Connective Tissue - Lesson 7 PDF

_____________LESSON 7 _____________ CONNECTIVE TISSUE I. INTRODUCTION The term “connective” means “serves to unite”. Therefore, as its name suggests, connective tissue forms a continuity with the other three types of tissue, epithelial, muscular and nervous, to keep the body integrated from a st...

_____________LESSON 7 _____________ CONNECTIVE TISSUE I. INTRODUCTION The term “connective” means “serves to unite”. Therefore, as its name suggests, connective tissue forms a continuity with the other three types of tissue, epithelial, muscular and nervous, to keep the body integrated from a structural and functional point of view. The connective tissue has a great morphological, topographic and structural diversity. II. BASIC FUNCTIONS Due to its morphological, topographic and structural diversity, it performs multiple and diverse functions: 1) Union: serves as a union between the different tissues (muscular, nervous and epithelial). 2) Structural support, framework: the supporting tissues are the bones, cartilage and ligaments that join the bones together, as well as the tendons that insert the muscles into the bones, the capsules that enclose the organs and the stroma that forms the structural network within organs. 3) Thermal regulation: carried out by adipose tissue. 4) Exchange medium: metabolic detritus, nutrient and oxygen between the blood and many cells of the body. 5) Defence, protection and repair: by a) phagocytic cells that engulf and destroy detritus, foreign particles and microorganisms; b) immunocompetent cells, which produce antibodies against antigens; c) cells that produce pharmacological substances that help to control inflammation; and d) as a physical barrier against the invasion and spread of microorganisms. 6) Fat storage: for energy needs. 1 III. EMBRYONIC ORIGIN Most of the connective tissues originate in the embryonic mesoderm, which gives rise to the mesenchyme, which contains the multipotential cells of the embryo. These cells migrate throughout the body and form the various types of connective tissue. IV. CONNECTIVE TISSUE COMPONENTS All connective tissues have two components (Figure 1): 1) The extracellular substance, which in the case of connective tissue is called extracellular matrix and 2) the cells. The extracellular matrix has the same components in the different types of connective tissues, only the proportion will vary in each one of them, while the cells vary from one to another. Figure 1: Scheme of the different elements that make up all connective tissues: 1) Extracellular matrix: fibers (collagen and elastic) and basic substance; 2) Cells: adipocytes, pericytes, fibroblasts, macrophages, mast cells, plasma cells, etc. Basic substance in gel form. First, we will study the extracellular matrix because it is similar in all types of connective tissues. IV.I. Extracellular matrix It consists of a basic or ground substance hydrated, gel-like, and fibers embedded in it. The properties that these components confer on the connective tissue are: 1) The hydrated basic substance resists compressive forces. 2 2) The fibers resist tensile or traction forces and provide elasticity. 1. Fibers The classical histologists distinguished 3 types of fibers according to their morphology and their reactivity with histological stains: 1) Collagen fibers; 2) Reticular fibers, and 3) Elastic fibers. Today is known that reticular fibers are fibers of collagen type III, but the term has been preserved for historical reasons and for convenience, to describe the organs that possess large amounts of this particular type of fibers of collagen. 1.1. COLLAGEN FIBERS The collagen fibers (Figures 2, 3 and 4) are abundant, and are present in most connective tissues, making up approximately 30% of the dry weight of the organism. Macroscopic appearance: when they are in large accumulations, as in tendons and ligaments, they have a bright white colour in the living organism. Optical microscope study: they measure 10 µm in diameter and are of indefinite length and are not isolated but in groups or bundles. With HE stain they are acidophilic and bright and with special histochemical techniques have the following colourings: blue with the techniques of the Methylene Blue and Masson Trichrome, red with the technique of van Gieson Picrofucsina, and green with the Light Green technique. Transmission electron microscope study: Figure 2. Scheme of the structure of a collagen fiber. • A collagen fiber is made up of collagen fibrils that measure between 10 and 300 nm (nanometers) in diameter and have a characteristic periodicity of electrondense bands and electron-lucent bands (striations) that alternate between each other approximately every 67 nm. • Each collagen fibril is a tropocollagen polymer measuring 280 nm in length and 1.4 nm in diameter. 