Connective Tissue
Components of Connective Tissue
Basic Cell Components
Fibroblasts:Stellate cells with a pale cytoplasm and larger paler nucleus relative to macrophages. These cells are responsible for secreting the tropocollagen that will be clipped into collagen fibres that are a extracellular matrix components to connective tissues.
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Mast Cells: Smaller, rounded cells whose dark cytoplasmic granules usually obscure the nucleus. The granules contain Histamine and the cells are involved in cell-mediated inflammatory response.
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Macrophages: can be fixed (stellate) or wandering (round). typically have a darker cytoplasm than fibroblasts and have a smaller darker nucleus.
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Plasma Cells: Small cells with an offset nucleus. Secrete antibodies.
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There are also specialized cells in different types of connective tissue
Extracellular Components
Collagen fibres: thick strong unbranching fibres. these fibres do not stretch and can serve as a framework for deposition of materials.
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Elastic Fibers: relatively thin and darkly staining straight if stretched and wavy when loose
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Reticular Fibers: specialized type III collagen fibers
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Ground Substance: includes proteoglycans ( core protein linked with glycosaminoglycans - hyaluronic acid, heparin sulphate, chondroitin sulphate, etc.). Ground substance binds water
Connective Tissue Proper
Embryonic types
Mesenchyme : all connective tissue is mesodermally derived from mesenchyme cells (small stellate cells similar to fibroblasts and are pluripotent precursors to all connective tissue cell types). The extracellular material is not well developed making mesenchyme a loose connective tissue. As an embryo develops the mesenchyme will develop into adult connective tissues.
Mesenchyme pic
Mucous "Whartons jelly" : Is found in the umbilical cord, surrounding and cushioning the arteries and veins. This connective tissue consists of few cells and fibres but has a lot of viscous fluid.
BV
BV
Mucous
Loose Areolar Tissue
This mesentery whole mount show the basic structure of loose areolar tissue. the cellular components of loose areolar tissue are Fibroblasts (F), Macrophages, Mast cells and plasma cells. The extracellular components of this tissue are elastic fibres (E), reticular fibres and collagen fibres (C).
The collagen fibres can be seen as pink broad unbranched bundles
The elastic fibres stained with orecin appear as slender wavy lines.
Reticular fibers can only be seen with a special silver stain.
The amorphous material surrounding the cells is rich with glycosaminoglycansand core protein to make proteoglycans
F
E
C
C
F
Mast Cell
Dense Irregular
When you have more fibres and less amorphous material is it classified as dense connective tissue, irregular refers to the patterning of the fibres in that they are in 3D lattice work with no orientation. Dense irregular Connective tissue is found in the submucosa and the dermis of the skin. This tissue allows for the mobility of the skin.
you can see the tightly packed collagen fibres with interspersed fibroblasts
Fibroblasts
Dense Regular
Dense regular connective tissue has similar components to dense irregular but the fibres are arranged in parallel bundles to reinforce.There are two kinds of dense regular connective tissue, Elastic and Collagenous.
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Collagenous : In the tendon, thick closely packed type I collagen (C) bundles are oriented to the long axis of the tendon. The only cells present are fibroblasts and are in spaces between the collagen bundles. This is flexible but resists stretch, the fibroblasts (F) can replace torn collagen in the event of an injury.
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Elastic : For ligaments the main fibrous material is elastic fibres (E) instead of collagen. Some ligaments are intended for elastic support e.g. Bovine ligament nuchae shown here takes some of the load off the muscle holding the head up. This is also found in the suspensory ligament of the penis.
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Ligaments anchor bone to bone while tendon anchor muscle to bone.
C
F
E
Special Tissue Types
Reticular Tissue
The Spleen section has been stained with silver to show the reticular fibres (R). These fibres are type III collagen fibres, these differ from other types of collagen fibres found in tendons and bones by having smaller bundles, branching, and by having greater mucopolysaccharide content. These fibres are flexible though they do not stretch.
