Pre-Medicine Program - Embryology

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Dr Mark Hill

Human development is one of the most exciting topics to study not only as a medical student, but also for our fundamental understanding of the human body.

This lecture and associated practical are very similar to the type of content that will be covered in the initial Foundations component of the UNSW Medicine program. This will then be expanded upon in Phase 1 BGDA (developmental timeline) and BGDB (specific systems) as well as Phase 2 BGD (clinical embryology and teratology). This lecture is going to take you briefly through key concepts in human development. I will be using simplified terms in the lecture slides (with developmental term in brackets).

Australian trends diabetes prevalence 19990-2008.jpg

Australian trends diabetes prevalence 1990-2008

Maternal diabetes, either pre-gestational or the gestational diabetes form can have major impacts upon not only the developing embryo and fetus, but also maternal health. For example, one impact can be damaging upon the developing pancreas, leading to later diabetic problems.

(Maternal Diabetes | Pancreas Development | Macrosomia)

In addition, new research suggests that the developmental environment can also affect life-long health, the Fetal Origins of Disease Hypothesis.

Remember also that development does not stop at birth, the early post-natal environment has a major role on growth, health and disease.

Pre-Medicine Links: Biology 1 - Cell Types | Biology 2 - Cell Compartments, Membranes | Biology 4 - Cell Export/Import | Biology 6 - Cell Cycle Biology 7 - Cell Filament Systems | Biology 8 - Embryology


  1. Purpose of learning embryology
  2. Basic facts about early human development
  3. Appreciate differences between the conceptus, embryo and fetus
  4. General understanding of the term “critical periods” of development

Concepts: Fertilization, Early conceptus, Germ layers, Embryo, Tissue origins, Timetable/stages of development, Fetus, Placenta

Background Lectures: Cell Structure (structure and function), Cell Division (mitosis, meiosis, lifespan, cell death), 4 Basic Tissues (Epithelial, Connective, Muscular, Nervous)

Animated overview

Human development 001 icon.jpg
 ‎‎Human Development
Page | Play
This animation begins at the two cell (blastomere) stage following fertilization and takes you through an overview of the entire 9 months of human development in just over a minute!

Note that in this animation specific features are neither labeled or identified,at this stage in your studies the many terms and concepts will not yet make sense.

Four Basic Tissue Types (How do they develop and what are their origins?)

  1. Epithelial
  2. Connective
  3. Muscular
  4. Nervous

(More? movie on separate page | annotated movie on separate page])

Human development timeline graph 01.jpg


There are many different excellent embryology textbooks, UNSW students have online access to the Embryology website and the textbook shown below. More Textbooks?

UNSW Embryology logo
Hill, M.A. (2020). UNSW Embryology (20th ed.) Retrieved May 18, 2024, from
The Developing Human, 9th edn.jpg Citation: Moore, K.L., Persaud, T.V.N. & Torchia, M.G. (2011). The developing human: clinically oriented embryology (9th ed.). Philadelphia: Saunders.

Links: NLM ID: 101293798 | publisher page | NLM Holding

The Developing Human: Clinically Oriented Embryology (10th edn) 
The Developing Human, 10th edn.jpg

UNSW Students have online access to the current 10th edn. through the UNSW Library subscription (with student Zpass log-in).

APA Citation: Moore, K.L., Persaud, T.V.N. & Torchia, M.G. (2015). The developing human: clinically oriented embryology (10th ed.). Philadelphia: Saunders.

Links: PermaLink | UNSW Embryology Textbooks | Embryology Textbooks | UNSW Library
  1. Introduction to the Developing Human
  2. First Week of Human Development
  3. Second Week of Human Development
  4. Third Week of Human Development
  5. Fourth to Eighth Weeks of Human Development
  6. Fetal Period
  7. Placenta and Fetal Membranes
  8. Body Cavities and Diaphragm
  9. Pharyngeal Apparatus, Face, and Neck
  10. Respiratory System
  11. Alimentary System
  12. Urogenital System
  13. Cardiovascular System
  14. Skeletal System
  15. Muscular System
  16. Development of Limbs
  17. Nervous System
  18. Development of Eyes and Ears
  19. Integumentary System
  20. Human Birth Defects
  21. Common Signaling Pathways Used During Development
  22. Appendix : Discussion of Clinically Oriented Problems