3 • Each tropocollagen molecule (monomer) is made up of three polypeptide chains (called alpha chains) wrapped around each other in a helical fashion. As they polymerize, they are arranged in a parallel fashion, overlapping each other for a quarter of their length. This arrangement is what gives rise to the periodic longitudinal striation every 67 nm. Each alpha chain contains about 1000 amino acids. The most abundant are glycine, proline, hydroxyproline, and hydroxylysine. But there are also others, which vary from one alpha chain to another. This variation gives rise to different collagen fibers. There are between 15 and 19 different types (depending on the texts), but the main ones are: - Type I fibers: dermis, tendons, bones, ligaments - Type II fibers: hyaline cartilage - Type III fibers: lymphatic system, spleen - Type IV fibers: basal lamina - Type V fibers: dermis, tendons Figure 3. Scheme of tropocollagen synthesis in fibroblasts and its polymerization at the extracellular level. Collagen fibers are synthesized in the rough endoplasmic reticulum (RER) as polypeptide chains (called alpha chains) (α). Inside the RER cisterns, these α chains are assembled 3 by 3 forming triple helices to form procollagen molecules which are transferred to the Golgi complex, packed into secretory vesicles and released by exocytosis to the extracellular environment. Procollagen is transformed into tropocollagen (monomer) by enzymatic cleavage, which polymerizes in the extracellular matrix to form collagen fibrils (tropocollagen polymer). Type I fibers are flexible and can adapt to movements and changes in size of the organs of which they are part. Collagen fibers are characterized by high tensile strength and low shear strength, being able to stretch only about 5% of their initial length. Therefore, they occur where high tensile strength is required, such as tendons, ligaments and organ capsules. 4 The reticular fibers, also known as reticulin fibers, are individual fibrils of collagen type III (they are coated with proteoglycans and glycoproteins). They measure 0.5 to 2 µm in diameter and are highly glycosylated, that is, they have a lot of carbohydrates. Under the light microscope, with HE, they are not distinguished from other collagen fibers, but they are stained with silver techniques in black and with PAS in pink-magenta due to their high carbohydrate content. The reticular fibers form a flexible and delicate mesh around capillaries, muscle fibers, nerves, adipose cells and hepatocytes, acting as a scaffold (stroma) to support cells or groups of cells in endocrine, lymphoid and hematopoietic organs. They are particularly common in: basal membrane, smooth muscle tissue, acini and alveoli of exocrine glands, follicles and cords of endocrine glands, hollow organs (urinary bladder, intestine and uterus) and in hematopoietic and lymphoid organs. 1.2. ELASTIC FIBERS The elastic fibers are responsible for the elasticity of the connective tissue and are present in normal organs whose functions require elasticity in addition to tensile strength. Therefore, its functions are to maintain the shape of numerous organs that are susceptible to being deformed, such as the skin and the lung. In blood vessels they are located mainly in the elastic lamina of largecaliber arteries, allowing the vessel to dilate when blood passes through and return to its initial state, thus helping to maintain blood flow. In animals that spend a long-time grazing (especially ruminants) the nape ligament is very rich in elastic fibers and helps to maintain an adequate head position. Elastic fibers are also abundant in other ligaments that stabilize the spine. Macroscopic appearance: in large quantities, they give a yellowish colour to the organs that contain them, such as the wall of the great blood vessels. Appearance under the light microscope: with the HE technique, they are more acidophilic and shinier than collagen fibers, and they form long and highly wavy bundles (Figure 4). Transmission electron microscope appearance: They have two components: 1) A central low electron density amorphous material, elastin. It is a protein rich in glycine and proline but containing other amino acids less frequent as the desmosine and isodesmosine, providing elasticity to the fibers. In fact, elastic fibers can stretch up to 150% in length and regain their normal shape without wasting energy. 2) A sheath around of fibrillin microfibrils, measuring about 10 nm in diameter. Elastin is synthesized by fibroblasts and smooth muscle fibers as tropoelastin, that is, unique polypeptide chains that are transformed into elastin cross-linked and assembled in the extracellular space. Microfibrils are secreted before elastin and constitute the scaffold in which elastin forms fibers and sheets. From a developmental point of view, elastic fibers are the last fibers to appear in organs (for example lung) or connective tissue. 