Morphological Changes During
Maturation of the gametes
• At the end of this session, the student should be
able to:
• a) Define and classify gametogenesis.
• b) Describe morphological changes during maturation of
oocytes.
• c) Describe morphological changes during maturation of
sperms .
• d) Describe abnormal gametes .
• Ref: Langman’s Medical Embryology, 11th edition, chapter
2, pages 21-26
Gametogenesis
• The process of formation and development of
specialized generative cells, gametes. This
process, involving the chromosomes and
cytoplasm of the gametes, prepares these sex
cells for fertilization. The chromosome
number is reduced by half and the shape of
the cells is altered.
– Spermatogenesis
– Oogenesis
• Spermatogenesis is the sequence of events by
which spermatogonia are transformed into
mature sperms.
• Oogenesis is the sequence of events by which
oogonia are transformed into mature oocytes.
Primordial germ cells(PGC)
• Gametes are derived from PGCs that are
formed in the epiblast during 2nd week and
that move to wall of the yolk sac.
• During 4th week these cells begin to migrate
from yolk sac towards developing gonads
where they arrive at the end of 5th week.
Oogenesis
Maturation of oocytes begins before
birth
• PGCs (in gonads)→Oogonia → undergo
mitotic divisions
• ↓5th month= 7 million germ cells (in ovary)
• Oogonia start degenerating and by 7 month
most have degenerated.
• All surviving germ cells (as primary oocyte)
have entered prophase of meiosis 1.
Maturation of Oocytes continue at
puberty
• At the time of birth= All oocytes in prophase
of meiosis 1 (arrested in its diplotene stage).
• Primary oocytes remain arrested in prophase
and do not finish meiosis 1 before puberty is
reached (OMI).
• Number of primary oocytes at birth= 6-8 lacs.
• Number at beginning of puberty= 40,000.
• Number being used during life= ˃ 500.
Maturation continues
• Primordial follicle: A primary oocyte
surrounded by flat epithelial cells.
• Pool of growing follicles.
• Every month 15-20 of growing follicle start
maturating → primary follicles →
secondary→ Preovulatory.
• Meiosis 1 is completed (surge in LH
stimulates preovulatory growrh)→ enters
meiosis 2 but arrests in metaphase about 3
hours before ovulation. M 2 is completed only
if oocyte is fertilized.
Spermatogenesis
It begins at puberty
• At birth, germ cells in male infant are
recognized in the testes sex cords as large,
pale cells surrounded by supporting(sertoli)
cells.
• Before puberty, the sex cords acquire a lumen
and become seminiferous tubules. PGCs give
rise to spermatogonial cells →type A →type B
→ primary spermatocytes (enters the meiosis
and form secondary spermatocyte and finally
spermatids.
Spermiogenesis
• It is the series of changes resulting in the
transformation of spermatids into spermatozoa.
• Time required: 74 days.
• Changes include:
– Formation of acrosome.
– Condensation of the nucleus.
– Formation of neck, middle piece and tail.
– Shedding of the most of the cytoplasm.
Clinical Correlates
• Abnormal gametes