CLASSIFICATION

• connective tissue proper:
  • loose irregular
  • dense irregular
  • dense regular
• skeletal connective tissues:
  • bone
  • cartilage
• special varieties of connective tissue:
  white adipose, brown adipose, pigmentary, mucous (jellyform), reticular
• BLOOD

LOOSE IRREGULAR CONNECTIVE TISSUE

Specifics:
cell-rich, poor in intercellular matter (fibers and amorphous matter [ground substance])

Localization:
forms stroma of many organs, vascular tunica adventitia; lies underneath epithelia - forms lamina propria of mucous membranes and submucosa, is abundantly represented among muscle cells and fibers

CELLS

• fibroblasts Five subtypes: young, mature, fibrocytes, myofibroblasts, fibroclasts; develop from poorly differentiated mesenchymal cells; stellate or spindle-shaped cells with little cytoplasm; functions - formation of collagen and elastic fibers, ground substance of connective tissue, synthesis of enzymes digesting fibers and ground-substance, as collagenase, elastase, hyaluronidase; elaboration of biologically active substances
• macrophages descendants of blood monocytes, large cells with round or bean-shaped nuclei and abundant cytoplasm, rich in lysosomes, phagosomes, irregular cytoplasmic membrane contour; functions - endocytosis, antigen presentation, production of a whole host of biologically active factors
• mast cells develop from a dedicated bone marrow predecessor; large cells, cytoplasm filled with basophilic granules containing histamine, heparin, serotonin, chymase, triptase; their function is related to release of granule contents and to effects of these substances on the organism, to secondary uptake of the substances, to elaboration of a number of bioactive compounds; mast cell granules on staining show what is known as metachromasia, i.e. they change color of the dye
• adventitious cells come from mesenchyme, represent a poorly differentiated component of the latter; they have numerous cytoplasmic membrane processes
• pericytes develop from poorly differentiated mesenchymal cells; cells of basal layer in capillaries
• endothelial cells are formed from poorly differentiated mesenchymal cells, line from the inside all of the blood vessels and lymphatics; produce many bioactive substances
• pigmentary cells come from neural crest, contain a pigment called melanin in their cytoplasm
• adipose cells develop from non-differentiated cells of mesenchyme; structure and function are discussed later
• plasma cells are from the B cell lineage, produce antibodies, are characterized by well-developed rough endoplasmic reticulum and Golgi complex in cytoplasm; Golgi complex is pale on regular staining - perinuclear halo
• leukocytes leukocytes that left circulation

INTERCELLULAR MATTER

FIBERS:

• collagen fibers are composed of a protein collagen. Structure - five levels of organization:
  1. polypeptide chain, consists of repeating sequences of 3 amino acids, 2 of them are proline or lysine and glycine, and the 3rd - any other
  2. molecule - three polypeptide chains form a collagen molecule
  3. protofibril - several collagen molecules, cross-linked by covalent bonds
  4. microfibril - a union of several protofibrils forms them
  5. fibril - a combination of bundles of protofibrils

  collagen is heterogeneous with regard to amino acid composition, number of cross-links, attached carbohydrate side-chains and degree of hydroxylation; on this basis collagen is divided into 15 subtypes
  collagen fibers are firm and do not stretch

• elastic fibers structure: on the fiber exterior there are microfibrils, composed of microfibrilary protein, the interior is formed primarily by a protein called elastin; elastic fibers stretch well, then easily regaining their original shape
• reticular fibers - a subtype of collagen fibers, are well impregnated with silver salts, hence the alternative term for them is argyrophilic fibers

GROUND (AMORPHOUS) SUBSTANCE:

• glycosaminoglycans (non-sulfated and sulfated) - hyaluronic acid
• proteoglycans (glycosaminoglycans with proteins) - chondroitin-4-sulfate, chondroitin-6-sulfate, dermatan sulfate, heparan sulfate, heparin
• glycoproteins - fibronectin, laminin, etc.

ground substance is jelly-like in consistence; cells and fibers are embedded in it

DENSE IRREGULAR CONNECTIVE TISSUE

Specifics: many fibers, few cells, fibers are irregular in orientation

Localization: reticular layer of dermis; periosteum, perichondrium

CELLS

the major component of the relatively scarce cellular population is fibroblasts; occasional mast cells and macrophages may also be found

