dermis, hypodermis, and connective tissue

dermis

• supports epidermis and binds to hypodermis

• divided into sublayers

  • papillary layer (loose CT)

  • reticular layer (dense CT)

hypodermis

loose connective tissue usually infiltrated with adipocytes

role of connective tissue in epithelium

carries nerves and blood vessels that provide innervation and blood supply to the epidermis

structural composition of connective tissue

consists of extracellular fibers, amorphous ground substance, and population of different types of CT cells

varieties of connective tissue

• loose connective tissue

• dense connective tissue

• supporting connective tissue

• CT with special properties (bone marrow with hematopoietic properties)

differentiation of connective tissue cells

1. mesenchymal cell (pale/euchromatin, embryonic/pluripotent)

2. -blast (fibro-, chrondro-, osteo-, etc.) (pale and dark/euchromatin and heterochromatin)

3. -cyte (fibro-, chondro-, osteo-, etc.) (dark, heterochromatin)

-blast connective tissue cells

• active

• making extracellular matrix and ground substance

-cyte connective tissue cells

• terminal differentiation of cell lineage

• not transcriptionally active

functions of connective tissue

• mechanical support for other tissues

• avenue for communication and transport among other tissues

mesenchyme

tissue from which connective tissue is derived

fibroblasts

secrete collagen and other components of the extracellular matrix

adipocytes

store fat

mast cells, macrophages, lymphocytes

cells with immune function

connective tissue matrix

composed of ground substance and fibers

ground substance

• consists of water stabilized by glycosaminoglycans, proteoglycans, and glycoproteins

• in bone, includes minerals (is solid)

• in blood, is fluid (plasma)

fibers

• principle type is collagen which confers tensile strength and flexibility

• elastic fibers confer resiliency

structural glycoproteins

10% sugars, 90% protein

proteoglycans

50% sugars, 50% protein

collagen type I

present in dermis and tendon

collagen type II

present in cartilage

collagen type III

present in reticular fibers

collagen type IV

present in basement membrane

collagen type V

present in fetal tissues

glycosaminoglycans

• chondroitin SO₄ + core protein

• dermatan SO₄ + core protein

• heparan SO₄ + core protein

• keratan SO₄ + core protein

synthesis of collagen

1. α1 and α2 genes are transcribed in the nucleus

2. synthesis and clipping of signalling peptide preprocollagen

3. hydroxylation of lysine and proline residues

4. attachment of glucosyl and galactosyl to hydroxylysyl residues

5. assembly of procollagen molecules (triple helix)

6. packaging and secretion in golgi apparatus

7. procollagen peptidases clip ends of procollagen

8. aggregation of tropocollagen in extracellular space

cartilage and bone

specialized form of connective tissues which produce extracellular matrix with firm consistency

extracellular matrix of cartilage

supports mechanical stress without distortion

lacuna

empty pocket/space

chondroplast

lacuna containing an isogenic group of chondrocytes

varieties of cartilage

• hyaline

• elastic

• fibrocartilage

composition of extracellular matrix of cartilage

• fibers

  • collagen type II

  • elastic fibers (elastic cartilage)

  • collagen type I (fibrocartilage)

• ground substance

  • proteoglycans (keratan SO₄, chondroitin SO₄)

  • hyaluronic acid

  • glycoproteins (chondronectin)

perichondrium

connective tissue sheath around hyaline and elastic cartilage

fibrous layer of perichondrium

contains fibers and fibrocytes, regular dense CT

chondrogenic layer of perichondrium

contains chondroblasts that produce ECM and differentiate into chondrocytes

appositional growth

results from division of chondroblasts in chondrogenic layer of perichondrium, most important type of growth for overall cartilage growth

chondrocyte

terminally differentiated cartilage cell, becomes trapped in ECM

interstitial growth

results from division of chondrocytes in chondroplasts