Ossification: (a) Intramembranous = mesenchymal cells converted directly to osteoblasts, which form woven bone eg. scull, clavical. (b) Endochonrial = mesenchymal cells converted into chondrocytes which form avascular cartilage model, blood vessels invade, osteoblasts form primary & secondary ossification centres, chondrocytes enlarge growth plates, die and are replaced by bone.

Bone fracture: Haemotoma (a blood 'clot') => hyaline cartilage forms => callus of woven bone => remodelled into secondary bone. Microfractures often form in bones under stress, remodelling repairs these.

Joints: C.T. holds bones together. (a) Fibrous: (eg. suture), virtually no movement, may fuse, eg. teeth to jaw, scull bones meet. (b) Cartilaginous: (i) primary = no movement, eg. growth plates in long bones, (ii) secondary = limited movement, eg. IV disks (Annulus fibrous surrounding Nucleus pulposis), pubic symphysis. (c) Fluid filled cartilaginous: synovial membrane & fluid, hyaline cartilage over bone ends, all held together by ligaments. Allows much movement. eg. knee, hip.

Epithelial specialisations

(a) Stomach: Cell adhesion: tight junctions, inermediate zonula adherants, desmosomes, gap junctions. Much protective mucous. Has 3 regions: (i) Cardia = shallow pits, long coiled glands, mostly mucous making cells. (ii) Fundus/body = branched tubular glands, 3 to 7 glands open into base of each gastric pit. Glands contain stem cells (replace dead cells), goblet (mucus), parietal (acid), and chief (pepsin) cells. (iii) Pylorus = long deep pits, short coiled glands. Mucus & endocrine (gastrin/somatostatin).

Endocrine cells need special stains.

ETC.

Transplant pathology

Definitions: Autograft = same person; Isograft = identical twins; Allograft = same species; Xenograph = different species (temporary measure); Orthotopic = Same site; Heterotopic = Different site.

Factors influencing graft survival: Immulogical matching, etc, etc.
 

Autoimmune disease

Self-tolerance: (a) Clonal deletion = T-lymphocytes with self-antigen receptors deleted in thymus; (b) Clonal anergy = APC present antigen, but not additional stimulation molecules; (c) Peripheral T-cell suppression = Suppressor T cells secrete inhibitory cytokines.

Causes of Autoimmunity: (a) Loss of helper T-cell tolerance, eg. self-antigen modified by drug, or infections cause APC to produce stimulatory signals; (b) Cross-reacting epitopes = Some bacteria share epitopes with self cells/tissue; (c) Sequestration of antigen, eg. thyroglobulin does not normally come into contact with blood or immune cells. (d) Polyclonal activation = endotoxin causes massive T-cell proliferation; (e) Imbalance of Suppressor/Helper T-cell function, much Th => more autoimmune.

Autoimmune atrophic gastritis => Vitamin B12 deficiency => Pernicious anaemia. B12 is needed for folate metabolism (and so DNA production) and nerve cell metabolism. AB made to parietal cells and Intrinsic factor, so B12 cannot be absorbed.

Rhumatic (Pan) Carditis: 2 to 3 weeks after group A streptococcal pharnygitis; AB made to some strep's M-protein cross-rects with glycoproteins of heart, joints & other tissues. Has 2 phases: (a) Acute phase (Rhumatic fever) => Aschoff body forms (AB/Antigen fibroid necrosis lesion) in cardiac CT, pericardium, and "vegetations" round valves. Also attacks joints, skin, sometimes lungs. (b) Chronic phase => Immune inflammation & fibrosic repair. Affect Mitral and Aortic valves most (ie. left side). => Atrial/Ventral hypertrophy, arrythmas, infective endocarditis, mural thrombi.