Thymus and Tonsil

Learning Objectives:

At the end of this lecture you will be able to identify the:

1. Histological features of thymus

2. The function of thymus

3. Locations of tonsils and

4. The histological features and functions of tonsils

THYMUS AND TONSIL

LECTURE OUTLINE

Thymus and Tonsil:

• These are different structures in the body mainly composed of lymph cells and reticular cells. • Lymphatic system gives organism immunity against injury by foreign substance or an organism like bacteria, viruses and parasites.

Thymus:

• It is the lymph organ situated in the thorax just behind the sternum. • It grows rapidly in size until the age of 2nd year then the rate of growth decreases till puberty. • Thymus then begins to involutes or decrease in size until old age.

Histological Features:

• It is highly lobulated organ covered by connective tissue capsule. • From the capsule septa penetrate the substance of the gland, forming lobes. • Each lobe is composed of two distinct regions cortex and medulla. • Cortex contains densely packed lymphocytes but no distinct nodule is found. • The dominant cells in the cortex are T-lymphocytes, in the outer cortex they are of large size while in the deeper cortex they are of small size. • In the medulla the lymphocytes enter in blood vessels. • They remain either in general circulation or populate I other lymphoid organs. • Medulla contains lymphocytes, epithelial reticular cells and a structure called Hassal’s corpuscle.

Epithelial Reticular Cells:

• The framework of thymus is made up of epithelial reticular cells. • These cells are derived from endoderm and they are not associated with fibers. • These cells are stellate in shape and their processes are joined together by desmosomes. • Epithelial reticular cells also form the perivascular sheath and Hassal’s corpuscles.

Blood Thymus Barrier:

• It is the barrier between blood and thymic lymphocytes so that macromolecules can not pass to the thymus. • It is formed by occluding junctions in the wall of capillaries and perivascular sheath of epithelial reticular cells. Functions of Thymus:

• Thymus produces hormones like thymosin and thymopioten. • T-lymphocytes are produced by thymus which are carried by blood to the other lymphoid organs.

Tonsil:

• These are masses of lymphoid tissue may be partially encapsulated as small organs. • There are three such masses which are paired structures, palatine, pharyngeal and lingual tonsil. • They all are located in the pharynx.

Palatine Tonsil:

• They are oval shaped bodies in oropharynx between glossopharyngeal and palatopharyngeal arches. • Free surface is covered by stratified squamous epithelium which is continuous with the epithelium of pharynx. • At various places on the surface of tonsil deep indentations or pockets occur which are known as crypts. • The crypts are 10~20 in number. • Several secondary crypts arise from the primary one. • Surrounding each crypt are lymphatic nodules as well as diffuse lymphatic tissue. • There is capsule of connective tissue over the attached or basal surface of lymphatic tissue. • Epithelium of the crypt is infiltrated with lymphocytes. • In tonsillitis other blood cells like neutrophil are also found in tonsillar tissue

Lingual Tonsil:

• These are spherical aggregation of lymphatic tissue present on the dorsal surface of tongue. • Wide mouth crypts are present which are lined by stratified squamous epithelium. • Ducts of some lingual glands frequently open in these crypts.

Pharyngeal Tonsil:

• These are lymphatic aggregation on the posterior wall of nasopharynx. • Lymphatic tissue is similar to that of palatine tonsil. • Surface is covered by pseudo-stratified ciliated columnar epithelium. • In children they are frequently enlarged and obstruct the nasal passages called Adenoids.