Human Reproductive Cycle

  • Meiosis in gonad produces haploid gametes (egg and sperm)


  • Menstrual Cycle a regular cycle of reproduction (28 days)
  • begins at puberty, release of 1 egg (oocyte) every cycle
  • Endocrine controlled (HPG axis) - Hypothalamus, Pituitary, Gonad
XXhpgaxis.jpg Menstrual cycle.png


  • begins at puberty, continuous production of spermatozoa, release millions of spermatazoa


Female Ovary and Uterus
Female Ovary and Uterus
  • Paired organs, lying in the peritoneal cavity
Human right ovary and tube 1.jpg Ovary10x.jpg


Human Menstrual Cycle
Ovulation icon.jpg
Follicle 001 icon.jpg
Page | Play


  • the process of the 2 haploid gametes (egg and sperm) fusing and combining genetic material.
  • conceptus - the entire product of fertilization
Fertilization 002 icon.jpg
Page | Play
Fertilization 001 icon.jpg
 ‎‎Mouse Fertilisation
Page | Play

Early Development

  • occurs during week 1 following fertilization
  • last menstrual period (LMP) week 3
  • mitosis to form solid ball of cells (morula), then hollow ball (blastocyst)
Early zygote.jpg Stage2.jpg
 ‎‎Day 3 to 6
Page | Play

Human-oocyte to blastocyst.jpg

Week 1 Development

Week1 001 icon.jpg
 ‎‎Week 1
Page | Play

Occurs freely floating in uterus

Week1 summary.jpg

Week 2 Development

  • Implantation
  • initial attachment to uterine wall
  • invasion of uterine wall
Week2 001 icon.jpg
 ‎‎Week 2 - Implant
Page | Play
Chorion 001 icon.jpg
 ‎‎Week 2 - Bilaminar
Page | Play


Detect Pregnancy

pregnancy test
Ovary - corpus luteum secretes hormone to support pregnancy.
  • Clinically can be detected following implantation (week 2)
  • Last Menstrual Period (LMP) - today ? ....... Birth Date -

Calculate a new Birth Date (I need to update calculator for 2010)

Gestation Calculation

  • First pregnancy (primipara) 274 days, just over 39 weeks
  • Subsequent pregnancies (multipara) 269 days, 38.4 weeks

Median duration of gestation assumed from ovulation to delivery

  • Historic - Franz Carl Naegele (1777-1851), first rule for estimating pregnancy length
  • Current - Ultrasound, the most accurate staging method


  • Divide the pregnancy into 3 "blocks" of about 3 months (trimesters)
  • First Trimester - embryonic period (organogenesis)
  • Second and Trimester - fetal period (growth)

Implantation Sites

Abnormal Implantation

Tubal pregnancy.gif
  • Ectopic Sites
    • external surface of uterus, ovary, bowel, gastrointestinal tract, mesentery, peritoneal wall
    • If not spontaneous then, embryo has to be removed surgically
  • Uterine - tubal pregnancy (most common ectopic)

Normal Implantation

  • Uterine body
    • posterior, anterior, superior, lateral (most common posterior)
    • inferior implantation - placenta overlies internal os of uterus Placenta Previa

Early Placenta

  • interaction between implanting conceptus and uterine wall (endometrium)
  • The uterine lining following implantation (Decidua)
    • forms 3 distinct regions, at approx 3 weeks
    • Decidua Basalis - implantation site
    • Decidua Capsularis - enclosing the conceptus
    • Decidua Parietalis - remainder of uterus
  • uterine cavity is lost by 12 weeks


  • Materno/fetal organ
  • No exchange of blood
  • Many different roles and can be "sampled" as part of a prenatal diagnostic test

Embryonic Development

  • Embryonic Period - Week 1 to 8 (first trimester)
  • Establish the basic structure of organs and tissues (Organogenesis)
  • development and growth of the placenta (Placentation)
Embryo stages 002 icon.jpg
 ‎‎Human Embryo
Page | Play

