The University of the West Indies,
at Mona

Development and Differentiation

Introduction to Embryology

“He who sees things grow from the beginning will have the finest view of them.”

­Aristotle 384 ­ 322 B.C.

Table of Contents

• Introduction
• How to use this guide
• Assessment
• Evaluation
• Aims
• Content
• Learning Objectives
• Teaching Method
• Resources
• Lecture Titles
• Glossary
• Self­Assessment Questions
• Answers and feedback

Introduction

This preliminary course in human development complements the Cell Biology and Molecular Medicine modules of the Development and Differentiation Course and is delivered early in Semester one. It is designed as a prelude to the detailed study of the structure and functioning of the body systems and provides a basis for an understanding of congenital abnormali­ties.

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How to use this guide

Read through the guide at least once to give you an idea of the format of the module. The ‘Aims ’ should be useful to assist you in defining the areas that need to be covered. Look at the lecture titles prior to attending lectures. They will give you a better idea of the upcoming topics and allow you the opportunity to do some preparatory reading.

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Assessment

Students will be assessed at the end of the semester in the integrated multi­ple choice examinations.

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Evaluation

The Curriculum Committee is particularly interested in finding out the views of students about their experience in the MBBS curriculum. A Semester 1 course evaluation form will be given to you near the end of the semester. The committee would appreciate a few minutes of your time to fill in this form, as the feedback will be used to assist in maintaining and improving the MBBS programme.

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Aims

To provide an understanding of how tissues and organs develop from a single cell into a complex multi­cellular organism and rational explanation of the relationships and positions of normal adult structures.

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Content

The important content areas dealt with by this course include:

• Gametogenesis
• Formation of the embryonic discand the chorionic sac
• Formation of germ layers and the early differentiation of tissues
• The organogenetic period
• The fetal period
• Development of the placenta and fetal membranes
• Development of the body cavities
• Development of the mesenteries and diaphragm
• Pharyngeal operators
• Birthdefects

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Learning Objectives

At the end of this module the student should be able to:

• Describe the process of spermatogenesis
• Explain the term non disjunction
• Describe the process of ovulation
• Describe the process of fertilization
• Describe cleavage of the zygote and implantation of the blastocyst
• Describe proliferation and differentiation of the trophoblast
• Describe the development of the amniotic cavity, bilaminar embryonic disc, yolk sac and the connecting stalk
• Describe the formation and growth of the primitive streak
• Define primitive knot, primitive groove and primitive pit
• Describe development of the notochord and neural tube
• Describe the development of the cardiovascular system and list the sites of blood formation
• Describe the allantois, oropharyngeal membrane and cloacal membrane
• Describe the general body form resulting from folding of the embryo and the effect of this process on the septum transversum, heart, foregut, midgut, allantois and yolk sac.
• List the significant differences between development during the embry­onic and fetal periods.
• Describe the differences in the vulnerability of embryos and foetuses to teratogenic agents
• List the possible effects of environmental agents and other factors on fetal growth and development.
• Describe the processes of intrauterine growth retardation, amniocente­sis, foetuscopy, fetal blood sampling and ultrasonography.
• List six common congenital anomalies and the embryonic basis for them
• Describe the genetic factors responsible for fetal development

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Teaching Method

The content of this module will be taught in a series of lectures.

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Resources

The Developing Human, Clinically Oriented Embryology.

Moore and Persaud, Saunders Co.

Study Guide and Review Manual of Human Embryology.

Moore and Persaud Saunders Co.

Notes on General Embryology.

Prof. T.L. The, Dept. of Anatomy, UWI, Mona.

Websites (optional)

1. http://www.med.uc.edu/embryology/
2. http://www.ucalgary.ca/UofC/eduweb/virtualembryo/
3. http://www.med.unc.edu/embryoimages/
4. http://www.ob­ultrasound.net/

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Lecture Titles

Session	Subject Matter	Resource
1	Cell: Structure/Mitosis and Meiosis Tissue: Epithel/Glands	Prof. Guldner
2	Gametogenesis	Dr. A.S. d’Souza
3	Fertilization, Cleavage, Implantation.	Dr. A.S. d’Souza
4	Formation and fate of germ layers.	Dr. A.S. d’Souza
5	Formation and fate of germ layers.	Dr. A.S. d’Souza
6	Formation of basic tissues of the body.	Dr. A.S. d’Souza
7	Notochord and neural tube formation.	Dr. A.S. d’Souza
8	Yolk sac and folding of the Embryo.	Dr. A.S. d’Souza
9	Foetal membranes and Placenta.	Dr. A.S. d’Souza
10	Congenital malformations.	Dr. A.S. d’Souza

