Animal embryology

For thousands of years, humans have wondered how they and other organisms came to be. By 340 BCE, Aristotle had described the development of the chicken in the egg, but since most early embryos are too small to be seen by an unaided eye, his and later descriptions of development started with larger, more formed embryos. That did not change very much until the late 1600s when the development of the microscope gave a glimpse of life too small to be seen unmagnified. By the early eighteenth century, the developmental patterns of many organisms had been observed and described. There was, however, still much disagreement about how the early stages progressed.

Most scientists believed in the theory of preformation, which said that a preformed embryo was present in the gametes. There were two main factions among the preformationists: the ovists believed that inside the egg was a tiny, fully formed organism that was stimulated to grow by the seminal fluid. Their opponents, the spermists, believed that the fully formed miniature organism was in the sperm and was nourished in its growth by the ovum. Thus, drawings of sperm and eggs from the seventeenth and eighteenth centuries often show fully formed bodies within.

By the end of the eighteenth century, more scientists were deserting preformation in favor of the theory of epigenesis, first proposed by Caspar Wolff in 1789, which stated that development occurs through the growth and remodeling of embryonic cells. Karl Ernst von Baer, who had published a collection of his observations and the observations of others, proposed that general features that are common to large groups of taxonomically related organisms appear earlier in development than more specialized features of individual species. After Darwin published his evolutionary theories, Johannes P. Müller, Ernst Haeckel, and other proponents of von Baer's law and of evolution proposed that the embryonic development of an organism (ontology) mirrored its evolution (phylogeny). Although this has been shown not to apply to all organisms or to all developmental sequences, it can be seen in the development of many embryos.

During the late nineteenth and early twentieth centuries, scientists’ understanding of embryonic development increased dramatically as they began applying recently discovered knowledge in evolution, genetics, and cell biology to embryology. Edwin Ray Lankester and Hans Spemann were two prominent scientists who studied comparative embryonic development at that time. Also at that time, the new science of experimental embryology began as Wilhelm Roux and G. Schmidt manipulated the cells of amphibian embryos and began to discover how and why development occurred. Further experimental embryology continued investigating why abnormalities and anomalies arise in development. Scientists often use young chicken embryos in this type of research because the embryos are similar to those of mammals but develop outside the mother. Chick embryo research concerning the development and progression of tumorigenesis and metastasis is particularly promising.

In the twenty-first century, new discoveries in biology and chemistry are applied to the study of embryonic development. For example, scientists are researching how the creation and study of artificial human embryos could aid in reversing genetic disorders. Modern technology aids in discoveries, such as the deep learning artificial intelligence model known as Dev-ResNet, which has been successfully used to measure the development of pond snail embryosa highly complex process.

Gametogenesis

The formation of gametes, eggs, and sperm is usually considered the beginning of embryology. In sperm formation, two things need to occur, reduction of chromosomes to the haploid state and maturation of the cytoplasm. During the first part of spermatogenesis, immature cells, called spermatogonia, form four haploid cells, called spermatids, by meiosis. Spermatids then go through a maturation process in which they become streamlined and motile. They also develop an acrosome that has enzymes needed to penetrate the egg. Like sperm, eggs must become haploid and mature, but both the timing and maturation are quite different. Maturation of the cytoplasm often begins before meiosis. All the cytoplasm of the early embryo comes from the egg, so immature ova are aided by various helper cells that increase each ovum’s cytoplasm and add food stores called yolk. The amount of yolk varies considerably, from mammals that have no yolk to birds that have huge amounts. Depending on the species, meiosis can begin at any time during cytoplasmic maturation and can be a continuous process or have one or more pauses. In sea stars and many other organisms, meiosis is complete before fertilization, while in others, such as nematodes, the egg matures fully and is released by the ovary before any meiosis begins. Sperm penetration then triggers the onset of meiosis.

