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Embryology/Embryogenesis Summarised Notes
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The lecture notes that follow are that of certain core competencies in general embryologywhich I could put together in the limited time available.
For diagrams and more information please refer any standard text book.
Errors if any ( may be pointed out) since it was done in a hurry
Topic – 76
Introduction to Embryology
This begins with the amazing progression of two cells to a little human being in about 9 months!
General considerations
• Embryology: Study of formation, growth and development of an
embryo.
• Embryo: Early stage of development up to 8 weeks after fertilization.
• Conceptus: Includes all the products of conception; meaning all the structures that develop
from the zygote, including the embryo and supporting structures like placenta needed for the
development of the embryo.
• Fetus: The developing organism from ninth week till birth.
• Prenatal: Before birth
• Postnatal: After birth
• Congenital: Present at birth
• Anomaly: Not normally present
• Pregnancy duration: 40 weeks.
o The duration of pregnancy or gestation is calculated from the first day of the last
menstrual period of the woman (easy to remember) and not from the day of
conception, which is about 2 weeks later.
o Hence the unborn baby spends about 38 weeks in the uterus.
Relevance of embryology to medical students
Often students ask: why embryology? What difference does it make?
Well, knowledge of embryology
• Can help understand the causes of congenital anomalies: a leading cause of infant
mortality.
• Can help adopt preventive measures to curtail such anomalies. For e.g., deficiency of folic
acid can cause neural tube defects. Administration of folic acid during pregnancy can
prevent conditions like anencephaly (absence of major part of brain, skull and scalp).
• Has resulted in the development of new techniques for prenatal diagnosis and treatment,
and overcome problems related to infertility.
• Can help understand the effects of teratogens (any agent that disturbs the development of
embryo or the fetus) and adopt preventive measures to avoid such exposure. For e.g., substances taken
by mother like alcohol or certain drugs can adversely affect prenatal development.
AN 76.1: Core – Written
• Describe the stages of human life
,STAGES OF HUMAN LIFE
• Infancy
• Childhood
• Adolescence
• Adulthood
• Old age
INFANCY
• First year of life
• Many physical milestones occur; infant gains control over his/her body; depends on others to
meet needs.
• Learns to trust others when needs are met.
• For proper physical and emotional development this security is essential
NEONATAL PERIOD (Neo – new, natus – born)
• First 28 days of life
• Infant undergoes transition from uterine environment to the external world.
• Weight, length, head circumference, pulse, and temperature are recorded: any abnormalities
that are observed are noted.
• Apgar score is determined: zero to ten, depending on the scores given for
o skin color
o pulse rate
o muscle tone
o breathing
o reflex irritability
• Apgar score is determined: zero to ten, depending on the scores given for
o Neonates delivered
▪ before 37 weeks: preterm (premature)
▪ after 42 weeks: post-term
• Fetus that dies and is delivered before 24 weeks: miscarriage
• Fetus that dies and is delivered after 24 weeks: stillborn
PERINATAL PERIOD (Peri – around, natus – born)
• From end of week 24 to 7 days after birth.
• Perinatology
o maternal-fetal medicine
o sub-speciality of Obstetrics (branch of medicine and surgery concerned with
childbirth)
o concerned with the care of mother and fetus with higher than normal risk of
complications
CHILDHOOD
• 2 to 12 years
• In developmental psychology, childhood is divided into
• Toddlerhood (learning to walk)
• Early childhood (play age)
• Middle childhood (school age)
• Adolescence (puberty through post-puberty).
, ADOLESCENCE
• Puberty to complete body maturation: Transitional period of growth and development
between childhood and adulthood.
• WHO defines an adolescent as any person between ages 10 and 19. This age range falls within
WHO’s definition of young people, which refers to individuals between ages 10 and 24.
• Puberty is the period of sexual maturation and achievement of fertility
• The time when puberty begins varies greatly among individuals. Both genetic and
environmental factors are involved in the timing of puberty.
o Usually occurs in girls between 10 and 14 years
o In boys: 12 and 16 years.
• Puberty is associated with the development of secondary sex characteristics and rapid growth.
o Breast development is usually the first sign of puberty in girls.
• Central precocious puberty (CPP) is puberty that occurs earlier than normal due to release
of hormones from the hypothalamus of the brain. Girls are more likely than boys to have
precocious puberty (early puberty).
• During puberty young bodies grow stronger and are infused with hormones that stimulate
desires appropriate to ensuring the perpetuation of the species.
• Puberty may also be accompanied by emotional and mood changes.
• Young persons experience numerous physical and social changes, often making it difficult
for them to know how to behave
o Stereotypes portray adolescents as rebellious, distracted, thoughtless, and daring.
• Problems in adolescence:
o Contrary to common belief, adolescents do not generally develop serious
psychological or social problems.
o Most problems reflect transitory experimentation, not enduring patterns of bad
behavior.
o Not all problems begin in adolescence (some have their roots in childhood)
o Most problems do not persist into adulthood (especially drug and alcohol use as well
as delinquency).
o Problems during adolescence are not biologically rooted or caused by the normative
changes of adolescence
o “Raging hormones” do not cause problem behaviors.
ADULTHOOD
• Period during which full physical and intellectual maturity are attained
• Commonly thought of as beginning at age 20 or 21 years.
• Adulthood: longest stage in human life cycle (19+ years).
• Middle adulthood: from about age 40 years to about 65 years.
• Physically, early and middle adulthood are marked by slow, gradual decline in body
functioning, which accelerates as old age is reached.
• Muscle mass continues to increase through the mid-20s, thereafter gradually decreases
• Skeletal mass increases until age 30 or so, and then begins to decrease, first in the central
skeleton (pelvis and spine), and last in the peripheral skeleton (fingers and toes).
• Throughout adulthood there is a progressive deposition of cholesterol in the arteries, and
the heart muscle eventually grows weaker even in the absence of detectable disease.
• Production of both male and female hormones diminishes with age, though this cannot be
directly related to the gradual diminution in sexual activity that occurs in both males and
females between 20 and 60.
OLD AGE (SENESCENCE)
• Final stage of the normal life span.
• Definitions of old age are not consistent from the standpoints of biology, demography
(conditions of mortality and morbidity), employment and retirement, and sociology.
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