3rd LectureEmbryology

Dr. Khaldoun Darwich

Developmental Disturbances with Ectoderm

►The Syndrome of Ectodermal Dysplasia involves the abnormal development of one or more ectodermal structures .

►This syndrome has a hereditary etiology and affected persons may have abnormalities of the teeth, skin, hair, nails, eyes, facial structure, and glands, because these are derived from ectoderm or associated embryological tissues

►Children with all the features of this syndrome resemble "little old men."

Developmental Disturbances with Ectoderm

►The Syndrome of Ectodermal Dysplasia

Developmental Disturbances with Ectoderm

►Persons with ectodermal dysplasia may suffer from partial or complete anodontia; the absence of some or all teeth in each dentition .

►The teeth present for both dentitions have frequent malformations.

►Partial or full dentures are used for both functional and cosmetic purposes and need to be reconstructed periodically as the jaws continue to grow.

► Implants may be considered after growth halts, if enough alveolar bone is present

Developmental Disturbances with Ectoderm

Fourth Week Of Prenatal Development

►During the fourth week of prenatal development, the disc undergoes embryonic folding into an embryo, placing the tissues in their proper positions for further embryonic development.

►The folding of the flat embryonic disc also results in a somewhat tubular embryo.

►This folding results from extensive proliferation of the ectoderm and differentiation of basic tissues.

►This occurs mainly at the cephalic end, where the brain will form.

►This tissue grows beyond the oropharyngeal membrane and overhangs the developing heart.

Fourth Week Of Prenatal Development

► Folding due to increased growth occurs not only at the cephalic end but also at the caudal end and at the sides of the embryo simultaneously.

►Because of this folding, the positions of the embryonic layers take on a more recognizable placement for the further development of the embryo.

Fourth Week Of Prenatal Development

►After folding of the disc, the endoderm lies inside the ectoderm, with mesoderm filling in the areas between these two layers .

►This forms one long hollow tube lined by endoderm from the cephalic end to the caudal end of the embryo, specifically from the oropharyngeal membrane to the cloacal membrane.

►This tube is the future digestive tract and is separated into three major regions: the foregut, the midgut, and the hindgut.

4th Week: Disc – Folding - Embryo

Fourth Week Of Prenatal Development

►The anterior portion of this tube is the foregut, which forms the primitive pharynx, or throat, and includes a portion of the primitive yolk sac as it becomes enclosed with folding

►The other more posterior portions, the midgut and hindgut , form the rest of the pharynx as well as the remainder of the digestive tract.

►During development of the digestive tract, four pairs of pouches form from evaginations on the lateral walls lining the pharynx.

►These are called the pharyngeal pouches

Fourth Week Of Prenatal Development

► Finally, during the fourth week, the face and neck begin to develop, with the primitive eyes, ears, nose, oral cavity, and jaw areas. (will be discussed later)

Developmental Disturbances During the Embryonic Period

►Because the beginnings of all essential external and internal structures are formed during the embryonic period, this is the most critical period of development.

►Developmental disturbances during this period may give rise to major congenital malformations of the embryo .

►Any teratogens can cross by way of the placenta. Thus, teratogens can be present during the active differentiation of an organ or tissue, possibly raising the incidence of these malformations.

Developmental Disturbances During the Embryonic Period

►An example of a: virus can be transmitted embryo from the pregnant woman.

►This infection of the embryo, can result in cataracts, cardiac defects, and deafness in the child.

►Another infective teratogen for the embryo is the syphilis spirochete , Treponema Pallidum, because it produces defects in the incisors and molars,• as well as blindness, deafness, and paralysis

Developmental Disturbances During the Embryonic Period

►An example of the result of a teratogenic drug effect during the embryonic period is fetal alcohol syndrome.

►Ethanol ingested by a pregnant woman easily crosses the placenta and can result in prenatal and postnatal growth deficiency, mental retardation, and other anomalies.

