4.2: The Zygote

4.2.1 Fertilization & Implantation

Fertilization occurs when a sperm and an oocyte (egg) combine and their nuclei fuse. Because each of these reproductive cells is a haploid cell containing half of the genetic material needed to form a human being, their combination forms a diploid cell. This new single cell, called a zygote, contains all the genetic material needed to form a human—half from each biological parent.

Recall that at the point of fertilization, the oocyte has not yet completed meiosis; all secondary oocytes remain arrested in the metaphase of meiosis II until fertilization. Only upon fertilization does the oocyte complete meiosis. The unneeded complement of genetic material that results is stored in a second polar body that is eventually ejected. At this moment, the oocyte has become an ovum, the female haploid gamete. The two haploid nuclei derived from the sperm and oocyte and contained within the egg are referred to as pronuclei. They decondense, expand, and replicate their DNA in preparation for mitosis. The pronuclei then migrate toward each other, their nuclear envelopes disintegrate, and the male- and female-derived genetic material intermingles. This step completes the fertilization process and results in a single-celled diploid zygote with all the genetic instructions it needs to develop into a human.⁹

Throughout this text, we will express embryonic and fetal ages in terms of weeks from fertilization, commonly called conception. The period required for full development of a fetus in utero is referred to as gestation (gestare = “to carry” or “to bear”). It can be subdivided into distinct gestational periods. The first 2 weeks of prenatal development are referred to as the pre-embryonic stage. A developing human is referred to as an embryo during weeks 3–8, and a fetus from the ninth week of gestation until birth. In this section, we’ll cover the pre-embryonic and embryonic stages of development, which are characterized by cell division, migration, and differentiation. By the end of the embryonic period, all of the organ systems are structured in rudimentary form, although the organs themselves are either nonfunctional or only semi-functional.¹⁰

Following fertilization, the zygote and its associated membranes, together referred to as the conceptus, continue to be projected toward the uterus by peristalsis and beating cilia of the epithelial cells of the Fallopian tube. During its journey to the uterus, the zygote undergoes five or six rapid mitotic cell divisions. Although each cleavage results in more cells, it does not increase the total volume of the conceptus. (Figure 4.2.1) Each daughter cell produced by cleavage is called a blastomere (blastos = “germ,” in the sense of a seed or sprout).¹¹

Figure 4.2.1 Pre-Embryonic Cleavages - Pre-embryonic cleavages make use of the abundant cytoplasm of the conceptus as the cells rapidly divide without changing the total volume. 
Image Source: Betts, J. Gordon, Kelly A. Young, James A. Wise, Eddie Johnson, Brandon Poe, Dean H. Kruse, Oksana Korol, Jody E. Johnson, Mark Womble, Peter DeSaix. Anatomy and Physiology. (2013). Open Stax, 2013 This work is distributed under a CC BY 4.0 license. Available from Open Stax.

Approximately 3 days after fertilization, a 16-cell conceptus reaches the uterus. The cells that had been loosely grouped are now compacted and look more like a solid mass. The name given to this structure is the morula (morula = “little mulberry”). Once inside the uterus, the conceptus floats freely for several more days. It continues to divide, creating a ball of approximately 100 cells and consuming nutritive endometrial secretions called uterine milk while the uterine lining thickens. The ball of now tightly bound cells starts to secrete fluid and organize themselves around a fluid-filled cavity, the blastocoel. At this developmental stage, the conceptus is referred to as a blastocyst. Within this structure, a group of cells forms into an inner cell mass, which is fated to become the embryo. The cells that form the outer shell are called trophoblasts (trophe = “to feed” or “to nourish”). These cells will develop into the chorionic sac and the fetal portion of the placenta (the organ of nutrient, waste, and gas exchange between a pregnant person and the developing offspring).¹²

The inner mass of embryonic cells is totipotent during this stage, meaning that each cell has the potential to differentiate into any cell type in the human body. Totipotency lasts for only a few days before the cells’ fates are set as being the precursors to a specific lineage of cells.¹³

As the blastocyst forms, the trophoblast excretes enzymes that begin to degrade the zona pellucida. In a process called “hatching,” the conceptus breaks free of the zona pellucida in preparation for implantation.¹⁴

Watch

Video 4.2

Cell Division

Source: MedlinePlus [Internet]. National Library of Medicine. (2020). Available from: Available from: https://medlineplus.gov/.

