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Vonnie D.C. Shields<sup>1</sup> \* and Thomas Heinbockel2 \*


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2 Department of Anatomy, Howard University College of Medicine, Washington, DC, USA

#### **References**


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**Author details**

14 Histology

Vonnie D.C. Shields<sup>1</sup>

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**Chapter 2**

**Provisional chapter**

**Epithelial Development Based on a Branching**

**Epithelial Development Based on a Branching** 

**Thymic Epithelium**

**Abstract**

epithelium

**1. Introduction**

**Thymic Epithelium**

Juan José Muñoz and Agustín G. Zapata

Juan José Muñoz and Agustín G. Zapata

http://dx.doi.org/10.5772/intechopen.81193

Additional information is available at the end of the chapter

Additional information is available at the end of the chapter

tions of common molecular signaling pathways.

**Morphogenesis Program: The Special Condition of**

**Morphogenesis Program: The Special Condition of** 

Numerous epithelia undergo tubulogenesis and branching morphogenesis during their development (i.e., lung, salivary gland, pancreas) in order to establish sufficient available surface for their proper functioning. The thymus is a primary lymphoid organ constituted by pharyngeal-derived epithelium necessary to produce immunocompetent lymphocytes whose mechanisms of development are not fully known. In the current chapter, we review histological, cellular, and molecular mechanisms governing early thymic epithelium development emphasizing its resemblance with the process of branching morphogenesis and tubulogenesis occurring in other epithelial organs in which epithelial-mesenchyme interactions determine the tissue patterning through specific combina-

**Keywords:** branching morphogenesis, epithelium, tubulogenesis, thymus, thymic

Many epithelia, particularly those derived from the gut, organize tubular structures (i.e., mammary gland, salivary glands, lungs, kidneys, pancreas) that repeatedly fold to reach an enlarged area necessary to perform their major functions (i.e., gas exchange, excretion, nutrient transport, etc.). Whereas a branching morphogenesis pattern of development is well established in the case of the respiratory system or in the exocrine glands, it appears to be less evident for other

endoderm-derived organs, such as the endocrine glands or the lymphoid organs.

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

© 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use,

distribution, and reproduction in any medium, provided the original work is properly cited.

DOI: 10.5772/intechopen.81193

#### **Epithelial Development Based on a Branching Morphogenesis Program: The Special Condition of Thymic Epithelium Epithelial Development Based on a Branching Morphogenesis Program: The Special Condition of Thymic Epithelium**

DOI: 10.5772/intechopen.81193

Juan José Muñoz and Agustín G. Zapata Juan José Muñoz and Agustín G. Zapata

Additional information is available at the end of the chapter Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/intechopen.81193

#### **Abstract**

Numerous epithelia undergo tubulogenesis and branching morphogenesis during their development (i.e., lung, salivary gland, pancreas) in order to establish sufficient available surface for their proper functioning. The thymus is a primary lymphoid organ constituted by pharyngeal-derived epithelium necessary to produce immunocompetent lymphocytes whose mechanisms of development are not fully known. In the current chapter, we review histological, cellular, and molecular mechanisms governing early thymic epithelium development emphasizing its resemblance with the process of branching morphogenesis and tubulogenesis occurring in other epithelial organs in which epithelial-mesenchyme interactions determine the tissue patterning through specific combinations of common molecular signaling pathways.

**Keywords:** branching morphogenesis, epithelium, tubulogenesis, thymus, thymic epithelium

## **1. Introduction**

Many epithelia, particularly those derived from the gut, organize tubular structures (i.e., mammary gland, salivary glands, lungs, kidneys, pancreas) that repeatedly fold to reach an enlarged area necessary to perform their major functions (i.e., gas exchange, excretion, nutrient transport, etc.). Whereas a branching morphogenesis pattern of development is well established in the case of the respiratory system or in the exocrine glands, it appears to be less evident for other endoderm-derived organs, such as the endocrine glands or the lymphoid organs.

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

In the present chapter, we will examine prior studies supporting the claim that the development of other branching organs as lung, salivary gland, pancreas, or kidney, despite their morphological and functional differences, follows common patterning programs under the control of epithelium-underlying mesenchyme interactions governed by a few families of molecules (FGF/FGFR, Wnt, BMP/TGFβ, Shh), and that the thymus, an epithelial primary lymphoid organ derived from the ventral endoderm of the third pharyngeal pouch, despite following the same pattern, constitutes a special case. Remarkably, its functions are not related to those of other epithelia of similar origin but rather to the establishment of a 3D epithelial network necessary for the functional maturation of thymocytes. Before acquiring their specific features, distinct epithelial organs, therefore, follow a common complex pattern of development which includes different processes. After a first step of specification from the original embryonic layer, they undergo a process of **tubulogenesis** consisting of outgrowth and extension of the epithelial primordium forming a tubular structure. A complex program of **branching morphogenesis helps to** increase the functional area of the organ. Finally, **terminal epithelial differentiation** prepares the primordium to become a functional organ.
