This page covers gametogenesis within the ovary. With the help of the tutors and other students you will work your way through identifying features described in the text.
Begin by looking at the ovary and the formation of the follicle containing the egg which matures and is released upon ovulation. The images are arranged in series so that progressive stages of the maturing follicle can be seen. The final image on this current page is a link to a movie showing follicle development and ovulation. Use the series of images of the cat ovary below to identify the key features described in the associated text.
Note: This should be a revision of the Ovary Histology Practical you have already completed. If you have trouble with the terms, there is a glossary at the bottom of each page.
The graph below shows the changes in human germ cell numbers in the ovary with age, peaking at about 7 million (occuring in early fetal development) and then decreasing by apopotic cell death. At puberty there remain only about 400,000 and only about 10% of these will be released through reproductive life. (More? Human Menstrual Cycle)
(Based on data from: Hassold, etal., Environ Mol Mutagen 1996. 28: 167-175)
Ovary (cat, cross-section) showing histology and maturation of follicle.
Image (low magnification) showing cortical primordial follicles with primary (preantral) and secondary (antral) follicles lying deeper. Mesovarium at lower right and blood vessels in medullary region.
At this magnification, the overall organization of the ovary can be observed, cortex/medulla organization and arrangement of the maternal blood vessels, but few specific follicle details can be seen.
The next image is of the ovarian cortical region.
Ovary cortex showing primordial follicles.
At the top of the image, is the outside of the ovary.
The thick connective tissue outer layer is the tunica albuginea. Over which a single layer of cells called the germinal epithelium (not visible) cover the surface of the ovary.
The next layer contains the earliest primordial follicles, single cells with pale cytoplasm and darkly stained nuclei.
The next layer contains many growing follicles at various stages of maturity and development. There is also evidence of degeneration as atretic follicles.
At the bottom of the image, is the medullary region of the ovary. Note the large number of maternal blood vessels which are the circulatory conduits for the estrogens and progesterones produced by the theca surrounding the ovarian follicles.
Note: germinal epithelium, tunica albuginea, primordial and atretic follicles. Note larger preantral follicle with (from the centre out) nucleus of maturing oocyte, oocyte cytoplasm, zona pellucida (pink ring), follicle cells, stromal cells.
View of cortical ovary region showing primordial follicles and a single preantral follicle, with atretic follicle to its left. Bottom of picture shows outer cells of antral follicle.
High power view of ovary cortical region showing primordial follicles and a single preantral follicle.
Features: germinal epithelium, tunica albuginea, preantral follicle, nucleus of oocyte, oocyte cytoplasm, zona pellucida, Call-Exner body, stratum granulosa, basement membrane, theca, blood vessels surrounding follicle in theca layer.
Low power view of ovary cortex and medullary region. Note 3 stages of follocle development (primordial, preantral and antral).
Features:
Movie (click image to play) showing process of ovulation (release of oocyte and follicular fluid). Click on movie to start.
Note that following ovulation the remnant of the follicle will degenerate if implantation does not occur (non-pregnant) forming a corpus albicans or following implantation (pregnancy) a corpus luteum which provides endocrine support to the uterus.
An endocrine signal (hCG human Chorionic Gonadotropin) from the implanting conceptus syncitiotrophoblasts maintains the corpus luteum, which in turn supports the uterine functional lining, preventing menstruation.
(Note Movie requires Quicktime to be installed on your computer)
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There are several different nomenclatures for the stages of follicle maturation.
Atresia
At any one time the majority of follicles are destined not to complete maturation and at any stage (from type 4-7) degeneration of the follicle can occur. This process is called ATRESIA.
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