Wk 5 Biomedical Science PT1

Topic 12 - Cells & Tissues II 

• List the cell and fibre types located in connective tissues 

• Classify connective tissues and provide examples of each 

• Understand the different arrangements of connective fibres and why the human body needs different types of connective tissues 

• List the functions and components of the skeletal system 

• Describe the structure and functions of each part of a long bone 

• Compare the properties of compact and spongy bone tissue 

• Describe the process and purpose of bone growth & remodelling 

• Describe the components and functions of blood 

• Understand the components of the lymphatic system and its basic function 

Connective tissue 

Connective tissue is found throughout the body but never exposed to the outside environment 

Functions include: 

• Establishing structural framework of the body 

• Transporting fluids & dissolved materials 

• Protecting delicate organs 

• Supporting, surrounding, and interconnecting other types of tissue 

• Storing energy reserves, especially in the form of lipids 

• Defending the body from invading microorganisms  

Main Cellular Components of Connective Tissue 

Connective tissue has several cellular components, each with specific functions. These cells help provide structure, defence, and support to the body. 

1. Fibroblasts (Builders) 🏗 

Most common cell in connective tissue – always present  

Function: Produce collagen, elastin, and other fibres that form the extracellular matrix. Secrete proteoglycans – interact with ECF to form ground substance 

 Location: Found in all types of connective tissue (e.g., skin, tendons, ligaments) 

2. Macrophages (Defenders) 🦠 

Function: Engulf and digest pathogens, dead cells, and debris (phagocytosis) 

Type of immune cell that helps with defence and tissue repair 

Example: Found in the skin, lungs, liver, and lymphatic tissue 

3. Mast Cells (Allergy Responders) 🚨 

Function: Release histamine and heparin to trigger inflammation and allergic reactions 

Location: Near blood vessels, skin, and mucosal surfaces (e.g., lungs, digestive tract) 

4. Adipocytes (Fat Cells) 🫙 

Function: Store energy as fat and provide insulation/protection 

Location: Found in adipose (fat) tissue under the skin and around organs 

5. Plasma Cells (Antibody Producers) 🛡 

Function: Produce antibodies to fight infections 

Develop from B lymphocytes (a type of white blood cell) 

Location: Found in lymphoid organs and connective tissues like the intestines 

6. Mesenchymal Cells (Stem Cells) 🌱 

Function: Act as stem cells that can differentiate into other connective tissue cells (fibroblasts, adipocytes, etc.) 

Location: Found in embryonic connective tissue and adult connective tissue for repair 

Connective tissue fibres  

1. Collagen 

• Long, straight & unbranched 

• Bundles of fibrous protein – E.g. tendons & ligaments  

2. Reticular fibres 

• Fine network of collagen bundles 

• Forms storma of many soft organs 

• Resist multi-directional forces 

3. Elastic fibres  

• Branched & wavy fibres of elastin 

• Rare – E.g. interconnect vertebrae  

Classification of Connective tissues  

Cells of connective tissues  

Chondrocytes – Found in cartilage, produce cartilage matrix. 

Osteocytes – Found in bone, maintain bone structure. 

Erythrocytes (Red Blood Cells) – Found in blood, transport oxygen. 

Leukocytes (White Blood Cells) – Fight infections, found in blood and connective tissue 

Loose Connective tissue  

Loose Connective tissue is a distributed tissue type that connects and supports other tissues and organs, characterized by a loose, irregular network of fibres and cells within a semi-fluid matrix 

• Lies immediately under epithelial coverings 

• Provides structural support & protection from invasions  

Dense Connective Tissue: Dense Regular 

Dense regular connective tissue is characterized by densely packed collagen fibres arranged in a parallel fashion, providing strong, unidirectional resistance to stress, and is found in structures like tendons and ligaments.  