5 Fibra de colágeno C F D G E H Fibra elástica A B Figure 4. Scheme of collagen (A) and elastic (B) fibers. Light microscope images with haematoxylin and eosin of collagen fibers (C), with silver staining of reticulin fibers (D) and with haematoxylin-eosin of elastic fibers (E). Electron microscope images of collagen fibers (F), reticulin fibers (G) and elastic fibers (H). 2. Ground substance The fibers and cells of the connective tissue are included in a substance of gelatinous consistency called ground substance. In the histological preparations stained with HE it is not seen. The ground substance has 3 components: a) Glycosaminoglycans (GAG, GAGs) b) Proteoglycans (PGs) c) Adhesive glycoproteins 2.1. GLYCOSAMINOGYCANS The glycosaminoglycans (other names: mucopolysaccharides) are long polysaccharides, inflexible and without ramifications, compounds of repeating disaccharide units. They are hydrophilic and form hydrated gels which, for their high-water content, are resistant to pressure. One of the disaccharides is always an amino sugar (N-acetyl-glucosamine or N-acetyl- galactosamine), 6 and the other one, a uronic acid (iduronic and glucuronic). There are two types of GAGs: 2.1.1. Sulfated: The amino sugar is sulphated. They are: keratan sulfate, heparan sulfate, heparin, chondroitin 4-sulfate, chondroitin-6-sulfate and dermatan sulfate. They are all made up of less than 300 repeated disaccharide units, and all are often covalently bounded to proteins. Its function is to retain liquid in the extracellular matrix, which confers resistance to compressive forces (that is negatively charged and attract cations as the Na+). The chondroitin 4 and 6 sulfate are abundant in cartilage, arteries, skin and cornea and a lesser amount in bone. The dermatan sulfate in skin, tendons, nuchal ligament, sclera and lung. The keratan sulfate is in cartilage, bone and cornea. Heparan sulfate in arteries and lung and heparin in mast cells, lung, liver and skin. 2.1.2. Not sulfated: the amino sugar is not sulfated. The only one of its kind is hyaluronic acid, made up of up to 25,000 repeated disaccharide units. It forms the skeleton of gigantic molecules whose function is to resist compression forces and act as a physical barrier to the spread of microorganisms and cells through the extracellular matrix. 2.2. PROTEOGLYCANS The proteoglycans are sulfated GAGs linked by covalent bonds to proteins. Both structures are shaped like a swab. Variable size of molecular weight, between 50,000 and 3 million daltons. Most are bound to hyaluronic acid (non-sulfated GAG) by binding proteins, forming molecules of several million daltons. The most abundant of these is: the aggrecan. Functions: (1) resistance to compression forces (due to its large volume); (2) physical barrier to the spread of microorganisms and metastatic cells (for the same reason); (3) molecular filters in the basal lamina (they select and delay the passage of molecules through them); (4) binding sites of signaling molecules (transforming growth factor beta, TGFβ, and fibroblast growth factor, FGF). The proportions of various proteoglycans in a certain type of tissue largely determine the morphological and functional properties of the tissue. 2.3. ADHESIVE GLUCOPROTEINS The adhesive glycoproteins are responsible for holding the various components of the extracellular matrix to each other and to the cells. Therefore, they have 3 adhesion domains: (1) to collagen fibers; (2) to proteoglycans; and (3) to the integrins of the cells. Types: (1) fibronectin, (2) laminin, (3) entactin, (4) tenascin, (5) chondronectin and (6) osteonectin. Examples: ➢ Fibronectin: plays an important role in several processes, such as cell adhesion, cell differentiation, cell development and phagocytosis. 7 ➢ Laminin, large glycoprotein. It is the main component of the basement membrane. It is synthesized by cells that are in contact with it. IV.II. Connective tissue cells Two types of cells are distinguished according to their presence in the tissue: o o Fixed cells or resident cells Transient, free or inmigrant cells IV. 2.a. Fixed or resident cells: these are stable cell populations, with a long half-life. They originate both from undifferentiated mesenchymal cells and from undifferentiated precursors of the bone marrow. They are: ▪ ▪ ▪ ▪ ▪ Fibroblasts and fibrocytes Pericytes Fat cells, adipose cells or adipocytes Mast cells Some macrophages Fibroblasts and fibrocytes Fibroblasts and fibrocytes are the same cell but in a different functional state. Fibroblasts are the metabolically active form, while fibrocytes are metabolically resting. These are