Embryology In
Medicine
What is embryology?
Animals
begin life as a single cell. That cell must produce new cells and form
increasingly complex structures in an organised and controlled manner to
reliably and successfully build a new organism (Figures 1.1 and 1.2). As an
adult human may be made up of around 100 trillion cells this must be an impressively
well choreographed compendium of processes.
Embryology is the branch of biology that studies the early formation and
development of these organisms. Embryology begins with fertilisation, and we
have included the processes that lead to fertilisation in this text. The human
embryonic period is completed by week 8, but we follow development of many
systems through the foetal stages, birth and, in some cases, describe how
changes continue to occur into infancy, adolescence and adult life (Figure
1.3).
Aims And Format
This book
aims to be concise but readable. We have provided a page of text accompanied by
a page of illustrations in each chapter. Be aware that the concise manner of
the text means that the topic is not necessarily comprehensive. We aim to be
clear in our descriptions and explanations but this book should prepare you to
move on to more comprehensive and detailed texts and sources.
Our
biological development is a fascinating subject deserving study for interest’s
sake alone. An understanding of embryological development also helps us answer
questions about our adult anatomy, why congenital abnormalities sometimes occur
and gives us insights into where we come from. In medicine the importance of an
understanding of normal development quickly becomes clear as a student begins
to make the same links between embryology, anatomy, physiology and neonatal
medicine.
The study of
embryology has been documented as far back as the sixth century bc when the
chicken egg was noted as a perfect way of studying development. Aristotle (384–322
bc) compared preformationism and epigenetic theories of development. Do animals
begin in a preformed way, merely becoming larger, or do they form from
something much simpler, developing the struc- tures and systems of the adult in
time? From studies of chickens’ eggs of different days of incubation and
comparisons with the embryos of other animals Aristotle favoured epigenetic
theory, noting similarities between the embryos of humans and other animals in
very early stages. In a chicken’s egg, a beating heart can be observed with the
naked eye before much else of the chicken has formed.
Aristotle’s
views directed the field of embryology until the invention of the light
microscope in the late 1500s. From then onwards embryology as a field of study
was developed.
A common
problem that students face when studying embryol- ogy is the apparent
complexity of the topic. Cells change names, the vocabulary seems vast, shapes
form, are named and renamed, and not only are there structures to be concerned
with but also the changes to those structures with time. In anatomy, structures
acquire new names as they move to a new place or pass another structure (e.g.
the external iliac artery passes deep to the inguinal ligament and becomes the
femoral artery). In embryology, cells acquire new names when they differentiate
to become more spe- cialised or group together in a new place; structures have
new names when they move, change shape or new structures form around them. With
time and study students discover these pro- cesses, just as they discover
anatomical structures.
If a student
can build a good understanding of embryological and foetal development they
will have a foundation for a better under- standing of anatomy, physiology and
developmental anomalies. For a medical student it is not difficult to see why
these subjects are essential. If a baby is born with ‘a hole in the heart’,
what does this mean? Is there just one kind of hole? Or more than one? Where is
the hole? What are the physiological implications? How would you repair this?
If that part of the heart did not form properly what else might have not formed
properly? How can you explain to the par- ents why this happened, and what the
implications are for the baby and future children? A knowledge of the timings
at which organs and structures develop is also important in determining periods
of susceptibility for the developing embryo to environmental factors and
teratogens.
We
appreciate that the subject of embryology still induces concern and despair in
students. However, if it helps you in your profession you should want to dig
deep into the wealth of understanding it can give you. We also appreciate that
you have enough to learn already and so this book hopes to represent embryology
in an accessible format, as our podcasts try to do.
One thing
that has not changed with the development of embryology as a subject is that
the more information that is gathered, the more numerous are the questions left
unanswered. For example, we barely mention the molecular aspects of development here. Should your interest in embryology and mechanisms of development be
aroused by this book, we hope that you ek out more detailed sources of
information to consolidate your learning.
