Kadasne’s Textbook of Embryology DK Kadasne
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EmbryologyChapter 1

Embryology is the study of intrauterine development of an individual. Total period of development is of 38 weeks. The development is divided into two stages, i.e. embryonic and fetal. Embryonic stage covers first two months while the fetal period of development runs from 3rd month to the birth. Embryonic period is important as the embryo obtains human look during the period. This is due to the development of organs and different systems of the body.
Organisms are added to the world as a continuous stream due to reproduction. One must remember that elimination (extinction) of species is prevented by adding new generations by reproduction. For reproduction in vertebrates, male and female are required. Sex cells are produced by sex glands known as gonads. Testes are the male gonads and the ovaries are the female.
Intrauterine development total period 38 weeks
0-2 months
3 months to birth
Let Us See Some of the Terms Used in the Embryology
  1. Ontogeny: It is an account of complete life cycle of an organism.
  2. Phylogeny: It includes evolutionary or ancestral history of a group of organisms. In the ascending order are the pisces, amphibians, reptiles, avians and the mammals at the top.
Let Us Get Familiar with the Terms often Used in Embryological Text
  1. Oocyte: Female germ cell produced by ovary.
  2. Sperm: Male germ cell produced by testes.
  3. Fertilization: Union of male and female gametes.
  4. Zygote: It is a cell formed after union of male and female gametes (see fertilization).
  5. Cleavage: Cell division by mitosis.
  6. Blastomere: Early embryonic cell, formed as a result of division of zygote, size of the zygote remains unchanged as the size of the cells formed after division, continues to become smaller.
  7. Morula: Means one which looks like a mulberry fruit. Morula is a compact mass of cells formed by 16 cell (The cells may vary from 12-32).
  8. Blastocyst: Fluid enters the morula forming a fluid lake inside the morula. Morula which is transformed into the fluid-filled cavity is called the blastocyst. Cells divide to form inner and outer cellular mass. The inner cell mass forms the embryo which is called the embryoblast.
  9. Primordium: It is the early form.
  10. Implantation: Attachment of blastocyst to the endometrium and its embedding in the endometrium.
  11. Gastrula: Formation of three germ layers, e.g. endoderm, mesoderm and the ectoderm.
  12. Neurula: Formation of neural tube from the neural plate.
  13. Conceptous: Structures derived from the zygote and the embryonic part.
  14. Abortion: Expulsion of embryo or the fetus before maturation.
  15. Habitual abortion: Spontaneous expulsion of the nonviable embryo or the fetus from the uterine cavity successively for more than three times.
  16. Missed abortion: Retention of the embryo or the fetus in the uterine cavity after death.
  17. Trimester: Duration of first three calendar months.
  18. Teratology: Deals with abnormal development.
It includes increase in cell number, cell size and the intercellular substance.
Cells of the epidermis and circulating erythrocytes are lost due to wear and tear. They are replaced by stem cells through mitosis. Thus, the cell population is maintained at the optimum level. Maintenance of steady state of cell population is called ‘Turn over’.
Auxetic Growth
It is observed in oocytes and some neurones. Large cell lies in the center surrounded by the small cells. Example: oocytes are surrounded by follicular cells while the neurons are surrounded by neuroglial cells. Accretionary growth is by increase in quantity of the intercellular substance, which is seen in bones and cartilages.
Following division of zygote (first cleavage) two cells are formed (two-cell stage). Now each cell can form separate embryo having three germ layers, e.g. ectoderm, endoderm and mesoderm. This explains formation of uniovular twins. Totipotent character of the cells exists only up to the 8-cell stage. After 8-cell stage, morula is formed (16-cell stage). Cells of the morula become pluripotent which are capable of producing specific types of tissues. This phase is also called the plastic phase.
It is the physiochemical event seen in the cells of the dorsal lip of the blastopore and the primitive streak of the higher vertebrates. This cellular zone of embryo is capable of inducing the process of tissue differentiation through chemical substances. Hence, these cellular zones are called organizers. Primitive streak becomes the primary organizer for inducing formation of notochord and the mesoderm. Notochord becomes the secondary organizer forming the brain and the spinal cord from the neuroectodermal plate.
Now the neural tube becomes the tertiary organizer and forms the somites (paraxial mesoderm). Chemodifferentiation is followed by histodifferentiation and later by the organogenesis. Hemodynamics of the circulation makes the walls of the arteries thicker. Thus, the structural change related to the function is called functional differentiation.
Importance of embryology in medicine:
  1. With the study of development, anatomical relations can be explained and better understood.
  2. One gets an insight regarding the abnormal development, their prenatal detection, prevention and treatment.
  3. Use of alcohol, smoking, drugs, viral infections and teratogens including the external envirnoment are blamed for the abnormal development. The incidence of the abnormal development can be reduced to the minimum by rendering advice and adopting preventive measures.
  4. Ex-utero surgery for congenital diaphragmatic hernias, removal of the cyst and repairing of the spinabifida is possible, only due to in-depth study of the embryology by the medical faculty.