Comparative Embryology

Comparative embryology is the branch of embryology that compares the development of embryos of two or more species. The observed similarities and differences may be used in taxonomic and phylogenic studies. Evolutionary embryology (evolutionary developmental biology or "Evo-Devo") is a related field that has been revitalized by molecular genetics and genomics.

Copies of Haeckel's drawings of vertebrate embryos published by Romanes (1892). Several of the embryos are inaccurately drawn, which leads to overestimation of the similarity of vertebrate embryos. Source: Wikimedia Commons.

Recapitulation Theory

Ernst Haeckel (1866) famously postulated that "Ontogeny recapitulates phylogeny", that is, the development of the individual repeats the development of the species. This is now known to not be literally true; however, species that are related evolutionarily are likely to share many embryological processes. Thus, insights into human development can be gained by studies of model organisms, even invertebrates such as C. elegans, Drosophila, sea urchins and Amphioxus (an invertebrate chordate). But caution is necessary when incorporating discoveries in other species into human embryology. For example, although vertebrate embryos are remarkably similar, it is surprising how widely divergent early embryological processes are in vertebrate development.

Fertilization

The nature of the female gamete varies between animal species.

• Most female gametes of vertebrates are in second metaphase of meiosis at the time of sperm entry, i.e., they are secondary oocytes.
• Few animals produce mature ova. A historically important exception is the sea urchin, which was commonly used in studies of fertilization and early embryogenesis.

Cleavage

The amount of yolk in the zygote influences cell division in the early vertebrate embryo.

• Vertebrate embryos with little yolk (e.g., mammals and Amphioxus) undergo complete (holoblastic) cleavage yielding equal-sized blastomeres.
• Vertebrate embryos with a moderate amount of yolk (e.g., amphibians and some fish) undergo complete (holoblastic) cleavage yielding small blastomeres in the animal pole and larger blastomeres containing more yolk in the vegetal pole.
• Vertebrate embryos with dense yolk (e.g., reptiles, birds and most fish) concentrated at one end (the future vegetal pole), undergo imcomplete (meroblastic) cleavage. Cleavage (discoidal cleavage) is limited to a disc at the developing animal pole. Thus the single yolk cell is much larger than the blastomeres.

Blastulation

Early vertebrate embryos composed of about 100 to 1000 cells are sometimes called blastulae. The term blastula seems to have been meant originally as an inclusive term to be applied to an early embryonic stage in all multicellular animals - the "segmented ovum", composed of a large number of relatively undifferentiated cells (blastomeres); however, the term blastula is often defined as a hollow spherical early embryo with a fluid-filled cavity called the blastocele, as seen in sea urchin development. At this embryonic stage, vertebrate embryos display marked differences.

• Amphioxus blastulas are similar to those of sea urchins - blastulas with large blastoceles surrounded by a unicellular layer of blastomeres.
• Amphibian blastulas have a multilayered wall with a small blastocele eccentrically located between the thin animal pole and the thicker vegetal pole.
• Blastulas of modern fish (teleosts), reptiles, and birds consist of the blastodisc (or blastoderm) situated atop a single large yolk cell. The blastodisc typically develops two layers (epiblast and hypoblast) separated by a blastocele.
• Mammalian embryos at this stage are called blastocysts. Blastocysts have a relatively large fluid-filled cavity (the blastocyst cavity) surrounded by an outer cell layer (trophoblast). The embryoblast is an inner cell mass located under the trophoblast at one end of the blastocyst. The distinction between blastula and blastocyst is important because blastocysts develop within the uterus and the outer cell layer forms extra-embryonic tissues that obtain O2 and nutrients in exchange for CO2 and waste products from maternal tissues.

Blastulation in Amphioxus and sea urchins. Cleavage produces multicellular embryos: a morula (1) and then a blastula (2).

Gastrulation in Amphioxus and amphibians. Cell and tissue movements convert a blastula (1) into a gastrula (2).

Gastrulation

Gastrulation is the process of conversion of a blastula into the next embryonic stage, the gastrula. Sea urchin gastrulas have an archenteron ("primitive gut"), formed by invagination of a part of the blastula wall. As part of this process of gastrulation, sea urchins develop three germ layers (ectoderm, mesoderm and endoderm). Most animals (except sponges, cnidaria and ctenophores) go through a variety of developmental processes during comparable stages of development to form embryos containing three distinct tissue layers. Therefore, although most vertebrates do not form true gastrulas, the process of formation of trilaminar embryos is often referred to as gastrulation. Gastrulation involves a variety of cell and tissue movements in differing proportions in different types of animals.

• Amphioxus and amphibian gastrulation includes invagination similar to that seen in sea urchins. The opening on the gastrula surface that leads into the archenteron is called the blastopore. This region of the gastrula is also important in subsequent developmental processes.
• Teleost fish gastrulation begins with epiboly, the growth of the blastoderm to surround the yolk. While the blastoderm is spreading over the yolk, it also thickens (partially by a process of involution of the leading edge) and differentiates into distinct tissue layers.
• Reptilian, avian, and mammalian "gastrulation" is characterized by formation of a primitive streak and ingression of some epiblast cells to form embryonic mesoderm and endoderm. The primitive streak is analogous to the amphibian blastopore.

Extra-embryonic Membranes

The conditions under which development occurs in vertebrates differs markedly, thus it is not surprising that the nature of the extra-embryonic membranes also differ.

• Fish and amphibians, which are usually oviparous and lay aquatic eggs, only have a single internal "extra-embryonic" membrane, a yolk sac. Fish have true yolk sac in that the yolk resides within the lumen of the developing gut. In amphibians, the yolk is located within the endodermal cells of the developing gut.
• Reptiles, birds and mammals are called amniotes because they have an amnion (and other extra-embryonic membranes) during development. These membranes appeared with the evolutionary development of the terrestrial egg. The amnion encloses the fluid within which the embryo develops.
  • Oviparous (egg-laying) amniotes (birds, most reptiles, and monotremes) lay eggs that have a large external yolk sac that contains nutritional stores, a large allantois to store waste products, and an outer membrane (chorion) that allows gas exchange.
  • Ovoviviparous amniotes (some reptiles) develop within the mother's body and are born alive, but do not derive nutritional support from the mother because there is sufficient yolk.
  • Viviparous (live-bearing) amniotes (most mammals and some reptiles) have little or no yolk, so they are dependent upon maternal support during development. Maternal-fetal relations are mediated by a placental organ composed of uterine endometrium and extra-embryonic membranes (chorion, yolk sac, or both).
    • Marsupials have a chorio-vitelline placenta, in which the fetal component is formed by the fusion of the chorion and yolk sac.
    • Most eutherian (placental) mammals have a chorio-allantoic placenta, in which the fetal component is formed by chorion and allantois. In humans, the contribution of the allantois is limited; it supplies the chorionic (or placental) circulation.