Embryology

Embryology literally means the study of embryos. But the term is more frequently used in a broader sense: the study of the growth and development of an individual from fertilization to the perinatal period, and of the fetal adnexa (placenta, fetal membranes, and umbilical cord). Thus embryology is covers both the embryonic and fetal periods of development of the conceptus. The subject matter of embryology is sometimes extended even farther in textbooks to include gametogenesis and postnatal development through puberty.

This photograph shows a life-sized model of a 8-week-old embryo in the hand of an adult. Source: WikiMedia Commons

Other Names

• Developmental biology
• Ontogeny

Types

• Descriptive Embryology
• Comparative and Evolutionary Embryology
• Experimental Embryology
• Teratology

Description

Descriptive Embryology

Descriptive embryology is a branch of microscopic anatomy dealing with the structure of embryos and fetuses. The discipline is usually subdivided into two fields:

• General embryology - the examination and description of embryos, fetuses, and fetal adnexa (placenta, fetal membranes and umbilical cord).
• Special embryology - the development of organ systems and body parts (e.g., cardiovascular system, digestive system, respiratory system, urinary system, genital systems, nervous system, head and neck, and limbs)

Descriptions of series of embryos provides insight into the three basic developmental processes: proliferation, differentiation and morphogenesis.
As with all anatomical sciences, standard reference axes and planes are defined, and used for clarity:

• Axes: Ventral - Dorsal, Cranial - Caudal, and Medial - Lateral.
• Planes: Frontal (or coronal), Sagittal, Transverse (or horizontal).

As growth and development are integral to the subject, embryology also utilizes standards for estimation of embryonic age and various gestation-related dates.

• Start date
  • Prenatal age, the age of the embryo or fetus, begins at Fertilization
  • Clinicians date a pregnancy (gestation) beginning with the first day of the last normal menstrual period (LMP), which is usually one to three weeks before ovulation and fertilization. Thus the dates of a pregnancy (in menstrual or "gestational" weeks) are about two weeks greater than the age of the embryo or fetus.
• Embryonic Period - first 8 weeks, beginning with fertilization
  • First week: Pre-implantation embryo (Fertilization, Cleavage and Blastocyst formation and attachment)
  • Second week: Implantation of blastocyst
  • Weeks 3-8: Embryogenesis
• Fetal Period - third month to birth
  • Second trimester: 4th-6th menstrual months
  • Age of viability: 20-26 weeks p.f. (22-28 menstrual weeks).
  • Third trimester - 7th-9th menstrual months
• Duration of prenatal life is about 38 weeks
  • Expected Date of Delivery (EDD or due date) is 40 weeks after the first day of the LMP. EDD is roughly calculated by Naegele's rule (add a year, subtract 3 months, and add 7 days).
  • Term: 35-40 weeks p.f. (37-42 menstrual weeks). Thus, preterm (premature or born early) is before 35 weeks p.f. (37 menstrual weeks), and post-term is over 40 weeks (42 menstrual weeks)

Comparative and Evolutionary Embryology

Comparative embryology compares the development of embryos of two or more species. The observed similarities and differences may be used in taxonomic and phylogenic studies.

Experimental Embryology

Experimental embryology is an important partner of descriptive embryology. Hypotheses about development are tested using genetic and embryo manipulations, e.g., lesions, transplantations, and transgenic organisms. Important concepts developed by experimental embryologists include induction, commitment, and programmed cell death (also known as apoptosis). Experimental embryology addresses, for example, questions about mechanisms underlying observed developmental changes, including whether these changes are autonomous or conditional.

Teratology

Teratology, literally the study of "monsters", is an interdisciplinary field of embryology and pathology that focuses on abnormal development and congenital malformations. Teratology is a basic medical science whose clinical counterpart is dysmorphology (branch of clinical genetics and pediatrics that also focuses on congenital malformations).

Importance of Embryology in Medicine

The study of human development is a fascinating topic and is of value to the student because it aids an understanding of anatomy and physiology,maternal-fetal relationships and congenital malformations.