temporary forms of the same cell type. They originate from undifferentiated mesenchymal cells. They are the most common cells of most connective tissues, and their function is to synthesize the components of the extracellular matrix (both the fibers and the ground substance). Morphology: they are spindle cells and are arranged parallel to the longitudinal axis of the collagen fibers. Under the light microscope, with the HE technique, its cytoplasm, which is acidophilic, is not usually distinguished from neighbouring collagen fibers, so only nuclei between fibers are seen (Figure 4C). The nucleus is ovoid and varies from one form to another: in fibroblasts is larger, granular, and with obvious nucleolus, whereas in fibrocytes is smaller, intensely basophilic and not granular (homogeneous) without apparent nucleoli. With the electron microscope, fibroblasts have abundant RER with dilated cisterns, Golgi complex and mitochondria. They also have abundant vimentin and actin filaments. Fibroblasts have some motility, and rarely can enter mitosis, except during wound healing. A variety of fibroblasts are myofibroblasts, which present characteristics in common with fibroblasts and smooth muscle cells, such as the presence of specific muscle actin that forms dense bodies, showing contractile activity. 8 However, they differ from smooth muscle fiber because they do not have a basal lamina. They are common in healing areas of wounds. With HE they are indistinguishable from fibroblasts. A B C Figure 5. Diagrams of fibroblast (A), fibrocyte (B), pericyte (C) and mast cell (D). Pericytes They are cells with spindle-shaped morphology and numerous cytoplasmic processes that partially surround the endothelial cells of the capillaries and smallcaliber venules and have their own basal lamina. They originate from undifferentiated mesenchymal cells. They have mixed endothelial cell and smooth muscle cell characteristics. Through their contractile activity, they regulate blood flow in these vessels. Fat cells, adipose cells, adipocytes They are cells specialized in synthesizing and storing lipids in the form of triglycerides. When needed, stored triglycerides are transformed into fatty acids and glycerol. Adipocytes originate from undifferentiated mesenchymal cells, although some histologists believe that they can also form from fibroblasts. Adipocytes are fully differentiated cells that never divide except in the immediate postnatal period. HISTOPHYSIOLOGY- The synthesis of triglycerides takes place in the adipocyte from glycerol phosphate and free fatty acids. Glycerol phosphate is synthesized by the fat cell. Free fatty acids are obtained from the blood through the action of its own enzyme, lipoprotein lipase, chylomicrons (from intestinal fat absorption), very low-density lipoproteins (VLDL) (from the liver) and fatty acids conjugated with albumin. The mobilization of triglycerides takes place after the action of different hormones on the adipocyte (insulin, growth hormone, adrenaline, norepinephrine), which stimulate the release of hormone-sensitive lipase. This enzyme breaks down triglycerides into glycerol and fatty acids. 9 D There are two types of adipocytes: the unilocular adipocyte and the multilocular adipocyte. Unilocular adipocyte is seen in white fat or white connective tissue and multilocular adipocyte in brown fat or brown connective tissue. Unilocular adipocyte: it is so named because triglycerides are stored forming a single large droplet (Figure 6A). Under the light microscope, it is a large round or polyhedral cell, up to 120 µm in diameter, with the cytoplasm occupied by a large vacuole (a rounded area with regular and clear boundaries, without content). The vacuole is the trace of the triglyceride drop, because the fat dissolves in the organic solvents used in the histological processing of tissues. The only thing really visible is the nucleus, which is displaced to one side of the cytoplasm, giving the adipocytes the shape of a signet ring (although it may not be seen depending on the level of cut). To see triglycerides, tissues must be frozen fixed, and then stained with special techniques such as Sudan III or Scarlet Red (red). Functionally, the unilocular adipocyte releases glycerol and fatty acids into the blood, and the latter are conjugated with albumin for circulation. A B Figure 6. Unilocular (A) and multilocular (B) adipocyte diagrams. Multilocular adipocyte: it is so called because triglycerides are stored forming several small droplets (Figure 6B). Under the light microscope, they are cells smaller than those of white fat and have numerous small vacuoles in the cytoplasm, but the nucleus remains in its central position. With electron microscope shows abundant mitochondria and absence of RER. This abundance of mitochondria