Human Carnegie stage 1-23.jpg

Week 3

  • 3 Key processes commence
Week3 folding icon.jpg
 ‎‎Week 3
Page | Play


Trilaminar embryo
Trilaminar embryo and tissue origins
  • the formation of the 3 layer embryo (trilaminar embryo)
    • All tissues of the body are formed from these 3 embryonic tissue layers (germ layers)
  1. Ectoderm (epithelium)
  2. Mesoderm (connective tissue)
  3. Endoderm (epithelium)
  • simplified explanation of the 3 layer contributions


  • forms the central and peripheral nervous system and epithelium of the skin


  • forms gastrointestinal tract organs and the epithelium of the gastrointestinal and respiratory tracts


  • forms the body connective tissues: blood, bone, muscle, connective tissue skin, gastrointestinal and respiratory tracts


  • segmentation of the mesoderm
  • forms the axial body plan


  • segmentation of the ectoderm
  • separates the neural tissue from the skin (epidermis)

Week 4

  • heart formation (cardiogenesis)
  • first functioning organ

Amnion 001 icon.jpg Chick Heart 001-icon.jpg Mouse CT E11.5 movie-icon.jpg

Week 4-8

  • early development of the other organs, tissues and limbs

Stage20-23 limbs.jpg

Fetal - Week 9 - 38

Fetal Length
Fetal Weight
  • Fetal Period - Second and Third Trimester
  • Continuing growth and differentiation of organs formed in embryonic period
    • some organs have a later development - neural, genital, respiratory, bones
    • some continue to develop after birth - neural, cardiovascular, genital, respiratory, bones

Second Trimester

  • growth in size, length
  • some organs begin to function
    • endocrine release hormones important for fetal development.
  • ossification, cartilage converted to bone.
  • neural growth
  • external genital development

Third Trimester

  • growth in size, length and weight.
  • some organs complete development, respiratory, testis descent

Fetal growth icon.jpg Ultrasound12wk 3D image2.jpg 19weeklabel1.jpg

Links: Fetal Development | Second Trimester | Third Trimester


Historic teaching model of birth
Birth MRI icon.jpg
 ‎‎Human Birth MRI
Page | Play
  • birth (parturition) is a complex physiological process between the fetus and mother
  • thought to be initiated by the fetus

Maternal Birth Stages

  1. Dilatation
  2. Expulsion
  3. Placental
  4. Recovery


Newborn infant (perinatal period)

Newborn (perinatal) needs to activate many systems and establish independent regulation (homeostasis)

  • Lung function - Fluid drainage, Gas exchange, muscular activity
  • Circulatory changes - Closure of 3 vascular shunts
  • Thermoregulation - metabolic rate, fat metabolism
  • Nutrition - gastrointestinal tract function, peristalsis
  • Waste - kidney function
  • Endocrine function - loss of placenta, maternal hormones

Critical Periods of Development

Abnormal Development

Three main causes:

  1. Genetic
  2. Environmental
  3. Unknown
  • First trimester most critical
  • Different effect depending on time of insult (teratogen)

Human-critical periods of development.jpg


  • Prenatal diagnosis - number of different techniques (non-invasive, invasive) for determining normal development
  • Neonatal diagnosis (APGAR test, Guthrie test)
  • Maternal diagnosis - often pregnancy will expose maternal health problems

Revision Notes

  • You don't need to know everything today, its an introduction.
  • Don't confuse "germ cell layers" (ectoderm, mesoderm, endoderm) with "germ cells" (egg, spermatazoa).
  • Remember the difference between "clinical weeks" (last menstral period, GA) and "embryonic weeks" (from fertilization, 2 weeks later).
  • Revise meiosis and the difference between mechanism and timecourse for oogenesis and spermatogenesis in generating haploid gametes.
  • With abnormalities, think about the types of prenatal dianostic techniques that are now available, the 2 major types (genetic and environmental) and the effect of maternal age/lifestyle.

Glossary Links

Glossary: A | B | C | D | E | F | G | H | I | J | K | L | M | N | O | P | Q | R | S | T | U | V | W | X | Y | Z | Numbers | Symbols | Term Link

Cite this page: Hill, M.A. (2024, May 18) Embryology Pre-Medicine Program - Embryology. Retrieved from

What Links Here?
© Dr Mark Hill 2024, UNSW Embryology ISBN: 978 0 7334 2609 4 - UNSW CRICOS Provider Code No. 00098G