15
Embryology of the
Genital Tract
Chapter 2
Erin M. Murphy, MD
❍ At what gestational age, do the primitive germ cells (oogonia) migrate to the gonadal ridge?
Approximately 5 weeks.
❍ Do germ cells induce gonadal development?
No. Germ cells do not induce gonadal development; however, absence of germs cell arrival to the gonadal ridge
results in no gonad development and the formation of a fibrous streak (gonadal agenesis).
❍ If the indifferent gonad is destined to become a testis, at what gestational age will differentiation occur?
6 to 9 weeks’ gestation.
❍ What is the factor that determines if an indifferent gonad will become a testis?
Testes-determining factor (TDF). TDF is a product of a gene located on the Y chromosome in the region of
SRY gene.
❍ How many oogonia are present throughout a female’s life?
This is a period of rapid mitotic activity. A maximum of 6 to 7 million is reached at 16 to 20 weeks’ gestational age.
This number is reduced to approximately 1 million at birth secondary to atresia. At puberty only 300,000 remain.
❍ At what point do oogonia become oocytes?
This begins at 11 to 12 weeks’ gestational age and is completed by birth. Oogonia enter the first meiotic division
and are arrested in prophase. These are referred to as primary oocytes.
❍ What is the mechanism of the loss of oocytes during the second half of pregnancy?
Oocytes are lost after 20 weeks’ gestation due to follicular growth and subsequent atresia, as well as degeneration of
oogonia not surrounded by granulosa cells. Also, germ cells that migrate to the surface of the ovary are lost in the
peritoneal cavity.
❍ After mitosis, what is required for an oocyte to become a single ovum?
Two meiotic divisions are required with the first at ovulation and the second at fertilization.