is related to their function: the multilocular adipocyte oxidizes the fatty acids in the mitochondria by means of a protein in the inner membrane of the mitochondria called thermogenin, producing heat. Mast cells Mast cells originate from undifferentiated cells in the bone marrow. With the light microscope, they are ovoid in shape and very variable in size (Figure 5D). With the HE technique, they present numerous basophilic granules in the cytoplasm. With the toluidine blue and Giemsa techniques, the granules change from blue to red, property called metachromasia. With the electron microscope, the granules are large and electron-dense, with very few cytoplasmic organelles. Their half-life is a few months, and sometimes can undergo cell division. 10 Distribution: the precursor cells leave the bone marrow and enter the blood, through which they circulate until they reach the connective tissue, where they differentiate into mast cells and acquire their characteristic granules. They are generally located around small blood vessels. A B C Figure 7. Images of mast cells under the light microscope stained with haematoxylin and eosin (A) with Giemsa (B) and with the electron microscope (C). C A The function of mast cells is involved in a type of inflammatory reaction known as immediate hypersensitivity reaction or anaphylactic reaction by releasing the content of their granules, which include heparin (anticoagulant), histamine (increases vascular permeability), and other chemical mediators of inflammation such as neutrophil and eosinophil chemotactic factors, arachidonic acid, proteases, and leukotrienes. The rat and mouse have serotonin and chemical mediators. Macrophages: Fixed or resident cells and transient cells Non-reactive connective tissue macrophages are normally fixed but become mobile in response to stimulation (Figure 8). They are cells that: 1) have lysosomes; 2) have phagocytosis capacity; and 3) have receptors for the Fc portion of immunoglobulins; and they are part of the so-called mononuclear phagocyte system. All macrophages originate from a common stem cell in the bone marrow that gives rise to monocytes, which pass into the blood, where they circulate until they are properly stimulated and go to the connective tissue. There, they mature into macrophages, which have a half-life of about two months. Some macrophages behave as transient connective tissue cells (free macrophages) and others as fixed connective tissue cells (resident macrophages). The free macrophages are those that develop as a result of an exogenous stimulus and migrate to that particular site. For example, virus entry zone. The resident macrophages are those who are always in the same locations of the body, regardless of there is or not external stimuli. These macrophages were given specific names before their origin was understood. These include Kupffer cells of the liver and alveolar macrophages and pulmonary intravascular macrophages of the lung. In addition, osteoclasts of 11 the bone tissue and the microglia of the central nervous system also belong to the mononuclear phagocyte system although its morphology is different. In general, whether free or resident, macrophages have an irregular morphology, and a large size, between 10 and 30 µm in diameter. They usually have cytoplasmic processes called filopodia, an ovoid or kidney-shaped, eccentric and vesicular nucleus, and a wide, basophilic and pale cytoplasm, sometimes vacuolized, in which numerous lysosomes, both primary and secondary, are observed. They also present highly developed RER and Golgi Complex. Under conditions of chronic inflammation, they can transform into epithelioid cells and foreign body giant cells, which are multinucleated giant macrophages. The functions of macrophages are: 1) Phagocytosis and digestion, both of microorganisms and foreign particles as well as cellular debris. 2) The synthesis and release of signalling molecules or chemical mediators, called cytokines, which are involved in the immune response, inflammation, and healing. 3) The processing and presentation of antigens to lymphocytes so that they can induce an immune response. A B C Figure 8. Scheme (A) and images with the light microscope with haematoxylin and eosin (B) and with the electron microscope (C) of the macrophages. IV. 2.b. Transient, free or immigrant cells: these are non-stable cell populations. They originate mainly from undifferentiated cells in the bone marrow and circulate in the blood. Under the appropriate stimulus or signal, they migrate from the blood to the connective tissue to carry out their functions. Therefore, they are motile cells, and most are short-lived (they are continually replaced by a large population of stem cells). They are: ▪ ▪ ▪ ▪ ▪ ▪ ▪ Plasma cells Lymphocytes Neutrophils