also called "loose."
Adipose : gel-like matrix, but very sparse. Large, close-packed cells. Found directly under the skin, around kidneys, eyeballs, in bones, abdomen, breasts. It provides a reserve food supply and insulates. Provides some support.
Reticular : loose ground substance, reticular fibers, reticular fibroblasts dominate. Reticular tissue is found in lymph nodes, bone marrow and spleen, and is responsible for support. It forms a soft, internal skeleton for those organs.
Fibrous (dense) : packed with collagen fibers, few elastin. Fibroblasts are the main cell type. Regular dense has a regular pattern and is found in tendons and ligaments. Irregular dense is "chunky" and is found in the dermis.
Cartilage
: firm, amorphous matrix formed of collagen fibers. Chondrocytes sit in small cavities called lacunae. In embryos, hyaline cartilage forms the skeleton, but in a mature individual, hyaline is found in joints, trachea, nose and ribs. It supports and reinforces.
Elastic : similar to hyaline, but has more elastin fibers. It’s found in the ear and epiglottis. Great flexibility, but also supportive.
Fibro : has a less thick, but similar, matrix to hyaline (thick collagen fibers). Found in the spinal disks and knee’s menisci. It is designed to have great strength, but also able to absorb shock.
Osseous (bone) : has a hard matrix containing calcium salts and many collagen fibers. Osteocytes sit in lacunae. It is extremely well vascularized. Function is to support and protect.
Blood : red (erythrocytes) and white (leukocytes) blood cells in a fluid matrix (plasma). Function is the transport substances to and from tissues.
Muscle Tissue
Highly cellular, well vascularized, responsible for movement.
Skeletal muscle : also voluntary or striated. Long, cylindrical cells that are multinucleate. They look striped due to the position of the actin and myosin filaments.
Smooth muscle : also involuntary. It has no visible striations. Spindle-shaped cells and uninucleate. Found in the walls of hollow organs.
Cardiac muscle : found only in the heart. It is striated, but uninucleate and branching. Cells fit together at junctions called intercalated disks.
Nerve Tissue
Regulates and controls body functions.
Neurons and glial (supporting) cells : neurons conduct the nerve impulses and glial cells support and insulate the neurons.