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Glossary

• Abortion ­ premature expulsion of a conceptus from the uterus or expulsion of an embryo or foetus before it is viable­capable of living outside the uterus.
• Blastocyst ­ (Gr. blastos, germ + kystis, bladder). A morula with a fluid­filled cavity.
• Cleavage ­ A series of mitotic cell divisions of the zygote that result in the formation of early embryonic cells, the blastomeres.
• Conceptus ­ (L. concepito, derivates of a zygote). Refers to the embryo and the associated membranes with the placenta.
• Embryo ­ (Gr. embryon). Developing human being during its early stages of development.
• Embryonic Period ­ This period extends from fertilization to the end of 8th week of pregnancy by which time the beginnings of all major struc­tures are present.
• Fertilization ­ The union of an oocyte and a sperm.
• Foetus ­ The developing human after the embryonic period (from 9 weeks after fertilization to birth).
• Gastrula ­ (Gr. gaster, stomach). A trilaminar embryonic (ectoderm, meso­derm and endoderm) disc that differentiates into the tissues and organs of the embryo.
• Implantation ­ The process during which the blastocyst attaches to the endometrium (lining of the uterus).
• Morula ­ (L. morus, mulberry). A mass of blastomeres on the 3rd to 4th day after fertilization just as the developing human enters the uterus.
• Neurula ­ (Gr. neuron, nerve). The embryo in the 4th week with the forma­tion of the neural tube, the primordium of the central nervous system (Brain and Spinal Cord).
• Oocyte ­ (L. ovum, egg). The female germ cell produced in the ovaries. Primordium ­ (L. primus, first + ordior, to begin). Refers to the first indi cation for the earliest stage of development of an organ or structure. Sperm ­ (Gr. sperma, seed). The male germ cell produced in the testes. Zygote ­ (Gr. zygotos, yoked). The beginning of a new human being result­ing from the union of an oocyte and a sperm.

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Self-Assessment Questions

Each of the following statements or questions is followed by five suggested responses or completions. Select the one best answer in each case.

1. The 8­day blastocyst
   1. has a single layer of trophoblast at the embryonic pole
   2. has lucanae in the syncytiotrophoblast
   3. is partially implanted in the endometrium
   4. is covered by the uterine epithelium
   5. has a primordial yolk sac.
2. The amniotic cavity develops
   1. initially on the 10th day
   2. within the exocoelomic cavity
   3. between the embryoic disc and the trophoblast
   4. in the extraembryonic mesoderm
   5. during the first week.
3. A 32 year old woman was admitted to hospital because of abdominal pain and hemorrhage. Her last menstrual period was about 7 weeks ear­lier. Which of the following is the most likely diagnosis?
   1. appendicitis
   2. ovarian cysts
   3. miscarriage
   4. ectopic pregnancy
   5. placenta previa
4. The wall of the chorionic sac is composed of:
   1. cytotrophoblast and syncytiotrophoblast
   2. two layers of trophoblast lined by extraembryonic somatic mesoderm
   3. trophoblast and the exocoelomic membrane
   4. extraembryonic splanchnic mesoderm and both layers of trophoblast
   5. none of the above is correct

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Answers and Feedback

1. c
   • The 8­day blastocyst is partially implanted in the endometrium.
   • The trophoblast at the pole opposite the embryo (abembryonic pole) remains relatively undifferentiated, consisting of a thin layer of flattened cytotrophoblastic cells.
   • The trophoblast consists of two layers only where it is in contact with the endometrium.
   • The primitive yolk sac is not usually present at 8 days, but the amniotic cavity is represented by a small, slitlike space.
2. c
   • The amniotic cavity appears as a small cavity in the inner cell mass on the eighth day after fertilization between the embryonic disc and the invading syncytiotrophoblast.
   • It does not develop in the exocoelomic cavity or in the extraembryonic mesoderm.
   • The amnion forms from cells derived from the epiblast.
3. d
   • The Clinical symptoms, as well as the missed menstrual period, suggest an ectopic pregnancy (i.e., the blastocyst is implanted outside the uter­ine cavity). More than 90% of ectopic implantation occur in the uterine tubes, the majority in the ampulla or infundibulum. The rupture of the uterine tube and haemorrhage are serious complications. To prevent this, the affected tube and conceptus are surgically removed.
4. b
   • The wall of the chorionic sac is composed of the chorion, which is formed by the combination of extraembryonic somatic mesoderm and the two layers of trophoblast (cytotrophoblast and syncytiotrophoblast).
   • The chorionic sac contains the embryo, which is attached to its wall by the connecting stalk.

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| Last Updated: August 24, 2004
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