At the same time that the oocyte is progressing through meiosis, corresponding changes are taking place in the follicular cells. Ovarian Follicle DevelopmentĪn ovarian follicle consists of a developing oocyte surrounded by one or more layers of cells called follicular cells. The other three cells are polar bodies and they degenerate. In oogenesis, only one functional fertilizable cell develops from a primary oocyte. In spermatogenesis, four functional sperm develop from each primary spermatocyte. Here again there are obvious differences between the male and female. If fertilization does not occur, the second meiotic division is never completed and the secondary oocyte degenerates. The first polar body also usually divides in meiosis I to produce two even smaller polar bodies. The smaller cell from this division is a second polar body. Again this is an unequal division with all of the cytoplasm going to the ovum, which has 23 single-stranded chromosome. If fertilization occurs, meiosis II continues. The secondary oocyte begins the second meiotic division, but the process stops in metaphase. The remaining half of the chromosomes go to a smaller cell called the first polar body. The large cell undergoes an unequal division so that nearly all the cytoplasm, organelles, and half the chromosomes go to one cell, which becomes a secondary oocyte. One of the primary oocytes seems to outgrow the others and it resumes meiosis I. By puberty the number of primary oocytes has further declined to about 400,000.īeginning at puberty, under the influence of follicle-stimulating hormone, several primary oocytes start to grow again each month. This is quite different than the male in which spermatogonia and primary spermatocytes continue to be produced throughout the reproductive lifetime. This is the lifetime supply, and no more will develop. Many of the primary oocytes degenerate before birth, but even with this decline, the two ovaries together contain approximately 700,000 oocytes at birth. The diploid (46 chromosomes) primary oocytes replicate their DNA and begin the first meiotic division, but the process stops in prophase and the cells remain in this suspended state until puberty. Oogonia then enter a growth phase, enlarge, and become primary oocytes.
Frog ovary parts full#
These divide rapidly to form thousands of cells, still called oogonia, which have a full complement of 46 (23 pairs) chromosomes. Early in fetal development, primitive germ cells in the ovaries differentiate into oogonia.
The sequence of events in oogenesis is similar to the sequence in spermatogenesis, but the timing and final result are different.
Oogenesisįemale sex cells, or gametes, develop in the ovaries by a form of meiosis called oogenesis. The medulla is a loose connective tissue with abundant blood vessels, lymphatic vessels, and nerve fibers. Each of the follicles contains an oocyte, a female germ cell. The cortex appears more dense and granular due to the presence of numerous ovarian follicles in various stages of development. The substance of the ovaries is distinctly divided into an outer cortex and an inner medulla. Underneath this layer is a dense connective tissue capsule, the tunica albuginea. This is actually the visceral peritoneum that envelops the ovaries. The ovaries are covered on the outside by a layer of simple cuboidal epithelium called germinal (ovarian) epithelium. They are held loosely in place by peritoneal ligaments. The ovaries are located in shallow depressions, called ovarian fossae, one on each side of the uterus, in the lateral walls of the pelvic cavity. Each ovary is a solid, ovoid structure about the size and shape of an almond, about 3.5 cm in length, 2 cm wide, and 1 cm thick. The primary female reproductive organs, or gonads, are the two ovaries.
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