Pinopodes (also known as pinopods and uterodomes) are protrusions on the apical cellular

membrane A membrane is a selective barrier; it allows some things to pass through but stops others. Such things may be molecules, ions, or other small particles. Membranes can be generally classified into synthetic membranes and biological membranes. ...

of uterine

epithelial Epithelium or epithelial tissue is one of the four basic types of animal tissue, along with connective tissue, muscle tissue and nervous tissue. It is a thin, continuous, protective layer of compactly packed cells with a little intercellu ...

cells. Pinopodes have a pinocytotic role (hence the name pinopode - Greek for "drinking foot"), as well as a secretory role. Their secretory

vacuole A vacuole () is a membrane-bound organelle which is present in plant and fungal cells and some protist, animal, and bacterial cells. Vacuoles are essentially enclosed compartments which are filled with water containing inorganic and organic ...

s reach towards the lumen; The contents within the vesicle may provide necessary nutrients to the

embryo An embryo is an initial stage of development of a multicellular organism. In organisms that reproduce sexually, embryonic development is the part of the life cycle that begins just after fertilization of the female egg cell by the male sperm ...

. This feature also assists in its ability to attach to the uterine endometrium. With the development of pinipodes, there is a decrease in epithelial cell contact, which prompts

blastocyst The blastocyst is a structure formed in the early embryonic development of mammals. It possesses an inner cell mass (ICM) also known as the ''embryoblast'' which subsequently forms the embryo, and an outer layer of trophoblast cells called th ...

attachment and penetration. The usefulness of pinopodes as a marker for endometrial receptivity has been debated in past literature,Quinn, CE and Casper, RF. Pinopodes: a questionable role in endometrial receptivity. Human Reproduction Update 2009 15(2):229-236 http://humupd.oxfordjournals.org/cgi/content/abstract/15/2/229 but is now generally accepted.

Structure and morphology

Pinopodes are usually measured to be 5–10 μm. Their structure is dependent upon the organism's current stage of the

menstrual cycle The menstrual cycle is a series of natural changes in hormone production and the structures of the uterus and ovaries of the female reproductive system that make pregnancy possible. The ovarian cycle controls the production and release of eg ...

, as pinopodes are regulated by the

sex hormone Sex hormones, also known as sex steroids, gonadocorticoids and gonadal steroids, are steroid hormones that interact with vertebrate steroid hormone receptors. The sex hormones include the androgens, estrogens, and progestogens. Their effects ...

estrogen Estrogen or oestrogen is a category of sex hormone responsible for the development and regulation of the female reproductive system and secondary sex characteristics. There are three major endogenous estrogens that have estrogenic hormonal a ...

and

progesterone Progesterone (P4) is an endogenous steroid and progestogen sex hormone involved in the menstrual cycle, pregnancy, and embryogenesis of humans and other species. It belongs to a group of steroid hormones called the progestogens and is the ma ...

. Studies in the

rat Rats are various medium-sized, long-tailed rodents. Species of rats are found throughout the order Rodentia, but stereotypical rats are found in the genus ''Rattus''. Other rat genera include ''Neotoma'' ( pack rats), ''Bandicota'' (bandicoot ...

have shown that progesterone stimulates the development of pinopodes, while estrogen is responsible for their regression. Generally, the first developmental stage occurs during the early

luteal phase The corpus luteum (Latin for "yellow body"; plural corpora lutea) is a temporary endocrine structure in female ovaries involved in the production of relatively high levels of progesterone, and moderate levels of estradiol, and inhibin A. It is ...

, around days 17-18 of the menstrual cycle. During this time, cellular bulging appears. The second mid-luteal stage of development, around days 20-22, is when the pinopodes are most prominent. During this peak phase, their structures look spherical and smooth, without microvilli. Towards the end of the secretory phase, around days 23-35, the regression period occurs. In this stage, the structure shrivels and appears wrinkly. Not every pinopode on the endometrial surface develops at the same time, affecting the consistency of distribution along the membrane. F-actin is found within pinopodes. The cytoskeleton is mainly made up of actin microfilaments. The structure varies between species. In rodents, pinopodes contain many vacuoles; Human pinopodes do not contain large vacuoles but do have secretory vesicles, a rough endoplasmic reticulum, and a

golgi apparatus The Golgi apparatus (), also known as the Golgi complex, Golgi body, or simply the Golgi, is an organelle found in most eukaryotic cells. Part of the endomembrane system in the cytoplasm, it packages proteins into membrane-bound vesicles ...

. Mitochondria and

glycogen Glycogen is a multibranched polysaccharide of glucose that serves as a form of energy storage in animals, fungi, and bacteria. The polysaccharide structure represents the main storage form of glucose in the body. Glycogen functions as one o ...

have been found in both rodent and human pinopodes.

Endometrial receptivity

Previous literature had questioned the validity of the pinopodes' role in endometrial receptivity and implantation. However, more recent literature has found correlations between the presence of pinopodes and endometrial receptivity. Women undergoing in-vitro fertilization with a high presence of pinopodes are more likely to have higher embryo implantation and pregnancy rates when compared to women with low pinopode coverage. The implantation window is known as the period of time in which the endometrium is receptive to blastocyst attachment. Due to pinopode development overlapping with the implantation window, blastocyst attachment takes place, which provides adhesion molecules necessary for the implantation process. Current trials and research have shown pinopodes to be a reliable endometrial receptivity marker.

References

{{Reflist Human female reproductive system