BIO NOTES - Spermatogenesis and Oogenesis in Humans
● In human males, spermatogenesis occurs within the testes; in females,
oogenesis occurs within the ovaries.
● Spermatogenesis
○ The testes contain stem cells called spermatogonia.
○ These cells keep the testes supplied with primary spermatocytes that
undergo spermatogenesis
○ Primary spermatocytes with 46 chromosomes undergo meiosis I to form
two secondary spermatocytes, each with 23 duplicated chromosomes.
○ Secondary spermatocytes undergo meiosis II to produce four sper.matids
with 23 daughter chromosomes.
■ Spermatids then differentiate into viable sperm (spermatozoa).
upon sexual arousal, the sperm enter ducts and exit the penis upon
ejaculation.
● Oogenesis
○ The ovaries contain stem cells, called oogonia, that produce many primary
oocytes with 46 chromosomes during fetal development.
○ They even begin oogenesis, but only a few continue when a female has
become sexually mature.
○ The result of meiosis I is two haploid cells with 23 chromosomes each
○ One of these cells, termed the secondary oocyte, receives almost all the
cytoplasm. The other is a polar body that may either disintegrate or divide
again.
○ The secondary oocyte begins meiosis II but stops at meta-phase II.
■ Then the secondary oocyte leaves the ovary and enters the uterine
tube, where sperm may be present.
■ If no sperm are in the uterine tube, or if a sperm does not enter the
secondary oocyte, it eventually disintegrates without completing
meiosis.
■ If a sperm does enter the oocyte, some of its contents trigger the
completion of meiosis II in the secondary oocyte, and another polar
, body forms.
● Spermatogenesis and oogenesis in mammals.
○ Spermatogenesis produces four viable sperm, whereas oogenesis
produces one egg and at least two polar bodies.
○ In humans, both sperm and egg have 23 chromosomes each; therefore,
following fertilization, the zygote has 46 chromosomes.
○ At the completion of oogenesis, following entrance of a sperm, there is
one egg and two or three polar bodies.
○ The polar bodies are a way to “dispose of” chromosomes while retaining
much of the cytoplasm in the egg.
○ Cytoplasmic molecules and organelles are needed by a developing
embryo following fertilization.
○ Some zygote components, such as the centrosome, are contributed by the
sperm.
○ The mature egg has 23 chromosomes, but the zygote formed when the
sperm and egg nuclei fuse has 46 chromosomes.
○ Therefore, fertilization restores the diploid number of chromosomes. The
production of haploid gametes and subsequent fusion of those gametes
into a diploid zygote complete a human life cycle.
● Changes in Chromosome Number and Structure
○ We have seen that crossing-over creates variation within a population and
is essential for the normal separation of chromosomes during meiosis.
○ Furthermore, the proper separation of homologous chromosomes during
meiosis I and the separation of sister chromatids during meiosis II are
essential for the maintenance of normal chromosome numbers in living
organisms.
○ Although meiosis almost always proceeds normally, a failure of
chromosomes to separate, or nondisjunction, may occur, resulting in a
gain or loss of chromosomes.
○ Errors in crossing-over may result in extra or missing parts of
chromosomes.
● In human males, spermatogenesis occurs within the testes; in females,
oogenesis occurs within the ovaries.
● Spermatogenesis
○ The testes contain stem cells called spermatogonia.
○ These cells keep the testes supplied with primary spermatocytes that
undergo spermatogenesis
○ Primary spermatocytes with 46 chromosomes undergo meiosis I to form
two secondary spermatocytes, each with 23 duplicated chromosomes.
○ Secondary spermatocytes undergo meiosis II to produce four sper.matids
with 23 daughter chromosomes.
■ Spermatids then differentiate into viable sperm (spermatozoa).
upon sexual arousal, the sperm enter ducts and exit the penis upon
ejaculation.
● Oogenesis
○ The ovaries contain stem cells, called oogonia, that produce many primary
oocytes with 46 chromosomes during fetal development.
○ They even begin oogenesis, but only a few continue when a female has
become sexually mature.
○ The result of meiosis I is two haploid cells with 23 chromosomes each
○ One of these cells, termed the secondary oocyte, receives almost all the
cytoplasm. The other is a polar body that may either disintegrate or divide
again.
○ The secondary oocyte begins meiosis II but stops at meta-phase II.
■ Then the secondary oocyte leaves the ovary and enters the uterine
tube, where sperm may be present.
■ If no sperm are in the uterine tube, or if a sperm does not enter the
secondary oocyte, it eventually disintegrates without completing
meiosis.
■ If a sperm does enter the oocyte, some of its contents trigger the
completion of meiosis II in the secondary oocyte, and another polar
, body forms.
● Spermatogenesis and oogenesis in mammals.
○ Spermatogenesis produces four viable sperm, whereas oogenesis
produces one egg and at least two polar bodies.
○ In humans, both sperm and egg have 23 chromosomes each; therefore,
following fertilization, the zygote has 46 chromosomes.
○ At the completion of oogenesis, following entrance of a sperm, there is
one egg and two or three polar bodies.
○ The polar bodies are a way to “dispose of” chromosomes while retaining
much of the cytoplasm in the egg.
○ Cytoplasmic molecules and organelles are needed by a developing
embryo following fertilization.
○ Some zygote components, such as the centrosome, are contributed by the
sperm.
○ The mature egg has 23 chromosomes, but the zygote formed when the
sperm and egg nuclei fuse has 46 chromosomes.
○ Therefore, fertilization restores the diploid number of chromosomes. The
production of haploid gametes and subsequent fusion of those gametes
into a diploid zygote complete a human life cycle.
● Changes in Chromosome Number and Structure
○ We have seen that crossing-over creates variation within a population and
is essential for the normal separation of chromosomes during meiosis.
○ Furthermore, the proper separation of homologous chromosomes during
meiosis I and the separation of sister chromatids during meiosis II are
essential for the maintenance of normal chromosome numbers in living
organisms.
○ Although meiosis almost always proceeds normally, a failure of
chromosomes to separate, or nondisjunction, may occur, resulting in a
gain or loss of chromosomes.
○ Errors in crossing-over may result in extra or missing parts of
chromosomes.