Eosinophils Basophils Monocytes Some macrophages 12 All types of blood leukocytes (lymphocytes, neutrophils, eosinophils, basophils and monocytes) can cross the vascular wall by diapedesis to reach loose connective tissue, in which they are called immigrant cells because they have the ability to move. Their number is highly variable (very abundant in the intestinal lamina propria). The morphological characteristics and functions of these cells will be studied in topic 14 (blood). V. CLASSIFICATION OF CONNECTIVE TISSUE 1. Embryonic connective tissue 1. a. Mesenchymal connective tissue or mesenchyme 1. b. Mucous connective tissue 2. Mature connective tissue 2. a. Connective tissue proper - Loose connective tissue (areolar) - Dense connective tissue - Dense irregular connective tissue - Dense regular connective tissue - Collagenic - Elastic - Reticular tissue - Adipose tissue 2. b. Specialized connective tissue - Cartilage - Bone - Blood 1. Embryonic connective tissue 1.a. Mesenchymal connective tissue (mesenchyme) It is a connective tissue found only in the embryo. It is the precursor tissue of all adult connective tissues. Components: extracellular matrix with abundant ground substance and reticular fibers, and mesenchymal cells, ovoid and irregular in shape, with extensions that contact each other forming a three-dimensional network, with few organelles and ovoid nuclei in which mitosis is common. 1.b. Mucous connective tissue (Figure 9) It is located exclusively in the embryonic hypodermis and in the umbilical cord and is known by the name of Wharton's jelly. In the adult, however, it persists in some locations, such as the glans of the bovine penis, the crest of birds, in the papillae of the reticular folds and in the lamina of the omasum. 13 It is made up of fibroblasts with a stellate morphology that form a threedimensional network; fibers in low amount, primarily of collagen types I and III; and ground substance (predominant component) rich in hyaluronic acid, which gives it the appearance of gelatin. Under the light microscope, it is slightly basophilic. B A C Figure 9. Mesenchymal embryonic connective tissue (A), mucosal embryonic connective tissue (B) and adult connective tissue proper (C). HE. 2. Mature connective tissue or proper (Figure 9) 2.a. Loose connective tissue (Other names: areolar) It is the most common type of connective tissue in adults. In some areas, it receives specific names. Thus, it is located under many lining epithelia, and when this lining epithelium is a mucosa, then it is called lamina propria (of the mucosa) (for example, in the digestive tract), in this case it offers vascular support and supply and constitutes the interstitial tissue of most hollow organs, allowing their easy movement and displacement. Loose connective tissue predominates in the pia mater and arachnoids. It is also located around blood vessels, forming the adventitia, and between muscle bundles and smooth muscle layers of hollow organs, between cells of many solid organs. Function: • Mechanical: as support and protection of biomechanical effects in various locations (for example, hypodermis). • Tissue repair and defensive activities (inflammation). Components: the ground substance is the predominant element, followed by resident cells: fibrocytes, fibroblasts and macrophages, followed by adipocytes and mast cells, and fibers, which are scarce and of all three types. 2.b. Dense connective tissue It is practically similar to the loose but varies the relative amounts of its components (here the predominant element is the fibers). It is classified into two large groups based on the orientation of the fibers: dense irregular connective tissue, in which the fibers are randomly oriented, and dense regular connective tissue, in which the fibers are oriented according to a regular pattern. 14 2.b.1. Dense irregular connective tissue: the most abundant fibers are those of collagen, and they are randomly oriented. They are usually organized in bundles that intersect each other at varying angles. In muscle fasciae, for example, the bundles are arranged in a single plane and resist stretching parallel to the orientation of the fibers. In muscle fasciae, organ capsule, dermis and the taste, the bundles overlap in various planes and interlock with each other in three planes: longitudinal, vertical and horizontal, allowing adaptation to change in organ size and muscle diameter. The capsules and muscle fasciae are continuous with the trabeculae or connective tissue between muscle bundles (perimysium). There may be elastic fibers, in less quantity. The predominant cells are fibrocytes and fibroblasts. Its main function is mechanical protection, presenting tensile strength in different directions due to the fact that the collagen fibers are oriented in multiple directions (dermis, nerve sheaths and capsules of organs such as the spleen, testis, ovary, kidney and lymph nodes, fasciae, aponeurosis, joint capsules, pericardium). 