LYMPHOID TISSUE II

Histology 2016-2017
Department of Anatomy &Histology: Dr.Rajaa Ali
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LYMPHOID TISSUE II
Classifi cation of Lymphoid Tissue
Lymphoid tissue may be broadly classified into:
A. Diffuse lymphoid tissue
It is constituted by a layer of diffusely arranged lymphocytes and plasma cells deep to epithelium in lamina propria of digestive, respiratory and urogenital tracts forming an immunological barrier against invasion of microorganisms.
B. Dense lymphoid tissue
• It is characterised by the presence of a large number of lymphocytes (plus few macrophages and plasma cells) ar ranged
• in the form of nodules.
• These nodules are found either in association with mucous membranes of viscera or as discrete encapsulated organs.
1. MALT (Mucosa associated lymphoid tissue; nonencapsulated)
In some places of the above tracts the lymphocytes agg _ regate to form conspicuous dense lymphatic nodules or follicles
in the lamina propria or submucosa forming MALT.
• Solitary nodules
• Aggregated nodules (Peyer’s patches)
• Lymphoid nodules in vermiform appendix
• Waldeyer’s lymphoid ring at the entrance of pharynx
2. Discrete lymphoid organs (encapsulated)
These include the following organs:
• Thymus
• Lymph node
• Spleen
• Tonsil (palatine tonsil – Part of MALT)
General Architecture of Lymphoid Organs
Lymphoid organs consist of supporting framework (connective tissue) and parenchyma (lymphocytes).
The supporting framework includes the capsule that covers the organ at the periphery; trabeculae/interlobular septae which enter into the organ carrying blood vessels and nerves, and reticulum which supports the cellular parenchyma.
THYMUS
GENERAL FEATURES
Thymus is a central lymphoid organ. It is responsible for the development of immune system of body and is essential for
the growth and development of other lymphoid organs.
It is a bilobed organ, the lobes being unequal in size, present in the superior and anterior mediastinum of thorax.
It has dual origin. Its lymphocytes arise from mesoderm, whereas the epithelial reticular cells arise from endoderm of III
pharyngeal pouch. So it is called a ‘lympho-epithelial organ’.
Thymus is larger and well-developed in foetus and in early childhood. It attains its peak development at puberty and
thereafter it starts involuting and is replaced by fi bro-fatty tissue. Its weight is 12–15 gm at birth, 30–40 gm at puberty and
10–15 gm at 60 years.
It has only efferent and no afferent lymphatic vessels.