Newsletter No. 387

4 No. 387, 19.11.2011 洞 明 集 電 視上常有拿幼兒排便來做文章的廣告,為求推銷 商品,內容也許略帶誇張,但排洩問題事關身體健 康,不可輕視,何況是最需要呵護照顧的小孩兒。事實上, 嬰兒呱呱墮地後,如二十四小時內仍未能排出胎糞,可能 是個警號,情況持續一兩個星期的話,更可能是患有先天 性巨結腸症了。這是小兒常見的先天性毛病,因為腸道無 法正常蠕動,影響排便功能,引致功能性腸梗塞,必須馬 上進行外科手術來治療。據統計,每千個嬰兒便有一個患 有這個症狀,男比女多,大概是四比一。 中大生物醫學學院 陳活彝 教授解釋,先天性巨結腸症的 成因有多種,其中一個是腸道賴以傳遞信息的神經系統出 了問題。「這個情況在胚胎時已出現,」他說,「胚胎細胞 在發育期間是會移動及改變位置,那些將會發展成腸道內 神經系統的細胞,若無法到達應到的位置,或者雖然到達 了,但因環境不適合發展而失去功能,胚胎卻繼續發育的 話,嬰兒誕生後,腸道會因為欠缺正常運作的神經系統而 不會蠕動,無法排便,形成先天性巨結腸症。」 陳活彝教授是發育生物學專家,主力研究胚胎發展,尤其 是與先天性疾病的關係。他特別感興趣的是神經系統,因 仍有頗多未能破解的神秘之處。他解釋說,很多神經細胞 都來自中樞神經系統,所以極受這個系統的發育所影響。 胚胎時期的中樞神經系統仍未發育,屬原始狀態,呈管 狀,稱為神經管,隨着胚胎發展,身體各部分所需的神經 細胞便會離開神經管,並開始四出「歸位」。要出問題的 話,往往也就在這個時候。 如上文所說,其中一個問題是這些神經細胞無法到達應到 的位置。陳教授打了個很有趣的比喻:「有些細胞根本連 走出門口也不懂,這是基本形成的問題。有些出了門,但可 能迷了路,或者路上遇到意外,結果去了不應該去的地方, 例如本來是腸道的神經細胞卻去了心臟,即是移動上出了 岔子。」 身負重任的神經細胞 先天性巨結腸症之謎 The Burden of Nerve Cells Mysteries of Congenital Megacolon 即使神經細胞順利抵達目的地,也要面對環境是否適合發 展的問題,這固然和母體的情況有直接關係,但也會受母 體所處身的大環境所影響。舉例說,母體身處的環境若污 染不堪,細胞的發展難免桎梏重重,也有可能受到刺激而 突變,畸形發展。總而言之,胚胎的神經細胞若出問題,最 壞的情況是胎死腹中,而就算胚胎發育了,誕下的嬰兒也 可能會有相應的先天性毛病。 胚胎存於母體內,要取出來進行離體研究一點也不容易。 陳教授一直以小鼠做實驗,他成功利用一套技術,把小鼠 胚胎取出來作體外培育兩、三天。小鼠的孕期是十九天, 其中五、六天是器官形成期,其餘是生長和強化功能期, 關鍵就在這五、六天,體外培育的時間剛好足夠進行實驗 研究。 科學研究是漫長的過程,需要極大的耐性。陳教授表示, 現時雖然已邁出了一步,確定了腸道缺少所需的神經細 胞,或者神經細胞無法正常運作,會造成腸道問題,但並 不是說腸道有問題便一定歸咎神經細胞。他說,下一步是 找出兩者的關係,最終目的是研究出治療的方法,例如移 殖正常細胞,以彌補先天不足。他說:「發育生物學屬於基 本生物醫學研究,與醫學的目標一致,都是找出治療疾病 的方法,」陳教授說。「不同之處是醫生處理的是生命中的 各種現象,發育生物學研究者着眼的則是生命的開端。」 M any advertisements for baby products claim to improve your offspring’s bowel movement. Marketing hype aside, proper excretion is crucial for health, in particular, that of infants. In fact a newborn who does not excrete meconium within 24 hours of birth is cause for alarm. And if the situation persists for a week or two, the child is likely to be suffering from congenital megacolon, a condition in which the large intestine is blocked due to improper muscle movement in the bowel. Waste builds up behind the blockage requiries immediate surgery. The disorder affects one in a thousand infants and roughly four times more often in boys than in girls. Prof. Chan Wood-yee of the School of Biomedical Sciences at CUHK explained that congenital megacolon has different causes and one of them is problem with the nerves in a part of the bowel. ‘The condition already exists in the embryo,’ he said. ‘During growth, embryonic cells move to new locations. If cells that will turn into nerves in the bowel fail to reach their proper location or if they do but lose their function due to unfavourable growth conditions, the infant’s intestine will not be able to move due to the absence of nerves that push material through.’ Professor Chan is an expert in developmental biology, specializing in embryonic development, in particular its relation to congenital diseases. He takes a special interest in the nervous system which still has many unsolved mysteries. He observed that many nerve cells come from the central nervous system (CNS) and are heavily influenced by its development. The embryo’s undeveloped CNS takes the rudimentary form of a neural tube, from which eventually many nerve cells required by the body leave for their respective destinations. This is the time when problems arise if they do. One of them, as described above, is when nerve cells fail to reach their destinations. Professor Chan remarked, ‘Some cells can’t even leave home. This is a problem with formation. Some leave home and lose their way or get into an accident along the way and end up somewhere else, such as nerve cells in the bowel ending up in the heart. This is a problem with migration.’

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