Haemolymphatic

• Haematopoiesis: development of erythrocytes, thrombocytes, granulocytes (neutrophils, eosinophils, basophils), monocytes and lymphocytes, occurs in the bone marrow. When there is excessive demand for haematopoiesis, extramedullary haematopoiesis can occur in the spleen and liver.
• Extramedullary haematopoiesis in the spleen normally ceases shortly after birth, but can occur in areas of nodular hyperplasia, or when there is increased demand e.g. severe anaemia. Extramedullary myelopoiesis occurs when there is excessive demand for neutrophils.
• In normal adult animals, the bone marrow has low cellularity and is dominated by fat, with a rim of thin active marrow at the periphery. In functional hyperplasia there is increased cellularity of the marrow to increase production by haematopoietic cells. Fatty white marrow is converted to cellular red areas. Hyperplasia may be a response to decreased peripheral cells or an increased demand in inflammatory conditions.
• Myeloproliferative disease is a non-specific term to describe disordered proliferation of one or more haematopoietic cell lines in the marrow (neoplasia). Proliferated marrow will sink in a jug of formalin as it is so dense. Normal marrow floats.
• Involution of bone marrow can happen for many reasons including plant and chemical poisons, radiation, bacterial toxins, ageing and viral diseases (e.g. FeLV). One or more cell lines may see hypoplasia or aplasia. Marrow fat is relatively resistant to lipolysis, but in extreme cases of cachexia or starvation it becomes watery, translucent and pink.

   

  Liberation of excessive amounts of haemoglobin can cause renal tubular damage
• Anaemia results from either:

  • Increased rate of destruction (haemolytic)  e.g. icterus and haemoglobinuria
  • Loss or haemorrhage (haemorrhagic)
  • Decreased rate of production (non-regenerative)
• The signs and lesions of anaemia are largely referable to reduced oxygen supply to the tissues (hypoxia)

  • Pallor – reduced haemoglobin
  • Cardiac changes – friable, flabby and pale (fatty degeneration)
  • Periacinar hepatic necrosis
  • Contracted spleen in haemorrhagic, large and meaty  if acute haemolytic
  • Pale muscles with less of myohaemoglobin
• Splenomegaly is enlargement of the spleen and can be categorised by morphological features:

  • Uniform enlargement, wet or bloody – from barbiturate euthanasia, anaesthesia, splenic torsion, acute haemolytic anaemia
  • Uniform enlargement, firm – chronic haemolytic anaemia, extramedullary haematopoiesis, chronic infectious disease
  • Nodular enlargement, wet or bloody – haematoma, haemangiosarcoma
  • Nodular enlargement, firm – nodular hyperplasia, primary neoplasia, inflammation (granulomas, abscesses)
• Nodular hyperplasia is very common in older dogs, producing demarcated, encapsulated swellings. The tissue comprises of hyperplastic foci of lymphoid cells or mixed lymphoid cells with areas extramedullary haematopoiesis. May be discrete or multifocal / coalescing.
• Primary haemangioma and haemagiosarcoma are common neoplasms of older dogs, especially large breeds. Frequently large with areas of haemorrhage and necrosis. Can rupture spontaneously or due to palpation, possibly causing massive internal haemorrhage. Metastasis to the lungs and wider parenchyma are frequent.

   

  Other neoplasms of mesenchymal origin e.g. fibrosarcoma, leiomyosarcoma, can arise in the spleen
• Atrophy of the spleen is seen with wasting conditions and ageing.

   

  Contraction of the spleen can be marked e.g. with catecholamine release, hypovolaemic shock or cardiogenic shock.

   

  Splenic rupture is common in the dog in RTA. Can be fatal, or additional splenic fragments may form in the surrounding tissues. Congenital accessory splenic can also be found in the omentum in some cases.
• Haemosiderosis is RBC breakdown and subsequent formation of haemosiderin (so normally seen in the spleen). Excessive amounts can occur in haemolytic anaemia or chronic heart failure.
• Periacinar necrosis describes the area around the central vein of a portal unit which is susceptible to hypoxia. Grossly this appears as necrosis, diffuse pinprick haemorrhage, fatty change and degeneration.
• Siderofibrotic plaques / siderofibrosis (gamna-gandy bodies) are common lesions in older dogs, caused by deposition of iron salts, calcification and fibrosis. These appear as yellow, dry, encrusted plaques on the splenic capsule, likely at sites of previous local haemorrhage.
• Splenic infarcts tendo to occur in peripheral zones of the spleen where perfusion is poorest, or in diseases which cause vasculitis. Caused by thrombus formation or emboli and is commonly seen in DIC. Also seen in swine fever.
• Lymphadenopathy is regional or generalised lymph node enlargement of unspecified cause.

   

  Changes to the lymph nodes can be reactive, lymphadenitis, diffuse infiltration or focal infiltration.
• Lymph node hyperplasia (reactive lymph node) occurs due to hyperplasia of B and/or T cell compartments, secondary to immunological reactions or infections. Hyperplasia of the monocyte population can also occur. Germinal centre hyperplasia can be generalised or localised, and can be followed by lymphocyte loss and exhaustion of GCs.
• Lymphadenitis can result when an infectious agent is present in the lymph node, so neutrophils aggregate in the peripheral sinuses and the body of the node. E.g. equine strangles. May be accompanied by lymphangitis (inflammation of lymphatic vessels. This is very painful, and can resolve, abscess or scar depending on severity.
• Lymph node neoplasia may be involved as part of a primary disease (e.g. lymphosarcoma), or the site of a metastasis (e.g. carcinoma). Infiltration may be diffuse or focal.
• Degenerative changes to the lymph nodes and atrophy is seen is association with old age and cachexia.

  Melanin pigment can be seen in association with pigmentary incontinence, where pigment is taken by macrophages to draining lymph nodes.

  Haemosiderin pigment is seen in draining lymph nodes from a congested area or haemorrhage.
• The thymus is composed of lymphoid and epithelial tissue, normally involutes with age.

  • Congenital issues arise e.g. equine combined immunodeficiency in arab foals, where the thymus is too small. Thymic cysts can arise, either in the thymus or in tissue remnants in cranial mediastinum
  • Thymitis is uncommon, seen in some viral diseases e.g. postweaning multisystem wasting syndrome in pigs
  • Degeneration – lymphocytolysis - viral diseases e.g. canine distemper virus.
  • Neoplasia of the thymus can be thymic lymphosarcoma (space occupying) or thymoma (less common, slow growing)