版權：Osmosis.org/learn/Ectoderm
審稿：Tanner Marshall, MS, Tanner Marshall, MS, Yifan Xiao M.D., Fergus Baird, Kyle Slinn, RN, BScN, MEd, Yifan Xiao M.D.
翻譯：Sherry 校對：daisy945
Hi各位新夥伴老夥伴，Osmosis是一個專業的醫學學習平臺，我們不僅僅發布COVID-19相關的知識哦，
最近小編想整理一個「生命之初」系列，詳細講講胚胎學最起頭兒的那些事兒。希望大家能喜歡！
When the embryo is a week old, it has two layers of cells: a dorsal epiblast layer and a ventral hypoblast layer.
當胚胎一周大時，它有兩層細胞：背側的上胚層細胞和腹側的下胚層細胞。
During week 3 of development, the embryo undergoes gastrulation where the cells in the epiblast layer form a three-layered trilaminar disc with ectoderm, mesoderm and endoderm layer.
So, imagine the embryo is like a strawberry birthday cake with the ectoderm as the candles, the mesoderm as the lime frosting, and the delicious sponge cake as the endoderm.
We can even put three candles on this cake to help you remember that gastrulation happens during week 3.
在發育的第3周中，胚胎開始發育管腔，原腸胚形成，其中的上皮胚層細胞形成了外胚層，中胚層和內胚層。
因此，想像胚胎就像草莓蛋糕，外胚層像蠟燭，中胚層像奶油糖霜，內胚層像美味的海綿蛋糕。
我們甚至可以在這塊蛋糕上放三支蠟燭，以幫助您記住在第3周開始原腸胚形成。
During gastrulation, some mesodermal cells start to differentiate and they form a structure called the notochord, a rod of cells that release different genetic transcription factors that help embryonic cells develop into the body's various organs and structures.
The notochord also kickstarts a process called neurulation, stimulating the cells in the nearby ectoderm layer to thicken and form a layer of cells called the neural plate.
As it forms, the neural plate starts to fold, and it dips down to form a neural groove with edges called neural folds.
在原腸胚形成的過程中，一些中胚層細胞開始分化並形成一種叫做脊索的結構，脊索細胞會分泌各種各樣的轉錄因子，從而幫助胚胎細胞發育成人體的各種器官和結構。
脊索還開啟了神經系統形成的過程，它刺激附近的外胚層細胞增厚並形成一層稱為神經板的細胞。
神經板逐漸開始摺疊，然後向下傾斜形成有邊緣的神經溝，被稱為神經褶。
As the groove continues to deepen, ventral to the ectoderm layer, the neural folds come together and pinch off from the surface of the ectoderm layer, forming the neural tube.
The neural tube now sits between the mesoderm and the ectoderm.
On the dorsal side of the neural tube where the neural folds fuse, there are special cells called neural crest cells that migrate out and form a new layer of cells between the ectoderm and neural tube.
隨著神經溝的凹陷，在外胚層的腹側，兩側的神經褶皺逐漸聚合併向外胚層的表面擠壓，形成神經管。此時的神經管是位於中胚層和外胚層之間。在神經管背側神經褶聚合的地方，有些細胞遷移出來並在外胚層和神經管之間形成新的細胞層，這些特殊的細胞稱為神經嵴細胞。
Neural crest cells are like little explorers: they migrate throughout the developing fetus to form a variety of tissues including the peripheral nervous system, melanocytes in the skin, specific parts of the facial bones, chromaffin cells of the adrenal glands, and parafollicular C cells in the thyroid.
神經嵴細胞就如探險家一樣：它們在整個發育中的胎兒中遷移形成各種組織，包括周圍神經系統，皮膚黑色素細胞，面部的特定骨頭，腎上腺的嗜鉻細胞和甲狀腺的濾泡旁細胞又稱C細胞。（小編註：新版FA613頁提及C細胞來源於內胚層，所以該處信息有誤）
In fact, neural crest cells are responsible for so many of the body's organs and tissues that you might think of them as the body’s unofficial fourth germ layer.
