1. Prepared by W.R.Jacinto DLSU-Dasmarinas

2.  According to Aristotle (384-322 B.C), the first major embryologist known to history, science begins with wonder. (superstition to observation)  “It is owing to wonder that people began to philosophize – the beginning of knowledge  Many primitive societies showed interest in our prenatal origins

3.  The development of an egg to an animal has been a source of wonder throughout human history.  Galen (130-200 A.D) – learned about advanced fetuses but the minute dimensions resisted analysis

4.  The minute structures caused a lag in the growth of knowledge about the embryo not until the development of microscope.  De Graaf in 1672 – described ovarian follicle  Hamm and Leeuwenhoek in 1677 – have seen the sperm cells  significance were not understood

5. Theory of Preformation  Spermists - contended that sperm contained the new individual in miniature and only nourished in the ovum  Ovists- thought the same way about the ovum and stimulated only in the seminal fluid.  Bonnet (1745) – discovered eggs of some insects undergoing parthenogenesis  Spallanzani (1729-1799) – demonstrated that both male and female sex products are necessary for the initiation of development

6.  Wolff (1733–1794) – wrote a thesis on epigenesis (embryological development occurs through progressive growth and differentiation)  Von Baer (1828) – discovered mammalian egg, first emphasized the fact that the more general basic features of any animal group appear earlier in the development than do special features of different members of the group  Demonstrated existence of germ layers

7.  The formulation of cell theory by Matthias Schleiden and Theodore Scwann laid down the foundation of modern embryology as a science.  Ernst Haeckel (1834 -1919) – drafted the Biogenetic Law – Haeckel’s Law of Recapitulation  Ontogeny recapitulates phylogeny

8.  Embryology- study of animal development between fertilization and birth  May include gametogenesis  Weismann (1834-1914) – distinguished between soma and germ cell

9.  Descriptive embryology (1880-1890) – serial sections and three dimensional wax plate reconstruction  Comparative embryology (late 1800s) – provided insights on recapitulation theory; started with invertebrates (evolution)

10.  Experimental – directed to factors that activate and regulate developmental processes.  Wilhelm Roux (1850-1924) – destroyed one cell of two-cell frog embryo; coined entwicklungsmechanik  Conrad Waddington – coined “epigenesis”, development is a result of a series of causal interactions between parts; reminds that genetic factors as among the important determinants of development

11.  Chemical embryology – information about the chemical and physiological events in the embryo.  Included the role of DNA and RNA - how it fabricates specific chemical and structural components of embryo  Teratology – concerned with the study of malformations  Reproductive biology – problems of conception and contraception  Developmental biology – approach, includes even postnatal processes.

12. In contemporary society  In vitro fertilization – developed for economic reasons  Involves fertility-enhancing drugs, laparoscopy  Embryo transfer  Surrogate mothers - $10000  Embryo banks  Cloning – 1902 salamander; 1951 frog; 1977 mice; 1986, mammals; July 5, 1996, Dolly; 1997 cloned mouse; 2001, ox and wild sheep.

14.  Adult mammary gland cell and enucleated ovum was fused by electric pulses – also to activate  Embryo was transferred in a pregnant sheep  Dolly out of 434 embryos

15.  Gourdon and colleagues use somatic cells of Xenopus

16. 30-60 mins

17. Different cell types, different length of cell cycle: 1) rapid cell division, continuous throughout life:  - epithelial cells of mucosal layer of stomach,  intestines, body cavities  - epithelial cells of stratum germinativum of skin  - hematopoetic stem cells  - spermatogonia  2) Do not divide  - mature nerve tissues  - mature muscle tissues  - mature red blood cells  3) Divide when induced  -liver cells in mature liver  -lymphocytes

18.  Normal length of cell cycle in adult tissues of Xenopus laevis: 20 hours  Cell cycle in cleaving embryos : less than 30  mins:  - S phase- less than 3 mins (adult tissue at  least 5 hours)  - no G1, short G2: large fertilized egg, large  cytoplasm

19.  cdk=cdc2=p34  Present throughout the cell cycle  Cyclin – undergo its synthesis at G1 and destroyed after mitosis  Combines to form MPF

20. Evidence for maturation promoting factor (MPF): mitotic cyclin-dependent kinase(Cdk)-cyclin complex

21. Other concepts  Totipotency – capability to form entire organisms  Restriction - reduction of developmental options  Determination – commitment to single developmental fate  Formation of cornea  Differentiation – actual morphological or functional expression of portion of genome of a group of cells

22.  Morphogenesis – entire group of processes that mold internal and external configuration of an embryo  Recapitulation -  Growth  Differential growth  Determinate growth  Indeterminate growth

23.  Apoptosis - genetically determined cell death  Induction – embryonic signal calling; effect of embryonic tissue on another

24. Figure 2. Drawings of the developing human head and face between the 4th and 5th week (adapted from Nelson, 1953). The top row are side views, and the bottom row are face views of the same stages. The face develops from extensions and fusions of the pharyngeal arches, structures which are found in all other vertebrates, and which are modified in different ways in different species. Abbreviations: m, maxillary process (upper jaw); j, lower jaw; h, hyoid; n, nasal pit.