口や鼻から吸い込んだ空気は気管を通って、その先の気管支（名前が変わっただけ）に入り、気管支の枝分かれを経て、肺胞に到達します。気管は体の中心にあるまっすぐな部分で、分岐した先からは「気管支」と呼ばれるようになるのだそうです。気管支の分岐の数は一番多いところで２３回にもおよぶそう。そして、先端部にある肺胞の数は左右の肺をあわせると10億個に上るのだそうです。下のNHKのリンク先では動画で、は、気管内視鏡が気管支の中に入っていく様子が示されています。さすがNHKが子供向けにつくった動画だけあって、とてもわかりやすい。
- ヒトの呼吸器のしくみ NHK for School
下のYOUTUBEの動画でも、気管内視鏡で気管支の内部を検査している様子が映し出されています。模式図などで気管支が枝わかれしているのは頭では理解できますが、こうやって内視鏡を中にいれて実際に分岐部に遭遇すると枝分かれが「実感」できます。
【聖マリアンナ医科大学病院】気管支鏡検査と肺癌診療の進歩 呼吸器内科森川 慶 聖マリアンナ医科大学病院 メディカルサポートセンター チャンネル登録者数 258人
声帯
声帯腫れテスト：声帯損傷を簡単にチェック Fauquier ENT チャンネル登録者数 61.8万人
腸管から気管が分化するときのシグナル
肺は消化管から芽を出すことで作られます。
Fig. 1. Overview of the stages of lung development. Lung endoderm specification begins at ∼E9.0 on the ventral side of the anterior foregut endoderm (yellow) where initiation of Nkx2.1 expression commences. https://europepmc.org/article/pmc/pmc3899811
気管の分岐の分子シグナル
Specification and early development of the lung endoderm. (A) The lung endoderm (marked by Nkx2.1 expression, blue) is first specified on the ventral side of the anterior foregut at E9.0. Wnt2/2b and Bmp4 signaling (indicated in orange???) from the surrounding mesoderm is required for this specification and for patterning of the anterior foregut in a ventral-dorsal manner. Wnt2 and Wnt2b signal via β-catenin to promote the expression of Nkx2.1 whereas Bmp4 represses the inhibitory action of Sox2 on Nkx2.1 expression. (B) During lung development, Fgf10 is essential for branching morphogenesis and is expressed in the mesenchyme surrounding developing branch points (Fgf10 expression indicated in green). This expression is restricted by epithelial expression of genes such Shh and Bmp4 (purple). Fgf10 also directs the orientation of epithelial cell division through regulation of Ras/Sprouty, setting up the appropriate direction of branch growth. https://europepmc.org/article/pmc/pmc3899811
FGF-Regulated ETV Transcription Factors Control FGF-SHH Feedback Loop in Lung Branching Developmental Cell Volume 35, Issue 3, 9 November 2015, Pages 322-332 Journal home page for Developmental Cell Article https://www.sciencedirect.com/science/article/pii/S153458071500653X
Figure 1. Epithelial-mesenchymal cross-talk and governing signalling during early development and branching morphogenesis of lung. Factors are represented only at sites where the expression is most abundant. Fibroblast growth factor-10 (FGF-10) is highly expressed in the distal mesenchyme and acts as a chemotactic focus for the epithelium during lung budding. FGF-10 also regulates Sox9 expression in the distal epithelial progenitors and induces bone morphogenetic protein (BMP)-4 expression. Sox2 expression in the proximal epithelium is under regulation of histone deacetylases 1/2 (HDAC1/2) signalling. FGF-10 expression in the mesenchyme is regulated by Wnt/β-catenin signalling (red arrow). A high concentration of BMP-4 signal also serves to locally inhibit endoderm proliferation, thereby inducing the lateral outgrowth of new airway branches. Sonic hedgehog (Shh) at the distal tips functions to downregulate FGF-10 expression in the mesenchyme, which limits local budding. Transforming growth factor-β (TGF-β) signalling also prevents local budding, by decreasing endodermal proliferation and by stimulating synthesis of matrix components at branch points. Solid arrows indicate sources from and influences on cells/molecules; dotted arrow indicates direction of patterning.
https://www.mdpi.com/1422-0067/17/1/128
FIGURE 1. A Wnt7b-Fgf10 epithelial-mesenchymal crosstalk maintains distal epithelial progenitors during lung development and becomes reactivated in the adult lung to regenerate injured airway epithelium. (A) During the branching stage of lung development, Fgf10 is expressed by mesenchymal progenitor cells, which depends on Wnt/β-catenin signaling, and acts on the distal epithelium to induce Bmp4 and Sox9 expression to keep them in an undifferentiated state. As the epithelial tube grows toward the Fgf10 source, Sox9 + progenitors acquire more proximal positions, switch on Sox2 expression and acquire bronchial epithelial fate. Simultaneously, distal Fgf10-expressing airway smooth muscle (ASMC) progenitors encounter epithelial Bmp4 and Shh (not shown) causing them to stop expressing Fgf10 and differentiate into mature ASMCs as they relocate proximally.
Fgf10 Signaling in Lung Development, Homeostasis, Disease, and Repair After Injury MINI REVIEW article Front. Genet., 25 September 2018 Sec. Stem Cell Research Volume 9 – 2018 | https://doi.org/10.3389/fgene.2018.00418 https://www.frontiersin.org/journals/genetics/articles/10.3389/fgene.2018.00418/full
https://link.springer.com/referenceworkentry/10.1007/3-540-29623-9_3330
