Tonsils:
Palatine Tonsils:are paired structures lie in the oropharynx between 2 arches (the palatoglossous & palatopharyngeal arch).
they are surrounded by a connective tissue capsule which separate this lymphatic aggregation from the remaining parts of the oropharynx, connective tissue trabeculae extend from the capsule to the inside of the tonsils in between the lymphatic aggregation called internodular septa.
covered by the same epithelium that lines this part of the pharynx which is non keratinized stratified squamous epithelium.
histological structure is formed from lymphatic tissue either diffuse form or arranged in the form of nodules.
The surface of the tonsils shows indentations or downward depressions lined by the same surface epithelium called "crypts" (primary crypts) which give rise to secondary crypts, the lymphatic nodules distributed regularly along these crypts, lymphocytes sometimes migrate to lie inside the cavity of the crypts & surrounded by saliva forming salivary corpuscles.
mucus alveoli in the oropharynx produce mucous secretions pass through their ducts which open on the surface epithelium of palatine tonsils & not in the bottom of the crypts so that food particles and bacteria are not washed continuously leading infection in tonsils (tonsilitis).
Function of palatine tonsils:
Responsible for immunity of the body because they contain B&T- lymphocytes thus they are important especially in children, but if recurrent tonsillitis occurs then tonsilectomy is indicated.
Pharyngeal Tonsil:
single structure lies in the nasopharynx.
covered by pseudosratified ciliated columnar epithelium with goblet cells similar to that of the respiratory system.
It is formed of randomly distributed lymphatic tissue with no crypts.
It may enlarge & interferes with the respiration so called "adenoid", removal of this adenoid is indicated.
connective tissue capsule is thin or absent.
mucus & serous alveoli are present around the tonsil.
Lingual Tonsil:
Several small aggregations of lymphocytes.
Lie on the back of the tongue between the circumvallate papillae behind the V- shaped line which divides the tongue into anterior 2/3 & posterior 1/3.
Lined by non- keratinized stratified squamous epithelium.
Contain deep crypts.
They are rarely infected because the duct of the mucus alveoli open in the bottom of the crypt so continuous washing of the crypt occur.
pharyngeal tonsilpalatine tonsil1- single1- pair2- lie in the nasopharynx2- lie in the oropharynx3-covered by pseudosratified ciliated columnar epithelium3-lined by stratified squamous non-keratinized4- no crypts4- 10-20 crypts5- thin capsule or absent5- thick capsule
Thymus:
(Thyme: is a plant, the thymus is similar in shape to the wings of that plant).is lymphoepithelial primary central organs of lymphoid tissue lies in the superior mediastinum & may extends down to the anterior mediastinum anterior to the pericardium & great vessels descending inferiorly as far as the 4th costal cartilage. Its upper ends extend to the lower end of the thyroid gland. In children it is pinkish grey in color, soft & finely lobulated consist of 2 pyramidal lobes connected by areolar tissue. The thymus varies in size with age, at birth it weighs (10-15gram) continues to grow to the age of puberty when it become (30-40g) after mid adult life it's weight is (10g) then it undergo gradual atrophy, decrease in size & replaced by fatty tissue. Emberyologically It is derived from the 3rd & 4th pharyngeal pouches.
Histological structure:
It is formed of 2 lobes, each lobe is surrounded by a delicate fibrous capsule from which septa penetrate each lobe dividing it into lobules (each of 0.5-2 millimeters in diameter), each lobule consist of darkly stained densely cellular cortex & light stained less cellular medulla.the histological structure of the thymus consists of 3 components:
Epithelial reticular cells: which areEndodermal in origin, it differ from the reticular cells of organs which are mesenchymal in origin.
not phagocytic & not associated with reticular fibers as reticular cells of other lymphatic organs.
stellate shape cells having numerous processes linked with the processes of other cells by desmosomes forming a meshwork lies in the cortex & medulla, covers the C.T trabeculae, lines the capsule & surround the blood vessel.
their cytoplasm contains lysosomes, vacuoles & dense secretory granules which secrete "Thymic hormone" which is important in lymphopoiesis (i.e. it stimulate the development of other lymphatic organs).
the reticular cells are separated from the capsule & vascular elements by the blood-thymic barrier which separate the blood inside the blood vessels from the developing T- lymphocytes, this barrier is important because the T-lymphocytes must develop in a germ-free medium thus the barrier is impermeable to certain circulating antigens but it allows nutrients, metabolites, & stem cells which are derived from the bone marrow to enter the thymus & the lymphocyte to enter & leave the thymus.
The barrier is formed from:
1-non fenestrated endothelium of the capillaries.
2-prominent basement membrane.
3-epithelial framework of the reticular cells.
There is no blood-thymic barrier in the medulla & the thymus has only efferent & no afferent lymphatic vessels thus it does not constitute a filter for the lymph.
Lymphocytes:
they lie between the epithelial reticular cells, they are mainly T-lymphocytes migrate from the bone marrow &enter thymus to continue their maturation then leave the thymus & circulate in the blood to reach other lymphatic organs where they accumulate in specific sites for months or years & share in the immunity process of the body. Other potential immature T-lymphocytes remain for a certain time 3-5 days then undergo degeneration & phagocytosed by macrophages.
3-Hassal's corpuscles ( thymic corpuscles ):
are spherical lamellated structures formed of central acidophilic hyalanized area surrounded by multiple layers of epithelial reticular cells, lie in the medulla of each thymic lobule, function of Hassal's corpuscle is unknown, but suggested to be phagocytosis of the gama globulins & antigens.
How Hassal's corpuscles are formed?
In the fetal life ,the medullary epithelial cells enlarge & become intensly acidophilic then undergo degeneration i.e. vaculation of their cytoplasm with fragmentation of their nuclei & nucleoli which are engulfed by the vacuoles in the cytoplasm, further epithelial cells enlarge & come to surround the central hyalanized cells then macrophages of the cortex migrate between the degenerated epithelial reticular cells & reach the center where they undergo hyalinization & solubilization forming spherical structures about 30-100 micrometer in diameter but they increase in size during involution of the thymus (i.e. after puberty).Functions of thymus:
It is important in immunity.
Secretion of (THF) which stimulates the development of other lymphatic organs.
T-lymphocyte continue their processing & maturation in the thymus during embryonic life & for few months after birth thus removal of thymus just after birth is dangerous& lead to death.