Embryology for Medical Students Sudhir Sant
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IntroductionCHAPTER 1

 
INTRODUCTION
The life begins at the time of fertilisation of the ovum. The zygote that is formed is a single cell which grows and develops into fully-formed adult. The part of this growth takes place inside the uterus (prenatal period) and part of it takes place outside the uterus after birth (postnatal period). The growth and development taking place in prenatal period is more dramatic, when a single cell is converted into fully-formed foetus which resembles the adult in the external form and all the systems and organs are present. The prenatal development involves the cell division, transformation or specialisation, migration and even the programmed cell death.
Embryology can be defined as study of growth and development in prenatal period. Any abnormal change in developmental process will produce congenital anomaly. Teratology is the science that deals with study of congenital anomalies, which include the abnormality of developmental process and the factors responsible for it.
Prenatal period can be divided in two parts. The first eight weeks of this period is called as embryonic period and the developing life form is called as embryo. From ninth week to full term (till childbirth) the developing life form is called as foetus and the period is called as foetal period.2
The postnatal period of growth and development extends from birth to about 25 years. The adulthood is reached by 25th year when the growth is complete and the bones are fully ossified. This postnatal period can be divided in certain parts.
 
Infancy
The period of about one year after birth is called as infancy, and newborn during this period is called as infant. First four weeks of this period is very critical for survival of the infant, this period is called as neonatal period, the newborn being called as neonate.
 
Childhood
This period extends from one year to 12 to 13 years.
 
Puberty
This is the period where there is very rapid growth, and development of secondary sexual characters. This period extends from 12 to 15 years in females and 13 to 16 years in males.
 
Adolescence
This period extends from 12 to 17 years. This period is characterised by rapid physical growth and sexual maturation.
 
Adulthood
This period extends from 18 to 25 years during which growth and development including ossification is completed.
During the embryonic period (first eight weeks after fertilisation), the single cell, i.e. zygote is converted into a form that externally resembles the adult and all the primitive organs and systems are formed. This developmental process is called as ‘general embryology’. It also includes the development of accessory structures like chorion, amnion, umbilical cord and placenta.
Further development and functional maturation of various organs and systems which take place in foetal period is called as‘systemic embryology’.3
 
Significance of Embryology
Embryology explains many facts in gross anatomy like relations and position of various organs, their blood supply, etc.
Embryology helps to understand and diagnose the various congenital anomalies. The knowledge of embryology is very helpful to the clinicians during the treatment of patients. One of the example is the subhepatic caecum and appendix, (a congenital anomaly) when inflamed, the symptoms can be very similar to cholecystitis. During appendicectomy the possibility of abnormal position of appendix is obviously very important to surgeon.
Knowledge about the congenital anomalies and factors responsible for them can be useful in preventing the occurrance of congenital anomalies.
Certain aspects of general embryology like gametogenesis, fertilisation and implantation are of immense importance to understand the causes and treatment of infertility.
In view of applications of embryology, a separate chapter on congenital anomalies and their aetiology is included in this book. Similarly, after describing the development of a particular structure or organ, the congenital anomalies of that structure or organ are discussed.