Video 4.3

Fertilization

Source: Hill, Mark A. "Embryology" Main Page. (2025) https://embryology.med.unsw.edu.au/e....php/Main_Page

For more information

More information about fertilization and early development before implantation can be found online in StatPearls Embryology.

Source: Khan YS and KM Ackerman. "Embryology, Week 1". In: StatPearls [Internet]. (April 17, 2023). Available from: https://www.ncbi.nlm.nih.gov/books/NBK554562/

At the end of the first week, the blastocyst comes in contact with the uterine wall and adheres to it, embedding itself in the uterine lining via the trophoblast cells. Thus begins the process of implantation, which signals the end of the pre-embryonic stage of development. (Figure 4.2.2)

Figure 4.2.2 Pre-Embryonic Development - Ovulation, fertilization, pre-embryonic development, and implantation occur at specific locations within the female reproductive system in a time span of approximately 1 week.
Image Source: Betts, J. Gordon, Kelly A. Young, James A. Wise, Eddie Johnson, Brandon Poe, Dean H. Kruse, Oksana Korol, Jody E. Johnson, Mark Womble, Peter DeSaix. Anatomy and Physiology. (2013). Open Stax, 2013 This work is distributed under a CC BY 4.0 license. Available from Open Stax.

Implantation can be accompanied by minor bleeding. The blastocyst typically implants in the fundus of the uterus or on the posterior wall. However, if the endometrium is not fully developed and ready to receive the blastocyst, the blastocyst will detach and find a better spot. A significant percentage (50–75 percent) of blastocysts fail to implant; when this occurs, the blastocyst is shed with the endometrium during menses. The high rate of implantation failure is one reason why pregnancy typically requires several ovulation cycles to achieve.¹⁵

When implantation succeeds and the blastocyst adheres to the endometrium, the superficial cells of the trophoblast fuse, forming the syncytiotrophoblast, a multinucleated body that digests endometrial cells to firmly secure the blastocyst to the uterine wall. In response, the uterine mucosa rebuilds itself and envelops the blastocyst (Figure 4.2.3).

Figure 4.2.3 Implantation - During implantation, the trophoblast cells of the blastocyst adhere to the endometrium and digest endometrial cells until the blastocyst is attached securely.
Image Source: Betts, J. Gordon, Kelly A. Young, James A. Wise, Eddie Johnson, Brandon Poe, Dean H. Kruse, Oksana Korol, Jody E. Johnson, Mark Womble, Peter DeSaix. Anatomy and Physiology. (2013). Open Stax, 2013 This work is distributed under a CC BY 4.0 license. Available from Open Stax.

The trophoblast secretes human chorionic gonadotropin (hCG), a hormone that directs the corpus luteum to survive, enlarge, and continue producing progesterone and estrogen to suppress menses. These functions of hCG are necessary for creating an environment suitable for the developing embryo. As a result of this increased production, hCG accumulates in the maternal bloodstream and is excreted in the urine. Implantation is complete by the middle of the second week. Just a few days after implantation, the trophoblast has secreted enough hCG for an at-home urine pregnancy test to give a positive result.¹⁶

Most of the time, an embryo implants within the body of the uterus in a location that can support growth and development. However, in one to two percent of cases, the embryo implants either outside the uterus (an ectopic pregnancy) or in a region of the uterus that can create complications for the pregnancy. If the embryo implants in the inferior portion of the uterus, the placenta may grow over the opening of the cervix, a condition called placenta previa.¹⁷

For more information

More information about fertilization and early development before implantation can be found online in StatPearls Embryology.

