USMLE Step 1 Lecture Notes 2021: 7-Book Set

Book Preface

GONAD DEVELOPMENT
Although sex is determined at fertilization, the gonads initially go through an indifferent stage weeks 4–7 when there are no specific ovarian or tes-ticular 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 epithe-lium which grow into the gonad that are populated by the migrat-ing primordial germ cells.
• Mesonephric (Wolffian) and the paramesonephric (Mullerian) ducts of the indifferent gonad contribute to the male and female genital tracts, respectively.

The indifferent gonads develop 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

Development of the ovary and female reproductive system requires estro-gen. Ovarian development occurs in the absence of the Sry gene and in the presence of the WNT4 gene.

GAMETOGENESIS
Meiosis
Meiosis, occurring within the testis and ovary, 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.
Two cell divisions take place in meiosis. 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.