(C1-12) Slide 87: Human ovary (H&E).

As you work through the female reproductive system laboratories, you will find many questions in the lab notes which are designed to help you integrate the lecture and lab material. Answer these questions as you go along. You can also use them for review questions at exam time. Answers follow the last description (after C55-57 for slide 89).

Before you begin, a word of warning. To see all the stages in follicle development, you will need to look at several slides since it would be extremely rare for all the different stages to appear in one section. Now let's get started...

Slide 87 shows a human ovary. At low power (C1), you can identify the germinal epithelium, a mesothelium, on the surface of the ovary and the underlying connective tissue layer, the tunica albuginea (ta). The cortex (c) is composed of a stroma in which the follicles are embedded. Note the remarkable concentration of cell nuclei (and hence, cells) in the stroma. The appearance of this connective tissue is quite different from any you have encountered thus far and it does not fit easily into any of the categories of connective tissue we have given you. Remember that cells from the stroma are constantly being recruited to form the thecae, interstitial glands and paralutein cells. The stroma is roughly demarcated from the more vascular medulla (m) by the purple line, representing an especially dense layer of stromal cells. A portion of the corpus luteum (cl) also appears on this slide. It looks like it is in the medulla due to the plane of section.

Looking at the cortex at higher power (C2), identify primordial follicles (arrows) and an early primary follicle (p) which is just beginning to grow. Take note that the follicular epithelium in the primordial follicle is simple squamous whereas it is simple cuboidal in the early primary follicle. (C3) shows more examples of these types of follicles Now, look around for examples of later primary follicles (like you see on C4) where the follicular epithelium is now stratified columnar and hence is called the membrana granulosa. You should be able to see a prominent zona pellucida in late primary follicles. Next, search for a secondary follicle (C5). Notice the presence of an antrum, the characteristic that distinguishes a secondary follicle from a late primary follicle. Also, identify the oocyte, zona pellucida, membrana granulosa, theca interna and externa. If you scrutinize this secondary follicle carefully enough, you will realize that it is actually an atretic follicle (see C9). Unfortunately, we were unable to find a good example of a secondary follicle that was not atretic. If you happen to find a better one, please let us know!

Moving on, look for a mature, or Graafian follicle (C6). Take note of the very large antrum, and the well defined cumulus oophorous which projects into the antrum and note the follicle embedded within the cumulus oophorous. Be advised that in many cuts through Graafian follicles on your slides, the plane of section will not have passed through the cumulus oophorus let alone the oocyte. But with any kind of luck, you'll find a Graafian follicle in which you can see the oocyte in the cumulus oophorous like in C6! A closer look at the wall of the follicle shows membrana granulosa (C7 mg), surrounding the antrum (a). The theca folliculi surrounds the membrana granulosa. It consists of the theca interna (i), and a poorly defined theca externa (e), that seems to fade off into the stroma. Note the vascularity of the theca interna.

Some questions for you:

· Q1 What substance is found in the antrum of the follicle ?

· Q2 Name two components of this substance?

· Q3 What hormone is produced by the follicular epithelium?

· Q4 To which pituitary hormone are the follicles in C5 responding?

The corpus luteum (C8) is a large structure, with remains of the antrum (a), surrounded by granulosa lutein cells. The paralutein cells are not clearly distinguishable in this slide. The granulosa lutein cells are large polyhedral shaped cells which stain very poorly.

· Q5 What function of the granulosa lutein cells contributes to the appearance of these cells?

· Q6 What structure found in the ovary would be the immediate predecessor of the corpus luteum?

· Q7 What process took place to change this structure in Q6 into the corpus luteum?

Although atretic follicles can develop from any stage follicle, the easiest way to find follicles undergoing atresia is to search for the larger secondary or Graafian follicles. In atretic follicles the membrana granulosa cells have shrunken. The nuclei of cells in an atretic follicle become condensed and stain very dark (these nuclei are called pyknotic nuclei, see C9 arrows). Sometimes the nuclei show karyorrhexis (i.e., nuclear fragmentation). Often you will see sloughing in the membrana granulosa cells into the antrum. There are several follicles at more advanced stages of atresia on most of your slides C10 shows an example of an atretic follicle. Atretic follicles show considerable disorganization (see C10). When the membrana granulosa is completely sloughed, the atretic follicle has transformed into an interstitial gland (C11). In interstitial glands, the theca interna is larger than that around healthy follicles and these thecal cells form columns radiating outward from the old antrum. Note the corpora albicantia (singular=corpus albicans, C12 arrows) derived from either corpora lutea or interstitial glands. These appear as knots of collagen within the stroma.