Source: Khan Yusuf S. and KKristin M. Ackerman. "Embryology, Week 1". In: StatPearls [Internet]. (April 17, 2023). Available from: https://www.ncbi.nlm.nih.gov/books/NBK554562/

4.2.2 Disorders of Embryo Development and Implantation

In the vast majority of ectopic pregnancies, the embryo does not complete its journey to the uterus and implants in the uterine tube, referred to as a tubal pregnancy. However, there are also ovarian ectopic pregnancies (in which the egg never left the ovary) and abdominal ectopic pregnancies (in which an egg was “lost” to the abdominal cavity during the transfer from ovary to uterine tube or in which an embryo from a tubal pregnancy re-implanted in the abdomen). Once in the abdominal cavity, an embryo can implant into any well-vascularized structure—the rectouterine cavity (Douglas’ pouch), the mesentery of the intestines, and the greater omentum are some common sites.¹⁸

Tubal pregnancies can be caused by scar tissue within the tube following a sexually transmitted bacterial infection. The scar tissue impedes the progress of the embryo into the uterus—in some cases “snagging” the embryo and, in other cases, blocking the tube completely. Approximately one half of tubal pregnancies resolve spontaneously. Implantation in a uterine tube causes bleeding, which appears to stimulate smooth muscle contractions and expulsion of the embryo. In the remaining cases, medical or surgical intervention is necessary. If an ectopic pregnancy is detected early, the embryo’s development can be arrested by the administration of the cytotoxic drug methotrexate, which inhibits the metabolism of folic acid. If diagnosis is late and the uterine tube is already ruptured, surgical repair is essential.¹⁹ Ectopic pregnancy is responsible for 5–10% of all pregnancy-related deaths.²⁰

Even if the embryo has successfully found its way to the uterus, it does not always implant in an optimal location (the fundus or the posterior wall of the uterus). Placenta previa can result if an embryo implants close to the internal os of the uterus (the internal opening of the cervix). As the fetus grows, the placenta can partially or completely cover the opening of the cervix (Figure 4.2.4). Although it occurs in only 0.5 percent of pregnancies, placenta previa is the leading cause of antepartum hemorrhage (profuse vaginal bleeding after week 24 of pregnancy but prior to childbirth).²¹

Figure 4.2.4 Placenta Previa - An embryo that implants too close to the opening of the cervix can lead to placenta previa, a condition in which the placenta partially or completely covers the cervix. 
Image Source: Betts, J. Gordon, Kelly A. Young, James A. Wise, Eddie Johnson, Brandon Poe, Dean H. Kruse, Oksana Korol, Jody E. Johnson, Mark Womble, Peter DeSaix. Anatomy and Physiology. (2013). Open Stax, 2013. This work is distributed under a CC BY 4.0 license. Available from Open Stax.

In some cases a tumor develops inside the uterus from tissue that forms after conception. This tissue is made of trophoblast cells. Sometimes there is a problem with the fertilized egg and trophoblast cells. Instead of a healthy fetus developing, a tumor forms. Until there are signs or symptoms of the tumor, the pregnancy will seem like a normal pregnancy. Most GTD is benign (not cancer) and does not spread, but some types become malignant and spread to nearby tissues or distant parts of the body. Gestational trophoblastic disease (GTD) is a general term that includes different types of disease including hydatidiform moles (complete and partial) and gestational trophoblastic neoplasia (GTN). Hydatidiform moles, the most common type of GTD, are slow-growing tumors that look like sacs of fluid. A hydatidiform mole is also called a molar pregnancy. The cause of hydatidiform moles is not known. Hydatidiform moles may be complete or partial. A complete hydatidiform moles forms when sperm fertilizes an egg that does not contain the mother’s DNA. The egg has DNA from the father and the cells that were meant to become the placenta are abnormal. A partial hydatidiform moles form when sperm fertilizes a normal egg and there are two sets of DNA from the sperm in the fertilized egg. Only part of the fetus forms and the cells that were meant to become the placenta are abnormal. Most hydatidiform moles are benign, but they sometimes become cancerous.

Gestational trophoblastic neoplasia (GTN) is a type of gestational trophoblastic disease (GTD) that is almost always malignant, and includes invasive moles, choriocarcinomas, placental-site trophoblastic tumors and epithelioid trophoblastic tumors. These conditions are treated with surgery, chemotherapy and radiation therapy.