Fertilization and Development

Once sperm reach the egg, the acrosomal enzymes must digest the various protective layers that surround the egg, and recognition structures on the surface of the sperm must be complementary to recognition structures on the egg cell membrane. The sperm’s nucleus then enters the egg and fuses with the haploid egg nucleus. This forms a diploid cell called the zygote. Interestingly, when a sperm first penetrates the egg, the polarity of the cell changes and chemicals are released by the membrane, which makes it impossible for other sperm of that species to enter the same egg.

Around twenty-four hours after fertilization, a period known as cleavage begins. During this time, cells divide rapidly with little or no growth between cell divisions. Cells become smaller and more numerous. At the end of the cleavage, a structure called the blastula is formed. In some animals, such as echinoderms, amphibians, and nonvertebrate chordates, the blastula is a hollow ball of cells. In higher vertebrates, the blastula is a flat, dish-shaped structure, often called the blastodisc. In mammals, the blastula is called a blastocyst and consists of a hollow ball of cells called the trophoblas, and a group of internal cells called the inner cell mass. During gastrulation, surface cells become internalized to form the three germ layers—ectoderm, mesoderm, and endoderm—that are seen in most animal embryos. A second internalization, this time of some ectodermal cells, forms the beginning of the central nervous system.

After this neurulation, the various body organs begin to form from the three germ layers. As these changes progress, cells become less general and more specialized, a process called differentiation. Once the major organs have differentiated, the embryo matures and grows, a process usually called gestation. The time it takes for embryonic development varies considerably. In chickens and small rodents, the process takes about three weeks; in humans, it takes approximately nine months, while in elephants, the process takes eighteen to twenty-two months. Some organisms emerge in very immature states that require more development. Amphibians and arthropods hatch as feeding larvae that must grow before they begin a metamorphosis that leads to the adult. Though their later development differs greatly from that of vertebrates, embryonic cells of arthropods and nematodes make decisions according to chemical signals received from other cells in the same way as vertebrate cells.

Marsupial embryology is unique in the animal kingdom. They are also born at a very immature stage of development called organogenesis, which begins just after gastrulation is complete. After climbing unaided to their mother's pouch, they must complete their embryonic development attached to the nipple, which swells in their mouth so they can not let go until they are more mature.

Principal Terms

Cleavage: cell division in the early embryo that, unlike division in adults, involves little or no growth between divisions

Fertilization: the process by which the egg and sperm unite to form the zygote

Gametes: the haploid cells, ova, and spermatozoa, that fuse to form the diploid zygote

Gastrula: the stage of development during which the endoderm (gut precursor) and the mesoderm (muscle and connective tissue precursor) are internalized

Haploid: having only one of each kind of chromosome

Zygote: the single cell formed when gametes from the parents (ova and sperm) unite, a one-celled embryo

Bibliography

Bronson, F. H. Mammalian Reproductive Biology. Chicago: University of Chicago Press, 1989.

Carlson, B. Patten’s Foundations of Embryology. 6th ed. New York: McGraw-Hill, 1996.

Devlin, Hannah. “Synthetic Human Embryos Created in Groundbreaking Advance.” The Guardian, 14 June 2023, www.theguardian.com/science/2023/jun/14/synthetic-human-embryos-created-in-groundbreaking-advance. Accessed 5 July 2023.

El-Bawab, Fatma. Invertebrate Embryology and Reproduction. Academic Press, 2020.

Hartl, Daniel L., and Elizabeth W. Jones. Essential Genetics. 2d ed. Sudbury, Mass.: Jones and Bartlett, 1999.

Hartl, Daniel L. Essential Genetics and Genomics. 7th ed., Jones & Bartlett Learning, 2020.

Kumé, Matazo, and Katsuma Dan. Invertebrate Embryology. Translated by Jean C. Dan. Belgrade, Yugoslavia: NOLIT Publishing House for the U.S. Department of Health and Human Services, 1968.

Ribatti, Domenico, and Tiziana Annese. “Chick Embryo in Experimental Embryology and More.” Pathology: Research and Practice, vol. 245, 2023, doi.org/10.1016/j.prp.2023.154478.