Developmental Disturbances During the Embryonic Period

►An affected child may have a cluster of facial changes such as small head circumference, a low nasal bridge, a short nose, a small midface, widely spaced eyes with epicanthic folds and eyelid fissures, an indistinct philtrum, and a thin Upper lip

►Oral changes such as anterior open bite and crowding of the dentition, mouth breathing, and related gingivitis may occur, possibly because of an increase in the habit of finger sucking.

Developmental Disturbances During the Embryonic Period

Developmental Disturbances During the Embryonic Period

Developmental Disturbances During the Embryonic Period

►Direct exposure to high levels of Radiation can act as an environmental teratogen during the embryonic period.

►Radiation may injure embryonic cells, resulting in celldeath, chromosome injury, and retardation of mental development and physical growth.

►The severity of embryonic damage is related to the absorbed dose, the dose rate, and the state of embryonic or fetal development at the time of exposure.

Developmental Disturbances During the Embryonic Period

►Human congenital abnormalities have not been proved to be caused by a diagnostic level of radiation, such as that used in dentistry.

►Scattered radiation from a radiographic examination of the oral cavity administers a dose of only a few millirads to a pregnant woman.

►This amount is not teratogenic to an embryo.

►Nevertheless, even this small dose should be avoided unless an emergency situation requires it. Proper protective precautions should be used with all patients and, with health care personnel.

Developmental Disturbances During the Embryonic Period

► Failure of fusion of the neural tube results in neural tube defects of the tissue overlying the spinal cord, such as the meninges, vertebral arches, muscles, and skin.

► ►One type of neural tube defect is spina

bifida characterized by defects in the vertebral arches and various degrees of disability.

Developmental Disturbances During the Embryonic Period

Developmental Disturbances During the Embryonic Period

Developmental Disturbances During the Embryonic Period

►Nutritional and environmental factors have an important role as teratogens causing neural tube defects.

► Folic acid supplements are now being recommended during pregnancy to help prevent this as well as to prevent cleft lip and palate.

FETAL PERIOD OF PRENATAL DEVELOPMENT

►The Fetal period of prenatal development follows the embryonic period.

►The fetal period encompass the beginning of the ninth week, or third month, to the ninth month.

►This is a period of time of maturation of existing structures as the embryo enlarges to become a fetus .

5 months

FETAL PERIOD OF PRENATAL DEVELOPMENT

►Although developmental changes are not as dramatic as those that occur during the embryonic period, they are important because they allow the newly formed tissues and organs to function.

►Even though the embryo has been breathing since the third week, by the end of the fourth month, the fetal heartbeat can be detected.

►By the end of the 4th month mark, fetal movements can be felt by the pregnant woman.

Developmental Disturbances During the Feta Period

►Congenital malformations can also occur during the fetal period.

► In amniocentesis , the most common invasive prenatal diagnostic procedure, amniotic fluid is sampled during the fourteenth to sixteenth weeks after the last missed menstrual period.

► It is performed in women of old age groups, when one or both parents have a chromosomal abnormality or neural tube defect, when previous child was affected, or when the parents are carriers of inborn errors of metabolism or X-linkei disorders such as hemophilia.

Developmental Disturbances During the Feta Period

►Systemic tetracycline antibiotic therapy of the pregnant woman can act as a teratogenic drug during the fetal period.

►This therapy of the woman can result in permanent tetracycline Staining of the child's teeth or primary teeth that, are developing at that time.

►This intrinsic yellow to yellow-brown staining of the teeth can occur in slight, moderate, or severe degrees: The antibiotic becomes chemically bound to the dentin for the life of the tooth, and because of the transparency of enamel, this stain is visible .

Developmental Disturbances During the Feta Period

►The adult teeth, or permanent teeth, may also be ^tedrft^ drug i, given during their development .

► If the permanent teeth are involved treatment may require full-coverage crowns or veneers to alter the appearance of the teeth Thus this type of antibiotic therapy should be avoided in pregnant women and children..