Look at this image and make sure you can recognise loose and dense connective tissue. This is an example of dense regular connective tissue. Can you identify the cells fibroblasts and collagen fibres? Notice how the fibres are all aligned. In this type of tissue, the collagen fibres are densely packed, and arranged in parallel. This type of tissue is found in ligaments which link bone to bone at joints and tendons connections between bones or cartilage and muscle.
These are powerfully resistant to axially loaded tension forces, but allow some stretch. Classification of Connective Tissue What is connective Tissue?
The other specialised types of connective tissue are covered in other topics. Loose connective Tissue. It provides protection to internal organs and supports the body. Both components of the matrix, organic and inorganic, contribute to the unusual properties of bone. Without collagen, bones would be brittle and shatter easily. Without mineral crystals, bones would flex and provide little support. Osteocytes, bone cells like chondrocytes, are located within lacunae.
The histology of transverse tissue from long bone shows a typical arrangement of osteocytes in concentric circles around a central canal. Bone is a highly vascularized tissue. Unlike cartilage, bone tissue can recover from injuries in a relatively short time. Cancellous bone looks like a sponge under the microscope and contains empty spaces between trabeculae, or arches of bone proper. It is lighter than compact bone and found in the interior of some bones and at the end of long bones.
Compact bone is solid and has greater structural strength. Blood and lymph are fluid connective tissues. Cells circulate in a liquid extracellular matrix. The formed elements circulating in blood are all derived from hematopoietic stem cells located in bone marrow Figure.
Erythrocytes, red blood cells, transport oxygen and some carbon dioxide. Leukocytes, white blood cells, are responsible for defending against potentially harmful microorganisms or molecules. Platelets are cell fragments involved in blood clotting. Some white blood cells have the ability to cross the endothelial layer that lines blood vessels and enter adjacent tissues. Nutrients, salts, and wastes are dissolved in the liquid matrix and transported through the body.
Lymph contains a liquid matrix and white blood cells. Lymphatic capillaries are extremely permeable, allowing larger molecules and excess fluid from interstitial spaces to enter the lymphatic vessels. Lymph drains into blood vessels, delivering molecules to the blood that could not otherwise directly enter the bloodstream. In this way, specialized lymphatic capillaries transport absorbed fats away from the intestine and deliver these molecules to the blood.
View the University of Michigan Webscope to explore the tissue sample in greater detail. Visit this link to test your connective tissue knowledge with this question quiz. Can you name the 10 tissue types shown in the histology slides? Connective tissue is a heterogeneous tissue with many cell shapes and tissue architecture. Structurally, all connective tissues contain cells that are embedded in an extracellular matrix stabilized by proteins.
The chemical nature and physical layout of the extracellular matrix and proteins vary enormously among tissues, reflecting the variety of functions that connective tissue fulfills in the body.
Connective tissues separate and cushion organs, protecting them from shifting or traumatic injury. Connect tissues provide support and assist movement, store and transport energy molecules, protect against infections, and contribute to temperature homeostasis. Many different cells contribute to the formation of connective tissues. They originate in the mesodermal germ layer and differentiate from mesenchyme and hematopoietic tissue in the bone marrow.
Fibroblasts are the most abundant and secrete many protein fibers, adipocytes specialize in fat storage, hematopoietic cells from the bone marrow give rise to all the blood cells, chondrocytes form cartilage, and osteocytes form bone.
The extracellular matrix contains fluid, proteins, polysaccharide derivatives, and, in the case of bone, mineral crystals. Protein fibers fall into three major groups: collagen fibers that are thick, strong, flexible, and resist stretch; reticular fibers that are thin and form a supportive mesh; and elastin fibers that are thin and elastic.
The major types of connective tissue are connective tissue proper, supportive tissue, and fluid tissue. Loose connective tissue proper includes adipose tissue, areolar tissue, and reticular tissue. These serve to hold organs and other tissues in place and, in the case of adipose tissue, isolate and store energy reserves.
The matrix is the most abundant feature for loose tissue although adipose tissue does not have much extracellular matrix. Dense connective tissue proper is richer in fibers and may be regular, with fibers oriented in parallel as in ligaments and tendons, or irregular, with fibers oriented in several directions.
Organ capsules collagenous type and walls of arteries elastic type contain dense irregular connective tissue. Cartilage and bone are supportive tissue. Cartilage contains chondrocytes and is somewhat flexible. Hyaline cartilage is smooth and clear, covers joints, and is found in the growing portion of bones.
