The internal female reproductive organs consist of the ovaries, uterine or fallopian tubes,
uterus, and vagina.
The ovaries are almond-shaped structures that produce steroid hormones like estrogen and
progesterone, as well as the female gametes or oocytes.
Each ovary is typically about 3 centimeters long, 1.5 centimeters wide, and 1 centimeter
thick, with a thin outer capsule.
The capsule is made up of a thin layer of simple cuboidal epithelium that’s only one
cell thick, and an underlying dense layer of connective tissue called the tunica albuginea.
The majority of the ovary consists of the cortex, which is the outer region where oocytes
and ovarian follicles develop.
Ovarian follicles are the fluid-filled structures that each contain an oocyte.
This cortex of the ovary has highly cellular connective tissue in comparison to the inner
region of ovaries called the medulla, which consists mostly of loose fibroelastic connective
tissue with many large, tortuous blood vessels, lymphatic vessels, and nerve fibers.
Alright, let’s take a closer look at the cortex.
The primordial follicles are the follicles found in the outer cortex that develop during
fetal life.
At this stage, they actually stop developing, which is called arrested development, and
they stay this way until they’re needed for ovulation.
The primary oocyte is the large round cell that makes up the majority of each primordial
follicle.
The primary oocytes have a prominent nucleus and a diameter that’s about 25 to 30 um.
The outermost layer of the primordial follicles is a single layer of flattened follicular
cells, which are pregranulosa cells.
The zona pellucida is a very thin, pink or eosinophilic layer of glycoproteins that sits
between the primary oocyte and the outer follicular cells.
When a primordial follicle continues developing, the next stage of follicular development is
called the primary follicle.
At this stage, the outer follicular cells from the primordial follicle have transformed
into cuboidal granulosa cells.
At first, the granulosa cells only form a single layer of cells, so these follicles
are called unilaminar primary follicles.
As the follicle size increases, the granulosa cells multiply and form multiple layers around
the oocyte, making their name change to multilaminar primary follicles.
At this stage, the eosinophilic zona pellucida is now more prominent and easier to identify,
with multiple filaments that can be seen branching off the zona pellucida.
The stromal cells that surround the follicle differentiate into a layer called the theca
folliculi or follicular theca.
This layer of highly vascular connective tissue also contains cells with endocrine functions.
The next stage of follicular development after the primary follicle is called the secondary
or antral follicle.
This stage can be identified by the formation of an antrum, which is a fluid-filled cavity
that increases in size as the follicle matures and is formed within the granulosa layer.
The granulosa cells will continue to proliferate and form a few structures like the corona
radiata, which surrounds the zona pellucida and oocyte.
The cumulus oophorus is also formed by granulosa cells and it anchors the corona radiata, zona
pellucida, and oocyte to the stratum granulosum.
The stratum granulosum is the inner wall of the follicle that consists of the remaining
granulosa cells.
If we take a closer look at the follicular theca is also further divided into the theca
interna or inner theca; and the theca externa or outer theca.
The theca interna is more vascular and also consists of cells that secrete androstenedione,
which is converted to estrogen by nearby granulosa cells.
The theca externa is a more fibrous layer that also contains smooth muscle cells.
The outer border of this layer will gradually merge with the surrounding stroma.