Lymphatic System and Lymphoid Tissue

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Lymphatic Systemand Lymphoid Tissue
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Lymphatic System

• Lymph Vessels - have the same layers as veins and
  similar composition except less well developed.
• Lymph Organs
• Lymph Nodes, Tonsils, Thymus, Spleen
• Lymph
• Circulates in one direction only (toward heart)
• Composition very similar to interstitial fluid

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Lymph Capillary, Cross Section Arrows
discontinuous basal lamina AF anchoring fibrils
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Lymph Organs

• Lymphoid Organization
• a) dense - compactly organized cell aggregates,
  lymph nodules/follicles.
• b) diffuse - loose and poorly defined cell
  collections.
• Occur in all loose CT.
• Mucosa associated lymphoid tissue (MALT)
• E.g. Peyers patches, appendix, respiratory
  tract, reproductive tract, urinary tract
• Lymph Nodules - structural units of lymph organs
  and solitary bodies may be homogeneous or have
  cortex and medulla (germinal center reaction
  center indicating dividing active Lymphocytes).

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Lymph Organs

• A. Lymph Node - occur generally in pre-vertebral
  region mesenteries, omenta, hypodermis.
• oval (bean) shaped organ with hilus.
• blood vessels and nerves enter and leave via
  hilus.
• organ is divided into a cortex (outer layer) of
  nodules, and medulla (center) of diffuse
  parenchyma.
• DICT capsule is penetrated by afferent lymph
  vessels.
• trabeculae or septae (areolar CT) penetrate the
  organ from the DICT capsule.

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1 macrophages 2 Reticular Cells 3 Trabecula
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Lymph Node

• Afferent lymph vessels penetrate parenchyma as
  lymph capillary sinuses
• a) marginal - outer cortex margin.
• b) cortical - perpendicular through cortex.
• c) medullary - throughout medulla.
• Medullary sinuses collect into efferent lymph
  vessels in medulla and exit via hilus.
• Lymph flow is uni-directional (valves)

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Lymph Organs

• B. Tonsils lymphoid masses ringing the
  oro-nasal pharynx. Three sets - paired
  palatines, pharyngeal, and lingual.
• All tonsil types have a DICT capsule on the
  embedded surface and some kind of epithelium on
  the oral cavity free surface. All are penetrated
  by deep crypts.
• No afferent lymphatic vessels no sinuses, only
  efferent vessels.
• No cortex no medulla.

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1 crypt 6 capsule 2 strat. Sq.
epith. 7 mucous glands 3 lymph nodules 4
diffuse lymphoid 5 germinative center
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Tonsils

• Palatines (faucals), oral cavity into pharynx,
  located on the sides of the soft palate covered
  externally with strat. Squamous epith.
• Lingual - in the root of the tongue strat.
  squamous.
• Pharyngeal - in the posterior wall
  pseudostratified columnar epith. in young, some
  strat. squamous later inflamed, swollen
  condition in youths adenoids.
• Tonsilectomy

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Lymph Organs

• C. Thymus - large organ varies in size with
  age
• 15 g at birth, 35 g at 14 years 25 g at 25
  years 12 - 15 g at 40 years, lt1 g at 70.
• Organ is of two lobes each lobe of several
  lobules (leaf shaped bodies) capsule of Areolar
  CT.
• Capsule forms trabeculae of areolar CT. organ
  has cortex and medulla
• Medulla penetrates into each lobule.
• No sinuses no afferent lymph vessels only
  efferent vessels
• No germinal centers in nodules

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Thymus

• Functional Cells T-Lymphocyte precursors or
  thymocytes.
• Paracrine secretion of hormones thymosin,
  thymopoetin and others
• Unique embryology dual origin lymphocytes
  (mesenchyme) stellate epithelio-reticular cells
  are derived from endoderm.
• Thymic Corpuscles (Hassell's Corpuscles) are
  calcified, refractile remains of primitive
  reticular epithelial stem cells. Function unknown

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Lymph Organs

• D. Spleen - largest lymph organ - has blood
  sinuses rather than lymph sinuses (filters
  blood).
• Only organ purely for filtering blood (liver does
  also) hematopoietic in embryo.
• Capsule of thin DICT which forms areolar CT
  trabeculae.
• Blood vessels enter parenchyma via trabeculae
  into nodules.

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Spleen

• Parenchyma of
• a) white pulp - (Splenic Follicle) lymph
  nodules around a central artery.
• b) red pulp Splenic Cords (Cords of Billroth)
  cords of diffuse parenchyma surrounded by blood
  capillary sinuses.
• Sinuses are completely lined by fenestrated
  endothelial cells.
• Spent rbc's and debris are removed here by
  macrophages.