Source: Kaplan Anatomy (Embryology)
SPERMATOGENESIS
At week 4, primordial germ cells arrive in the indifferent gonad and remain
dormant until puberty.
• When a boy reaches puberty, primordial germ cells differentiate into
type A spermatogonia, which serve as stem cells throughout adult life.
• Some type A spermatogonia differentiate into type B spermatogonia.
• Type B spermatogonia enter meiosis I to form primary spermatocytes.
• Primary spermatocytes form 2 secondary spermatocytes.
• Secondary spermatocytes enter meiosis II to form 2 spermatids.
• Spermatids undergo spermiogenesis, which is a series of morphological
changes resulting in the mature spermatozoa.
OOGENESIS
At week 4, primordial germ cells arrive in the indifferent gonad and differentiate
into oogonia. Oogonia enter meiosis I to form primary oocytes. All
primary oocytes are formed by month 5 of fetal life; they are arrested the first
time in prophase (diplotene) of meiosis I and remain arrested until puberty.
• Primary oocytes arrested in meiosis I are present at birth.
• When a girl reaches puberty, during each monthly cycle a primary
oocyte becomes unarrested and completes meiosis I to form a secondary
oocyte and polar body.
• The secondary oocyte becomes arrested the second time in metaphase
of meiosis II and is ovulated.
• At fertilization within the uterine tube, the secondary oocyte completes
meiosis II to form a mature oocyte and polar body.
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