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The Uterine Cycle: Secretory Phase

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  • 0:07 Secretory Phase
  • 1:33 Uterine Glands
  • 3:21 Progesterone
  • 4:35 Fertilization
  • 5:47 Lesson Summary
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Lesson Transcript
Instructor: Heather Adewale

Heather has taught reproductive biology and has researched neuro, repro and endocrinology. She has a PhD in Zoology/Biology.

The third (and final) phase of the uterine cycle is the secretory phase. During this phase, the uterine tissue regrowth is completed in preparation for the possibility of pregnancy and the implantation of a fertilized egg.

Secretory Phase

Ah, here we are, the third and final phase of the uterine cycle: the secretory phase. The secretory phase of the uterine cycle follows the proliferative phase. And, just like the proliferative phase, the changes that occur during the secretory phase take place in the functional zone of the endometrium.

While the proliferative phase jumpstarted the process of rebuilding the uterine tissue, the secretory phase completes this process by putting on a few finishing touches. What do I mean by finishing touches? Well, if you think of the proliferative phase as the building of a house, the secretory phase would be like painting the walls and picking out the carpet, or maybe hanging a few pictures. You know, all that stuff that makes a house feel more homey and inviting. More technically speaking, the secretory phase is when the maximal thickness of the functional zone is reached and the uterine glands complete their growth.

Okay, so now that you know what happens, let's take a look at how and why. The secretory phase begins after ovulation - that's the release of the mature egg from the ovary. The reason for this is that it requires a certain hormone from the ovary that isn't produced until after ovulation.

Uterine Glands

Uterine glands change shape to increase their surface area.
Uterine Glands Diagram

Once this hormone reaches the uterus, it helps stimulate the glands within the endometrium to complete their growth. This involves some small changes in shape or structure. These changes are aimed at increasing the amount of surface area each gland has. What do I mean by surface area? Well, the more surface area a gland (or cell) has, the more space it has for interacting with substances around it or for secreting substances of its own.

Think about it this way: which object has more of its surface exposed, a straight wire with a length of 12 inches or a coiled wire, like a slinky, whose coiled up length is also 12 inches? Not sure? Well, what if you were to straighten out the coiled up wire? Then which would be longer? The coiled one, right? Or what about a line, if you painted a straight line, like below (that was about 12 inches long) and then painted a line that looked like the green one below (but was also 12 inches long), which line do you think used up more paint? The second one, right?

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That's the same idea that the uterine glands use as they undergo changes in their shapes. First, they coil up to increase surface area - kind of like a slinky. Then, along their edges, they form small indentations - kind of giving it a tooth-like appearance - to increase the amount of area exposed. That way, they have more space to release their secretions into the surrounding tissue.

Progesterone

Okay, so next I want you to think about what structure appears in the ovary after ovulation. Or, more importantly, what this structure produces. Any guesses? Well, if you said the corpus luteum, you would be correct. And, if you said that it produces the hormone progesterone, you would be doubly correct!

It's progesterone that stimulates the finishing touches in the uterus. Increased progesterone causes increased secretions from glands in the uterus. These secretions contain nutrients like proteins, lipids and glycogen essential for the survival of the developing embryo. And, it's these secretions that help make the uterus a welcoming environment for the fertilized egg - soon to be embryo, fetus and then a fully developed baby! You know, like adding some extra fabric softener to bed sheets to make them just a little bit softer and cozier.

The corpus luteum in the ovary produces progesterone.
Corpus Luteum Diagram

Even more importantly, without these glands, research has shown that the survival of the fertilized egg, or embryo, is compromised. That means it's a lot less likely to survive or develop normally.

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