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3
Lecture
Embryology
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
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.
► Although
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..