✅ EMBRIOLOGÍA del SISTEMA INMUNITARIO 🤜💥🦠

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In this section, the speaker introduces the topic of the embryology of the immune system and discusses the primary function of the immune system as body defense. The speaker also mentions that the immune system is composed of various structures and organs linked to lymphatic vessels and blood vessels.

Development of Lymphatic System

• Lymphatic vessels are unidirectional ducts through which lymph circulates, composed of plasma and lymphocytes. They develop as saccular invaginations from the endothelium of veins.

• Six primary lymphatic sacs are formed: 2 jugular, 2 iliac, 1 retroperitoneal, and a cistern of chyle dorsal to the region of the retroperitoneal sac.

• The right and left thoracic ducts join the jugular sacs with the chyle cistern and establish an anastomosis between these ducts.

• Numerous conduits connect the sacs to each other and drain lymph from different parts of the body.

• The thoracic duct is formed from the distal portion of the right thoracic duct and cranial portion of the left thoracic duct. The right lymphatic duct derives from the cranial segment of the right thoracic duct.

• Both ducts retain their connections with the venous system and drain at confluence with internal jugular vein and subclavian vein.

Formation of Lymph Nodes

• Lymph nodes start forming from around third month gestation in both lymphatic sacs and along developing lymphatic vessels.

• Mesodermal cells invade walls of sacs/vessels, developing capsule, trabeculae or connective partitions, and reticular fiber mesh in ganglia.

• Lymphatic sinuses are interconnected spaces between lymphatic tissue and connective tissue capsules/septa where lymph circulates.

• Lymphocytes from bone marrow (B lymphocytes) and thymus (T lymphocytes) are sustained in the mesh of reticular fibers.

• Part of the lymphocytes begins to be produced in the lymph nodes themselves, contributing to their hemopoietic function.

• Lymph nodes in the wall of hollow organs of respiratory, digestive, urinary, and genital systems form mucosal-associated lymphatic tissue (MALT).

• MALT is mostly composed of lymph nodes. Examples include pharyngeal tonsil and tubal tonsils in the respiratory system, palatine tonsils and Peyer's patches in the digestive system.

Development of Tonsils

• Pharyngeal tonsils or adenoids develop from mesoderm of roof of nasopharynx where a mesh of reticular fibers develops for nesting and proliferation of lymphocytes from bone marrow and thymus.

• Tubal tonsils develop from mesoderm surrounding first pair of pharyngeal pouches and form aggregations around pharyngeal openings of pharyngotympanic tubes or eustachian tubes.

• Lingual tonsil develops in mesoderm at root of tongue similar to pharyngeal tonsil.

• Palatine tonsils develop from endoderm of second pair of pharyngeal pouches and nearby mesenchyme.

For more information on embryology related to head and neck, refer to the second part video mentioned by the speaker.

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In this section, the speaker discusses how some parts of the immune system reside in the walls of hollow organs such as respiratory, digestive, urinary, and genital systems. This sector is known as mucosal-associated lymphatic tissue (MALT), which includes various types of lymph nodes.

Mucosal-associated Lymphatic Tissue (MALT)

• MALT is an important sector of the immune system that resides in the walls of hollow organs.

• Most of MALT occurs in the form of lymph nodes.

• In the respiratory system, nodules are located mainly in the pharyngeal tonsil and tubal tonsils.

• In the digestive system, they are found in palatine tonsils, lingual tonsil, and Peyer's patches of the small intestine.

• Lymph nodes associated with these tissues are compact and numerous.

The speaker mentions that more information on this topic can be found in the second part video about embryology of head and neck.

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This section discusses the development and structure of the mesogastrium or dorsal mesentery.

Mesogastrium Development

• The mesogastrium is a group of mesodermal cells that evolves in a manner similar to lymph node buds but around blood vessels.

• In later development, cells generate the capsule, connective septa, and mesh of reticular fibers where the red pulp and white pulp of the spleen are held.

• The white pulp is colonized by lymphocytes from the bone marrow and thymus, which reproduce in the white pulp itself throughout an individual's life.

• The red pulp between the third and sixth month of gestation generates a part of the erythrocytes, leukocytes, and megakaryocytes of the fetus.

Thymus Development

• The third pharyngeal bag gives rise to the thymus and lower parathyroid glands.

• Thymic primordia appear as endodermal proliferations of the third pharyngeal bags at around four weeks.

• These proliferations form hollow tubes that transform into solid branching cords invading underlying mesenchyme, which become primordia of thymic lobules.

• Between four and seven weeks, these primordia lose their connections with the pharynx and migrate caudally and medially to their final location inferior and ventral to developing thyroid and dorsal to sternum.

• They join together through connective tissue to form a single bilobed thymic gland that quickly infiltrates neural crest cells to form capsules and septa.

• During the third month, lymphocytes and dendritic cells infiltrate the thymus, and Hassall's corpuscles are believed to arise from ectodermal cells of the third pharyngeal cleft.

• The thymus is very active during the perinatal period, continues to grow during childhood, and reaches its maximum size at puberty. After puberty, it rapidly regresses and is represented by adipose vestiges in adults.

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Conclusion

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