The Thymus and T Lymphocyte Development

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Questions and Answers

What is the primary function of the thymus?

Supply of B lymphocytes

Supply of T lymphocytes (correct)

Production of antibodies

Storage of erythrocytes

From which embryonic structure does the thymus develop?

Mesodermal tissue of the superior mediastinum

The endoderm of the third pair of pharyngeal pouches (correct)

The first pair of pharyngeal pouches

The ectoderm of the facial region

What type of cells do thymic epithelial cells differentiate into?

Lymphoblasts

Thymic epithelial cells (correct)

Bone marrow stromal cells

Hematopoietic stem cells

Where is the thymus located in the body?

In the superior mediastinum Signup and view all the answers

What happens to lymphocyte precursors in the thymus?

They mature into immunocompetent T lymphocytes Signup and view all the answers

What is the primary function of the thymus in the immune system?

To educate T cells Signup and view all the answers

What happens to T cells that fail thymic cell education?

They are phagocytosed Signup and view all the answers

What is the significance of positive selection in T cell development?

Survival based on recognition of foreign antigens Signup and view all the answers

What structure serves to prevent blood-borne antigens from entering the thymus?

Blood-thymus barrier Signup and view all the answers

Which of the following statements about lymph nodes is true?

They are involved in filtering lymph Signup and view all the answers

Where do lymphocyte precursors come from during fetal life?

Liver Signup and view all the answers

What do T cells retain to become helper T lymphocytes?

CD4 Signup and view all the answers

What is the size range for lymph nodes?

1 mm to 20 mm Signup and view all the answers

What type of selection tests T cells for potential autoimmunity?

Negative selection Signup and view all the answers

Which part of a lymph node contains lymphatic nodules?

Cortex Signup and view all the answers

What is the primary function of type VI thymic epithelial cells in the thymic medulla?

Forming thymic corpuscles and producing cytokines Signup and view all the answers

What distinguishes the wall of the cortical capillaries in the blood-thymus barrier?

Thick basal lamina with numerous pericytes Signup and view all the answers

Which component of the blood-thymus barrier is responsible for phagocytosing antigenic molecules?

Macrophage-rich perivascular connective tissue Signup and view all the answers

What is the diameter range of the thymic corpuscles formed by type VI TECs?

30 to 100 µm Signup and view all the answers

What is a notable structural feature of the type I thymic epithelial cells in the blood-thymus barrier?

Continuous layer bound by occluding junctions Signup and view all the answers

What role do cytokines produced by thymic corpuscles play in the thymus?

Supporting T cell education Signup and view all the answers

How do arteries penetrate the thymus to distribute blood supply?

Through the thymic capsule Signup and view all the answers

What kind of cells are found at the corticomedullary junction in the thymus?

Arterioles giving rise to fenestrated capillaries Signup and view all the answers

What is the primary purpose of the blood-thymus barrier?

Preventing T lymphocytes from contacting blood-borne antigens Signup and view all the answers

Which part of the thymus contains immature T lymphocytes densely packed?

Cortex Signup and view all the answers

Approximately what percentage of thymocytes become immunocompetent after education?

5% Signup and view all the answers

Which type of thymic epithelial cell is primarily responsible for forming the blood-thymus barrier?

Type I Signup and view all the answers

What occurs to the thymus during adulthood?

It undergoes involution Signup and view all the answers

Which type of thymic epithelial cell is known as thymic nurse cells?

Type II Signup and view all the answers

Which of the following thymic epithelial cells has an unknown function?

Type VI Signup and view all the answers

During which life stage does the thymus actively grow and perform its functions?

Childhood and puberty Signup and view all the answers

Which structure in the thymus isolates the cortex from the medulla?

Thymic epithelial cells type III Signup and view all the answers

What role do macrophages play in the thymus?

Engulfing apoptotic cells Signup and view all the answers

Study Notes

The Thymus

The thymus is a primary lymphoid organ
It is responsible for the supply of T lymphocytes to the body
It provides a special microenvironment in which the precursors of the T cells develop, differentiate, and undergo the unique process of "T cell education"
The thymus is located in the superior mediastinum, consists of two lobes, and is enclosed within a capsule of dense irregular connective tissue
Knowledge of the development of the thymus is important to understand the histological structure and functions of this organ.
The thymus develops from the endoderm of the third pair of pharyngeal pouches.
The right and left pharyngeal pouches give rise to the superior mediastinum
Cells are called thymic epithelial cells (TECs), or epithelialoreticular cells.
Precursors (lymphoblasts) originating in the bone marrow reach and invade the developing thymus
These cells are destined to develop into immunocompetent T lymphocytes
T lymphocytes occupy the space between the thymic epithelial cells, and thus the thymus is converted into a lymphoepithelial organ
The thymus is well-developed before birth and continues to enlarge in childhood and puberty
Thymus activity declines and it undergoes a steady process of regression (involution) as a result of the involutionary process, the thymus gradually decreases in size and most of the lymphatic tissue is replaced by fatty tissue
The capsule of the thymus extends connective tissue septa (trabeculae) into the substance of each lobe and subdivides the thymus into a number of incomplete lobules
Each lobule consists of a cortex and medulla.
The cortex contains a very high population (about 95%) of thymocytes, which are immature T lymphocytes, greatly increasing the number of small lymphocytes
The medulla contains a much lesser number of T lymphocytes but lodges a large number of macrophages
The cortex of each thymic lobule is densely packed, immature thymocytes lie within a network called thymic epithelial cells
The peripheral region of the thymic cortex contains cells which are actually lymphoblasts that originate in the bone marrow, pass through the blood vessels to reach the thymus, proliferate, and give rise to the thymocytes
The thymic epithelial cells of the cortex have sheet-like cytoplasm and are joined by desmosomes, isolating the cortex
The type II TECs, also called thymic nurse cells, surround T cells and play a very important role in T cell education
The type III thymic epithelial cells are located at the junction of the cortex and medulla, these cells also possess cytoplasmic processes and are present at the corticomedullary junction
The type IV thymic epithelial cells are bound to each other and to type III TECs by occluding junctions, so that a barrier is formed
The type V thymic epithelial cells form the cytoreticulum of the medulla; these cells are joined by desmosomes and therefore stain much lighter than the lymphocytes
The type VI thymic epithelial cells form thymic corpuscles, which are isolated masses composed of densely packed type VI cells

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Description

This quiz covers the thymus, a primary lymphoid organ essential for T lymphocyte supply and education. Explore its developmental origins, histological structure, and the unique microenvironment it provides for T cell differentiation. Understanding the thymus is crucial for grasping its role in the immune system.