USMLE Step 1 Lecture Notes 2018: 7-Book Set (Kaplan Test Prep)
Although sex is determined at fertilization, the gonads initially go through an indifferent stage weeks 4–7 when there are no specific ovarian or testicular characteristics. The indifferent gonads develop in a longitudinal elevation or ridge of intermediate mesoderm called the urogenital ridge. The components of the indifferent gonads are as follows:
•Primordial germ cells provide a critical inductive influence on gonad development, migrating in at week 4. They arise from the lining cells in the wall of the yolk sac.
•Primary sex cords are finger-like extensions of the surface epithelium which grow into the gonad that are populated by the migrating primordial germ cells.
•Mesonephric (Wolffian) and the paramesonephric (Mullerian) ducts of the indifferent gonad contribute to the male and female genital tracts, respectively
TESTIS AND OVARY
The indiﬀerent gonad develops into either the testis or ovary.
Development of the testis and male reproductive system is directed by the following:
•Sry gene on the short arm of the Y chromosome, which encodes for testis-determining factor (TDF)
•Testosterone, which is secreted by the Leydig cells
•Müllerian-inhibiting factor (MIF), which is secreted by the Sertoli cells
•Dihydrotestosterone (DHT): external genitalia
Meiosis occurs within the testis and ovary. This is a specialized process of cell division that produces the male gamete (spermatogenesis) and female gamete (oogenesis). There are notable diﬀerences between spermatogenesis and oogenesis.
Meiosis consists of 2 cell divisions. In meiosis I, the following events occur:
•Synapsis: pairing of 46 homologous chromosomes
•Crossing over: exchange of segments of DNA
•Disjunction: separation of 46 homologous chromosome pairs
(no centromere-splitting) into 2 daughter cells, each containing 23 chromosome pairs
In meiosis II, synapsis does not occur, nor does crossing over. Disjunction does occur with centromere-splitting.
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