UNSW Embryology

Foundations

Practical - Introduction to Human Development

© Dr Mark Hill (2009)

Acknowledgements

6. Critical Periods

Introduction

Critical periods of development refer to times when genetic or materal effects can impact upon the developmental process. The timing of these effects will impact on different systems at different times.

Systems with long periods of development or complicated structural origins and differentiation are more susceptible to developmental abnormalities. Which systems take a long time to develop?

The Fetal period is also potentially sensitive to maternal effects that impact upon interuterine growth. Along these lines there is growing evidence that low birth weight, and therefore inhibited fetal growth, may have postnatal effects on lifelong health outcomes. (More? Fetal Origins Hypothesis)

Comments

We have now had a very quick trip through more than 9 months of development in nearly 2 hours. Before we finish it is worth thinking about times in development when things may go wrong. In UNSW Embryology in every section of System Notes Page 2 refers to abnormalities associated with that system.

These notes and linked materials have been prepared for Educational purposes only. Please email Dr Mark Hill if you wish to make a comment about this current project.

Key Systems

Look at the following linked sections of notes and consider why and when development of these two systems are also critical.

Respiratory Development

Heart Development

Birth APGAR Score

A birth scoring system developed by Dr Virginia Apgar, first published in 1953 and now used internationally (More? Normal Development - Birth - Apgar | Dr Virginia Apgar). A score is given for each sign at 1 and 5 minutes after the birth (if problems with baby an additional score given at 10 minutes).

• 7 - 10 is considered normal.
• 4 - 7 might require some resuscitative measures.
• 3 and below requires immediate resuscitation.

Birth Defects

How do birth defects occur?

Abnormal Development

How are birth defects monitored in Australia?

National Perinatal Statistics Unit (NPSU) | Congenital malformations, 1981-1997 (an example of one of the units reports)

Finished!

This is the Last Page of the current Practical. Remember these pages are always available for revision, there are many more online detailed notes and we will come back to some concepts in Beginnings, Growth and Development. (More? Exam Revision)

Glossary

Use the alphabetical list below to find definitions of terms that are new to you.

Terms

absolute risk - Term used in describing the effect of environmental teratogens on development. This epidemiological statistical term describes the high likely rate of occurrence of an abnormal phenotype among individuals exposed to the agent (e.g. fetal alcohol syndrome). The other risk descriptor term used to describe the likelyhood of an abnormality is relative risk.

critical period - (critical period of development) The term used to describe a developmental time when exposure to a teratogen can lead to a developmental abnormality, which can be further divided into an early major and later minor developmental abnormality. The defined critical period will differ in timing and length for different systems. (More? Abnormal Development - Environmental)

fetal period - In humans, the development week 9 to 36 is the fetal stage (second and third trimester) and during this time organs formed in the embryonic period continue to develop and the fetus grows in size and weight. The first 8 weeks of development is considered the embryonic period and is divided into 23 Carnegie stages based upon developmental milestones. Note when searching an alternate spelling "foetal". (More? Fetal | Embryo Stages)

fetal weight - The measurement of the weight of the developing fetus. The measurement is obtained by ultrasound calculation or clinically estimated by palpatation. The greatest addition of fetal weight occurs during and towards the end of the third trimester. (More? Fetal Weight | Fetal | Ultrasound Measurements | Ultrasound Movies)

fetotoxicant - A chemical that adversely affects the developing fetus, resulting in low birth weight, symptoms of poisoning at birth or stillbirth (fetus dies before it is born). (More?Abnormal Development - Environmental | Abnormal Development - Drugs)

intrauterine growth restriction - (IUGR) Term used to descibe clinically a fetus that has not reached its growth potential because of genetic or environmental factors. Abnormal development measured as less than 10th percentile for gestational age, not easy to detect before 32 weeks. This poor fetal growth can have fetal, placental or maternal causes. (More?Abnormal Notes IUGR | Human Fetal Weight | Fetal Length | Fetal Notes | Fetal Programming Hypothesis | Medline Plus - IUGR)

relative risk - Term used in describing the effect of environmental teratogens on development. The ratio of the rate of the condition among the exposed and the non-exposed. (e.g. smokers risk of having a low birth weight baby compared to non-smokers) A high relative risk may indicate a low absolute risk if the condition is rare. (More? Abnormal Development - Environmental)

teratogen - (Greek, teraton = monster) any agent that causes a structural abnormality following fetal exposure during pregnancy. The overall effect depends on dosage and time of exposure. Absolute risk - the rate of occurrence of an abnormal phenotype among individuals exposed to the agent. (e.g. fetal alcohol syndrome) Relative risk - the ratio of the rate of the condition among the exposed and the non-exposed. (e.g. smokers risk of having a low birth weight baby compared to non-smokers) A high relative risk may indicate a low absolute risk if the condition is rare. (More? Abnormal Development)

Quick Movie Links

Movie of Human Embryo Growth (this shows a human embryo growing, all images are to scale)

Movie of Mouse Embryo Growth (this shows a mouse embryo growing)

Quick Links

UNSW Embryology ISBN: 978 0 7334 2609 4

UNSW CRICOS Provider Code No. 00098G