For more information

More information regarding ectopic pregnancy and ultrasounds for ectopic pregnancy can be found online in "Ectopic Pregnancy," StatPearls.
Source: Baker, Mark and Jonathan dela Cruz. "Ectopic Pregnancy, Ultrasound." In StatPearls [Internet]. (January 16, 2023). Available from: https://www.ncbi.nlm.nih.gov/books/NBK482192/

More information about placenta abnormalities, placenta accreta, and placenta previa can be found online in "Placenta Abnormalities," StatPearls.
Source: Rathburn, Kimberly M. and Jason P. Hildebrand. "Placenta Abnormalities". In StatPearls. [Internet] (October 17, 2022). Available from: https://www.ncbi.nlm.nih.gov/books/NBK459355/

Additional resources for gestational trophoblastic disease and treatment can be found online in "Gestational Trophoblastic Diesease Treatment," (PDQ) - Health Professional Version, National Cancer Institute.

4.2.3 Creation and Organization of the Embryonic Membranes

During the second week of development, with the embryo implanted in the uterus, cells within the blastocyst start to organize into layers. Some grow to form the extraembryonic membranes needed to support and protect the growing embryo: the amnion, the yolk sac, the allantois, and the chorion.²²

At the beginning of the second week, the cells of the inner cell mass form into a two-layered disc of embryonic cells, and a space—the amniotic cavity—opens up between it and the trophoblast (Figure 4.2.5).

Figure 4.2.5 Development of the Embryonic Disc Formation of the embryonic disc leaves spaces on either side that develop into the amniotic cavity and the yolk sac.
Image Source: Betts, J. Gordon, Kelly A. Young, James A. Wise, Eddie Johnson, Brandon Poe, Dean H. Kruse, Oksana Korol, Jody E. Johnson, Mark Womble, Peter DeSaix. Anatomy and Physiology. (2013). Open Stax, 2013. This work is distributed under a CC BY 4.0 license. Available from Open Stax.

Cells from the upper layer of the disc (the epiblast) extend around the amniotic cavity, creating a membranous sac that forms into the amnion by the end of the second week. The amnion fills with amniotic fluid and eventually grows to surround the embryo. Early in development, amniotic fluid consists almost entirely of a filtrate of maternal plasma, but as the kidneys of the fetus begin to function at approximately the eighth week, they add urine to the volume of amniotic fluid. Floating within the amniotic fluid, the embryo—and later, the fetus—is protected from trauma and rapid temperature changes. It can move freely within the fluid and can prepare for swallowing and breathing out of the uterus.²³

On the ventral side of the embryonic disc, opposite the amnion, cells in the lower layer of the embryonic disk (the hypoblast) extend into the blastocyst cavity and form a yolk sac. The yolk sac supplies some nutrients absorbed from the trophoblast and also provides primitive blood circulation to the developing embryo for the second and third week of development. When the placenta takes over nourishing the embryo at approximately week 4, the yolk sac has been greatly reduced in size and its main function is to serve as the source of blood cells and germ cells (cells that will give rise to gametes). During week 3, a finger-like outpocketing of the yolk sac develops into the allantois, a primitive excretory duct of the embryo that will become part of the urinary bladder. Together, the stalks of the yolk sac and allantois establish the outer structure of the umbilical cord.²⁴

The last of the extraembryonic membranes is the chorion, which is the one membrane that surrounds all others. The development of the chorion will be discussed in more detail shortly, as it relates to the growth and development of the placenta.

For more information

More information about amniotic fluid can be found online in "Placenta Previa," StatPearls.

More information about the yolk sac can be found online in "Embryology, Yolk Sac," StatPearls.

A review of the beginning of pregnancy can be found online at "The surprising science of how pregnancy begins," Shots Health News from NPR.

4.2.4 The Placenta

4.2.4.1 Development of the Placenta

During the first several weeks of development, the cells of the endometrium—referred to as decidual cells—nourish the nascent embryo. During prenatal weeks 4–12, the developing placenta gradually takes over the role of feeding the embryo, and the decidual cells are no longer needed. The mature placenta is composed of tissues derived from the embryo, as well as maternal tissues of the endometrium. The placenta connects to the conceptus via the umbilical cord, which carries deoxygenated blood and wastes from the fetus through two umbilical arteries; nutrients and oxygen are carried from the pregnant person to the fetus through the single umbilical vein. The umbilical cord is surrounded by the amnion, and the spaces within the cord around the blood vessels are filled with Wharton’s jelly, a mucous connective tissue.

