Lecture 3 Connective Tissue (2025) PDF

Connective Tissue  Connective Tissue Connective tissue provides structural support for the body by binding cells and tissues together to form organs. It also provides metabolic support by creating a hydrophilic environment that mediates the exchange of substances betw...

Connective Tissue  Connective Tissue Connective tissue provides structural support for the body by binding cells and tissues together to form organs. It also provides metabolic support by creating a hydrophilic environment that mediates the exchange of substances between the blood and tissue. Connective tissue is of mesodermal origin. Structurally connective tissues is formed by three classes of components: 1) Cells 2) Fibers 3) Ground substance  Connective tissue is classified based on the characteristics of its cellular and extracellular components. The main criteria are the type of cells, arrangement and type of fibers, and composition of the extracellular matrix. Unlike the other tissues which are formed mainly by cells, the major constituent of connective tissues is the extracellular matrix (fiber and ground substance). A) Connective Tissue Cells - A variety of cells are found in connective tissue, which differ according to their origin and function. Some cells differentiate from mesenchymal cells, such as adipocytes and fibroblasts; these cells are formed and reside in the connective tissue and are called fixed cells. - Other cells, which arise from hematopoietic stem cells, differentiate in the bone marrow and migrate from the blood circulation into connective tissue where they perform their functions; these mast cells, macrophages, plasma cells, and leukocytes are called wandering cells. A. Fixed cells B. Free cells Fibroblast Macrophage (histiocyte) Mesenchymal cells Mast cell Adipocyte Plasma cells Leucocytes 1) Fibroblast cells They are the most common cells in connective tissue. Their nuclei are ovoid or spindle shaped and can be large or small in size depending on their stage of cellular activity. Fibroblasts produce and maintain the extracellular matrix.. These cells originate locally from undifferentiated mesenchymal cells & spend all their life in this tissue. 2) Mesenchymal cells Undifferentiated cells. Mesenchymal cells are small cells with large nuclei, prominent nucleoli and fine chromatin. These are multipotent stem cells that differentiate as progenitor cells for all types of connective tissue, such as fibroblasts, osteoblasts, chondroblasts and adipocytes.. 3) Mast cells They are of bone marrow origin and are circulate in the bloodstream, and migrate into connective tissue, where they proliferate and differentiate into mature mast cells (granular). They are motile, ovoid cells. The cytoplasm is filled with large granules & contain heparin (is an anticoagulant) and histamine ( increase the permeability of the vessel). 4) Macrophages (tissue histiocytes) Are highly phagocytic cells that are derived from blood monocytes. They engulf and digest microbes, cellular debris, and foreign substances. Monocytes develop in bone marrow, circulate in the bloodstream, and migrate into connective tissue, where they differentiate into macrophages. 5) Plasma cells Are derived from B lymphocytes. They are oval shaped and have the ability to secrete antibodies that are antigen specific. Their histological features include an eccentrically placed nucleus, a cartwheel pattern of chromatin in the nucleus, and basophilic-staining cytoplasm due to the presence of abundant RER. Another histological features include a small, clear area near the nucleus. This cytoplasmic clear area (Golgi zone [GZ]) marks the position of the Golgi apparatus. Portion of a chronically inflamed intestinal villus. Electron micrograph of a plasma cell Ultrastructure of a plasma cell 6) Pericytes They are contractile cells of mesenchymal origin. Wrap around the endothelial cells that line the capillaries, arterioles and venules throughout the body. These cells are contractile & may control the size of the vascular lumen. 7) Adipose cells (adipocytes-fat cells) Arise from undifferentiated mesenchymal cells. They gradually accumulate cytoplasmic fat, which results in a significant flattening of the nucleus in the periphery of the cell. Adipocytes are found throughout the body Their function is to store energy in the form of triglycerides and to synthesize hormones such as leptin. 8) Leukocytes White blood cells, are considered the transient cells of connective tissue. They migrate from the blood vessels into connective tissue by the process of diapedesis. This process increases greatly during various inflammatory conditions. B) Connective tissue fibers Three types of fibers are found in connective tissue: collagen, elastic, and reticular. The amount and type of fibers that dominate a connective tissue are a reflection of the structural support needed to serve the function of that particular tissue. These three fibers all consist of proteins that form elongated structures, which, although produced primarily by fibroblasts, may be produced by other cell types in certain locations. For example, collagen and elastic fibers can be produced by smooth muscle cells in large arteries and chondrocytes in cartilages. Collagen & Reticular fibers are formed by the protein collagen, and Elastic fibers are composed mainly of the protein elastin. These fibers are distributed unequally among the types of connective tissue. 