Today we know that ontogeny does not exactly recapitulate phylogeny, nevertheless, an individual's ontogeny or development constrains, or sometimes opens evolutionary pathways. For this reason, we must establish some basic knowledge about chordate development. We start with the zygote or fertilized ovum, the individual's first manifestation.
Cleavage from A. S. Romer. 1977. The Vertebrate Body.
Polarity: In all ova yolky matrial tends to concentrate at one end, yielding:
- "Animal" pole, the non-yolky side
- "Vegetal" pole, the yolky part.
Cleavage Phase of rapid cell division with little overall growth. The zygote transforms into a hollow sphere of cells, the blastula. The space in the middle is the blastocoel
The blastula has two cell types:
- Smaller cells nearer the "animal" pole.
- Larger, yolky cells nearer the "vegetal" pole.
Gastrulation from A. S. Romer. 1977. The Vertebrate Body.
- Cells of the "vegetal" hemisphere form a flat plate then invaginate inward, yielding a two layered hemisphere.
- The rim of this concavity quickly closes into a small opening, the blastopore.
- Endoderm: Destined to give rise to the gut and associated structures. The space enclosed by the endoderm is the archenteron, the precursor to the cut cavity.
- Ectoderm: Destined to give rise to the outer surface.
In cnidarians, the blastula assumes the form of the planktonic planula larva which then directly develops features of an adult with no front, back, left, or right. For bilaterians, however, it is more complicated.
Bilaterian Development:Mesoderm: As the illustration indicates, a third basic cell type, mesoderm, is present in the blastula, forming a collar around the large cells of the vegetal pole. They are fated to give rise to a third germ layer. Like the endoderm, they invaginate into the interior, but do so asymmetrically, extending along one side of the archenteron. This extension marks the animal's plane of bilateral symmetry. From there, mesoderm cells proliferate into the space between the endoderm and ectoderm, giving rise to a great range of three-dimensional structures and organs.
Eucoelomate coelom schematic from A Review of the Universe
- increased gut expansion,
- hydrostatic structure against which muscles may act, facilitating purposeful locomotion
- use of coelomic fluid in gas and nutrient transport, breaking dependence on simple diffusion and allowing larger body size.
Eucoelomate coelom schematic from Fueleducation
- Purposeful locomotion
- Burrowing into the substrate
Specialized organs: These capabilities came at a price. Animals with only endoderm and ectoderm don't need to worry about gas exchange and elimination of nitrogenous waste, because no living cell is so far from the body surface that simple diffusion can't do the trick. Bilaterians, in contrast, usually require specialized organs for functions like:
- Gas exchange
- Nitrogenous waste elimination
- Coordination of the nervous system
- Sensing the environment (useful to motivate and manage locomotion)
Protostomous and deuterostomous development from Nature Volume 409 Number 6816
Deuterostome Development:Bilateria breaks down into two major groups that were first distinguished by developmental characters (although molecular phylogenies have supported them):
Protostomia: Includes arthropods, mollusks, annelid worms, brachiopods, and bryozoans plus some minor groups. Synapomorphies include:
- Blastopore gives rise directly to mouth and anus by elongating and "pinching off." (right - a)
- Development of the coelom is schizocoelomate, in which the coelom forms from direct cavitation of the mesoderm.
- Development is determinate, in which cell fates are absolute.
- Cleavage of embryonic cells is spiral, or arguably derived from a spiral pattern.
- Nerve cord is ventral to gut tube in adults.
Deuterostomia: Includes chordates and echinoderms plus some minor groups. Synapomorphies include:
- A distinct deuterostomous pattern of development (right - b) where the blastopore becomes the anus and the mouth develops from a secondary opening.
- Development of the coelom is enterocoelomate, in which the coelom originates as an out-pouching of the gut tube.
- Development is indeterminate development: i.e. the fate of a given cell is fluid and based on inductive relationships with neighboring cells and tissues.
- Cleavage of embryonic cells is radial , giving embryos the ability to twin.
- Nerve chord forms dorsal to gut tube in adults.
Neurulation in Branchiostoma from A. S. Romer. 1977. The Vertebrate Body.
Chordate Development:Neurulation: The indeterminate nature of deuterostome development, in which the fates of specific cells are influenced by inductive relationships with other cells, is illustrated by the next big step chordate development - the formation of the neural tube that gives rise to the central nervous system. In this process, the activity of mesoderm cells triggers a cascade of events.
- Cells on the mid line of the mesoderm form a cylinder that becomes the notochord.
- Mesoderm to either side folds outward and expands down the side of the gastrula's inner surface.
- An inductive relationship causes the ectoderm directly above the notochord to sink downward, forming a trough, and ectoderm to either side to rise up forming crests to either side. The trough collapses into a hollow tube lying above the precursor to the notochord. This is the neural tube - the precursor to the brain and spinal cord.
- In vertebrates and urochordates, cells slough off of the interior of the crests to either side of the neural tube to form neural crest ectoderm. These cells migrate in amoeboid fashion to distant locations in the body where they give rise to a large list of other things.
- The peripheral nervous system: Note that in Branchiostoma, which lacks neural crest, muscle cells communicate with the central nervous system by little "tails" that extend to touch the spinal cord. There are no peripheral nerves.
- In urochordates, neural crest cells migrate to distribute pigmentation around the surface of the body.
At this stage we have three distinct flavors of ectoderm:
- Non-neural ectoderm of the body surface
- Neural-tube ectoderm
- Neural crest ectoderm
Stupid mesoderm tricks: While neurulation is happening, mesoderm is also busy. Aside from the gut tube (endoderm), skin, and nervous system (ectoderm), most of the body derives from mesoderm. Here are a few major features:
- Intermediate and Lateral Plate Mesoderm: Mesoderm extending down the sides of the embryo in which the coelom forms coalesces into zones with distinct fates, including kidneys and gonads, the coelom, and somites (see below.)
Romer, A. S. The Vertebrate Body, 1977.
- Somites: Dorsal to (above) the lateral plate, rectangular pouches of mesoderm bud off to form little hollow capsules on either side of the notochord. These are the somites - precursors to body segments from which dermis, muscles, and much of the skeleton forms. The first one forms near the front of the embryo, with successive somites budding off behind it. Animation of zebra-fish somite development.
Mesenchyme cells from UCSF School of Medicine
- Mesenchyme: Mesoderm gives rise to a network of star-shaped cells capable of amoeboid movement. These are the mesenchyme cells - the connective tissue of the embryo. They give rise to the circulatory system and (in vertebrates) most of the skeleton. We will meet them again.