University:
Kentucky State University
Course:
BIO 111 | Principles of Biology
Academic year:

2019
Views:

240

Pages:

15
Author:

ENROBREPRAILA6p9

Chapter 20 The endoderm: Tubes and organs for digestion and respiration Endoderm The embryonic endoderm has several functions: 1- To induce the formation of several mesodermal organs. 2- To construct the lining of two tubes within the vertebrate body: digestive and respiratory tubes. • The region of the digestive tube anterior to the point where the respiratory tube branches off is called the pharynx. 3- To form the epithelium of several glands. The pharynx The anterior endodermal portion of the digestive and respiratory tubes begins in the pharynx. The digestive tube and its derivatives • The endodermal cells generate only the lining of the digestive tube and its glands • The mesenchymal cells from the splanchnic portion of the lateral plate mesoderm surround the tube and provide the future smooth muscles that generate peristaltic contractions. Specification of the gut tissue • The production of the endoderm is specified by the expression of Sox17 gene which encodes for the transcription factor Sox17. • Although Sox17 helps specify the digestive tube, it does not give the tube its polarity. What tells the endodermal tube to become particular tissues at particular places? • The polarity starts with the specifications of the pharynx followed by the specifications of the remainder of the gut tube. • Pharynx can develop only in the regions containing little or no retinoic acid. Specification of the gut tissue • The second phase of gut specifications is through signals from splanchnic mesoderm-derived mesenchyme surrounding the endodermal tube. • The initial “default” specification of the entire gut tube is thought to be anterior (stomach, esophagus). Wnt (posterior mesoderm) Instructed by RA and FGF gradients Cdx1 and Cdx2, Indian Hedgehog (gut endoderm) Cdx1 and Cdx2 (high) Large intestine Cdx1 and Cdx2 (low) Small intestine Specification of the gut tissue • In the anterior portion of the tube (which forms the thymus, pancreas, stomach and liver), Wnt signaling is blocked. Soluble frizzeled related proteins sFRP1 and sFRP2 -5 Specification of the gut tissue • The regional differentiation of the mesoderm (into smooth muscle types) and regional differentiation of the endoderm (into different functional units such as the stomach, duodenum, and small intestine) are synchronized. -5 Pyloric Specification of the gut tissue • The dorsal mesentery: is a fibrous membrane that connects the endoderm to the body wall. • The looping of the intestinal tube is driven by a combination of two factors: the intrinsic growth of the intestinal endoderm and the connection of the intestinal tube to the dorsal mesentery. Accessory organs: The liver, pancreas, and gallbladder • The endoderm that forms the hepatic diverticulum originates from two populations of cells: 1. Lateral endoderm cells that exclusively form liver cells. 2. Ventral-medial endoderm cells that form several midgut regions, including the liver. • A branch from the drainage duct of the liver produces the gallbladder. • As the dorsal and ventral pancreatic buds grow, they come close to each other and eventually fuse to form the pancreas. The respiratory tube • The respiratory diverticulum (laryngotracheal groove) extends ventrally from the center of the pharyngeal floor, between the fourth pair of pharyngeal pouches. • Tracheal- esophageal fistula: the separation between the gut tube and respiratory tube is not completed and the baby is born with a connection between the two tubes. The respiratory tube • In the absence of Barx1, Wnt signals from the mesenchyme cause β- catenin to accumulate in the region of the gut tube that will become the lungs and trachea. • This results in the expression of NKx2-1 transcription factors in this region and the differentiation of the ciliated respiratory epithelium of the trachea. β- catenin NKx2-1 transcription factors The respiratory tube • Soluble Frizzled- related proteins (sFRPs) binds to Wnt proteins and prevent them from reaching to their cell membrane receptors. • This blocks the Wnt activity and helps to specify the squamous esophageal epithelium. The respiratory tube • In the developing mammals, the respiratory epithelium grows straight, forming the trachea in the neck region. • However, after entering the thorax, the respiratory epithelium branches, forming the two bronchi and then the two lungs. • Under the influence of the lung mesenchyme, the right bronchus proliferated and branched, whereas under the influence of tracheal mesenchyme the left bronchus continued to grow in an unbranched manner The respiratory tube • The lung must be able to draw in oxygen at the first breath of the new born infant. • Surfactant: a secretion of specific proteins and phospholipids such as sphingomyelin and lecithin produced by the type II alveolar cells of the lungs very late in gestation. The surfactant enables the alveolar cells to touch one another without sticking together. • The embryonic lung may signal the mother to start delivery.

Chapter 20 The Endoderm Tubes and Organs for Digestionand Respiration

Related Documents

Recommended Documents

Description

Related Documents

Free up your schedule!

Take 5 seconds to unlock