• Connects two (or more) tissues together 

• Tendons – attach skeletal muscle to bone 

• Ligaments – connect bones or stabilise organs  

• Parallel fibres aligned with forces applied to tissue 

• Large number of fibroblasts scattered among fibres  

Skeleton  

Skeletal system: 

• Provides support 

• Protects the internal organs E.g. brain & heart  

• Assists body movements (in conjunction with muscle) 

• Stores & releases salts of calcium and phosphorous 

• Participates in blood cell production (haemopoiesis) 

• Stores triglycerides in adipose cells of yellow bone marrow  

Components of skeletal system:  

• Bones 

• Ligaments – dense, fibrous tissue. They connect the ends of bones together in order to form a joint 

• Tendons – tough, flexibleband of fibrous connective tissue that connects muscles to bones 

• Joints 

Bones are made of: 

• Several different types of connective tissue 

• Bones or osseous tissue – compact or spongy 

• Blood – red marrow & blood vessels 

• Dense connective tissue – endosteum & periosteum 

• Adipose tissue – yellow bone marrow 

• Cartilage  

Cells of Bone Tissue  

Bone tissue contains several types of cells, each playing a specific role in bone formation, maintenance, and remodelling.  

The main types of bone cells are: 

Osteoblasts: These are the bone-forming cells. They produce the bone matrix (collagen and other proteins) and facilitate the mineralization of bone, turning it hard and dense. When they become surrounded by the matrix they produce, they mature into osteocytes. 

Osteocytes: These are mature bone cells that are embedded in the bone matrix. They maintain the bone tissue and communicate with other bone cells to help regulate the balance between bone formation and resorption. 

Osteoclasts: These are large, multinucleated cells responsible for breaking down and resorbing bone tissue. They help in the process of bone remodelling by removing old or damaged bone, allowing for new bone formation. 

Bone Lining Cells: These are flat cells that cover the surface of bones. They are thought to be derived from osteoblasts and play a role in regulating the exchange of calcium between bone and blood. 

Matrix of bone tissue  

• Collagen with calcium salts deposited on it  

Compacts & spongy bone under the microscope  

Compact bone 

• Hard external layer of all bones and surrounds the medullary cavity 

• Consists of osteons 

Spongy bone 

• Consists of trabeculae 

• Surrounded by an outer layer of osteoblasts 
   

Osteoblasts & Osteoclasts 

How bones develop 

It begins when neural crest-derived mesenchymal cells differentiate into specialized, bone-forming cells called osteoblasts.  

• Osteoblasts group into clusters and form an ossification centre.  

• Osteoblasts begin secreting osteoid, an unmineralized collagen-proteoglycan matrix that can bind calcium 

Hyaline Cartilage  

• Most common 

• Matrix contains small bundles of densely packed collagen fibres (not obvious by light microscopy) 

• Ends of long bones, anterior ends of ribs, nose, trachea  

Remodelling of skeletal structures  

• Bone tissue is constantly being torn down & built up 

• About 10 percent of bone is replaced every year in adult skeleton  

Bone growth & remodelling 

Normal bone metabolism depends on several factors: 

• Minerals – especially calcium & phosphorous 

• Vitamins – Vitamin A: stimulates osteoblasts; Vitamin C: collagen; Vitamin D: Ca2+ absorption in gut 

• Hormones – insulin- like growth factors (+ve), thyroid hormones (+ve), oestrogen (+ve), Glucocorticoids (-ve)   

Bone tissue responds to load  

This has implications... 

• Increased weight bearing => Sports & incorrect posture 

• Lack of weight bearing => Bed rest & astronauts 

• Ageing => Shifts in balance of osteoblasts & osteoclasts  

Blood  

Functions of blood:  

Transportation  

• Gases, nutrients, hormones, waste  

Regulation  

• PH, temperature, fluid balance 

Protection  

• Against disease & blood loss  

Blood cells begin as a common progenitor  

Lymphatic system 

• Assists in circulating body fluids and defence against pathogens and abnormal cells 

Consists of: 

• Lymph fluid – a fluid connective tissue 

• Lymphatic vessels  

• Lymphatic tissue – a type of reticular connective tissue 

• Lymphocytes within a filtering tissue  

The lymphatic system is closely associated with the cardiovascular system  

Functions of Lymphatic System 

• Drains excess interstitial fluid 

• Returns approx. 3L per day to blood 

• Maintenance of fluid balance 

• Transports dietary lipids from GI tract to blood 

• Immune response – initiates highly specific responses against microbes or abnormal cells