INTERCELLULAR MATTER

FIBERS: numerous collagen and elastin fibers

GROUND (AMORPHOUS) SUBSTANCE: moderate amounts of glycosaminoglycans and proteoglycans

DENSE REGULAR CONNECTIVE TISSUE

Specifics: many fibers, scarce cells, fibers are regularly oriented - arranged in bundles

Localization: tendons, ligaments, capsules, fasciae, and fibrous membranes

CELLS

very few cells are present, most of them are fibroblasts; some mast cells and macrophages as well

INTERCELLULAR MATTER

FIBERS: in abundance, collagen and elastic; have a regular orientation, form thick bundles (fascicles)

GROUND (AMORPHOUS) SUBSTANCE: glycosaminoglycans and proteoglycans in very limited amounts

TENDON
in tendons collagen fiber fascicles are invested with thin layers of loose irregular connective tissue; the thinnest (first-order) fascicles are surrounded by endotendineum; second-order fascicles are surrounded by peritendineum, the tendon itself is a third-order fascicle

CONNECTIVE TISSUES WITH SPECIAL PROPERTIES

ADIPOSE TISSUE

Specifics:
basically composed of fat cells and thin layers of loose irregular connective tissue

white adipose tissue

Localization: present everywhere

CELLS
white adipose cells (white adipocytes)
cytoplasm is dominated by a single large fat vacuole; cell nucleus and other organelles are pushed by the vacuole to the periphery

there is loose irregular connective tissue between groups of adipocytes

INTERCELLULAR MATTER

FIBERS: small numbers of collagen and elastic fibers

GROUND SUBSTANCE: glycosamino- and proteoglycans in small amounts

brown adipose tissue

Localization: between scapulae, around kidneys and thyroid gland

brown adipose tissue is more widely represented in fetuses, its quantity is greatly reduced after birth

CELLS
brown adipose cells (brown adipocytes)
many small fat droplets are scattered throughout the cytoplasm; nucleus and organelles are in the cell center; multiple mitochondria are present

the brown color of cells is due to significant amounts of iron-containing pigments - cytochromes; in brown adipocyte mitochondria the free energy released from the oxidation of fatty acids and glucose is not stored in the form of ATP, rather it is dissipated as heat; therefore the function of this tissue is heat production and regulation of body's thermal balance

there are also small numbers of fibroblasts and other cell types of loose irregular connective tissue

INTERCELLULAR MATTER

FIBERS: small amounts of collagen and elastic fibers

GROUND SUBSTANCE: glycosamino- and proteoglycans in small amounts

PIGMENTARY TISSUE

Specifics: ordinary loose or dense connective tissue with large numbers of pigment cells

Localization: vascular tunic of the eye, dermis under mammary gland areolae, birthmarks, and nevi

MUCOUS CONNECTIVE TISSUE

Specifics: few cells and fibers, lots of ground substance

Localization: umbilical cord (Wharton's jelly)

CELLS
in general, poorly differentiated fibroblasts in small numbers

INTERCELLULAR MATTER

FIBERS: scarce and thin collagen fibers

GROUND SUBSTANCE: generally contains hyaluronic acid

RETICULAR TISSUE

Specifics: provides a soft structural framework (stroma) for organs of hemopoiesis and immunity

Localization: spleen, lymph nodes, tonsils, lymphoid follicles, and red bone marrow

CELLS

reticular cells (a subtype of fibroblasts) these cells possess processes which they connect to each other through forming thereby a network; other cell types of loose connective tissue are also present in small quantities: macrophages, mast cells, plasma cells, fat cells (adipocytes)

INTERCELLULAR MATTER

FIBERS: reticular fibers - a subtype of collagen fibers, are well stained with silver salts; that's why they are also known as argyrophilic fibers; they form a meshwork

GROUND (AMORPHOUS) SUBSTANCE: extracellular fluid

BONE

CELLS:

• osteoblasts - take origin from poorly differentiated mesenchymal cells; reside in the internal layer of periosteum, during bone development osteoblasts are on the periosteal surface and around interosseous blood vessels; these cells are cuboidal, columnar and polygonal in shape, have a well-developed rough endoplasmic reticulum
  function - production of bone intercellular matter
• osteocytes - a mature form of osteoblasts, they lie in lacunae within a bone and extend protoplasmic processes into small canaliculi in the intercellular matrix
  function - weak secretion of bone intercellular matter components
• osteoclasts - macrophages of bone tissue, blood monocytes being their precursors; large multinucleated cells; a zone of cytoplasm adjacent to osseous surface is referred to as ruffled border, multiple cytoplasmic processes and lysosomes are found here
  function - destruction and resorption of bone fibers and ground substance

