Introduction to Embryology Dr. Khaldoun Darwich Ph.D. in Oral and Maxillofacial Surgery

► Syllabus

Why do we study Embryology?

“the one who sees things from the beginning will have the finest view of them” Aristotle, B.C.

► It is important for dental professionals to understand the major events of prenatal development to understand better the development of the structures of the face, neck, and oral tissue and the underlying relationships among these structures

► What is EMBRYOLOGY?

Embryology ► Definition: ► Is the study of prenatal development

► Prenatal period: In Humans, prenatal development begins at the start of pregnancy and continues until the birth of the child. 38 weeks from conception to birth

Terminology Different to that used in gross anatomy

Prenatal development : Consists of three distinct periods

► Periods of Prenatal development :  Preimplantation period  Embryonic period  Fetal period

Preimplantation Period

Developmental Disturbances during prenatal development ► These developmental problems can include congenital malformations or birth defects, which are developmental problems evident at birth.

Developmental Disturbances during prenatal development ► Most of these occur during both the preimplantation period and the embryonic period and thus the first trimester of the pregnancy

Developmental Disturbances during prenatal development ► Statistics show that such malformations occur with an incidence of 1 in 700 live births. ► This does not include anatomical variants which are common, such as variation in the lesser details of a bone’s shape.

Developmental Disturbances during prenatal development ► Malformations can be due to genetic factors such as chromosome abnormalities or enviromental factors. ► These enviromental factors can include infections, drugs, and radiation and are called teratogens.

Teratogens ► Enviromental influences or agents that can damage the developing organism

Teratogens ► Women of reproductive age should avoid teratogens at the time of their first missed menstrual period and thereafter to protect the developing human

Influence of Teratogens  Larger doses, over time, more negative  Genetics can help or hinder teratogens  Several negative factors increase impact  Sensitive periods especially important  Embryonic period most susceptible  Physical and psychological effects

Drugs ► Thalidomide – sedative ’s ► Thalidomide – sedative ’s ► Aspirin - low birth weight, infant death Poor motor develop., lower IQ ► Heavy caffeine use - miscarriage Premature births, irritability

Illegal Drugs ► Cocaine, heroin, methadone  Prematurity, low birth weight  Physical defects, breathing problems,  Drug addiction & death ► Cocaine - lasting difficulties  Genital, urinary tract, kidney, heart deformities  Brain seizures

Tobacco use ► Low birth weight and premature births ► Impaired breathing ► Miscarriage & infant death ► Placenta doesn’t work correctly ► Greater concentration of carbon monoxide ► Passive smoking also injurious

Alcohol ► Fetal alcohol syndrome (FAS)  Large amounts of alcohol, whole pregnancy  Mental retardation  Slow physical growth  Facial abnormalities ► Alcohol interferes with cell duplication  Especially neural cells ► Draws oxygen away from organism

Fetal Alcohol Syndrome

Hormones ► Diethylstilbestrol (DES),  Vaginal cancer  Malformation of the uterus  Miscarriage, low birth weight

Radiation ► Hiroshima & Nagasaki  Miscarriage, slow physical growth  Underdeveloped brains  Malformation of skeleton & eyes ► X-Rays - avoid during pregnancy

Environmental Pollutants (Chemicals) ► Mercury  Mental retardation, speech problems  Difficulty chewing & swallowing  Uncoordinated movements ► Polychlorinated Biphenyl (PCB)  Lower birth weight, smaller heads  Poor memory, lower verbal IQ

Infections ► Viruses - Most not harmful ► Rubella  Heart defects, cataracts, deafness  Genital, urinary & intestinal problems ► HIV & AIDS  20-30% Mothers pass to infants  Most survive 5 to 8 months

Malformations ► In the face, neck, and oral cavity range from serious clefts in the face or palate region to small deficiencies of the soft palate or cysts underneath an otherwise intact oral mucosa.

It is important for dental professionals to remember that : ► Any orofacial congenital malformations found when examining a patient are understandable and traceable to a specific time in the embryological development of the individual.

Preimplantation period of prenatal development: ► It takes place during the first week. ► At the beginning of the first week a woman’s ovum is penetrated by and united with a man’s sperm during fertilization

FERTILIZATION Week 1

Passage of sperm through corona radiata surrounding the zona pellucida Hyaluronidase from sperm acrosome Changes the makeup of zona pellucida FERTILIZATION Week 1

► Zona reaction occurs – lysosomal enzymes released by cortical granules near plasma membrane of oocyte Esterases, acrosin, and neuramidase released from acrosome ► Completion of second meiotic divison occurs with formation of male and female pronucleus FERTILIZATION Week 1

Preimplantation period of prenatal development: ► This union of the ovum and sperm subsequently forms a fertilized egg, or zygote ► during fertilization the final stage of meiosis occur in the ovum.

► At conception in fallopian tube, maternal and paternal genetic material join to form a new human life (zygote)

Meiosis ► The result of this process is the joining of the ovum’s chromosomes with those of the sperm. ► This joining of chromosomes from both biological parents forms a new individual with shuffled chromosomes

Meiosis ► this process enables the ovum and sperm to reduce by one half the normal number of chromosomes ( to a haploid number of 23 ) ► Thus the zygote has received half its chromosomes from the female and half from the male with the resultant genetic material a reflection of both biological parents

Week 1 post conception ► Zygote ► Morula ► Blastocyst

Mitosis ► After fertilization the zygote then undergoes Mitosis or individual cell division or cleavage

Morula ► After initial cleavage the solid ball of cells is known as a morula

Blastula ► Because of the ongoing process of mitosis and secretion of fluid by the cells within the morula the zygote becomes a vesicle known as a blastocyst (Blastula) ► The rest of the first week is characterized by further mitotic cleavage, in which the blastocyst splits into smaller and more numerous cells as it undergoes successive cell divisions by mitosis

Mitosis and Meiosis ► What is Mitosis? ► a type of cell division that results in daughter cells each with the same number and kind of chromosomes as the parent cell. ► This type of cell division allows multicellular organisms to grow and repair damaged tissue

Mitosis and Meiosis ► What is Meiosis? ► a type of cell division that results in daughter cells each with half the number of chromosomes of the parent cell ► Meiosis enables organisms to reproduce sexually. sexually

Blastocyst stage ► As it grows by cleavage the blastocyst travels from the site where fertilization took place to the uterus ► By the end of the first week the blastocyst stops travelling and undergoes implantation and thus becomes embedded in the prepared endometrium the innermost lining of the uterus.

Blastocyst stage ► After 7 days of cleavage the blastocyst consists of Two distinct types of cells:  Inner cell mass (embryoblast layer)  Trophoblast layer

Endometrium of uterus Implanted blastocyst

Blastocyst stage ► The trophoblast layer gives rise to important prenatal support tissues ► The embryoplast layer gives rise to the embryo during the next prenatal period