Chapter 46.4, 47.1 Book notes
In humans and other mammals, a complex interplay of hormones regulates gametogenesis.
, is based on meiosis, but details differ in females and males.
is the development of mature ova (egg cells), and
, is the production of
mature sperm cells, and it occurs in the seminiferous tubules of the testes.
Oogenesis differs from spermatogenesis in three major ways:
During the meiotic divisions of oogenesis, cytokinesis is unequal, with almost all the
cytoplasm monopolized by a single daughter cell, the secondary oocyte. This large cell
can go to become the ovum; the other products of meiosis, smaller cells called polar
bodies, degenerate. By contrast, in spermatogenesis, all four products of meiosis develop
into mature sperm.
Although the cells from which sperm develop continue to divide by mitosis throughout
the male’s life, this is thought not to be the case for oogenesis in the human female.
Oogenesis has long “resting” periods, in contrast to spermatogenesis, which produces
mature sperm from precursor cells in an uninterrupted sequence.
Two different types of cycles occur in female mammals:
Humans and certain other primates have
Other mammals have
Ovulation occurs at a time the both cycles after the endometrium has started to thicken and develop a rich
blood supply, preparing the uterus for the possible implantation of an embryo.
In menstrual cycles, the endometrium is shed from the uterus through the cervix and vagina in a bleeding
In estrous cycles, the endometrium is reabsorbed by the uterus and no extensive bleeding occurs.
In some mammals, the only time the vagina permits mating is called
The term menstrual cycle refers to changes that occur in the uterus; therefore it is also called the
. It is caused by cyclic events that occur in the ovaries called the
Oogenesis begins in the female embryo with differentiation of primordial germ into
specific stem cells.
At prophase I,
, remain quiescent within small follicles (cavities lined with protective
coverings) until puberty, when hormones reactivate them.
Follicale stimulating hormone (FSH) periodically stimulates a follicle to grow and induces its primary
oocyte to complete meiosis I and start meiosis II. Meiosis then stops again. Arrested at metaphase II, the
is released at ovulation, when its follicle breaks open.