Clinical_Rounds_in_Endocrinology_Volume_II_-_Pediatric_Endocrinology

296

3. What is the difference between mosaicism and chimerism?

Mosaicism is characterized by coexistence of two or more cell lines (e.g.,

45,X/46,XY or 46,XY/47,XXY) originating from a single zygote as a result of

mitotic nondisjunction, while chimerism is characterized by coexistence of two

or more cell lines having different genetic origin in a same individual.

Chimerism occurs as a result of double fertilization (dispermy) of a binucleate

ovum or fusion of two zygotes before implantation. The ovotesticular disorders

are the example of chimerism (e.g., 46,XX/46,XY).

4. What is the difference between sex determination and sex differentiation?

The development of bipotential gonad into either ovary or testis is called

sex determination and is genetically determined. Sex differentiation is the

process of development of internal and external genitalia of a male or

female, as a result of appropriate function of the respective gonad. Both

these events occur during the critical period of embryogenesis during sev-

enth to twelfth week of intrauterine life. The sequence of events that consti-

tute sex determination and differentiation are shown in the fi gure given

below (Fig. 9.2 ).

+ TDF

+ DHT - MIF - Testosterone

Testis

+MIF

Regression of
Mullerian ducts

Growth of

seminal vesicles

and vas deferens

Differentiation and growth

of fallopian tubes, uterus,

upper 2/3rd of vagina

Regression of

Wolffian ducts

+Testosterone

Fusion of labial scrotal folds

growth of phallus and prostate

Undifferentiated

gonad

Ovary

Fig. 9.2 Sex determination and differentiation in primitive gonad

9 Disorders of Sex Development