Fibrocartilage is tough because of extra collagen fibers and forms, among other things, the intervertebral discs. Elastic cartilage can stretch and recoil to its original shape because of its high content of elastic fibers. The matrix contains very few blood vessels. Bones are made of a rigid, mineralized matrix containing calcium salts, crystals, and osteocytes lodged in lacunae. Bone tissue is highly vascularized.
Cancellous bone is spongy and less solid than compact bone. Fluid tissue, for example blood and lymph, is characterized by a liquid matrix and no supporting fibers. Under the microscope, a tissue specimen shows cells located in spaces scattered in a transparent background. Ligaments connect bones together and withstand a lot of stress.
What type of connective tissue should you expect ligaments to contain? One of the main functions of connective tissue is to integrate organs and organ systems in the body. Discuss how blood fulfills this role. Blood is a fluid connective tissue, a variety of specialized cells that circulate in a watery fluid containing salts, nutrients, and dissolved proteins in a liquid extracellular matrix.
Blood contains formed elements derived from bone marrow. Erythrocytes, or red blood cells, transport the gases oxygen and carbon dioxide. Leukocytes, or white blood cells, are responsible for the defense of the organism against potentially harmful microorganisms or molecules. Some cells have the ability to cross the endothelial layer that lines vessels and enter adjacent tissues.
Nutrients, salts, and waste are dissolved in the liquid matrix and transported through the body. Why does an injury to cartilage, especially hyaline cartilage, heal much more slowly than a bone fracture? A layer of dense irregular connective tissue covers cartilage. No blood vessels supply cartilage tissue. Injuries to cartilage heal very slowly because cells and nutrients needed for repair diffuse slowly to the injury site.
Skip to content The Tissue Level of Organization. Learning Objectives By the end of this section, you will be able to: Identify and distinguish between the types of connective tissue: proper, supportive, and fluid Explain the functions of connective tissues. Functions of Connective Tissues Connective tissues perform many functions in the body, but most importantly, they support and connect other tissues; from the connective tissue sheath that surrounds muscle cells, to the tendons that attach muscles to bones, and to the skeleton that supports the positions of the body.
Embryonic Connective Tissue All connective tissues derive from the mesodermal layer of the embryo see Figure. Classification of Connective Tissues The three broad categories of connective tissue are classified according to the characteristics of their ground substance and the types of fibers found within the matrix Figure. Connective Tissue Proper Fibroblasts are present in all connective tissue proper Figure. Connective Tissue Proper. Fibroblasts produce this fibrous tissue.
Connective tissue proper includes the fixed cells fibrocytes, adipocytes, and mesenchymal cells. Connective Tissue Fibers and Ground Substance Three main types of fibers are secreted by fibroblasts: collagen fibers, elastic fibers, and reticular fibers. Loose Connective Tissue Loose connective tissue is found between many organs where it acts both to absorb shock and bind tissues together. Adipose Tissue. This is a loose connective tissue that consists of fat cells with little extracellular matrix.
It stores fat for energy and provides insulation. In some tendons, the tendon sheath extends along the tendon, while in others it is found only in the binding parts of the bone. The parietal synovial layer is found only under the paratenon in the body regions where tendons are exposed to high friction. This is called the epitenon and surrounds the fascicles. In regions where friction is less, tendon is surrounded by paratenon only.
At the tendon-bone junction, the collagen fibers of endotenon continue into the bone and become a peritendon.
The regions of the tendon bonding to the bone consist of a dense connective tissue, which is able to adhere to the hard bone from the dense connective tissue and is resistant to movement and damage. Although they occupy a small area in size, the areas of adhesion to the bone have a complex structure that is much different from that of the tendon itself. According to the size of the load they carry, they show a different proportion of collagen bundles [ 18 ]. The tendons cling to the bone is a complex event; collagen fibers mix into fibrocartilage, mineralize, and then merge with the bone.
Sticking to the bone is done in two ways. In the first type, the adhesion of many collagen fibers is direct to the bone, while the second type indirectly adheres to the periosteum. In other words, the tendon is attached to the bone in the form of fibrous or indirect adhesion to the metaphysics and diaphysis of long bones or fibrocartilaginous or direct adhesion to the epiphyses of the bone. In fibrous adhesions, while the collagen fibers of the tendon are permanently adhered to the periosteum during bone development, fibrocartilaginous adhesions have a gradual transition from tendon to bone.
