Embryology (in Greek εμβρυολογία) is the branch of biology that studies the processes by which organisms grow and develop before birth. Today embryology studies all those stages that from a female gamete (egg or egg cell) fertilized by a male gamete (spermatozoon) lead to a mature adult individual. The term embryology, therefore, refers to the study of organisms between the unicellular stage (generally, the zygote) and the onset of independent life.
Embryology was originally and until the 20th century a primarily descriptive science. From the visual and clinical observation it then moved to the use of microscopy and then electron microscopy, to the cellular and subcellular (including chromosomes) and macro-microenvironmental study, arriving through the progress of biochemistry, biophysics and physiology up to the current genetics and then to the connections with molecular biology, with proteomics (study of biological molecules, their structure, properties, functioning and interactions) and genomics.
The study of molecular, genetic and physiological mechanisms through which the embryonic and fetal phases follow one another, is the object of study of the scientific discipline called developmental biology. It is useful to remember the distinction between development (understood as differentiation/succession of structural and organizational phases of increasing complexity) and growth (understood mainly as ‘quantitative’ growth).
Animal embryology studies the embryonic development of the various animal species that multiply both by somatic or vegetative reproduction and by sexual reproduction through gametes. Other chapters of embryology deal with metamorphosis, regeneration phenomena, and teratology.
Chemical embryology is the study of the chemical processes that occur during embryonic development. In particular it researches the chemical and physical determinants that act in the early stages of this development in order to regulate the subsequent differentiation, the metabolic reactions that accompany the latter and the mechanisms by which the exchanges of respiration, nutrition and elimination of metabolic wastes between mother and child take place. The most used methods in chemical embryology are cytochemical, histochemical, chromatographic, spectrophotometric, radioactive.
The comparative embryology studies the embryonic development by comparing the similarities or differences found in the various classes of vertebrates, according to a phylogenetic process that proved gradual from the simplest organism (amphioxus) to the most complex (placental mammal) and therefore explains and justifies the formulation of a systematic scale and the concept of evolution.
It is the study of the causes that contribute to the development of organisms according to particular patterns and of the morphological and physiological significance of the various structures that appear during the different stages of embryonic development. It is also called causal embryology or “developmental mechanics”. The most widely used technique for this type of observation is the microscope, but other methods are employed in specialized laboratories. Among these, the techniques of transplantation of part of an embryo on different areas of the same embryo, or a different embryo of the same species or another species, more or less close in the evolutionary scale; the technique of grafting parts of embryo on extraembryonic membranes of Amnioti (chorioallantoic membrane) and that of tissue-cultures with which embryonic cells are grown in vitro, using nutrient media natural or synthetic suitable.
Experimental tests have shown, among other things, that the egg does not need to be fertilized by the spermatozoon to give rise to an embryo (experimental parthenogenesis); varying the amount of cytoplasm in the egg-cell can have in some cases a regular development (merogamy); following an abnormal fertilization occurs an irregular distribution of chromosomes at the first segmentation of the egg (two blastomeres stage) and then you get individuals that are out of normality. With microsurgery experiments on morula and blastula it has been demonstrated the existence of embryos with mosaic development and embryos with regulatory development: typical example of the latter is that of Amphibians.
It studies the processes of formation and development of plant organisms that originate from an agamic cell as well as from a zygote, and in particular from products of fertilization of the ovule of Embryophytes. It was born with the work of G. B. Amici who discovered the existence of fertilization in plants and described (1823-30) the formation of the pollen tube and its penetration into the ovule. Subsequently W. Hofmeister made fundamental researches on the embryo of Phanerogams and discovered the alternation of generations.
The foundations of modern plant embryology were laid towards the end of the last century by E. Strasburger, who was the first to study mitosis and clarified the notions previously acquired about fertilization, ascertaining that the pollen tube, penetrating inside the embryonic sac, pours two sperms, of which only one joins the nucleus of the oosphere to give origin to the zygote. This mode led him to believe, since 1884, that the nucleus is the carrier of hereditary characters and that (1888) the male and female nucleus have the same number of chromosomes. In 1898 S. C. Navašin discovered the fertilization of the secondary nucleus of the embryonic sac that gives rise to the formation of the endosperm; in the same period H. O. Juel and S. Murbeck discovered the mechanism of parthenogenesis. With the development of genetics, already at the beginning of the 20th century plant embryology became more and more tributary of this science.