This tissue is found in lymph nodes, spleen, liver and thymus where it also provides a framework for these large loose organs.
Specialized fibroblasts called Reticular cells are responsible for synthesizing the reticular matrix.
R
Elastic Tissue
Found in the Tunica adventitia of arteries and veins, as well as in lungs alveoli. These fibres can stretch and recoil back to their original length.
Adipose
White adipose
White (Unilocular) Fat : found normally in the hypodermis of the skin. The cytoplasm is rich in lipid droplets that are fused into one large droplet with only a thin rim of cytoplasm. White adipose serves many functions; storage, insulation, cushioning.
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Brown fat : is multilocular in that is has numerous small droplets and more cytoplasm. Its found in the intrascapular region in neonates and in hibernating animals like rodents and bats in a thermogenic process. These cells have a central nucleus and a wealth of mitochondria that use the stored lipids as source to generate energy. Mitochondria in the brown adipose cells have a protein called thermogenin that short circuits the oxidative phosphorylation pathway and favours heat production.
Cartilage
Cartilage is normally avascular,
1)Its cellular components consists of chondrocytes which are derived from mesenchymal cells. These cells secrete chondroitin sulphate, type II collagen and elastic fibres.
2)Cells become separated by matrix but retain mitotic activity for some time and will occupy a space called a lacunae.
3)Has a fibrous outer layer called perichondrium which contains fibroblasts and collagen fibres,
4)The cells can convert to chondroblasts secreting more matrix and enlarging the cartilage (appositional growth)
Hyaline
L
C
M
The matrix is semitransparent and stains with a bluish tint. The matrix binds a lot of water which is important for cellular components so oxygen, sugars, amino acids can diffuse to the cells.
The type II collagen fibres are thin and loosely arranged
Cells sit in lacunae, in the section they appear shrunken but in life they fill the lacunae.
This cartilage is stiff and rigid. it can be found in the epiphyseal plate, trachea etc.
Articular cartilage is a special kind of hyaline cartilage found in the joints, there is no perichondrium in this connective tissue instead nutrients are delivered in the synovial fluid.
P
Elastic
E
C
Cells appear closer together abundant with elastic fibres in matrix. This is found in the pinna of the ear.
Must have special stain orecin to see elastic fibres.
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Fibrocartilage
Fibrocartilage is an intermediate between elastic and hyaline cartilage.
This can be found in invertebral discs
This cartilage lacks a perichondrium and the chondrocytes are typically arranged in "fibres" of tissue
Has a dense network of collagen to withstand stress.
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Blood
Neutrophil
Erythrocyte
Lymphocyte
Monocyte
Eosinophil
Granular Cytoplasm
Blood is a connective tissue with many cellular components and the matrix as plasma.
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Erythrocytes : red blood cells. Typically 7.5 um in diameter. these cells are anucleate and packed with hemoglobin, carbonic anhydrase (for exchange of CO2), and metabolic enyzymes to generate ATP. There are no organelles. Cytoskeletal elements are present to maintain the biconcave shape. With no organelles these cells will die and be degraded routinely making it imperative that these cells be remade.
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Platelets : smaller fragments 2-4u shed from megakaryocytic and are involved in blood clotting and repair.
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Agranular leukocytes:
Monocytes : largest leukocytes (10-12 um) with an eccentric oval or kidney bean shaped nucleus. Has an a-granular basophilic cytoplasm. No known function in circulating blood, they quickly migrate into tissues where they are active in phagocytizing dead cells and foreign material, in the inflammatory response and in processing and presenting antigens. 3-8% of circulating leukocytes.
Lymphocytes : these cells typically 6-8um but may reach up to 18 um. they possess a spherical, offset nucleus and a thin crescent of agranular basophilic cytoplasm. Lymphocytes include antibody secreting B cells (humorally mediated immune system), T cells (the cellularly mediated immune system) which kill forge in or virally altered cells and regulate the immune system, and null cells. Lymphocytes make up 20-25% of circulating white blood cells.