2.b.2. Dense regular connective tissue: the only difference with the previous one is the orientation of the collagen fibers, which is ordered here, and the type of fibers that predominate. There are two varieties: 1) the dense regular collagenous connective tissue: predominate collagen fibers arranged in parallel bundles to each other (tendons, fascia, ligaments and aponeurosis), or perpendicular bundles to each other (as in the cornea, which provides light transparency and tensile strength), 2) the dense regular elastic connective tissue: predominate elastic fibers, which are arranged in parallel to each other (in large-caliber blood vessels, in the yellow ligament of the spine and in the suspensory ligament of the penis). The high tensile strength of the tendons and ligaments is reflected in its structure, which is formed by parallel bundles of collagen fibers. These bundles are linked together by sparse loose connective tissue that forms a protective sheath around the blood vessels and nerves of the tendon and continues with the peritendon. Repair of severed tendons is performed by fibroblasts from loose connective tissue. The typical structure of the tendon is disturbed at the points of attachment to the bone or cartilage or at the places where the tendons run around the bone. Whenever tendons insert into bone or cartilage, the regular dense tendon tissue gradually transforms into fibrocartilage and then mineralized fibrocartilage. Its function is to gradually transmit biomechanical force from a flexible fibrous unit to a rigid bone unit. Elastic ligaments are made up of interconnected parallel and branching elastic fibers surrounded by loose connective tissue. The nape ligament and the elastic fascia of the abdominal musculature of herbivores are some examples. 2 C. Reticular tissue It is a connective tissue in which the predominant element is reticular fibers or type III collagen fibers. The cells that synthesize these fibers are fibroblasts, but they have a different morphology from the typical fibroblast, which is why they are called reticular cells: they are stellate in shape, with numerous long and thin cytoplasmic processes. 15 Cells and fibers stick together and form a delicate network that forms the structure (stroma) of the spleen, lymph nodes, hemolymphatics, hemal, bone marrow, liver sinusoids, adipose tissue, smooth muscle tissue, and islets of Langerhans in the pancreas. 2.d. Adipose tissue Adipose or fatty tissue is a loose connective tissue in which the predominant cells are adipocytes (predominate over the fibers and over the ground substance). It is classified according to the type of adipocyte it has in white adipose tissue (unilocular adipocyte) and brown adipose tissue (multilocular adipocyte). Other differences between both adipose tissues are color (as the name suggests), vascularity and metabolic activity. Functions: mechanics, isolation and organic metabolism 2.d.1. White adipose tissue (white fat) (Figure 10): When fresh, it is whitish in color, although if the diet is rich in foods that contain carotenoids, such as carrots, it may have a yellowish color. The unilocular adipocytes are densely packed together, and each group is separated from the next by reticular fibers partitions with abundant blood vessels, which gives a morphology honeycomb (lobes). It is located mainly in the subcutaneous tissue, where it forms the adipose panniculus (especially developed in pigs). It is also widely distributed in other locations, such as the mesentery, between skeletal muscle fibers, in the pericardium, etc. The function of adipocytes in white fat is to store energy in the form of lipids (triglycerides), which are used to produce chemical energy (ATP, GTP) when energy needs require it. They also provide mechanical protection to certain organs such as perirenal fat or bone marrow fat, in these cases lipids are not mobilized when there are energy needs. 2.d.2. Brown adipose tissue (brown fat) (Figure 10): When fresh, it is brownish in color and highly vascular. It is typical of hibernating animals, rodents, monkeys and newborn animals, and its function is to maintain body temperature by storing triglycerides that can be metabolized in the mitochondria to produce energy in the form of heat thanks to the presence of an enzyme (thermogenin) which prevents the formation of ATP. It is localized mainly in the subcutaneous tissue and in certain organic regions as the armpit, the mesentery, mediastinum, thoracic aorta and perirenal fat. Function: thermal. 16 A Figure 10. Diagrams of white adipose tissue (A) and brown adipose tissue (B). 17 B