Lymphatics

• Lymph nodes


  All lymph passes through at least one lymph node, where foreign material is exposed to and removed by phagocytes, and fresh lymphocytes are recruited from the cortex
• The lymphatic system can provide a pathway for spread of infections and neoplasia
• Haemolymph nodes


  Normal lymph nodes which contain some red blood cells

  
• Haemal nodes


  Dark in colour, sit across blood vessels and which can be found in the roof of the abdomen and thorax of the sheep, presumed to have a similar function to the spleen

  
• The pattern of lymph nodes is variable in the adult animal

  
• Lymphocentres of the head

  • Retropharyngeal
  • Parotid
  • Mandibular
• Lymphocentres of the neck

  • Superficial cervical
  • Deep cervical
• Lymphocentres of the forelimb

  • Axillary

  
• Lymphocentres of the thorax

  • Dorsal thoracic
  • Ventral thoracic
  • Mediastinal
  • Bronchial
• Lymphocentres of the abdomen

  • Lumbar
  • Coeliac
  • Cranial mesenteric
  • Caudal mesenteric

  
• Lymphocentres of the hindlimb, pelvis and abdominal wall

  • Popliteal
  • Ischial
  • Deep inguinal
  • Superficial inguinal
  • Iliosacral

  
• Palpation of lymph nodes should form part of every good clinical examination

  
• Palpable lymph nodes in the normal dog and cat

  • Parotid
  • Mandibular
  • Retropharyngeal (sometimes)
  • Superficial cervical (prescapular)
  • Axillary
  • Accessory axillary (sometimes)
  • Popliteal
  • Superficial inguinal
  • Femoral (sometimes)

  
• Palpable lymph nodes in the normal horse

  • Submandibular

  
• Palpable lymph nodes in cattle

  • Prefemoral
  • Superficial cervical (prescapular)
• Enlarged thoracic lymph nodes are often visible on radiographs, as are enlarged abdominal lymph nodes which can also be seen by ultrasound
• Lymph node centres and their afferent and efferent drainage

  • Parotid
  • Mandibular
  • Retropharyngeal
  • Superficial cervical
  • Deep cervical
  • Axillary
  • Dorsal thoracic
  • Ventral thoracic
  • Mediastinal
  • Bronchial
  • Lumbar
  • Celiac
  • Cranial mesenteric
  • Caudal mesenteric
  • Popliteal
  • Ischial
  • Deep inguinal
  • Superficial inguinal
  • Iliosacral

  
• Knowledge of the lymph nodes and their drainage pathway is very important clinically, as the lymphatic system provides a route for metastasis of many neoplastic diseases

  
• Superficial inguinal node


  Drains groin, external male genitalia or female mammary glands
• Subiliac node


  Drains skin & deep structures between midflank and thigh
• Iliosacral centre


  Roof of caudal abdomen, pelvic cavity. Medial iliac, sacral, hypogastric, anorectal. Adjacent structures, hindlimb and pelvic organs as 2ndry stage.
• Lumbar ducts


  Drainage pathway
• Understanding the likely routes of metastasis enables accurate staging of disease and proper planning for surgery – e.g. in performing a mammary strip for removal of mammary tumours
• Tracheal ducts


  Follow the course of the trachea in the neck. Originate in the retropharyngeal nodes and receive tributaries from deep cervical nodes, thereby receiving the lymphatic drainage from the head, neck and proximal forelimb
• Left duct


  Joins the thoracic duct
• Right lymphatic duct


  Drains the right thorax and receives the right tracheal duct. Terminates in a similar way to the thoracic duct
• Cisterna chyli


  Sits between the aorta and vertebrae at thoracolumbar junction and may surround the caudal vena cava and the origin of the coeliac artery at this level. Receives the drainage from the abdominal lymphocentres, and from the hindlimbs and pelvis via the lumbar ducts. Gives rise to the thoracic duct.
• Lumbar ducts


  Form a plexus on the roof of the abdomen and drain into the cisterna chyli
• Thoracic duct


  Originates from the cisterna chyli. Passes cranially through the aortic hiatus to the mediastinum, where it runs cranioventrally to drain into the cranial vena cava in most cases.
• Chyle


  The fluid in the thoracic duct which is often white in colour and has a high proportion of chylomicrons, particularly following a meal.
• Embryology of lymphoid vessels

  1. Lymphoid vessels start to form in the mesoderm after the cardiovascular system is established

  2. Initially, there are six lymph sacs; the paired jugular sacs, the cisterna chyli and retroperitoneal sac, and the paired iliac sacs

  3. The lymph sacs (except the cisterna chyli) develop into lymph nodes as lymphoid tissue aggregates around them

  4. Mesenchymal cells infiltrate to form the lymph channels and other lymph nodes develop along the lymphatic vessels

  5. These nodes are then seeded with B lymphocytes from the bone marrow (or from the Bursa of Fabricius in birds) and T lymphocytes from the thymus

  
• Spleen


  Develops from mesoderm in the dorsal mesogastrium. In the adult animal the spleen sits in the left cranial abdomen and is attached to the greater curvature of the stomach by the gastrosplenic ligament. It is a very vascular organ which stores red blood cells in addition to its lymphoid functions.

  
• Thymus


  Arises from the third pharyngeal pouch, from endoderm, mesoderm and possibly also ectoderm. It is a paired organ initially, which gives off buds that grow down the neck to invade the mediastinum where they form a single organ which extends to the pericardium. The cervical portions regress in the dog before birth. The thoracic portion may be seen on thoracic xrays of the young animal, but should regress around sexual maturity.
• Lymphatic system's role in fluid balance

  • There is net filtration of water from the circulation into the ISF, which must be removed to prevent formation of oedema
  • The hydrostatic pressure gradient leads from the ISF into the lymphatic capillaries, so water is absorbed
  • Lymphatic capillaries remove protein from the ISF, thereby raising the oncotic (colloid osmotic) pressure inside the lymphatic capillary and favouring further water uptake from the ISF
  • Increased flow of lymph favours increased protein uptake – this means that when ISF volume is increased, the lymphatic system takes up increased amounts of protein which both increases the lymphatic absorption of water and reduces the oncotic pressure in the ISF thereby reducing filtration of water out of the plasma
• Factors affecting lymph flow

  • Movement of lymph within lymphatic vessels is due to hydrostatic pressure differences – bulk flow
  • External pressure from skeletal muscle propels lymph along the vessels, and also smooth muscle in the wall of larger lymphatic vessels enables them to contract
  • Lymphatic vessels contain valves, preventing backflow of lymph
  • Drainage into the venous system is reliant on the venous pressure being less than the hydrostatic pressure in the lymphatic vessels
• Causes of oedema or effusions

  • Disease of the lymphatic system (e.g. lymphangitis)
  • Capacity is overwhelmed (e.g. congestive heart failure (CHF) leads to excessive ISF volume)
  • Derangement of hydrostatic pressure gradient preventing drainage (e.g. a tight bandage)
• Structure of lymphatic vessels

  • Lymphatic capillaries are very thin-walled and highly permeable to facilitate the removal of large molecules like proteins, and foreign material
  • Larger, transporting lymphatic vessels may have some smooth muscle in the tunica media and resemble veins, but have pale coloured contents and more delicate walls
  • Lymphatic vessels possess endothelial valves to prevent the backflow of lymph