The placenta develops throughout the embryonic period and during the first several weeks of the fetal period; placentation is complete by weeks 14–16. As a fully developed organ, the placenta provides nutrition and excretion, respiration, and endocrine function. (Figure 4.2.6).

Figure 4.2.6 Placenta - This post-expulsion placenta and umbilical cord (white) are viewed from the fetal side.
Image Source: Betts, J. Gordon, Kelly A. Young, James A. Wise, Eddie Johnson, Brandon Poe, Dean H. Kruse, Oksana Korol, Jody E. Johnson, Mark Womble, Peter DeSaix. Anatomy and Physiology. (2013). Open Stax, 2013 This work is distributed under a CC BY 4.0 license. Available from Open Stax.

It receives blood from the fetus through the umbilical arteries. Capillaries in the chorionic villi filter fetal wastes out of the blood and return clean, oxygenated blood to the fetus through the umbilical vein. Nutrients and oxygen are transferred from maternal blood surrounding the villi through the capillaries and into the fetal bloodstream. Some substances move across the placenta by simple diffusion. Oxygen, carbon dioxide, and any other lipid-soluble substances take this route. Other substances move across by facilitated diffusion. This includes water-soluble glucose. The fetus has a high demand for amino acids and iron, and those substances are moved across the placenta by active transport.

The maternal portion of the placenta develops from the deepest layer of the endometrium, the decidua basalis. To form the embryonic portion of the placenta, the syncytiotrophoblast and the underlying cells of the trophoblast (cytotrophoblast cells) begin to proliferate along with a layer of extraembryonic mesoderm cells. These form the chorionic membrane, which envelops the entire conceptus as the chorion. The chorionic membrane forms finger-like structures called chorionic villi that burrow into the endometrium like tree roots, making up the fetal portion of the placenta. The cytotrophoblast cells perforate the chorionic villi, burrow farther into the endometrium, and remodel maternal blood vessels to augment maternal blood flow surrounding the villi. Meanwhile, fetal mesenchymal cells derived from the mesoderm fill the villi and differentiate into blood vessels, including the three umbilical blood vessels that connect the embryo to the developing placenta (Figure 4.2.7).

Figure 4.2.7 Cross-Section of the Placenta - In the placenta, maternal and fetal blood components are conducted through the surface of the chorionic villi, but maternal and fetal bloodstreams never mix directly. 
Image Source: Betts, J. Gordon, Kelly A. Young, James A. Wise, Eddie Johnson, Brandon Poe, Dean H. Kruse, Oksana Korol, Jody E. Johnson, Mark Womble, Peter DeSaix. Anatomy and Physiology. (2013). Open Stax, 2013 This work is distributed under a CC BY 4.0 license. Available from Open Stax.

Maternal and fetal blood does not commingle because blood cells cannot move across the placenta. This separation prevents the pregnant person's cytotoxic T cells from reaching and subsequently destroying the fetus, which bears “non-self” antigens. Further, it ensures the fetal red blood cells do not enter the pregnant person's circulation and trigger antibody development (if they carry “non-self” antigens)—at least until the final stages of pregnancy or birth. This is the reason that, even in the absence of preventive treatment, an Rh​−​ person doesn’t develop antibodies that could cause hemolytic disease in their first Rh+ fetus.

Although blood cells are not exchanged, the chorionic villi provide ample surface area for the two-way exchange of substances between maternal and fetal blood. The rate of exchange increases throughout gestation as the villi become thinner and increasingly branched. The placenta is permeable to lipid-soluble fetotoxic substances: alcohol, nicotine, barbiturates, antibiotics, certain pathogens, and many other substances that can be dangerous or fatal to the developing embryo or fetus. For these reasons, pregnant people should avoid fetotoxic substances. Alcohol consumption by pregnant people, for example, can result in a range of abnormalities referred to as fetal alcohol spectrum disorders (FASD). These include organ and facial malformations, as well as cognitive and behavioral disorders.

