Body-Buidling - Vertebrate Development

John Merck


The "evolution" of Babylon 5's Jason Ironheart from Not Entirely Stable

Growth and evolution:

Remember, ontogeny pertains to the individual, evolution to the lineage. (TV sci-fi plots notwithstanding.) That doesn't mean they aren't connected in any way. Now that we know something about the evolutionary pattern of Vertebrata, let's fill in their developmental pattern.

Heterchrony:

Subtle changes over evolutionary time in an organism's developmental timetable are a potent source of overall evolutionary change. This is an idea with a history as long as the study of evolution.

General patterns of heterochrony:



Eumetazoan Development:


Cleavage from A. S. Romer. 1977. The Vertebrate Body.
Consider the basic steps by which we get a proper embryo with a top, bottom, front, and back from a zygote:

Polarity: In all ova yolky matrial tends to concentrate at one end, yielding:

The amount of yolk greatly influences developmental dynamics. For now, we consider an ovum with relatively little yolk.

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:



Gastrulation from A. S. Romer. 1977. The Vertebrate Body.
Gastrulation:

The embryo is now a gastrula. It possesses these basic germ layers: The blastopore becomes an opening from the gut to the outside. In cnidarians and ctenophores, this is simple, as there is only one "mouth." In bilaterians, the blastopore will become either the mouth or the anus, depending on the taxon.

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
The Coelom: A characteristic feature of bilaterians is the presence of a coelom or body cavity. This feature allows for:


Eucoelomate coelom schematic from Fueleducation
The evolution of the coelom opened many vistas for animal evolution, including significantly expanded locomotor strategies. Cnidarians show the limits of what a hydrostatic skeleton can do for an animal with a single module. Bilaterians, however, display body segmentation in which separate modules of the hydrostatic skeleton can lengthen and shorten, facilitating much more complex movement. This makes possible activities like: Indeed, non-bilaterians are deemed incapable of burrowing.

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:

Fortunately, the presence of mesoderm and a coelom seems to bestow the developmental plasticity needed to allow these to evolve. Indeed, the gut tube, kidneys, and gonads are ancestrally suspended inside the coelom.



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:

Deuterostomia: Includes chordates and echinoderms plus some minor groups. Synapomorphies include:


Cleavage and blastula in a shark from A. S. Romer. 1977. The Vertebrate Body

Yolk and cleavage:

So far our examples have been of animals whose ova contain relatively little yolk. In fact, among chordates yolk quantity can range from little to huge (consider a hen's egg, in which the yolk greatly outweighs the ovum's cytoplasm.) Because yolk is not metabolically active, its presence hinders cleavage, yielding distinct patterns: In highly yolky eggs, development is significantly altered. For example, in a shark blastula (right) cleaving cells are restricted to a disk, the blastodisc (D) on top of a metabolically inert yolk mass. The active cells of the blastula are, thus, not a hollow sphere, but a cap enclosing a fluid-filled blastocoel (E).

Gastrulation in a meroblastic embryo occurs not at a distinct blastopore, but along a furrow called the primitive streak.


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.


Mesoderm schematic from Studyblue
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:


Craniate Development:

In some ways, Branchiostoma is distinctly lacking. It has no: The last item is surprising for an active animal. Instead of motor neurons extending axons to muscle fibers, Branchiostoma's muscles project tails that directly contact the central nervous system. Why are craniates so different?


Romer, A. S. The Vertebrate Body, 1977.
Neural Crest: Beside endoderm, mesoderm, and ectoderm, craniates have a fourth major tissue type, neural crest. To understand it, consider a craniate twist on neural tube formation. Among craniates, neural tube formation progresses as in other chordates, but with one difference: As the neural tube forms, masses of ectodermal cells break away from the crests to either side of the neural fold and aggregate as neural crest cells. Some of these give rise to the peripheral nervous system. Others migrate to various locations in the body, giving rise to:



Inductive formation of the craniate eye from
Anatomical Foundations of Neuroscience - University of Western Ontario
Special sense organs: The special sense organs develop through the inductive interaction of neural tube ectoderm and ectodermal placodes - thickened regions on the body surface.

