Q2 LONG ANSWER 10M ★ High-yield

Describe the cells, fibres and ground substance of connective tissue. Classify connective tissue with examples.

Anatomy · 1st Professional — Pre-Clinical

Definition and Components

Connective tissue is a tissue of mesodermal origin in which the cells are widely separated by an abundant extracellular matrix, and which supports, binds and nourishes other tissues.

Three components — cells, fibres and ground substance
Unlike epithelium it is highly vascular (except cartilage) and the matrix predominates over the cells

Cells

Cell	Origin	Function
Fibroblast	Mesenchyme	The most numerous; synthesises collagen, elastin and ground substance; central in wound healing
Macrophage (histiocyte)	Blood monocyte	Phagocytosis and antigen presentation; part of the mononuclear phagocyte system
Mast cell	Bone marrow	Granules of histamine and heparin; degranulates in type I hypersensitivity
Plasma cell	B lymphocyte	Secretes antibody; eccentric nucleus with a cartwheel (clock-face) chromatin pattern
Adipocyte	Mesenchyme	Stores fat; the nucleus is pushed to one side — the "signet-ring" appearance
Mesenchymal (stem) cell	Mesenchyme	Can differentiate into other connective tissue cells
Leucocytes	Blood	Migrate in during inflammation

Fibres

Fibre	Collagen type	Staining	Properties	Sites
Collagen (white)	Type I	Pink with eosin; blue with Masson	Great tensile strength; inelastic	Tendon, ligament, dermis, bone
Reticular	Type III	Black with silver (argyrophilic); PAS positive	Fine branching network	Framework of liver, spleen, lymph node, bone marrow; basement membrane
Elastic (yellow)	Elastin with fibrillin	Verhoeff or orcein	Stretch and recoil	Aorta and large arteries, lung, ligamenta flava, skin

Types of collagen worth knowing — I bone, skin, tendon; II hyaline and elastic cartilage; III reticular fibres; IV basement membrane; VII anchoring fibrils
Mnemonic — "Be (I) So (II) Tough (III), Be (IV) There"

Ground Substance

An amorphous gel of glycosaminoglycans, proteoglycans and adhesive glycoproteins
GAGs — hyaluronic acid, chondroitin sulphate, dermatan sulphate, keratan sulphate, heparan sulphate
Adhesive glycoproteins — fibronectin in connective tissue, laminin in basement membrane
Functions — a medium for diffusion of nutrients and metabolites; resists compression; a barrier to the spread of infection
Hyaluronidase, produced by some bacteria and by spermatozoa, breaks down hyaluronic acid and is therefore a "spreading factor"

Types of Connective Tissue

Type	Features	Sites
Loose areolar	All cell types; loosely arranged fibres; abundant ground substance	Superficial fascia, around vessels and nerves, submucosa
Dense regular	Parallel collagen bundles; few fibroblasts	Tendon, ligament, aponeurosis
Dense irregular	Interwoven bundles resisting stress from all directions	Dermis, capsules of organs, periosteum, dura
Adipose	White (unilocular) and brown (multilocular, rich in mitochondria)	Subcutaneous; brown fat in the neonate for non-shivering thermogenesis
Reticular	Type III fibres forming a network	Lymphoid organs, bone marrow, liver
Elastic	Predominantly elastic fibres	Ligamenta flava, suspensory ligament of the penis
Mucoid (Wharton jelly)	Abundant ground substance, few fibres	Umbilical cord; vitreous humour

Functions of Connective Tissue

Structural support and binding of tissues and organs
Packing and padding — filling the spaces between structures
Nutrition — every capillary lies in connective tissue, and nutrients diffuse through the ground substance
Defence — macrophages, mast cells and plasma cells are all resident here; inflammation is essentially a connective tissue reaction
Repair — fibroblasts lay down the collagen of a scar
Storage — of fat, and of water and electrolytes in the ground substance

Applied Aspects

Scurvy — vitamin C is needed for hydroxylation of proline and lysine in collagen; deficiency gives defective collagen with bleeding gums, poor wound healing and subperiosteal haemorrhage
Osteogenesis imperfecta — a defect of type I collagen; brittle bones, blue sclerae and deafness
Ehlers–Danlos syndrome — defects of collagen synthesis; hyperextensible skin, hypermobile joints and fragile vessels
Marfan syndrome — a defect of fibrillin, the scaffold of elastic fibres; tall stature, arachnodactyly, lens dislocation and aortic dissection
Keloid — excessive type III then type I collagen in a scar, extending beyond the original wound; commoner in dark skin
Mast cell degranulation underlies anaphylaxis, asthma and urticaria; the histamine released causes vasodilatation and increased permeability
Wound healing passes through inflammation, proliferation with granulation tissue, and remodelling; the tensile strength of a scar never exceeds about 80% of the original
Fibrosis is excessive collagen deposition replacing functional tissue — the final common pathway of chronic injury in liver, lung and kidney
Oedema collects in the ground substance of loose connective tissue, which is why it is most obvious where that tissue is abundant and lax — the eyelids, scrotum and ankles
Vitamin C is required by prolyl and lysyl hydroxylase, both of which need it as a reducing agent — the biochemical reason scurvy is a connective tissue disease
Adipose tissue is an endocrine organ, secreting leptin, adiponectin and inflammatory cytokines, which links obesity to insulin resistance
Liposarcoma, fibrosarcoma and osteosarcoma are the malignant tumours of connective tissue; sarcomas spread by blood rather than by lymphatics

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