INTERCELLULAR MATTER

FIBERS: collagen fibers

GROUND (AMORPHOUS) SUBSTANCE: the predominant component is calcium phosphate mostly in the form of hydroxyapatite crystals and some in amorphous state; small amounts of magnesium phosphate and very scarce glycosaminoglycans and proteoglycans are also found

THERE ARE TWO TYPES OF THE BONE:

fibrous (immature) bone

• ossein fiber orientation is random
• cells are embedded into the intercellular matrix, reside on bone surfaces and around vessels penetrating the bone

lamellar (mature) bone

• ossein fibers are regularly arranged, forming bone lamellae
• fibers in a lamella have identical orientation
• fibers in adjacent lamellae lie in the same plane but perpendicular to each other
• cells reside between lamellae in specialized lacunae as well as around penetrating vessels
• cells have processes, which they can connect to each other by
• apart from osseous lamellae in lamellar bone there are special structures - osteons
• osteon forms around a vessel, therefore there is always a vessel in the center of osteon, and circular lamellae with some cells scattered in-between are present around a vessel
• an osseous channel through which a blood vessel traverses is referred to as Haversian canal

PERIOSTEUM
has 2 layers:
external - fibrous; made of dense irregular connective tissue
internal - cellular (osteogenic); contains many osteoblasts and blood vessels, some osteocytes as well

functions: bone nutrition, longitudinal and transverse growth of bone, its regeneration

ENDOSTEUM
a lining covering a bone from the marrow side, made of loose irregular connective tissue with osteoblasts and osteoclasts in addition to more common cell types of this tissue

CARTILAGE

THREE TYPES OF CARTILAGE ARE DISTINGUISHED: HYALINE, ELASTIC AND FIBROUS

what distinguishes them is primarily intercellular matter composition

CELLS

• chondroblasts - less differentiated cartilage cells, originate from non-differentiated mesenchyme; have a flattened shape; a well-developed rough endoplasmic reticulum in a basophilic cytoplasm;
  function - elaboration of cartilage intercellular matter; under certain circumstances chondroblasts are capable of producing matrix-degrading enzymes - collagenase, elastase, hyaluronidase
  reside in the internal layer of periosteum and in the depth of matrix - within lacunes
  chondroblasts mature into chondrocytes
• chondrocytes - differentiated cartilage cells; of round or angular shapes, with advancing cellular age chondrocytes progressively lose their rough endoplasmic reticulum;
  function - elaboration of cartilage intercellular matter; under certain circumstances chondroblasts are capable of producing matrix-degrading enzymes - collagenase, elastase, hyaluronidase
  reside in the depth of matrix - within minute special cavities lacunes
  sometimes the number of cartilage cells in one lacune is more than one, it is the consequence of cell division; quite often the division id accomplished through amitosis; such cellular groups are called isogenic groups
  

hyaline cartilage
Localization: trachea and bronchi, articular surfaces, larynx, costosternal junctions

INTERCELLULAR MATTER

FIBERS: collagen fibers

GROUND SUBSTANCE: glycosaminoglycans and proteoglycans

elastic cartilage

Localization: pinna of the ear, corniculate and cuneiform cartilages of the larynx, nasal cartilages

INTERCELLULAR MATTER

FIBERS: elastic and collagen fibers

GROUND SUBSTANCE: glycosaminoglycans and proteoglycans

fibrous cartilage

Localization: attachments of tendons and ligaments to hyaline cartilage, intervertebral disks, semimovable joints, and symphysis

INTERCELLULAR MATTER

FIBERS: parallel fascicles of collagen fibers; more fibers than in other types of cartilage

GROUND (AMORPHOUS) SUBSTANCE: glycosaminoglycans and proteoglycans

intervertebral disk
consists of:

• anulus fibrosus - contains predominantly circular fibers
• nucleus pulposus - is composed of glycosaminoglycans and proteoglycans with cartilage cells in them

PERICHONDRIUM

has 2 layers:
external - fibrous; made of dense irregular connective tissue
internal - cellular (chondrogenic); contains many osteoblasts and blood vessels

functions: cartilage nutrition, appositional growth, and regeneration

there are no blood vessels in cartilage