UNSW Embryology

Embryology for K12 Students

© Dr Mark Hill (2008)

Acknowledgements

Arms and Legs

The arms and legs (we call them "limbs") begin as small buds on the sides of the embryo body.

From week 5 to week 8 of embryo development major changes occur in the shape and structure of the embryo limbs.

The arms begin to develop before the legs and if you look at the whole embryo the legs always look younger (less developed) than the arms (almost a stage behind, about 2 days difference).

Look below at the series of images and see what changes you can see in the shape, features and position of the arm. Towards the end of this period (9 weeks) the embryo will also begin to move the limbs. So far we have only put the arm images on this page and not the legs.

Questions

• What animals have more than 2 arms and legs?
• What are fish and birds "arms" and "legs"?
• We have a thumb that is opposite (opposable thumb) our fingers. What does this allow?
• Why do your elbows bend forward and your knees backward?

Facts

• The arm begins to form before the leg.
• Cells within the developing arm and leg come from round balls of cells (somites) that occur beside the developing spinal cord in the body.
• Snakes used to have legs and have lost them, by loosing the signals that form their limbs.
• You first have paddles for hands, and cells die between your fingers so that you have separate fingers "digits".
• Some people have cells that have not been totally lost "webbing" between their fingers or toes.

Human Arm (Upper Limb, week 5 to 8)

The pictures below show how the arm (upper limb) changes in shape between week 5 and week 8, from a small bud (in the first picture) to something that looks like your own arm in the last. At the bottom there is also a movie showing you these changes.

Arm Movie Page or Movie on new page (179Kb)

Human Embryo development during week 5 to week 8

click the small picture to see a big picture.

Stage 15 (week 5)

Stage 16 (week 6)

Stage 18 (week 7)

Stage 19

Stage 20

Stage 21

Stage 22

Stage 23 (week 8)

Ultrasound

Movie of 8 week embryo ultrasound showing the limb buds, heart beating and spine.

Quicktime movie 1.8 Mb

Ultrasound uses a beam of sound waves (frequency 3 to 10 MHz) reflecting back off diffferent structures and analysed by a computer this can then be shown as a picture on a screen.

Detailed Notes

A detailed coverage of limb development is given in the Systems Notes covering the Musculoskeletal system - Limb development notes/P>

Words Used

• bone- the skeleton of the body. This gives your body a shape so that you are not a blob lying on the ground.
• embryo- the name given to the baby during the early time of growth.
• limb buds- these are the early arms and legs of the embryo. At this time they look like paddles either side ou the embryo.
• muscle- part of the body that moves the skeleton and allows you to stand up.

Quick Movie Links

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

Movie of Limb Growth(this shows development of the human arm from bud to paddle, to hand with arm)

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

Movie of Rotating Embryo(this shows an early embryo from all angles)

All Moviesthis shows many different ways of looking in at changes in different embryos.

Quick Links

Week 1 | Week 2 | Week 3 | Week 4 | Week 5 to 8 | Week 9 to 12 | Week 13 to 15 | Week 16 to 20 |

Development Notes ----------------- Menstrual Cycle Week 1 Week 2 Week 3 Week 4 Placenta Serial Images Genes/DNA Molecular Dev Abnormal Dev Postnatal Dev Notes Index

System Notes ----------------- Heart/Cardiovascular Respiratory Coelomic cavity Gastrointestinal Tract Head and Neck Neural Neural Crest Eye Ear Nose Endocrine Urogenital Musculoskeletal Integumentary

Movies ----------------- Embryo Growth Ultrasound Week 1 Week 2 Week 3 Week 4 Stage 13/14 Stage 22 Neural Head and Neck Heart Vasculature Gastrointestinal Urogenital Other Embryos Larsen Internet

Other Embryos ----------------- Chicken Fly Frog Mouse Rabbit Rat Worm Zebrafish Guinea Pig

UNSW Embryology ISBN: 978 0 7334 2609 4

UNSW CRICOS Provider Code No. 00098G