Lecture 26 Notes

Lecture 26

Derived from endoderm: airways and gut. Derived from mesoderm: blood vessels. Chapt. 22 p. 1272-1283

Lungs • Lungs are a system of branched tubes; the smallest terminate with tiny air-filled sacs – alveoli. Fig. 22-17. • Alvioli walls are formed with two kinds of cells: allowing gas exchange and secreting surfactant. Blood capillaries are within the alveoli walls. • Lining of alveoli has two kinds of cells: Type I alveolar cells: thin flat cells that allow gas exchange Type II alveolar cells secrete surfactant, which forms a film and reduces surface tension, so that alveoli won’t collapse • Larger airways are kept clean by respiratory epithelium Fig. 22-18. • It’s is formed by 4 types of cells: o Ciliated cells that move mucus with the cilia o Endocrine cells, that secrete serotonin and signal molecules, that regulate the rate of ciliary beating and mucus secretion o Basal cells (stem cells). • Cleaning the alveoli o There is no mucociliary escalator in alveoli o Macrophages roam the lungs and engulf foreign matter, kill and digest bacteria Squamous epithelium. o Upper end of the respiratory tract is covered by squamous epithelium o Squamous epithelium consists of many layers of flattened cells densely packed with keratin (like epidermis). o Cells in the squamous respiratory epithelium retain their nucleus and they are kept moist (differs from epidermis) o The boundary between mucous and squamous epithelium is abrupt

Gut • • •

Aggressive acid pH in the stomach, where food is digested is separated from neutral pH of small intestine, where nutrients are absorbed Small intestine lining consists of absorptive cells with microvilli, goblet cells (mucus secretion), enteroendocrine cells and Paneth cells (immune defense) Small intestine cells are replaced frequently – turnover time about 1 week.

Fig. 22-19. New cells are generated by stem cells, which are located at the bottom of crypts. They give rise to rapidly dividing cells, which move up and differentiate and also less frequently to Paneth cells, which move down to the bottom of crypts.

Liver • Functions of the liver Synthesis (proteins, cholesterol) Degradation (poisons, drugs, cholesterol) Storage (vitamins, cholesterol) Homeostasis (maintains balance in the body) of glucose, proteins, fat, cholesterol, vitamins o Defense system o o o o

• • o o o o o o

Liver allows an exchange between the gut and bloodsteam. Fig. 22-21 Liver can regenerate Hepatocytes can divide, even though they are fully differentiated cells Liver can grow back to the normal size if part of it is destroyed. Hepatocytes switch from resting state to proliferation. One of the important signals for that – hepatocytes growth factor When liver of a small dog is grafted into a large dog, it grows to the size appropriate to the large dog. If a liver of a large dog is crafted into a small dog, it shrinks. Shrinking is guided by a survival control: hepatocytes depend on signals from other cells for their survival. If liver is damaged by alcohol or hepatitis it scars and it’s ability to regenerate is compromised

Tissues that derive from mesoderm • • •

Cartilage, bone, fibrous tissue, including dermis Muscles and entire vascular system, including heart, blood vessels and blood cells Tubules, ducts and supporting tissues of kidneys and gonads

Blood vessels and endothelial cells • • • • • •

Endothelial cells line the blood vessels Fig. 22-22 They can divide and migrate, forming new blood vessels Pericytes are wrapped around small blood vessels; smooth muscles surround arteries and veins. Fig. 22-24 New blood vessel formation is called angiogenesis. Fig. 22-25 Capillary can form in vitro. Fig. 22-26. Example of blood vessels growth control in response to oxygen shortage. New blood vessels form, induced by vascular endothelial growth factor, in response to hypoxia inducible factor. Fig. 22-28.

Practice questions 1. 2. 3. 4. 5.

What is the mechanism that prevents alveoli from collapsing? What cells are inside the alveoli walls? What is the way to clean large airways? How are the small airways cleaned? What are the differences between squamous cells epithelium in lungs and epidermis? 6. Where do the stem cells locate in the lining of small intestine? 7. What is the purpose of rapidly dividing cells? 8. What kinds of differentiated cells can be found in the lining of small intestine? 9. Can hepatocytes divide? 10. Are hepatocytes stem cells? 11. Can liver regenerate? 12. Can endothelial cells divide? 13. What cell is wrapped around small blood vessels? 14. What is one of the possible signals for the capillary to invade the tissue?

Terms to know Alveoli Surfactant Ciliated cells Endocrine cells Basal cells mucociliary escalator respiratory epithelium Squamous epithelium Microvilli absorptive cells microvilli goblet cells enteroendocrine cells Paneth cells Homeostasis Hepatocytes Endothelial cells Pericytes Angiogenesis vascular endothelial growth factor hypoxia inducible factor