4.2.4.2 Functions of the Placenta

• Nutrition and Digestion

  • Mediates diffusion of maternal glucose, amino acids, fatty acids, vitamins, and minerals
  • Stores nutrients during early pregnancy to accommodate increased fetal demand later in pregnancy
  • Excretes and filters fetal nitrogenous wastes into maternal blood

• Respiration

  • Mediates maternal-to-fetal oxygen transport and fetal-to-maternal carbon dioxide transport

• Endocrine Function

  • Secretes several hormones, including hCG, estrogens, and progesterone, to maintain the pregnancy and stimulate maternal and fetal development
  • Mediates the transmission of maternal hormones into fetal blood and vice versa

For more information

More information about the role of the placenta, how the placenta forms, and ongoing research about the importance of the placenta in human development can be found online in "Embryology, Placenta," StatPearls.

4.2.5 Development of Twins

Most of the time, a person releases a single egg during an ovulation cycle. However, in approximately 1 percent of ovulation cycles, two eggs are released and both are fertilized. Two zygotes form, implant, and develop, resulting in the birth of dizygotic (or fraternal) twins. Because dizygotic twins develop from two eggs fertilized by two sperm, they are no more identical than siblings born at different times.

Much less commonly, a zygote can divide into two separate offspring during early development. This results in the birth of monozygotic (or identical) twins. Although the zygote can split as early as the two-cell stage, splitting occurs most commonly during the early blastocyst stage, with roughly 70–100 cells present. These two scenarios are distinct from each other, in that the twin embryos that separated at the two-cell stage will have individual placentas, whereas twin embryos that form from separation at the blastocyst stage will share a placenta and a chorionic cavity.²⁵

For more information

More information about twin births can be found online in "Twin Births (Archived)," StatPearls

Glossary Terms

 

Allantois

An extraembryonic membranous sac derived from the YOLK SAC of REPTILES; BIRDS; and MAMMALS. It lies between two other extraembryonic membranes, the AMNION and the CHORION. The allantois serves to store urinary wastes and mediate exchange of gas and nutrients for the developing embryo.

Amnion

The innermost membranous sac that surrounds and protects the developing embryo which is bathed in the AMNIOTIC FLUID. Amnion cells are secretory EPITHELIAL CELLS and contribute to the amniotic fluid.

Barbituates

A class of chemicals derived from barbituric acid or thiobarbituric acid. Many of these are GABA MODULATORS used as HYPNOTICS AND SEDATIVES, as ANESTHETICS, or as ANTICONVULSANTS.

Choriocarcinoma

A malignant metastatic form of trophoblastic tumors. Unlike the HYDATIDIFORM MOLE, choriocarcinoma contains no CHORIONIC VILLI but rather sheets of undifferentiated cytotrophoblasts and syncytiotrophoblasts (TROPHOBLASTS). It is characterized by the large amounts of CHORIONIC GONADOTROPIN produced. DNA analyses can determine tissue origins: placental (fetal) origin or non-placental origin.

Chorionic sac

The fetal part of the placenta.

Chorionic villi

Projections of the chorionic membrane that burrow into the endometrium and develop into the placenta.

Cytotophoblast

Cells lining the outside of the BLASTOCYST. After binding to the ENDOMETRIUM, trophoblasts develop into two distinct layers, an inner layer of mononuclear cytotrophoblasts and an outer layer of continuous multinuclear cytoplasm, the syncytiotrophoblasts, which form the early fetal-maternal interface.

Diploid

A cell containing two matched sets of chromosomes.

Ectopic pregnancy

The implantation of an embryo outside of the uterus.

Endometrium

The inner lining of the uterus, part of which builds up during the secretory phase of the menstrual cycle and then sheds with menses.

Facilitated diffusion

Diffusion of a substance with the aid of a membrane protein.