Note: many other structures, including:

form from similar inductive interactions.

Ectoderm inventory: We have now discussed four distinct types of ectoderm:


Origin of the head:

Beyond neural crest, what do all of these taxa have in common that the outgroups (Urochordata and Cephalochordata) lacks? Heads - a radical departure.

The appearance of the head is one of the major enigmas of chordate evolution because it is so sudden. No fossil chordate appears to have a partial head. In fossil non-craniate deuterostomes like Pikaia, Yunnanozoon, and vetulicolians, the front of the body is occupied by a mouth and pharynx with no brain or recognizable special sense organs, or even room to put them. In craniates, the head is present in front of it, and contains the special sense capsules. Is there a connection between heads and neural crest, which appears at the same point in evolution?

First, a digression.

Regulatory genes


Wikimedia Commons
The structure of DNA was discovered in 1953, and its role as the physical repository of genes illuminated in the following decades. How information encoded as nucleic acid is expressed as proteins is well-known.

The magic of protein synthesis is that DNA is only transcribed into mRNA when the protein it codes for is needed. E.G.: The bacterium Escherichia coli freely metabolizes glucose, but if glucose is lacking, and lactose is present, it can metabolize it by producing an enzyme, beta-galactosidase, that breaks lactose down in to glucose and galactose. But how does it know when to make beta-galactosidase?

The work of François Jacob and Jacques Monod established the answer (Nobel Prize in 1965): Adjacent to the gene for beta-galactosidase is a small gene to which a protein, the lac repressor binds. This blocks RNA polymerase from unzipping the DNA and making mRNA. But, lactose, itself, binds to the lac repressor causing it to fall off of the DNA strand and allowing RNA polymerase to do its thing. Once the lactose has been metabolized, the lac repressor reattaches and transcription ceases. Animation.

Of course, the lac repressor protein is coded for by a repressor gene whose activity may be regulated by other repressors. Indeed, the expression of genes as proteins is the result of complex interactions of regulatory and structural genes and their protein products.

A similar system appears to govern the differentiation of regions of the body.

HOX genes


The Biology Corner
Hox genes: During the late 20th century it became known that segmentation in bilaterians is governed by a special class of regulatory genes. The story:

Fruit flies are a favorite model for geneticists, with short generation spans and interesting mutations that often effect entire sections of the body (modifying or eliminating body segments and/or the appendages that grow from them). Investigation into these segmentation-altering mutations revealed that they can be caused by mutations to eight genes. What makes this interesting:

The homeobox codes for a proteins called the homeodomain that is functionally similar to regulatory proteins that block or allow transcription of other genes. It appears that the cluster of eight genes controls the identity of body segments in fruit flies. These are called Hox genes, after the homeoboxes they contain.

But it gets better: The search for homologs to fruit fly Hox genes found them in almost every animal surveyed. Mammals, for example, have four clusters of Hox gene homologs, in each of which the genes occur in the same order on the chromosome as the regions for which they code.

This is huge for two reasons:

Why do the numbers of Hox genes and Hox gene clusters differ? - Errors in transcriptions result in duplications in which paralogous copies of the genes are generated - i.e. genes coexisting within the same genome that trace their descent to a common ancestor genes.

What happens to an organisms phenotype (physical form) when a gene or gene cluster that controls body segmentation is duplicated?

Remember, Hox genes control segmentation and fundamental orientation of embryo. They are conservative gene clusters found throughout the animal realm. Besides controlling orientation and segmentation, each gene influences a specific region of the body.

Gene transcription errors have profound effects. One common consequence of such errors is the duplication of genes, where two paralogous copies of a gene are generated. (As opposed to homologous versions of physical structures. ) Once present, they can each evolve independently and ultimately code for different proteins.

Putting it together: In 2008, the genome of Branchiostoma was sequenced. We now know:


Additional reading: