1. Biology 1110 Principles of Biology
Biology 1110 Laboratory
Laboratory # 10
Embryology – The Study of the Early Embryo
Support/Review Materials
All of the micrographs and model pictures in this presentation were photographed in the MnWest Biology laboratory unless otherwise noted.

2. Study of the Early Embryo
In this presentation we will study the development of an animal from the fertilized egg (zygote) to the embryonic stage known as the Gastrula.
We take our study to the level of the Gastrula as to be able to demonstrate the beginnings of differentiation.
The development that you will see is very similar to the development of the human embryo.

3. Fertilization
When the sperm cell fertilizes the egg it delivers half of the genetic information required for the new life.
The fertilization also initiates the early cell divisions called cleavage or segmentation divisions.
Unfertilized Egg or Ovum
Sperm cell

4. The Early Embryo
In the fertilized egg the pronuclei of the sperm cell and egg cell are not visible.
The micrograph on the right contains a fertilized egg and the embryo that results from the first cell division. Note the fertilization membrane visible around the 2 cell embryo.

5. Early Embryos
The micrographs below contain the embyos that develop during the first few days of life.
This development takes place in the oviduct as the embryo is traveling towards the uterus.
Eight Cell Stage
Two Cell Stage
Four Cell Stage
Morula

6. Blastomeres The cells of the early embryo are pretty much identical.
These undifferentiated cells of the early embryo are called blastomeres.
The four cell stage contains four balstomeres; the eight cell stage contains eight blastomeres.
A morula is a solid cluster of blastomeres.

7. Identify the Early EmbryosC D B
Check your answers on the next slide.

8. Identify the Early EmbryosC D B
A – a morula
B – the fertilized egg
C – Two cell stage
D – another morula
E – Eight cell stage

9. Blastula or Blastocyst
The rapid cleavage of the embryonic morula cells continues.
Through a process called blastulation the cells begin to organize themselves around a fluid-filled cavity.
This fluid-filled cavity is called the blastocoel.
This process results in the formation of the blastula or blastocyst as it is called in mammals.
Two Blastulas
Blastocoel

10. Blastula or Blastocyst
In the human, the embryo arrives at the uterus in about 5 days.
During the next few days the blastula (blastocyst in mammals) develops.
It is at this stage when the developing embryo begins the process of implantation.
Implantation involves the embryo embedding itself in the endometrium (lining of the uterus).

11. Gastrula
All continued development of the embryo takes place within the endometrium.
The process of gastrulation leads to the formation of the gastrula.
An inner cell mass begins to develop into the fluid-filled cavity (the blastocel).
This marks the beginning of the process of differentiation.

12. Differentiation
Differentiation is the process through which relatively unspecialized cells become specialized. It is through differentiation that specific cell and tissue types develop.
With the development of this inner cell mass we are seeing the formation of two of the three embryonic germ (tissue) layers.
An Early Gastrula

13. Differentiation in the Gastrula
The three embryonic germ are the ectoderm, endoderm, and the mesoderm. From these germ layers all other tissues arise.
The developing inner cell mass is the endoderm, the remaining cells form the ectoderm.
Ectoderm
Endoderm

14. Gastrula
As the gastrula continues to develop the endoderm develops a cavity called the archenteron.
The archenteron is the primitive gut. The opening to the primitive gut is the blastopore.
Differentiation is well underway.
Two Late Gastrulas

15. The late Gastrula Archenteron Ectoderm Ectoderm Blastopore Blastopore
Endoderm
Endoderm

16. Differentiation of germ layers
Ectoderm - the outer body covering; skin and associated structures, nervous system, sensory organs such as the eye and ear.
Mesoderm - bone, cartilage, muscle, blood & blood vessels, lymphatics, kidneys, gonads, & other. The mesoderm develops a little later between the endoderm and ectoderm.
Endoderm - epithelial linings of the digestive, urinary, and respiratory tract, & other.

17. Embryology Models Two Cell Stage The fertilized egg (zygote)
A Blastomere

18. Embryology Models Four Cell Stage Eight Cell Stage A Blastomere

19. Embryology Models
Morula – 16 cell
Later Morula

20. Embryology Models
Blastula - hemisection
Blastula
Blastocoel

21. Embryology Models
Identify the following: ectoderm, endoderm, blastopore,
Archenteron, and blastocoel.
Early Gastrula
Later Gastrula

22. The End