Embryonic Development VARIATIONS IN EMBRYONIC GERM LAYERS AND BODY CAVITY

Formation of the Digestive Cavity  At the Blastula stage, the embryo forms a fluid filled ball of cells  During Gastrulation, the blastula caves in on one end to form a Gastrula. The cavity formed by gastrulation will form a digestive cavity or digestive tract.  If gastrulation is incomplete, the gastric cavity will have only one opening. This arrangement is characteristic of Cnideria and Platyhelminthes  If gastrulation is complete, the digestive tract will have two openings. This arrangement has 2 variations, protostomes and deuterostomes

Protostomes vs. Deuterostomes In Protostomes, the blastopore forms the mouth. In Deuterostomes, the blastopore forms the anus and the mouth is the second opening

Embryonic Germ Layers  Organisms that undergo gastrulation fit into 2 categories: diploblasts and triploblasts  Diploblasts only have 2 embryonic germ layers  Gastrulation forms an inner layer of cells (endoderm) and an outer layer (ectoderm)  This diagram shows the endoderm in blue and the ectoderm in black

Embryonic Germ Layers  Triploblasts develop a third germ layer in between the endoderm and the ectoderm  The middle layer is called mesoderm  In the diagram, mesoderm is represented in yellow  Variations in the presence, absence and arrangement of mesodermal tissue is one of the most important distinguishing features of animal phyla

Phylogeny based on Mesodermal Arrangement  Phylum porifera lack a true digestive cavity  Cnideria are diploblastic. They have a digestive cavity but lack mesodermal tissue  The triploblastic phyla vary in regard to the arrangement of mesoderm and body cavity

Development of Mesodermal Tissue  Mesoderm is formed by a migration of endodermal cells into the blastocoel.  Mesoderm forms by one of 2 possible mechanism  The diagram at the right shows mesoderm development in acoelomates and protostomes  The formation of mesoderm initiates near the blastopore. Dividing mesodermal cells migrate into the blastocoel

Acoelomate phyla  Acoelomates do not form a body cavity  Flatworms (Platyhelminthes) are acoelomate  Mesodermal cells fill the entire blastocoel  Internal organs form, but are not separate and distinct

Pseudocoelomate Phyla  Pseudocoelomates have a body cavity, but mesoderm is associated with the outer body wall (ectoderm).  Nematodes and rotifers are pseudocoelomate  All pseudocoelomates have a true digestive tract (2 openings) and are protostomes (blastopore = mouth)  The digestive tract lacks mesodermal tissue, thus is not muscular

Cross Section of a Pseudocoelomate  The diagram shows a cross section through a nematode  Notice the space between organs (body cavity), but note that there is only mesoderm on the outer boundary, associated with the body wall. The Intestine is a simple layer of epithelium (no muscle)  The organs are separate and distinct, but the body cavity is “false” – not fully lined with mesoderm

Coelomate Protostomes  A true body cavity is fully lined with mesodermal tissue.  In coelomate protostomes, the body cavity is “schizocoelous” (note the descriptive root word “schizo”)  The solid mass of mesoderm filling the blastocoel splits due to the programmed cell death (“apoptosis”) of some centrally located cells  The diagram shows the coelom in pink

Coelomate Deuterostomes  All deuterostomes have a true body cavity  In deuterostomes, the blastopore forms the anus (so the “second opening” forms the mouth”)  In deuterostomes, the mesoderm forms from outpouching of the gut  Since it is “caving in” it is “enterocoelous”  Because the coelom is formed from pouches, it is completely surrounded by mesodermal tissue

Schizocoelous vs Enterocoelous