Anatomy chapters
Connective tissue
The cells, fibres and ground substance of connective tissue, the main types of connective tissue and where they are found, and the diseases that follow when collagen goes wrong.
- Connective tissue has three parts: cells, fibres and ground substance.
- The fibres are collagen (strength), elastic (stretch and recoil) and reticular (fine supporting network).
- Collagen types: I bone, tendon, skin; II cartilage; III reticular fibres; IV basement membrane.
On this page
Overview
Connective tissue supports, binds and protects the other tissues of the body.
It is made of cells, fibres and ground substance. Unlike epithelium, it has much more material between its cells than cells, and most types have a blood supply.
Cells of connective tissue
Resident cells
- Fibroblasts: the main cells. They make collagen, elastin and ground substance. Spindle-shaped with an oval nucleus.
- Adipocytes (fat cells): store fat in one large droplet that pushes the nucleus to the edge ("signet ring").
- Mesenchymal cells: stem cells that can turn into other connective tissue cells.
Wandering (immune) cells
- Macrophages: eat debris and microbes and present antigens. They come from blood monocytes.
- Mast cells: full of granules containing histamine and heparin. They release them in allergic reactions.
- Plasma cells: make antibodies. They come from B lymphocytes and have a "clock-face" or "cartwheel" nucleus.
- White blood cells (neutrophils, eosinophils, lymphocytes) move in during inflammation.
Mast cells carry IgE on their surface. When the allergen binds, they release histamine, causing allergy and anaphylaxis.
Fibres
Collagen fibres
Collagen is the most common protein in the body. Fibres are thick, strong and do not stretch. They stain pink with H&E.
| Type | Where found |
|---|---|
| I | Bone, tendon, ligaments, skin (dermis), dentine, fibrocartilage, scars |
| II | Hyaline and elastic cartilage, vitreous body |
| III | Reticular fibres: spleen, lymph nodes, bone marrow, liver; also blood vessels and early scars |
| IV | Basement membranes |
A widely used memory aid: type one is bone; type two is cartwolage; type three is for three-D networks (reticular); type four is the floor (basement membrane).
Hyaline cartilage contains type II, but fibrocartilage contains mainly type I collagen.
Elastic fibres
Elastic fibres stretch and recoil. They are made of elastin around a scaffold of fibrillin microfibrils.
They are plentiful in the walls of large arteries, the lungs, the skin, elastic cartilage and the ligamenta flava.
Reticular fibres
Thin fibres of type III collagen. They form a fine supporting mesh in lymphoid organs, bone marrow and the liver. They stain black with silver stains, not well with H&E.
Ground substance
A clear gel that fills the spaces between cells and fibres. It is made of glycosaminoglycans (such as hyaluronic acid), proteoglycans and glycoproteins.
It holds water, lets nutrients and waste diffuse and resists compression.
Types of connective tissue
| Type | Features | Sites |
|---|---|---|
| Loose (areolar) | Few fibres, many cells, lots of ground substance | Under epithelia, around vessels and nerves |
| Dense irregular | Thick collagen bundles in all directions | Dermis, capsules of organs |
| Dense regular | Collagen bundles in parallel rows with fibroblasts between | Tendons, ligaments, aponeuroses |
| Elastic | Mainly elastic fibres | Ligamenta flava, walls of large arteries |
| Reticular | Network of reticular fibres | Lymph nodes, spleen, bone marrow |
| Adipose | Mostly fat cells | Under the skin, around organs |
| Mucous | Jelly-like, few cells | Wharton's jelly of the umbilical cord |
Cartilage, bone and blood are specialised connective tissues (see bone and cartilage).
White and brown fat
- White fat: one big fat droplet per cell; stores energy.
- Brown fat: many small droplets and many mitochondria. It makes heat using the protein thermogenin (UCP1). Plentiful in newborns, between the shoulder blades and around the neck and kidneys.
Brown fat keeps newborns warm by making heat without shivering.
Clinical relevance
Scurvy
How it happens: lack of vitamin C, which is needed to hydroxylate proline and lysine when collagen is made.
What you see: weak collagen: bleeding gums, loose teeth, bruising, poor wound healing and bleeding around hair follicles.
Marfan syndrome
How it happens: a defect in the fibrillin-1 gene, so elastic fibres are abnormal.
What you see: a tall person with long limbs and fingers, dislocated lens of the eye and widening of the aorta, which can tear (aortic dissection).
Ehlers-Danlos syndrome
How it happens: inherited defects in collagen.
What you see: very stretchy skin, loose joints and easy bruising. Some types affect blood vessels.
Osteogenesis imperfecta
How it happens: a defect in type I collagen.
What you see: bones that fracture easily, blue sclerae and hearing loss.
Keloid
How it happens: too much collagen is laid down in a healing wound.
What you see: a raised scar that grows beyond the edge of the original wound.
Common questions: which collagen type is in a given tissue, which cell releases histamine, which fibre stains with silver, which protein is faulty in Marfan syndrome, and why vitamin C deficiency weakens collagen.
Quick revision
- Fibroblast: main cell; makes fibres and ground substance.
- Mast cell: histamine and heparin. Plasma cell: antibodies, clock-face nucleus.
- Collagen I bone and tendon; II cartilage; III reticular; IV basement membrane.
- Elastic fibres: elastin on a fibrillin scaffold.
- Dense regular connective tissue: tendons and ligaments.
- Brown fat: heat production in newborns.
- Scurvy (vitamin C), Marfan (fibrillin), osteogenesis imperfecta (type I collagen).
Written by The House Job from standard histology references (Junqueira's Basic Histology).
FAQ
Which collagen is found in basement membranes?
Type IV collagen.
What type of connective tissue forms tendons?
Dense regular connective tissue, with parallel bundles of type I collagen.
What is the role of brown fat?
It produces heat, mainly in newborn babies.
Which protein is defective in Marfan syndrome?
Fibrillin-1, the scaffold for elastic fibres.