| MadSci Network: Anatomy |
Hi Paul!
The condition you are describing is not that uncommon in newborns, with some 1 in 5000 having some form of imperforate anus (slightly more common in males than females).
There are different grades of severity of the condition and it is sometimes associated with more complex syndromes, but that doesn't sound like your case.
I don't know what your embryology is like, but the condition started way back during the development of your son - the gut starts to form from about 4 weeks after fertilisation. The last bit of the hindgut at this early stage is a cavity called the cloaca.
Slightly later in development, this cloaca gets invaded by a sheet of cells that cut it in half dorsoventrally. This sheet of cells that divide the cloaca into the future rectum and urogential sinus is called the urorectal septum.
By week 7, the urorectal septum fuses with the cloacal membrane covering the future bottom bit of the embryo, dividing where the anal opening will be from where the urogenital opening will be. The tissue around this fusion proliferates, therefore the anal membrane ends up as being slightly depressed. This dimple is called the proctodeum.
The anal membrane covering this dimple usually erupts at around the end of week 8, thereby giving an open anus.
Most imperforate anuses are caused by the abnormal development of the urorectal septum, such that there is an incomplete separation of the anorectal and urogenital portions of the cloaca.
The most common forms of malformation are known as High Anorectal Malformations, accounting for about two-thirds of cases. The most common is Anorectal Agenesis, where the rectum (bottom bit of the guts) ends somewhere inside the body cavity slightly above the anus. The exact term for the condition your son has will depend upon exactly where all the final bits of anatomy came to rest.
If you want to know more about the development of the urorectal septum, I suggest you hit some graduate texts on developmental biology. For most of the above I went back to an old University favourite of mine, 'The Developing Human' by Keith L Moore. I had the 4th edition but it's probably moved on since those days.
You can also do a search on 'urorectal septum' and come up with all sorts of stuff, mainly university lecture notes (I particularly liked this for the clear visual of separation of the cloaca).
Hope that's some use overall.
Happy New Year!
Adrian