Embryological origins of the mouth and anus
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This article needs to be clarified and may contain errors.
The embryological origin of the mouth
and anus
is an important characteristic, and forms the morphological basis for separating bilateria
n animals into two natural grouping
s: the protostome
s and deuterostome
s.
In animals at least as complex as an earthworm
, a dent forms in one side of the early, spheroidal
embryo
. This dent, the blastopore
, deepens to become the archenteron
, the first phase in the growth of the gut
.
In deuterostome
s (including humans), the original dent becomes the anus, while the gut eventually tunnels through the embryo until it reaches the other side, forming an opening that becomes the mouth.
It was originally thought that the blastopore of the protostome
s formed the mouth, and the anus was formed second when the gut tunneled through the embryo. More recent research has shown that our understanding of protostome mouth formation is somewhat less secure than we had thought. The edges of the dent appear to move together and close up in the middle, leaving openings at the ends which become the mouth and anus.
However, this idea has been challenged, because the Acoelomorpha
, which form a sister group to the rest of the bilaterian animals, have a single mouth which leads into a blind gut (with no anus). The genes employed in the embryonic construction of the flatworm mouth are the same as those expressed around the protostomes', which suggests that the two structures are equivalent, and that the older ideas about protostome
mouth formation were correct.
s, which are basal to protostomes and deuterostomes. The story is a little more complex, because the blastopore itself does not go on to become the mouth. However, flatworms, the sister group to the rest of the bilaterians, branched off from the ancestral stock before the bilaterian group originated, and have a mouth at one end and a blind gut. This suggests that the last common ancestor of bilaterians had a similar gut configuration, and that the anus evolved after the mouth.
Exactly how a through gut formed from this blind gut is somewhat harder to tell. The genetic mechanisms responsible for anus formation are quite variable, which might suggest that the anus evolved several times in different groups. Scientists are currently looking into this matter to generate a more complete picture.
The embryological origin of the mouth
Mouth
The mouth is the first portion of the alimentary canal that receives food andsaliva. The oral mucosa is the mucous membrane epithelium lining the inside of the mouth....
and anus
Anus
The anus is an opening at the opposite end of an animal's digestive tract from the mouth. Its function is to control the expulsion of feces, unwanted semi-solid matter produced during digestion, which, depending on the type of animal, may be one or more of: matter which the animal cannot digest,...
is an important characteristic, and forms the morphological basis for separating bilateria
Bilateria
The bilateria are all animals having a bilateral symmetry, i.e. they have a front and a back end, as well as an upside and downside. Radially symmetrical animals like jellyfish have a topside and downside, but no front and back...
n animals into two natural grouping
Clade
A clade is a group consisting of a species and all its descendants. In the terms of biological systematics, a clade is a single "branch" on the "tree of life". The idea that such a "natural group" of organisms should be grouped together and given a taxonomic name is central to biological...
s: the protostome
Protostome
Protostomia are a clade of animals. Together with the deuterostomes and a few smaller phyla, they make up the Bilateria, mostly comprising animals with bilateral symmetry and three germ layers...
s and deuterostome
Deuterostome
Deuterostomes are a superphylum of animals. They are a subtaxon of the Bilateria branch of the subregnum Eumetazoa, and are opposed to the protostomes...
s.
In animals at least as complex as an earthworm
Earthworm
Earthworm is the common name for the largest members of Oligochaeta in the phylum Annelida. In classical systems they were placed in the order Opisthopora, on the basis of the male pores opening posterior to the female pores, even though the internal male segments are anterior to the female...
, a dent forms in one side of the early, spheroidal
Sphere
A sphere is a perfectly round geometrical object in three-dimensional space, such as the shape of a round ball. Like a circle in two dimensions, a perfect sphere is completely symmetrical around its center, with all points on the surface lying the same distance r from the center point...
embryo
Embryo
An embryo is a multicellular diploid eukaryote in its earliest stage of development, from the time of first cell division until birth, hatching, or germination...
. This dent, the blastopore
Blastopore
A blastopore is an opening into the archenteron during the embryonic stages of an organism. The distinction between protostomes and deuterostomes is based on the direction in which the mouth develops in relation to the blastopore...
, deepens to become the archenteron
Archenteron
The primitive gut that forms during gastrulation in the developing blastula is known as the archenteron. It develops into the digestive tract of an animal....
, the first phase in the growth of the gut
Gut (zoology)
In zoology, the gut, also known as the alimentary canal or alimentary tract, is a tube by which bilaterian animals transfer food to the digestion organs. In large bilaterians the gut generally also has an exit, the anus, by which the animal disposes of solid wastes...
.
In deuterostome
Deuterostome
Deuterostomes are a superphylum of animals. They are a subtaxon of the Bilateria branch of the subregnum Eumetazoa, and are opposed to the protostomes...
s (including humans), the original dent becomes the anus, while the gut eventually tunnels through the embryo until it reaches the other side, forming an opening that becomes the mouth.
It was originally thought that the blastopore of the protostome
Protostome
Protostomia are a clade of animals. Together with the deuterostomes and a few smaller phyla, they make up the Bilateria, mostly comprising animals with bilateral symmetry and three germ layers...
s formed the mouth, and the anus was formed second when the gut tunneled through the embryo. More recent research has shown that our understanding of protostome mouth formation is somewhat less secure than we had thought. The edges of the dent appear to move together and close up in the middle, leaving openings at the ends which become the mouth and anus.
However, this idea has been challenged, because the Acoelomorpha
Acoelomorpha
The Acoelomorpha are a disputed phylum of animals with planula-like features that were considered to belong to the phylum Platyhelminthes. In 2004 molecular studies demonstrated that they are a separate phylum, although their position in the tree of life is contentious; most researchers believe...
, which form a sister group to the rest of the bilaterian animals, have a single mouth which leads into a blind gut (with no anus). The genes employed in the embryonic construction of the flatworm mouth are the same as those expressed around the protostomes', which suggests that the two structures are equivalent, and that the older ideas about protostome
Protostome
Protostomia are a clade of animals. Together with the deuterostomes and a few smaller phyla, they make up the Bilateria, mostly comprising animals with bilateral symmetry and three germ layers...
mouth formation were correct.
Evolutionary origin
Bilaterians evolved from a jellyfish-like ancestor, which was radially symmetrical. Because the development of the embryo typically reflects lineages' evolutionary origin, this has led to suggestions that the blastopore started out as the digestive surface on a radial organism, which became elongated (and thus bilaterially symmetrical) before its sides closed over to leave a mouth at the front and an anus at the rear. This matches with the "flaps-folding-over" model of gut formation, but an alternative view is that the original blastopore migrated forwards to one end of the ancestral organism, before deepening to become a blind gut. This is consistent with living flatwormFlatworm
The flatworms, known in scientific literature as Platyhelminthes or Plathelminthes are a phylum of relatively simple bilaterian, unsegmented, soft-bodied invertebrate animals...
s, which are basal to protostomes and deuterostomes. The story is a little more complex, because the blastopore itself does not go on to become the mouth. However, flatworms, the sister group to the rest of the bilaterians, branched off from the ancestral stock before the bilaterian group originated, and have a mouth at one end and a blind gut. This suggests that the last common ancestor of bilaterians had a similar gut configuration, and that the anus evolved after the mouth.
Exactly how a through gut formed from this blind gut is somewhat harder to tell. The genetic mechanisms responsible for anus formation are quite variable, which might suggest that the anus evolved several times in different groups. Scientists are currently looking into this matter to generate a more complete picture.