實際上，神經嵴細胞負責人體的許多器官和組織，所以可以被認為是人體第四胚層。
At this point, like any hollow tube, the neural tube still has openings at both ends: a large opening at the top end called the cranial neuropore, and a smaller opening at the bottom end called the caudal neuropore.
The cranial neuropore seals up around day 25, while the caudal neuropore seals up a few days later, around day 28.
Once the cranial and caudal neuropores have closed, the neural tube is fully formed.
接下來介紹神經管，就像任何中空管有兩個開口一樣：神經管在頂端的大開口稱為顱神經孔，而在底端的較小開口稱為尾神經孔。
顱神經孔大概在胚胎發育25天時閉鎖，而尾神經孔則會晚幾天，大概在28天時閉鎖。
一旦顱神經孔和尾神經孔關閉，神經管就完全徹底地形成了。
Meanwhile, ectoderm (known as surface ectoderm now that the neural tube has closed) starts to form the ears and eyes.
Two bumps called the otic placode and the lens placode form near the cranial end of the ectoderm.
Over time, the otic placode will form otic vesicles, which develop into the cochlea and the inner ear.
At the same time, the lens placode develops into the lens and cornea of the eyes.
Finally, many of the ectoderm cells go on to form sensory epitheliums like those in the nose and mouth.
Others will go on to form the epidermis layer of the fetus』 skin, as well as structures associated with the skin like hair, nail, sweat glands, and mammary glands.
同時，外胚層開始形成耳朵和眼睛。
在外胚層的顱骨末端附近形成兩個隆起，稱為耳突斑和晶狀體斑。
隨著時間的推移，耳道斑將形成耳道囊泡，並最終發展成耳蝸和內耳。
同時，晶狀體斑塊發展成眼睛的晶狀體和角膜。
最後，許多外胚層細胞繼續形成感覺上皮，也包括鼻子和嘴中的那些感覺上皮。
其餘部分將繼續形成胎兒皮膚的表皮層，以及與皮膚相關的結構，如頭髮，指甲，汗腺和乳腺。
Here’s a story to help you remember the important structures derived from the ectoderm:
a man touches the ectoderm candle on the strawberry cake.
His skin (epidermis) and nails get burned and turn black (melanocytes), and he feels a sharp pain (in his central and peripheral nervous system).
He pulls his burning finger back, but pokes himself in the face, setting his head on fire.
His eyes, ears, nose, hair, and even his teeth get set on fire!
In a last ditch effort to put the fire out, his body starts sweating and lactating (sweat and mammary glands) and even tearing up (pitui-「teary」 gland).
這有一個故事可以幫助記憶外胚層衍生的重要結構：一個人觸摸草莓蛋糕上的外胚層蠟燭。
他的皮膚（表皮）和指甲被燒傷並變黑（黑素細胞），並且他感到劇烈的疼痛（中樞和周圍神經系統）。
他向後撤回燃燒的手指，但不小心戳到了自己的臉，接著他的頭部開始著火。
他的眼睛，耳朵，鼻子，頭髮，甚至牙齒都著火了！
最後為了撲滅大火，他的身體開始出汗，哺乳（汗腺和乳腺），甚至流淚（垂體）。
All right, as a quick recap… The ectoderm is the dorsal most germ layer, and through a process called neurulation, these cells form the neural tube.
From the neural tube, neural crest cells migrate to help form the central nervous system, the peripheral nervous system, the sensory epithelium of the ears, nose, eyes, and mouth, as well as glands such as sweat glands, mammary glands, and the pituitary gland.
結束時讓我們快速回顧下……
外胚層來源於最靠背側的胚胎層，通過神經發育，形成了神經管。
神經嵴細胞從神經管遷移幫助形成中樞神經系統，周圍神經系統，耳朵，鼻子，眼睛和嘴巴的感覺上皮以及汗腺，乳腺和垂體等。
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