Gestational trophoblastic neoplasia

A group of diseases arising from pregnancy that are commonly associated with hyperplasia of trophoblasts (TROPHOBLAST) and markedly elevated human CHORIONIC GONADOTROPIN. They include HYDATIDIFORM MOLE, invasive mole (HYDATIDIFORM MOLE, INVASIVE), placental-site trophoblastic tumor (TROPHOBLASTIC TUMOR, PLACENTAL SITE), and CHORIOCARCINOMA. These neoplasms have varying propensities for invasion and spread

Haploid

the presence of a single set of chromosomes in an organism’s cells. Sexually reproducing organisms are diploid (having two sets of chromosomes, one from each parent). In humans, only the egg and sperm cells are haploid

Hemolytic disease

Hemolytic disease of the newborn (HDN) is a blood disorder in a fetus or newborn infant. In some infants, it can be fatal.

Hydatidiform mole

Trophoblastic hyperplasia associated with normal gestation, or molar pregnancy. It is characterized by the swelling of the CHORIONIC VILLI and elevated human CHORIONIC GONADOTROPIN. Hydatidiform moles or molar pregnancy may be categorized as complete or partial based on their gross morphology, histopathology, and karyotype.

Implantation

The process by which a blastocyst embeds itself in the uterine endometrium.

In utero

In the uterus, before birth.

Lipid-soluble

A drug's solubility in fatty or oily solutions.

Menses

The shedding of the inner portion of the endometrium out though the vagina; also referred to as menstruation.

Methotrexate

An antineoplastic antimetabolite with immunosuppressant properties. It is an inhibitor of TETRAHYDROFOLATE DEHYDROGENASE and prevents the formation of tetrahydrofolate, necessary for synthesis of thymidylate, an essential component of DNA.

Neoplasia

New abnormal growth of tissue. Malignant neoplasms show a greater degree of anaplasia and have the properties of invasion and metastasis, compared to benign neoplasms.

Persistalsis

A movement, caused by sequential muscle contraction, that pushes the contents of the intestines or other tubular organs in one direction.

Prenatal

Having to do with the time a female is pregnant, before birth occurs; also called antenatal.

Progesterone

A sex hormone important in regulating the reproductive cycle in those with ovaries and the maintenance of pregnancy.

Syncytiotrophoblast

Superficial cells of the trophoblast that fuse to form a multinucleated body that digests endometrial cells to firmly secure the blastocyst to the uterine wall.

Trophoblast

The fluid-filled shell of squamous cells destined to become the chorionic villi, placenta, and associated fetal membranes.

Trophobalstic tumor

Trophoblastic growth, which may be gestational or nongestational in origin. Trophoblastic neoplasia resulting from pregnancy is often described as gestational trophoblastic disease to distinguish it from germ cell tumors which frequently show trophoblastic elements, and from the trophoblastic differentiation which sometimes occurs in a wide variety of epithelial cancers. Gestational trophoblastic growth has several forms, including HYDATIDIFORM MOLE and CHORIOCARCINOMA.

Yolk sac

The membrane associated with primitive circulation to the developing embryo; source of the first blood cells and germ cells and contributes to the umbilical cord structure.

Footnotes

9. Betts, J. Gordon, Kelly A. Young, James A. Wise, Eddie Johnson, Brandon Poe, Dean H. Kruse, Oksana Korol, Jody E. Johnson, Mark Womble, Peter DeSaix. Anatomy and Physiology. (2013). Open Stax, 2013 This work is distributed under a CC BY 4.0 license. Available from Open Stax.
10. Betts et al., 2013
11. Betts et al., 2013
12. Betts et al., 2013
13. Betts et al., 2013
14. Betts et al., 2013
15. Betts et al., 2013
16. Betts et al., 2013
17. Betts et al., 2013
18. Betts et al., 2013
19. Betts et al., 2013
20. Houser M, Kandalaft N, Khati NJ. Ectopic pregnancy: a resident’s guide to imaging findings and diagnostic pitfalls. Emerg Radiol 2022; 29(1): 161–172
21. Betts et al., 2013
22. Betts et al., 2013
23. Betts et al., 2013
24. Betts et al., 2013
25. Betts et al., 2013

Image Acknolwedgements

Betts, J. Gordon, Kelly A. Young, James A. Wise, Eddie Johnson, Brandon Poe, Dean H. Kruse, Oksana Korol, Jody E. Johnson, Mark Womble, Peter DeSaix. Anatomy and Physiology. (2013). Open Stax, 2013 This work is distributed under a CC BY 4.0 license. Available from Open Stax.