adipose & cartilage

Adipose tissue is a connective tissue 

Types of adipocytes 

• White (Unilocular): 1 large fat droplet 

• Brown (multilocular) 

  • Smaller, store many droplets of fat 

  • Primarily for heat production 

Functions of adipocytes 

• Store and synthesise triglycerides 

• Functions as an endocrine organ 

  • Secretes leptin 

    • Hormone that binds to cells in the hypothalamus to signal satiety 

Mobilization and storage of triglyceride 

• Where does the fat come from? 

  • Chylomicrons: circulating fats from intestines 

  • Very low-density lipoproteins: from liver 

  • Both broken down into fatty acids and taken into adipocytes where they are reorganised into triglycerides and stored in fat droplets 

• How is fat mobilised 

  • Autonomic nervous system and hormones 

    • When stressed/threatened, brain must stimulate sympathetic nervous system (norepinephrin) to put energy into bloodstream from triglycerides 

  • Lipase breaks down triglycerides  

• Regulation: 

  • Short term (daily) 

    • Controls appetite and metabolism 

      • Ghrelin appetite stimulate: stimulates eating 

      • Peptide YY appetite suppressant: decreases appetite and indicates to stop eating 

  • Long term: 

    • Leptin: Hormone that binds to cells in the hypothalamus to signal satiety 

    • Insulin: moves glucose out of the bloodstream and into the cells 

Characteristics of White Adipose 

• Unilocular 

• Nucleus and cytoplasm displaced against cell membrane 

• Is an energy storage site 

• Synthesizes and stores triglycerides 

• At birth more uniformly distributed, but sex hormones will lead to redistribution 

Characteristics of Brown Adipose 

• Multilocular 

• Smaller, small many droplets of fat 

• Central nuclei 

• Primarily for heat production 

  • Seen around internal organs to keep organs warm 

• Numerous mitochondria 

Cartilage 

Structure and characteristics of cartilage 

• connective tissue that has a specialised extracellular matrix (ECM) that allows tissue to withstand mechanical stress and not deform 

• Its firm but pliable 

• Cartilage consists of chondroblasts which start to produce ECM 

• When completely surrounded by matrix, chondroblasts are called chondrocytes 

• Chondrocytes reside in lacunae (cell nest) 

• Characteristics: 

  • >95% is ECM and consists of ground substance, collagen and elastic fibers 

  • Avascular, lacks nerves and lymphatic vessels 

  • Provides support and scaffold for bone formation 

  • Surrounded by sheath of dense CT, perichondrium 

How does cartilage develop? 

1. Differentiation of mesenchyme into chondroblasts 

   1. Mesenchymal cells first condense and then differentiate outward 

2. Secretion of matric separates the chondroblasts which become chondrocytes 

3. Chondrocytes appear in the centre 

4. Interstitial growth and appositional growth 

Types of cartilage, their characteristics, where do you find them 

• Hyaline cartilage:  

  • homogenous amorphous matrix 

  • Mostly type II collagen (helps hold it together) 

  • Most common type 

• Elastic cartilage: 

  • Elastic fibers and elastic lamellae 

  • Does not calcify 

• Fibrocartilage: 

  • Most of the time will be replaced by bone 

  • Matrix in dense CT 

  • Abundant in type I collagen 

  • Lacks perichondrium 

 Function of each type of cartilage 

• Hyaline cartilage: 

  • Aggrecan interacts with collagen, chondronectin binds cells to ECM 

  • Chondrocytes reside in lacunae 

  • Found in tracheal rings, bronchi, larynx, nose, articular surfaces (long bones), and ends of rids (costal cartilages) 

  • Primarily in type II collagen 

  • Role in bone development and growth 

    • Growth occurs at epiphyseal growth plate 

    • Site of interstitial growth 

    • In adult bones, hyaline cartilage is only found at articular surfaces 

  • Facilitates the lengthening of long bones 

  • Forms the growth plates at the proximal and distal ends of long bones 

• Elastic cartilage: 

  • Matrix contains elastin as well as everything in hyaline cartilage but has less mineral 

  • Located in external ear, external auditory meatus, eustachian tube, epiglottis, parts of larynx 

  • Chondrocytes are large, hypercellular 

  • Surrounded by perichondrium 

• Fibrocartilage: 

  • Fewer chondrocytes than other cartilage 

  • Intermediate between cartilage and dense fibrous connective tissue with large bundles of collagen fibres 

  • Alternating layers of hyaline cartilage (chondrocytes) and thick layers of dense connective tissue (type 1 and collagen) 

  • Acts more as a shock absorber: fibres oriented in the direction of functional stresses 

  • Presence indicated resistance to both compression and shear forces 

  • Limited distribution in the body 

  • Found in areas of high stress: 

    • Intervertebral discs 

    • Mandibular condyle 

    • Symphysis pubis 

    • Sternoclavicular joint 

    • Menisci of knee 

   

Hyaline cartilage formation and repair 

• Growth and repair: 

  • Growth occurs by interstitial or appositional growth 

  • Calcifies before it is replaced by bone 

  • Repair is limited bc its avascular 

    • Occurs only by perichondral proliferation, with few cartilage cells 

    • Aging process involves calcification, at times bone deposition instead of cartilage repair 

Structure of the epiphyseal growth plate 

• Cartilage that’s present during growth at the ends of long bones, is then ossified by the epiphyseal line once growth has terminated. 

Describe the perichondrium 

• Dense connective tissue layer that surrounds the cartilaginous bone 

  • Also functions in the growth and repair of cartilage 

  • Found around the perimeter of elastic cartilage and hyaline cartilage 

  • Fibrocartilage and articular cartilage both lack perichondrium 

• Composed of 2 separate layers 

  1. inner cellular/chondrogenic layer: 

     1. Source of cartilage cells, has progenitor cells which make new cartilage 

  2. Outer layer is fibrous, contains fibroblasts which produce new chondroblasts