The Preface (cont.)

The placenta is one of the least understood and most understudied organs in the human body. For centuries, while the anatomy, histology, and physiology of other organs were studied in detail, the placenta was ignored. With the advent of medical sciences and in conjunction with efforts to practice medicine based on scientific evidence, pathological conditions associated with all other systems were intensely studied. Mostly due to lack of available knowledge about the normal placenta and its development, whether its histology or physiology or developmental pathways at biochemical or molecular level, this organ once again was left behind.

Numerous factors complicated the efforts to study this exotic organ.

To name a few:

1. The placenta is a transient structure that begins with the implantation of the embryo and completes its function. It is discarded after parturition, so it is not available to study in a non-pregnant woman.
2. It is a challenging organ whether one studies its anatomy, histology, physiology, or developmental biology. It constantly changes throughout the pregnancy, making it difficult to investigate and make generalizations.
3. It is difficult to create an efficient in situ model of the human placenta to learn the basics of its function and structure. Animal models, although helpful, are not ideal. Placentas vary in various species of animals. Primate placentas most resemble that of humans, but their use is limited.

Since the human fetus completes most of her/his early development in utero and completely depends on this organ to survive and interact with her/his mother and the environment, common sense dictates that no organ is more significant than the placenta during this most crucial period of development.

In past decades a number of dedicated scientists contributed significantly to this neglected field, and recently, the number of investigators who "discovered" this organ has increased exponentially. These scientists use new molecular tools to illuminate the basics of human reproduction. This is partly due to the advancement of artificial reproductive technologies (ART) and the demands placed on these laboratories to improve their success rates.

Although multiple well-written texts are available on the pathology of the placenta, few sources specifically focus on the normal development, anatomy and histology of this complex organ. Consequently anybody who is trying to understand and learn about the normal development and structure of placenta must browse large volumes devoted to the pathology of this organ or a variety of basic science journals.

Initially my purpose was only to summarize the recent information about the normal condition of this organ. Rapid accumulation of information in recent years coupled with the complexity of the basic anatomy and histology made the task much more challenging than I expected. During the time it took to complete the initial draft, sufficient new data were published that required multiple revisions and additions to the manuscript.

In this treatise I have tried to summarize recent information about the normal developmental biology of the human placenta. It is intended to be an introduction and a systematic review of a challenging topic. I hope that it will provide scientists and practitioners who study the placenta an introductory source. A generous number of references are provided to help the reader beyond the presented material.