• It gives an understanding of how different organs and tissues develop from a single cell (a fertilized oocyte) and how they acquire functional capacity. This understanding gives a rational explanation of the relationships of many normal adult structures.
• Embryology also includes the development of the fetal adnexa (placenta, fetal membranes, and umbilical cord), which connect the fetus to the mother. This is of importance in obstetrics and prenatal diagnosis.
• Malformations and certain pathological conditions can be explained as failures in growth, development or differentiation. Normal and abnormal embryology is an important component of various medical specialties, for example, surgery, pediatrics, pathology, and obstetrics.

History

The history of embryology from Aristotle to the 19th century is largely that of descriptive embryology of non-human vertebrates, and of comparative embryology^[1] because of the difficulty of obtaining human embryos for examination. Wilhelm His, Sr. (1831-1904) was the first to systematically study a collection of human embryos^[2]. His students, Franz Keibel (1861-1929) and Franklin P. Mall (1862-1917), continued the work of collecting and describing human embryos. Mall started the Carnegie Human Embryo Collection in 1887, and collected about 2000 embryos during his career. Keibel published "Normal Plates of the Human Embryology" (1908)^[3] as one of sixteen volumes of his studies of vertebrate embryology^[4]. Together, Keibel and Mall (1910-12) published an important handbuch of human embryology^[5]
George Streeter (1873-1948), Mall's successor at the Carnegie Institution, outlined and started detailing the current system of staging human embryos^[6]. Rohan O'Rahilly completed the staging system in 1973^[7]. With Fabiola Müller, he revised the system in 1987^[8], and the estimated ages of embryos at various stages in 2001^[9].

How Embryology Was Named

The English word embryology is derived from the Greek words έμβρύον (embryon, "the fruit of the womb before birth") and λόγος (logos, "word" or "treatise"), probably through the medieval Latin word embryologia or the French word embryologie.

Other Resources

O'Rahilly R and Müller F. Human Embryology & Teratology, 3rd Ed. New York: Wiley-Liss, 2001.
Skandalakis JE and Gray SW. Embryology for Surgeons, 2nd Ed. Baltimore: Williams & Wilkins, 1994.

References

1. ↑ Needham J. A History of Embryology. Cambridge: University Press, 1934. (2nd Ed., 1959)
2. ↑ His W. Anatomie menschlicher Embryonen. 3 parts and atlas. Leipzig: FCW Vogel, 1880-85.
3. ↑ Keibel F and Elze C. Normentafel zur Entwicklungsgeschichte des Menschen. Jena: VVG Fischer, 1908.
4. ↑ Hopwood N. A history of normal plates, tables and stages in vertebrate embryology. Int. J. Dev. Biol. 51: 1 - 26 (2007) pdf
5. ↑ Keibel FKJ and Mall FP. Manual of Human Embryology. 2 vols. Philadelphia:JP Lippincott, 1910-12.
6. ↑ Streeter GL. Developmental horizons in human embryos. Contributions to Embryology, Washington 30: 211-245 (1942); 31: 27-63 (1945); 32: 133-203 (1945); 33: 149-169 (1949); 34: 165-196 (1951).
7. ↑ O'Rahilly R. Developmental Stages in Human Embryos. Part A. Embryos of the First Three Weeks (Stages 1 to 9). Carnegie Institution of Washington, 1973.
8. ↑ O'Rahilly R and Müller F. Developmental Stages in Human Embryos. Carnegie Institution of Washington, 1987.
9. ↑ O'Rahilly R and Müller F. Human Embryology & Teratology, 3rd Ed. New York: Wiley-Liss, 2001.

External Links

Human Developmental Anatomy Center (HDAC) of the National Museum of Health & Medicine

• O'Rahilly R & Müller F. Developmental Stages in Humans. Carnegie Institution of Washington, 1987.
  

Embryology.ch - an online course in embryology for medical students
The Visible Embryo
The Virtual Human Embryo Project
Edinburgh Human Development Anatomy Database
University of New South Wales Embryology - an educational resource
Gilbert SF. Developmental Biology, 6th Ed. Sunderland, MA: Sinauer Associates, 2000. Searchable text at NCBI Bookshelf.