UNSW Embryology

Embryology for K12 Students

© Dr Mark Hill (2008)

Acknowledgements

Week 4

Introduction

In the fourth week of embryo development organs begin to form within the embryo and surface features begin to form.

Embryo at 4 weeks (Carnegie stage 13)

On the embryo surface limb buds and special sensoryregions on and near the embryo head appear. Sensory placodes will form parts of the ear, eye and nose. Limb buds first form from 2 layers of the embryo are the "paddle-like" projections from the body. The arms appear before the legs. This period of organ development continues to 8 weeks of development. Folding of the embryo continues and the earliest functioning organ is the heart. Other systems such as the blood vessels, gut and nervous system are beginning to all take shape. (Do all animals have eyes, ears and noses?)

(More? Year 7 -12 Detailed Embryology Notes - week 4)

Human Embryo development during week 4

click the small picture to see a big picture.

Stage 10

The Embryo has now begun to form small balls of cells along the middle that will make all the muscle and bone. The dark line in the middle is the beginning of the nervous system.

In the picture the head is at the right, tail is at the left.

Stage 11

The Embryo now has an early heart and simple brainand spinal cord region. More small balls of cells can now be seen along either side of the embryo.

In the picture the head is at the right, tail is at the left.

The heart is the white bulge (right) on the bottom of the embryo.

Stage 12

The Embryo now also has an early liver and limb buds. (mainly the arms).

The brain continues to grow very fast and there is still no "head" covering it yet.

Notice the long "tail" on the embryo on the left of the picture.

Stage 13

The Embryo now also has arm and leg limb buds. The brain continues to grow very fast and the head has begun to form as 2 folds.

Notice the "tail" on the embryo on the left of the picture. The placenta is the white part near the end of the tail pointing up (towards 12 o'clock).

Next

Go to Week 5 to 8

Facts

• Our arms used to look more like our legs.
• Snakes once had legs but lost them due to changes in development.

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.
• blastocyst- a solid ball of cells that then forms a hollow ball of cells with a clump on the inside on one wall.
• embryo- the name given to the baby during the early time of growth.
• egg- the cell from the mother that contributes half the genes of the embryo.
• fertilization- A process where a single male (father) and female (mother) cell are joined together.
• genes- information in the cell that gives it rules on how to grow and what cell to become. This information is stored in DNA in the cell nucleus.
• muscle- part of the body that moves the skeleton and allows you to stand up.
• placenta- all mammals provide support for the embryo growth through the placenta. It connects the embryo with the mother. It carries food and oxygen to the embryo and takes waste and carbon dioxide from the embryo.
• sperm- the cell from the father that contributes half the genes of the embryo.
• stages- are like different years at school. Each year something new will occur, each stage something new will happen. Stages are a way of measuring the changes that occur in an early embryo and are very similar for all animals. The time that these changes occur will be different for different animals, using stages means that we can compare the development of different animals. The stages used here were originally identified and named last century at a research institue that gives the stage names "Carnegie Stages". (More? Year 7-12 Embryo Carnegie Stages).
• uterus- the muscular space within the mother where the embryo grows. It also grows when the embryo does and has lots of blood to feed the embryo and strong muscles in its walls for childbirth.
• zygote- The first cell formed after fertilization.

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