Connective Tissue
Connective tissue serves the purpose of connecting and providing support to other tissues. Unlike epithelial tissue, it consists of cells that are dispersed throughout an extracellular matrix. Loose Connective Tissue
Loose connective tissue, which is the most common type of connective tissue in vertebrates, has two primary functions: holding organs in place and attaching epithelial tissue to other underlying tissues.
This type of connective tissue can be classified based on its “weave” and the fibers it contains. There are three main types of loose connective tissue:
- Collagenous Fibers: These fibers are made up of collagen and consist of bundles of coiled collagen molecules known as fibrils.
- Elastic Fibers: Made up of elastin, these fibers have the ability to stretch.
- Reticular Fibers: These fibers serve to connect connective tissues with other tissues.
Another type called fibrous connective tissue is found specifically in tendons and ligaments. It is characterized by densely packed collagenous fibers.
Specialized Connective Tissues
Adipose
Adipose tissue is a type of loose connective tissue that functions as a storage for fat.
Cartilage
Cartilage is another specialized form of connective tissue made up of tightly packed collagenous fibers in chondrin, which is a gelatinous substance with rubbery properties. Sharks and human embryos possess skeletons made of cartilage. In adults, cartilage provides flexible support to structures like the nose, trachea, and ears.
Bone
Both bone and blood are forms of connective tissue. Bone is composed of collagen and calcium phosphate, making it strong due to mineralization. In contrast, although serving a different function, blood is also classified as connective tissue. It contains plasma as its extracellular matrix, with suspended erythrocytes, leukocytes, and platelets.
The types and relative abundance of cells, fibers, and ground substance, as well as the organization of fibers, classify connective tissues. The accompanying images introduce different types of connective tissues. In specific Blocks, there will be an opportunity to study some of these connective tissues in more detail.
Areolar connective tissue, also known as loose connective tissue, is a cellular type of connective tissue that contains an ample amount of ground substance and thin fibers that are relatively sparse. It has a consistency similar to gel and plays a vital role in the exchange of oxygen and nutrients through small blood vessels, as well as the transportation of metabolites back to these vessels. This particular tissue is mainly found underneath the epithelia that line the internal surfaces of the body, along with its presence around small blood vessels and in association with glandular epithelia. Its function includes being the primary defense mechanism against antigens, bacteria, and other harmful substances that penetrate an epithelial surface.
Dense irregular connective tissue is distinguished by its abundance of collagenous fibers, which confer strength to the tissue. Fibroblasts typically constitute the sole cell type found in this tissue, with a minimal amount of ground substance. The collagenous fibers are grouped into bundles that run in diverse orientations, enabling the tissue to withstand various types of stress. This variety of connective tissue can be observed on the external surface of numerous organs, within the dermis layer of the skin, and as an independent layer known as the submucosa in different organs.
Dense regular connective tissue is composed of tightly packed collagenous fibers arranged in regular patterns, with rows of cells distributed throughout. This type of tissue can be located in tendons, ligaments, and aponeuroses. Tendons link muscles to bones, while ligaments connect bones to other bones. Some ligaments also possess elastic fibers and are referred to as elastic ligaments. Aponeuroses are wide and flattened tendons that consist of multiple layers of regularly organized fibers.
Specialized Connective Tissues
Cartilage is a connective tissue made up of specialized cells called chondrocytes. These cells release a matrix that contains GAGs like hyaluronic acid, chondroitin sulfate, and keratan sulfate, leading to basophilia in the matrix. The matrix also has collagen (type II) fibrils, but they are not easily distinguishable from the ground substance. Chondrocytes are found in lacunae, which are spaces within the tissue that they occupy. However, during tissue preparation, chondrocytes often shrink and become dislodged, resulting in partially filled or empty lacunae. For more information about cartilage, refer to the MSK Block.
Bone is a connective tissue that has a mineralized extracellular matrix. Osteocytes, which are cells, secrete this matrix. It mainly consists of mineralized collagen fibers organized in lamellae. The small amount of ground substance present is also mineralized. More information about bone can be found in the MSK Block.
Blood
Blood is a type of fluid connective tissue that circulates throughout the body. Its primary functions include providing nutrients and oxygen to tissues, removing waste products, transporting various substances such as hormones and immunogenic agents, and maintaining stability. The accompanying images depict the presence of white cells within blood.
Animals are mostly composed of muscle tissue, which contains cells that can contract. This tissue includes actin and myosin, contractile proteins that create microfilaments.
There are three main types of muscle tissue:
Cardiac Muscle is located in the heart and gets its name from its location. It is a branched and striated type of muscle, with intercalated discs connecting the cells to allow for coordinated heart contractions.
Skeletal Muscle
Skeletal muscle, responsible for voluntary movements, is connected to bones via tendons. Unlike cardiac muscle, skeletal muscle cells are not branched. Additionally, there exists another type of muscle called visceral or smooth muscle.
Visceral muscle, commonly referred to as smooth muscle, is found in various organs such as arteries, bladder, and digestive tract. Unlike skeletal muscle, it does not have cross striations. While visceral muscle contracts at a slower pace compared to skeletal muscle, it can sustain contraction for a longer period of time.
The nerve tissue, also referred to as nervous tissue, controls both the central and peripheral nervous systems. It comprises neurons and their processes, as well as other specialized or supporting cells and extracellular material.
Nervous tissue is responsible for sensing stimuli and transmitting signals throughout an organism. Neurons, which are essential components of nervous tissue, play a crucial role.
As previously mentioned, the structure of a neuron is closely linked to its function within nervous tissue.
Two main components make up a neuron:
The central cell body of a neuron, which includes the nucleus, associated cytoplasm, and other organelles, also contains nerve processes that resemble fingers. These processes are capable of conducting and transmitting signals. They can be classified into two types.
The primary function of axons is to transmit signals away from the cell body, while dendrites are responsible for transmitting signals towards the cell body.
Typically, neurons possess a solitary axon that can branch out. The axon concludes at a synapse, which acts as the pathway for transmitting signals to the subsequent cell, primarily through a dendrite.
Dendrites generally possess more branches and are shorter in length compared to axons, although there may be deviations from this pattern, as is the case with other structures found in organisms.
The term “nerves” is used to describe bundles of axons and dendrites, which can be categorized as sensory, motor, or mixed based on their composition. Sensory nerves consist only of dendrites, while motor nerves are made up entirely of axons. In comparison, mixed nerves contain both axons and dendrites.
