Foundations Lecture - Introduction to Human Development

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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. Of all health issues in Medicine, fertility and reproduction is a topic that will affect everyone. This lecture is going to take you briefly through key biological concepts in human development, these will later be explored in more detail through the BGD course. I will be using simplified terms in the lecture slides (with developmental term in brackets).

Australian Statistics On 23 April 2013 at 08:02:20 AM (Canberra time), the resident population of Australia is projected to be: 22,999,392

The lecture will be followed by a practical class introducing online resources for independent study and working through similar embryology concepts.

Links: Printable Lecture Page | 2012 | PDF version (873 KB, 6 pages)


  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)

Links: 2013 Practical | Embryology Textbooks

Human Development Timeline

Human development timeline graph 02.jpg

UNSW Embryology Online


<mediaplayer width='360' height='410' image="">File:Human development 001.mp4</mediaplayer>
Human development 001 icon.jpg
 ‎‎Human Development
Page | Play


Using these resources (online navigation, organization and printing) will be covered in the introduction to the associated Practical class.

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


  • There are many different excellent embryology textbooks
  • I have included 2 that cover the clinical topics as well. More Textbooks?

Links: More Embryology Textbooks

Four Basic Tissue Types

Stomach Wall containing all 4 basic tissues.

Tissues and organs of the body consist of combinations of 4 basic tissue organisations:

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

  • Where do they come from?
  • How do they develop?

Human Reproductive Cycle

  • Meiosis in gonad produces haploid gametes
    • testis the sperm (spermatozoa)
    • ovary the egg (oocyte)
  • there are several differences in when and how gametes are formed in the male and female gonad.


  • 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 sperm (spermatozoa, human male 2,000/second)
  • release millions of spermatozoa (require activation, capacitation)


  • Paired organs
  • lying in the peritoneal cavity

Human right ovary and tube 1.jpg Ovary10x.jpg


Human Menstrual Cycle

Human ovulation 01.jpg

Human ovulation

Ovulation icon.jpg Follicle 001 icon.jpg


  • the process of the 2 haploid gametes (egg and sperm) fusing and combining genetic material.
  • conceptus - the entire product of fertilization

Fertilization 001 icon.jpg Fertilization 002 icon.jpg

Early zygote.jpg

Early Development

Human Zygote
  • occurs during week 1 following fertilization
  • last menstrual period (LMP) week 3
  • mitosis to form solid ball of cells (morula), then hollow ball (blastocyst)

Human-oocyte to blastocyst.jpg

Human blastocyst day 1-6.jpg

Week 1 Development

  • occurs freely floating in uterus

Week1 001 icon.jpg

Week1 summary.jpg

Week 2 Development

  • Implantation - initial attachment to uterine wall, and then invasion of the uterine wall.

Week2 001 icon.jpg Chorion 001 icon.jpg


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 - January 30, 2014



  • 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
    • 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)

Human Carnegie stage 1-23.jpg

Embryo stages 002 icon.jpg

Week 3

  • 3 Key processes commence

1. Gastrulation

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

2. Somitogenesis

  • segmentation of the mesoderm into somites
  • forms the axial body plan

3. Neuralation

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

Week3 folding icon.jpg

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

Week 9 - 38

  • 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, genital, respiratory, bones
  • growth in size, length (Second Trimester)
  • growth in weight (Third Trimester)

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


Historic teaching model of birth
MRI Birth
  • 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

Australian Birth Rate 1998-2007


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

Human critical periods of 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

Human development timeline
  • You don't need to know everything today, this is an introduction.
  • Use the glossary to help understand new terms.
  • Don't confuse "germ cell layers" (ectoderm, mesoderm, endoderm) with "germ cells" (egg, spermatazoa).
  • Remember the difference between "clinical weeks" (last menstral period) and "embryonic weeks" (from ovulation/fertilisation, 2 weeks later).
  • With abnormalities
    • think about the types of prenatal dianostic techniques that are now available
    • the 2 major types (genetic and environmental)
    • the effect of maternal age/health/lifestyle.


Logo.png Hill, M.A. (2013) UNSW Embryology (13th ed.). Sydney:UNSW.
The Developing Human, 9th edn.jpg Keith L. Moore, T.V.N. Persaud, Mark G. Torchia. (2011). The Developing Human: clinically oriented embryology (9th ed.). Philadelphia: Saunders.
Larsen's human embryology 4th edn.jpg Schoenwolf, G.C., Bleyl, S.B., Brauer, P.R. and Francis-West, P.H. (2009). Larsen’s Human Embryology (4th ed.). New York; Edinburgh: Churchill Livingstone.

The following chapter links only work with a UNSW connection and can also be accessed through this UNSW Library connection.

Foundsmall.jpg Foundations Practical - Introduction to Human Development

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. (2021, April 20) Embryology Foundations Lecture - Introduction to Human Development. Retrieved from

What Links Here?
© Dr Mark Hill 2021, UNSW Embryology ISBN: 978 0 7334 2609 4 - UNSW CRICOS Provider Code No. 00098G