1. Collagen fibers - ▪ Contain mature collagen (most are type I collagen (most abundant protein in the body). ▪ Tensile strength - resistance to stretching. ▪ Skin. 2. Elastic fibers - ▪ Contain elastin and fibrillin. ▪ Elasticity - can be stretched, yet still, return to its original length. ▪ lungs, aorta, and skin. 3. Reticular fibers - ▪ Contain immature collagen. ▪ Support - network of thin fibers. ▪ Occur around nerve fibers, fat cells. In capsules of liver, kidney & endocrine glands. C) Ground substances Ground substance is a clear, viscous substance with a high water content, but with very little morphologic structure. Ground substance is primarily composed of water and large organic molecules, such as glycosaminoglycans (GAGs), proteoglycans, and glycoproteins. GAGs are polysaccharides that trap water, giving the ground substance a gel-like texture. Provides medium through which substances are exchanged between blood and cells.  TYPES OF CONNECTIVE TISSUE 1‐ Embryonic connective tissue Mesenchymal Connective Tissue Mucous Connective Tissue 2- Connective tissue proper A‐ Loose connective tissue 1. Areolar loose C.T 2. Adipose loose C.T 3. Reticular loose C.T B –Dense connective tissue 1. Dense regular connective tissue 2. Dense irregular connective tissue 3‐ Specialized connective tissue A‐ Cartilage B‐ Bones C‐ Blood 1- Embryonic connective tissue A. Mesenchymal Connective Tissue It is found in the embryo and fetus and contains considerable ground substance. It contains scattered reticular fibers and star or spindle -shaped mesenchymal cells. Mesenchymal connective tissue is capable of differentiating into different types of connective tissues. B. Mucous Connective Tissue Mucous connective tissue is a primitive (embryonic) connective tissue that persists in the umbilical cord. Mucous connective tissue is composed of fibroblast cells, which can be spindle or stellate shaped. They produce the surrounding, abundant, gelatinous ground substance (Wharton’s jelly) and collagen fibers. This type of connective tissue does not differentiate beyond this stage. 2-Connective tissue Proper A) Loose connective tissue: Is characterized by abundant ground substance, with numerous connective tissue cells and fewer fibers (more cells and fewer fibers) compared to dense connective tissue. It is richly vascularized, flexible, and not highly resistant to stress. 1) Areolar connective tissue A loose packing tissue found in most organs & tissue, serves to bind body parts together. Wraps small blood vessels and nerves, surrounds glands, and forms subcutaneous tissue. 2) Reticular connective tissue This tissue contains a network of branched reticular fibers, reticulocytes (specialized fibroblasts), macrophages, and parenchymal cells, such as pancreatic cells and hepatocytes. This tissue provides the architectural framework for parenchymal organs, such as lymphoid nodes, spleen, liver, bone marrow, Reticular fibers are shown in black with and endocrine glands. a silver stain 3) Adipose tissue Adipose tissue is a special form of connective tissue, consisting mainly of adipocytes and it has a rich neurovascular supply. Morphologically, there are two types of adipose tissue: A) White adipose tissue: ▪ A single lipid droplet occupies almost the entire cell (Unilocular fat cell). The nucleus is flat. It is found throughout the adult human body. B) Brown adipose tissues: ▪ Multiple individual lipid droplets are found in the cytoplasm (Multilocular fat cell). The nucleus is round or oval. It is mainly found around the adrenal glands, great vessels, and in the neck region. Unilocular and Multilocular adipose tissue Brown adipose tissues Brown adipose tissue is found in all mammals, but is best developed in animals that hibernate. The main function of brown adipose tissue is to supply the body with heat. mitochondria nucleus Fat droplets nucleolus B) Dense connective tissue: 1- Dense irregular connective tissue: Dense irregular connective tissue consists of few connective tissue cells and many connective tissue fibers, the majority being type I collagen fibers, interlaced with a few elastic and reticular fibers. These fibers randomly arranged without a definite orientation. Locations: Capsule of some organs, dermis, hard palate. Section of dense irregular connective tissue. Skin 2- Dense regular connective tissue Dense regular connective tissue also consists of fewer cells and more fibers but the fibers are arranged into a definite linear pattern. Fibroblasts are arranged linearly in the same orientation. a) White fibrous connective tissue: Tendon 1- Tendon: Is cord like- structure that join muscle to bones. 2- Ligament: Is similar to a tendon, but fibers are less regularly arranged than those of tendon. It joins bone to bone. Ligament b) Elastic (yellow) connective tissue: It consists of bundles of parallel elastic fibers. The bundles are bounded together by loose connective tissue. Locations: It occurs in the walls of the large elastic arteries such as aorta. Thick bundles of elastic lamellae are arranged in parallel wavy sheets, with the smooth muscle cells and collagen fibers insinuated between alternating lamellae. Elastic connective tissue is able to recoil after stretching. Overview of connective tissue proper  3‐ Specialized connective tissue 1. Cartilage ❖ Cartilage is a firm and avascular specialized form of connective tissue. ❖ Produced and maintained by chondrocytes. ❖ Most cartilage is covered by a layer of dense irregular connective tissue called perichondrium, which contains a rich blood supply and is innervated by nerve fibers conveying pain. ❖ Perichondrium is important for the growth and maintenance of cartilage; it has two layers. The outer fibrous layer of the perichondrium contains connective tissue fibers, fibroblasts, and blood vessels. These perichondrial vessels represent an essential blood supply for cartilage. Because cartilage itself is avascular, these vessels are the route through which nutrients access the matrix by diffusion. The inner cellular layer of the perichondrium consists of chondrogenic cells, which are able to differentiate into chondroblasts. ❖ The functions of cartilage include the support of soft tissues, the facilitation of smooth movement of bones at joints, and the mediation of growth of the length of bones during bone development. ❑ Cartilage Cells The main types of cells in cartilage are: 1. Chondrogenic cells; are derived from mesenchymal cells. These cells can differentiate into both chodroblast and osteoprogenitor cells. 