Embryology: Early Development and Key Theories

What is Embryology?

Embryology is the branch of biology that studies the development of an organism from a fertilized egg. It encompasses several key concepts:

• Ontogeny: The transformations that the embryo undergoes during development.
• Phylogeny: The evolutionary history of a species.

Historical Theories of Development

Historically, two main theories attempted to explain embryonic development:

• Preformationism: This theory argued that a miniature version of the organism, called a "homunculus," was present in either the sperm or the egg. Scientists who believed in this were called "animalculists." In 1699, Danlenpatius proposed this theory.
• Epigenesis: This theory proposed that adult body structures were not preformed in the gametes but developed gradually. By the mid-nineteenth century, it was shown that body proportions were formed only during embryonic development.

Key Features of Human and Primate Development

The development of humans and other primates is characterized by the early appearance of embryonic membranes: the chorion, amnion, and allantois.

Asexual vs. Sexual Reproduction

• Asexual Reproduction: Does not involve gametes. It is essentially cloning, where an individual is genetically identical to its parent.
• Sexual Reproduction: Involves the fusion of two gametes from genetically different individuals.

The Zygote and Egg Classification

The egg or zygote is an undifferentiated cell that, through mitosis, gives rise to all new living systems.

Eggs can be classified based on the amount of yolk:

• Oligolecithal or Microlecithal: Eggs with little yolk, such as those of sea urchins.
• Mesolecithal or Heterolecithal: Eggs with a medium amount of yolk, as in amphibians.
• Polylecithal: Eggs with a large amount of yolk, like those of birds.

Eggs can also be classified based on yolk distribution:

• Isolecithal: Yolk is distributed homogeneously throughout the egg.
• Centrolecithal: Yolk is concentrated in the center of the egg.
• Telolecithal: Yolk is clumped together at the vegetative pole of the egg.

Milestones in the History of Embryology

Aristotle made careful observations of chicken embryonic development, but limited scientific understanding provided few useful ideas until the 16th century.

• 1592: Fabricius D'Acquapendente published the first detailed work on the fetal anatomy of chickens, fish, and mammals. His work is considered a precursor to modern embryology.
• 1651: Harvey published work suggesting that sperm fertilizes a woman's entire body, but only the uterus can develop the fetus.
• 1672: Regnier de Graaf observed fluid-filled cavities in the ovaries of animals, which he thought were ovules. These are now known as Graafian follicles.
• 1677: Anton van Leeuwenhoek observed with his microscope how sperm and eggs from fish and amphibians unite.
• 1679: Jan Swammerdam observed a tiny, preformed animal, supporting the "preformist" theory.
• 1768: Kaspar Friedrich Wolff observed the embryonic development of chickens and concluded that the embryo is not preformed, supporting the "epigenesis" theory.
• 1828-1834: Karl Ernst von Baer discovered the mammalian egg and identified four germ layers in mammals.
• ~1848: Robert Remak clarified that there are three germ layers and named them ectoderm, mesoderm, and endoderm.
• 1847: Albert von Kolliker provided evidence that sperm are produced in the testes and fertilize the egg.
• 1849-1922: Herwig Oscar observed the fusion of male and female pronuclei following fertilization.
• 1866: Haeckel introduced the terms blastula, gastrula, and morula and proposed that the embryonic development of organisms recapitulates their evolution.