This gradual transition in fibrocartilaginous adhesions includes the tendon, decalcified fibrocartilage, calcified fibrocartilage, and four zones of bone, so that the uniform distribution of the load at the adhesion site and the joint movement and the coordination of the collagen fibers are ensured.
However, changes in the fibrocartilaginous structure due to compressive loading vary depending on the adhesion sites of the tendons. This ensures better protection against compressive forces. The bones of the tendons are composed of four regions within the bone; at the end of the tendon region 1 , collagen fibers enter the fibrocartilage fibrous cartilage—region 2.
As the fibrocartilage progresses, it becomes mineral fibrocartilage area 3 and then integrates with cortical bone fourth region. This transformation, which is more bone structure than tendon structure, leads to gradual increase of mechanical properties of the tissue [ 3 , 19 , 20 , 21 ].
In general, they pass through the joints and adhere to their distal. In this way, they increase the effectiveness of the muscles on the joints. At the same time, similar to bones, mechanical properties vary depending on the load carrying place. For this reason, knowing where they are helps us understand the structure. In fact, not every muscle has a tendon. While some tendons are involved in some muscles that play an active role in joint movements, the presence of some tendons is to increase muscle movement distances rather than the movement of the joint.
For example, Achilles tendon is a very special tendon for the body carrying the loads by centralizing the strength of a few muscles. In contrast, some tendons, such as the posterior tibial tendon, act by distributing the load to several bones. Although it is known that most tendons originate from the muscle and adhere to the bone, some tendons may be the starting point for muscles, or two muscles are connected to each other through a tendon [ 22 , 23 ].
They can be very small and very long, and they can be very large and very short. Tendons are very variable according to their shape, long, round, rope-shaped such as Achilles tendon , or short; flat tissue adhesion such as bicipital aponeurosis can be seen. In other words, tendons may change from flat to cylinder, from fan shape to ribbon shape. However, round tendons such as flexor digitorum profundus or flat tendons such as rotator cuff, bicipital aponeurosis are more involved in the body.
In this simple classification, tendons are divided into round and flat and are very different from each other as structural and functional. For example, while round tendons respond equally to tensile loads with parallel collagen patterns, flat tendons such as rotator cuffs can respond microanatomically in the form of compression and shear forces due to longitudinal, oblique, and transverse collagen sequences.
However, in round tendons, the section area is proportional to the maximum isometric strength of the muscle. In other words, due to parallel collagen sequences, flat tendons are resistant to compression and shear forces due to flat, longitudinal, and oblique collagen sequences in comparison to round tendons that respond equally to the tensils [ 3 , 24 ]. Tendons can be classified in many ways according to their location, but the most logical one is the tendon classification in relation to the functions they see as the intraarticular biceps long head and popliteus tendon and the extraarticular Achilles tendon.
Most tendons are non-articular, but the intra-articular ones lack the ability to repair after injury as in the same intra-articular ligaments an example of anterior cruciate ligament tear. At the same time, although most tendons adhere to the bone, some tendons form the origo point for the muscles lumbrical muscles originate from the flexor digitorum profundus or connect two muscles such as omohyoid and digastric muscle. In addition, the large part of the tendon may originate from the muscle itself gastrocnemius and soleus.
For example, in some muscles tendons move into the muscle joint and tendon sticks at an angle. This allows a high proportion of muscle fibers to adhere to the tendon, thereby increasing the strength of the muscle-tendon unit but reducing the range of motion. According to their anatomy, the tendons can also be classified as sheathed or synovial-coated such as the long flexor of the fingers or unsealed or paratenon-coated such as Achilles tendon.
In other words, these tendons, which are separated by intrasynovial and extrasynovial, have a higher slippage resistance compared to the intrasynovial tendon structure, when examined more closely. At the same time, the soft tissue protection and vascularity of these two tendons are different [ 20 ].
According to its functions, tendons can be classified as energy storage or positional tendons Table 1. In general, the muscles tend to tendon to shorten the stress load; the affected tendon is stretched and the muscle can relax again when relaxed. This makes the tendon a structure that stores elastic voltage energy. The best example of energy storage tendons is Achilles tendon. Tibialis anterior tendons in human are examples of positional tendons, and they can never extend relatively. Positional tendons are rarely injured because they extend less [ 25 , 26 , 27 ].
In conclusion, tendons are composed of multiple bundles, fibroblast, and dense linear collagen fibrils, which form the macroscopic structure of tendons and give the fibrous appearance.
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