2. Chondroblasts; are derived from mesenchymal cells and chondrogenic cells. 3. Chondrocytes. They are surrounded by lacuna. ❑ Cartilage Matrix The matrix of cartilage is nonmineralized and consists of fibers and ground substance. Collagen fibers are mainly type II in the matrix, although some cartilage may also contain type I or elastic fibers. The major components of ground substance include chondroitin sulfate, glycosaminoglycans (GAGs), proteoglycans, and glycoproteins. ❑ Types of Cartilage Cartilage can be classified into three types based on the characteristics of the matrix. 1- HYALINE CARTILAGE Is characterized by the presence of a glassy, homogeneous matrix that contains type II collagen, which is evenly dispersed within the ground substance. It is found in the nose, larynx, trachea, bronchi, and the distal ends of ribs. Hyaline cartilage covers the smooth surface of joints, providing for free movement, and is also involved in bone formation and long bone growth. Hyaline cartilage, bronchus(H&E) 2- ELASTIC CARTILAGE Is similar to hyaline cartilage except for its rich network of elastic fibers, arranged in thick bundles in the matrix. This type of cartilage has a perichondrium and it also contains type II collagen in the matrix. Elastic cartilage is located in areas where elasticity and firm support are required, such as the epiglottis , auditory canal and tube, and the pinna of the ear, which is able to recover its shape after deformation. elastic cartilage (epiglottis) 3- FIBROCARTILAGE Does not have a perichondrium. It has type II collagen. It is characterized by thick, coarse bundles of type I collagen fibers that alternate with parallel groups of columns (or rows) of chondrocytes within the matrix. Fibrocartilage is resistant to tearing and compression. It is found in areas where support and tensile strength are required, such as intervertebral disks and pubic symphysis. fibrocartilage (intervertebral disk)  2. Bone Bone is a specialized hard connective tissue. It is different from cartilage in that bone is calcified and, hence, is harder and stronger than cartilage. In addition, it has many blood vessels penetrating the tissue. In general, the external surface of the bone is covered by periosteum, a layer of connective tissue containing small blood vessels, osteogenic cells, and nerve fibers conveying pain information. The inner surface of the bone is covered by endosteum, a thin connective tissue layer composed of a single layer of osteoprogenitor cells and osteoblasts that lines all internal cavities within bone; this lining represents the boundary between the bone matrix and the marrow cavities. ❑ Bone matrix Bone is primarily characterized by a hard matrix, which contains organic materials (mainly type I collagen fibers chondroitin sulfate and keratin sulfate) and inorganic materials (calcium in the form of hydroxy apatite and phosphate). Compared to cartilage, bone contains only about 25% water in the matrix, whereas cartilage matrix contains about 75% water. This combination makes bone hard, firm, and very strong. ❑ Bone cells The main types of cells in bone are osteoprogenitor cells, osteoblasts, osteocytes, and osteoclasts. These cells contribute to bone growth, remodeling, and repair. ❑ The general structure of compact bone includes: (1) the osteon; is the basic unit of the compact bone structure. It has concentrically arranged laminae (concentric lamellae) surrounding a centrally located Haversian canal. (2) interstitial lamellae, lamellae layers in between the osteons. (3) outer circumferential lamellae, outer layers of lamellae located beneath the periosteum and surrounding the outside of the entire compact bone. (4) inner circumferential lamellae, layers of lamellae located beneath the endosteum and forming the innermost layer of compact bone. The Haversian canal is a central space through which blood vessels pass; the Volkmann canal is the space that sits perpendicularly to the Haversian canals and forms the connection between two Haversian canals. Overview of bone structure, long Compact bone, finger bone Cancellous bone (spongy bone), nasal bone Bone Development Bone development can be classified as intramembranous ossification and endochondral ossification, according to the mechanism of its initial formation. (1) Intramembranous ossification is the process by which a condensed mesenchyme tissue is transformed into bone. A cartilage precursor is not involved; instead, mesenchymal cells serve as osteoprogenitor cells, which then differentiate into osteoblasts. Osteoblasts begin to deposit the bone matrix. (2)Endochondral ossification is the process by which hyaline cartilage serves as a cartilage model precursor. This hyaline cartilage proliferates, calcifies, and is gradually replaced by bone. Endochondral ossification involves several event. The development of long bone is a good example of endochondral formation. In this particular case, the hyaline cartilage undergoes proliferation and calcification in the epiphyseal plates. This epiphyseal cartilage can be divided into five recognizable zones: reserve zone, proliferation zone, hypertrophy zone, calcification zone, and ossification zone. Bone Development A representation of the development of a long bone 3. Blood