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PubMed This is a summary of 25 peer-reviewed journal articles Updated
Gynecology

The Biology and Subtypes of Endometriosis

At a Glance

Endometriosis is a complex biological disease categorized into three main subtypes: superficial peritoneal, ovarian endometriomas, and deep infiltrating. These active tissue lesions can grow their own nerves and blood vessels, causing severe pain and inflammation throughout the pelvic cavity.

To understand endometriosis, it helps to think of it not just as a “period problem,” but as a complex biological process where tissue behaves in ways it shouldn’t. While your uterine lining stays inside the uterus, endometriosis tissue travels elsewhere and acts like a cellular invader.

The “Perfect Storm”: Why Does This Happen?

Scientists are still uncovering the exact cause of endometriosis, but they believe it involves a “perfect storm” of several biological factors:

  • Retrograde Menstruation: This is the most common theory, suggesting that during a period, some menstrual blood flows backward through the fallopian tubes and into the pelvic cavity [1][2].
  • Immune Dysregulation: In most people, the immune system cleans up this misplaced tissue. In those with endometriosis, the immune system fails to recognize these cells as “out of place,” allowing them to survive and grow [3][4].
  • Cellular Transformation: Some researchers believe that certain cells in the abdomen can actually transform into endometrial-like cells, or that cells were misplaced during fetal development [5].
  • Epigenetic Switches: “Switches” in your DNA can be turned on or off, making certain cells more likely to invade tissue [6].

The Three Main Subtypes

Not all endometriosis is the same. Doctors categorize the disease into three main “subtypes,” which behave differently and have different impacts on your body.

1. Superficial Peritoneal Endometriosis (SUP)

This is the most common form. It consists of thin, flat lesions on the peritoneum (the lining of your abdomen).

  • The “Invisible” Disease: Because these lesions have no depth, they are often invisible on ultrasounds and MRIs [7].
  • The Pain Factor: Don’t let the word “superficial” fool you. These lesions can be intensely painful because they release high levels of inflammatory chemicals into the pelvic cavity [8].

2. Ovarian Endometriomas (OMA)

Often called “chocolate cysts,” these are fluid-filled sacs that grow deep inside the ovaries.

  • Appearance: They are filled with old, dark blood that looks like melted chocolate [9].
  • Impact: These cysts are easily seen on ultrasounds. Because they grow inside the ovary, they can affect your ovarian reserve (the number of eggs you have left) [10][11].

3. Deep Infiltrating Endometriosis (DIE)

This is the most aggressive form of the disease. It occurs when lesions grow more than 5 millimeters deep into organs like the bowel, bladder, or the ligaments that support the uterus [12][13].

  • Structural Damage: DIE often creates significant internal scarring called fibrosis, which can “glue” organs together (adhesions) [13][12].
  • Imaging: Unlike superficial disease, DIE can usually be seen and mapped using specialized MRI or expert-guided ultrasound [14][15].

How the Tissue Behaves

Endometriosis tissue is uniquely “active.” It doesn’t just sit there; it interacts with your body in three specific ways:

  1. Adhesion and Invasion: The cells use specialized “glue” molecules (like ICAM1) to stick to your organs and enzymes to burrow deep into the tissue [16][17].
  2. Neuro-angiogenesis: Perhaps most significantly, the lesions grow their own blood vessels (angiogenesis) and their own nerve fibers (neurogenesis) [18][19]. This means the lesions “plug in” to your nervous system, allowing them to send pain signals directly to your brain [18].
  3. Progesterone Resistance: Some lesions become less responsive or “deaf” to progesterone, the hormone that normally keeps the uterine lining in check. While hormonal therapies remain highly effective for the majority of patients, this mechanism explains why birth control doesn’t fully stop the pain for everyone [20][21].

The Link to Adenomyosis

Many people with endometriosis also have adenomyosis. In this condition, the tissue grows into the muscular wall of the uterus itself rather than outside of it [22].

  • Co-occurrence: Adenomyosis is especially common in patients who have the Deep Infiltrating (DIE) subtype [23][24].
  • Symptoms: It often causes a heavy, “enlarged” feeling in the uterus and very heavy menstrual bleeding [22]. Because it involves the same biological mechanisms as endometriosis, they are often managed together [22][25].

Common questions in this guide

What are the different subtypes of endometriosis?
The three main subtypes are superficial peritoneal endometriosis (thin lesions on the abdomen lining), ovarian endometriomas (fluid-filled cysts on the ovaries), and deep infiltrating endometriosis (aggressive lesions growing deep into organs).
Why does my endometriosis not show up on an MRI or ultrasound?
Superficial peritoneal endometriosis consists of very thin, flat lesions that lack the depth and mass needed to be visible on standard imaging like MRIs and ultrasounds. Despite being invisible on scans, these lesions can still cause severe pelvic pain.
What is a chocolate cyst?
A 'chocolate cyst,' medically known as an ovarian endometrioma, is a fluid-filled sac that grows inside the ovary. It gets its name because it is filled with old, dark blood that resembles melted chocolate. These cysts can potentially impact your ovarian reserve.
How does endometriosis cause nerve pain?
Endometriosis lesions can undergo neuro-angiogenesis, a process where they grow their own blood vessels and nerve fibers. This allows the lesions to connect directly to your nervous system, sending pain signals to your brain even when you are not on your period.
What is the difference between endometriosis and adenomyosis?
In endometriosis, tissue similar to the uterine lining grows outside the uterus. In adenomyosis, this tissue grows directly into the muscular wall of the uterus itself, often causing a heavy feeling in the pelvis and severe menstrual bleeding.

Questions to Ask Your Doctor

Curated prompts to bring to your next appointment.

  1. 1.Based on my imaging, do I have signs of Ovarian Endometriomas (OMA) or Deep Infiltrating Endometriosis (DIE)?
  2. 2.Could my symptoms be explained by Superficial Peritoneal Endometriosis, even if my scans are currently clear?
  3. 3.Did my imaging show any thickening of the uterine wall that might suggest adenomyosis is also present?
  4. 4.Does my specific subtype of endometriosis typically respond well to hormonal suppression, or should we consider surgery for lesion removal?
  5. 5.Can you explain how 'neuro-angiogenesis' might be contributing to the specific type of nerve pain I'm feeling?

Questions For You

Tap a prompt to share your answer — we'll use it plus this page's context to start a tailored conversation.

References

References (25)
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    Cellular Origins of Endometriosis: Towards Novel Diagnostics and Therapeutics.

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    From Retrograde Menstruation to Endometrial Determinism and a Brave New World of "Root Treatment" of Endometriosis: Destiny or a Fanciful Utopia?

    Guo SW, Habiba M, Benagiano G

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This page provides educational information about the biology and subtypes of endometriosis. It is for informational purposes only and does not replace professional medical advice, diagnosis, or treatment.

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