Embryology Lecture note 1

1. Embryology Lecture note 1 History of embryology Ndodo Nnaemeka B.Sc Human Anatomy (UNICAL), M.SC Human Anatomy (ABU), M.Sc. Molecular Bioengineering ( Graz, Austria) Human Anatomy Department UDUS sokoto Nigeria dndodo@yahoo.com

3. Embryology • the science that deals with the formation and development of the embryo and fetus, • can be traced back to the ancient Greek philosophers. • Originally, embryology was part of the field known as "generation’. • a term that also encompassed studies of reproduction, development and differentiation, regeneration of parts, and genetics.

4. History of embryology • Generation described the means by which new animals or plants came into existence. • The ancients believed that new organisms could arise through • sexual reproduction • asexual reproduction • spontaneous generation

5. History of embryology • As early as the sixth century B.C., Greek physicians and philosophers suggested using the developing chick egg as a way of investigating embryology.

6. History of embryology • According to preformationist theories, an embryo or miniature individual preexists in either the mother's egg or the father's semen and begins to grow when properly stimulated. • Some preformationists believed that all the embryos that would ever develop had been formed by God at the Creation.

8. History of embryology • Aristotle actually favored the theory of epigenesis, which assumes that the embryo begins as an undifferentiated mass and that new parts are added during development.

9. Aristotle thought that the female parent contributed only unorganized matter to the embryo. He argued that semen from the male parent provided the "form," or soul, that guided development and that the first part of the new organism to be formed was the heart.

10. Aristotle (384–322 B.C.) described the two historically important models of development known as • preformation • epigenesis.

11. Naturalists who favoredpreformationist theories of generation were inspired by the new mechanical philosophy and by the microscope, • Microscope helped to see the embryo at earlier stages of development. • Some naturalists produced very unreliable observations of early embryos, • but Marcello Malpighi (1628–1694) and Jan Swammerdam (1637–1680), two pioneers of microscopy, provided observations that seemed to support preformation

12. In his studies of embryology, as in his research on the circulation of the blood, • Harvey was inspired by the work of his teacher, Girolamo Fabrici (ca.1533–1619). • Fabrici is thought to be founder of modern embryology because of the importance of his embryological texts: On the Formed Fetus and On the Development of the Egg and the Chick.

13. Histroy • Respected scientists of the time, including Albrecht von Haller (1708–1777), • Charles Bonnet (1720–1793), • LazzaroSpallanzani (1729–1799), • René Antoine Ferchault de Reaumur (1683–1757), • They supported preformation. • Bonnet's studies of parthenogenesis in aphids were regarded as strong support of ovistpreformationism.

14. Ovist preformation • some naturalists argued that the whole human race had preexisted in the ovaries of Eve • while others reported seeing homunculi (tiny people) inside spermatozoa.

15. Other eighteenth century naturalists rejected both ovist and spermist preformationist views. • One of the most influential was Casper Friedrich Wolff (1733–1794), • who published a landmark article in the history of embryology, "Theory of Generation," in 1759.

16. Wolff argued that the organs of the body did not exist at the beginning of gestation, • but formed from some originally undifferentiated material through a series of steps. • Naturalists who became involved in the movement known as nature philosophy found Wolff's ideas very attractive.

17. During the nineteenth century, • cell theory, • the discovery of the mammalian ovum by Karl Ernst von Baer (1792–1876), • and the establishment of experimental embryology by Wilhelm Roux (1850–1924) and Hans Driesch (1867–1941) • transformed philosophical arguments about the nature of embryological development.

18. Harvey's On the Generation of Animals was not published until 1651, but it was the result of many years of research. • Although Harvey began these investigations in order to provide experimental proof for Aristotle's theory of epigenesis, • his observations proved that many aspects of Aristotle's theory of generation were wrong.

19. History of embryology • But even in the 1700s it was still thought that perhaps every embryo started from a very small version of a complete organism. • In the 1800s, studies revealed the progressive development of complexity in the growth of an embryo. • From the 1930s and before, it was known that different genes are involved in different aspects of embryo development. • And with the advent of gene manipulation methods in the 1970s and 1980s, the genetic control of development in organisms such as fruit flies in tremendous detail. • Among the important discoveries made were the homeobox genes .

20. At the end of the 1800s experiments based on removing or modifying parts of early embryos began • by the 1920s it had been discovered that there were definite pieces of embryos that were responsible for inducing various aspects of development to occur.

21. concentrations of diffusing chemicals might define where in an embryo different elements would form was first suggested in the early 1900s, • but it was not until the 1970s and 1980s - after it was emphasized by Lewis Wolpert in 1969 under the name "positional information“ • that there was clear experimental investigation.

22. Summary of historical highlight

28. CONCEPTS IN EMBRYOLOGY

30. GENE ACTIVATION • Genes are typically not active in the zygote. • they are lightly complexed with basic proteins called histones. • The chromosomal DNA plus its enveloping histones is called chromatin; • densely staining heterochromatin, seen within the nucleus represents inactivated, or repressed genetic material. • As development begins certain groups of genes become activated or derepressed by being freed from their associated histones.

31. Derepressed DNA represents potentially functional genes. • The first genes to become derepressed are those involved with proliferative and general metabolic activity of the cell.

32. Restriction and Determination • The fertilized ovum has the capacity to form an entire organism. • Cells resulting from the first few divisions after fertilization retain this capability called totipotence. • with further development cells gradually lose the ability to form all cell types found in the body. • The reduction of the developmental options permitted to a cell is called restriction. • The mechanism of restriction is not yet well understood.