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Granular lymphocytes
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Eosinophils : these cells are 9-11um in diameter, characteristic bilobed nucleus and few organelles. Cytoplasm is filled with acidophilic granules. Eosinophils granules contain parasitic destroying enzymes like major basic protein and eosinophilic cationic protein. Make up 2-4% of circulating leukocytes.
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Neutrophils : medium sized 8-9um and posses and multi lobed nucleus. Younger neutrophils have 3-4 lobes while older may have up to 5 lobes. Female cells have an additional drumstick shaped Barr body. Neutrophil granules contain antimicrobial agents. The kamikaze cells are sometimes known as macrophages. In killing bacteria they destroy themselves forming pus. Most common leukocytes 60-70%.
Blood formation
Blood is a connective tissue by virtue of its cellular components (erythrocytes, leukocytes) and matrix (plasma). Blood formation or haemopoisis occurs in bone marrow primarily. can identify the basic cell types involved : hemocytoblasts, megakaryocytes, basophilic erythroblasts, normoblasts, myelocytes.
Bone
Bone has a high tensile strength.
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3 basic cell types:
1) Osteoblasts : bone forming
2) Osteocytes : mature bone cells
3) Osteoclasts : bone reabsorbing cells, large, multinucleate cells derived from fusion of monocytes. they have a ruffled border and found in Howships lacunae.
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The matrix is type I collagen arranged in orthogonally oriented sheets that serve as a focal point of ossification. Hydroxyapatite spicules are added. Bone is highly vascularized and the matrix retains less water.
Osteoblasts form the bone matrix and in the process they become trapped in lacunae.
Mature bone cells (osteocytes) are surrounded by matrix and have long microvilli which traverse the matrix in channels called cannaliculi.
The processes from adjacent osteocytes contact each other via gap junctions. They share ions, metabolites, hormones so that even cells deep in the matrix can respond even though they are not in immediate vicinity of blood vessels.
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Cannaliculi
Compact bone
Lacunae
Compact bone has an outer surface invested with fibrous periosteum attached via Sharpey's Fibres; osteoprogenitor cells in periosteum. The organization is molded around blood vessels; functional units called Haversian Systems which consist of several layers or Lamellae composed of mineralized matrix and type 1 collagen orthogonally arranged. Mature osteocytes found in lacunae. The central canal is called the Haversian Canal which contains blood vessels.
Haversian canal
Lamllae (Layers)
Endochondral Bone Formation
Zone of Maturation :
chondrocytes relax and synthesize matrix and spread out
Zone of proliferation :
Chondrocytes multiply forming flattened stacks of cells
Zone of Quiescence :
Typical Hyaline Cartilage
Zone of Calcification :
cartilage matrix is provisionally calcified killing the chondrocytes which depend on the matrix for transport of nutrients.
Zone of Hypertrophy :
chondrocytes accumulate glycogen and swell tremendously Matrix between cells decreases as lacunae enlarge
zone of ossification :
Numerous osteoblasts and osteoclasts can be identified as calcified cartilage matrix is covered and replaced with bone matrix. This region is well vascularized, as blood supplies osteogenic cells.
See if you can identify the zones.
Spongey Bone
Comprised of 3D lattice of branching trabeculae which also have lamellae. Trabeculae are thin, not usually penetrated by blood vessels
Intramembranous Bone Formation
For Intramembranous bone the mesenchymal cells are transformed by embryonic induction; they condense in elongate clusters, secrete tropocollagen that assembles into delicate collagen fibrils which then enlarge.
Vascularization occurs, and mesenchyme cells take on a more cuboidal shape, becoming osteoblasts. These osteoblasts continue to lay down an organic base of collagen, followed by the deposition of hydroxyapatite onto the type 1 collagen fibrils, completing the process of ossification and creating a primitive system of lacunae and canaliculi. Smaller spicules of intramembranous bone fuse to form larger plates. Some of the fact bones of the skull are formed by the intrammebranous process.