Prepared by W.R.JacintoDLSU-Dasmarinas

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

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

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

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

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

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

Embryology- study of animal development between fertilization and birth May include gametogenesis

Weismann (1834-1914) – distinguished between soma and germ cell

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)

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

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.

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.

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

Gourdon and colleagues use somatic cells of Xenopus

30-60 mins

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

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

cdk=cdc2=p34 Present

throughout the cell cycle

Cyclin – undergo its synthesis at G1 and destroyed after mitosis

Combines to form MPF

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

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

Morphogenesis – entire group of processes that mold internal and external configuration of an embryo

Recapitulation - Growth

Differential growth Determinate growth Indeterminate growth

Apoptosis - genetically determined cell death

Induction – embryonic signal calling; effect of embryonic tissue on another

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.