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EMBRYONIC DEVELOPMENT & STEM CELL COMPENDIUM

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I. Anomalies of deve lopment

A. ETIOLOGIC FACTORS: malformations deve lop as a result of 3

mechanisms

1. Noxious influence of ext ernal factors during the first phase of

development. This type of teratogenic agent has been studiedexperimentally

2. Transmission of a gene tic abnormality by the parent s: is

inscribed in the genet ic code of one or both parents and

transmitted according to the laws of he redity

a. The fe tus may be the carrier of a purely molecular

abnormality, such as hemophilia, due to the absence of

a globulin necessary for coagulation

This molecular abnormality also may be translated onthe cellular scale: an abnormalhemoglobincan bringabout a visible RBC deformity

b. More rarely, certain metabolic anomalies bring about a

morphologically visible malformation, such as the

dwarfism and body deformities of achondroplasia

c. Chromosome aberration existing in one o f the gametes

or appearing during the first divisioni. The mechanism can result in an abnormality of

chromosome constitution at the time of

gametogenesis

ii. At fert ilization, the zygot e resulting from the

above gamete will have an abnormal karyotype

which will be transmitted to all cells of the

embryo. Thus, the abnormality can involve

either an autosome or a sex chromosome

B. AUTOSOMAL ABERRATIONS

1. The most frequent is mongolism or Down's syndromewhich is

caused by trisomy 21; that is, chromosome pair 21 consists of

3 chromosomes instead of One gamete, t he ovum (most

frequently abnormal) carries two 21 chromosomes, w hereas

the other, the spermatozoon, normally carries only on The

zygote thus cont ains three 21 chromosomes and 47

chromosomes in t otal

a. Characte ristics: mental deficiency, brachycephaly, flat

nasal bridge; slant to eyelid fissures; protruding tongue,

simian crease, congenital heart defects

2. Trisomy 17-18 results in ment al retardation, congenital heart

defect s, low-set ears, and flexion of the fingers and hands

a. Frequent ly see micrognathia, renal anomalies,

syndactyly, and skeletal malformations

b. Seen in about 0.3 of 1000 births, and infants usually die

by the age of 2 months

3. Trisomy 13-15 results in ment al retardation, congenital heart

defects, deafness, and cleft lip and palat Also shows

microphthalmia, anophthalmia, and coloboma

a. Incidence is about 0.2 pe r 10,000 newborns. Most die

by 3 months

31. Congenital Malformations And Their Causes

Review of MEDICAL EMBRYOLOGY Book by BEN PANSKY, Ph.D, M.D.

PREV NEXTTABLE OF CONTENT30. Multiple Pregnancies 32. Congenital Malformations And Their Causes: Human Malf

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8/10/2019 Chapter 31=Congenital Malformations And Their Causes

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1/19/2015 Chapter 31. Congenital Malformations And Their Causes - Review of Medical Embryology Book - LifeMap Discovery

http://discovery.l ifemapsc.com/l ibrary/review-of-medical -embryology/chapter -31-congeni tal -malformations-and- their -causes

C. SEX CHROMOSOME ABERRATIONS

1. Appear to be more frequent than autosomal abnormalities, but

this may be due to t he fact t hat the latte r usually lead to

precocious death of the embryo

2. Initiated by a chromosomal anomaly of the ovum; 2 major sex

chromosome aberrations are described

a. Turner's syndrome: the ovum does not carry any sex

chromosome; t here is a female phenotype

(appearance), and one sees dwarfism, malformation,

and gonadal aplasia or dysgene sis

All cells are sex-chromatin negative, and cells have only45 chromosomes with an XO chromosomal complement

Usually due to nondisjunction in male gamete duringmeiosis

b. Klinefelter's syndrome: the ovum carries 2 sex

chromosomes; the phenotype is male, and one no tes

sterility, testicular atrophy, hyalinization of the

seminiferous tubules, and usually gynecomastia

Cells have 47 chromosomes with a sex chromosomalcomplement of XXY type, and a sex chromatin body isseen in 80% of cases

Seen in 1 of 500 male births; caused by nondisjunctionof the XX homologs

c. Triple-X syndrome: from fertilization of an XX oocyte by

an X sperm

Patients are infantile with scanty menses and a degreeof mental retardation

Have 2 sex chromatin bodies in their cells and are called"superfemale"

Some are of proven fertility, and offspring may benormal

PREV NEXTTABLE OF CONTENT30. Multiple Pregnancies 32. Congenital Malformations And Their Causes: Human Malf

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