HG&D INTERACTIVE Block two Objectives

Review of Placenta Formation
March 29, Lectures 1 and 2==

Definitions

 * Stromal Cells and Decidual cells of developing endometrium: Stromal cells differentiate into decidual cells during pregnancy.  The decidual cells serve an immunoregulatory function during pregnancy.


 * Basal and functional layers of endometrium: The basal layers of the endometrium has its own blood supply and is not sloughed off during menstruation.  The functional layer is made of the compact and spongy layers, which are both sloughed off during menstruation and after parturition.l


 * Spiral arteries, uterine glands: know what these are

A blastocyst forms from the morula. It has a fluid-filled space called the blastocystic cavity. There is a thin outer cell layer called the trophoblast which gives rise to the embryonic part of the placenta. There is also a group of centrally located blastomeres called the inner cell mass, which is referred to as the embryoblast. It is the primordium of the embryo. Later, with implantation, the trophoblasts differentiate into the cytotrophoblasts and syncytiotrophoblasts to allow for implantation. Implantation of the blastocyst occurs at the end of the first week and is completed by the end of the second week. Rapid proliferation and differation of the trophoblast are important features of the second week. Various endometrial changes follow, known as the decidual reaction. Concurrently, the primary yolk sac forms the extraembryonic coelum develops. The extraembryonic coelom forms from spaces that develop in extraembryonic mesoderm. The extraembryonic coelom later becomes the chorionic cavity. The primary yolk sac becomes smaller and gradually disappears as the secondary yolk sac appears. The amniotic cavity appears as a space between the cytotrophoblast and embryoblast. The embryoblast differentiates into a bilaminar embryonic disc consisting of epiblast, related to the amniotic cavity, and hypoblast, adjacent to the blastocyst cavity. The prechordal plate develops as a localized thickening of the hypoblast, which indicates the future cranial region of the embryo and the future site of the mouth; the prechordal plate is also the important organizer of the head region. The conversion of the bilaminar embryo into a trilaminar embProxy-
 * Blastocyst embryo (blastula), bilaminar embryo, trilaminar embryo:


 * primitive streak
 * This is a thickening of the epiblast layer to the median plane of the disc. The epiblasts invaginate giving rise to mesenchymal cells  that migrate ventrally, laterally and cranially between the epiblast and hypoblast.  As soon as the primitive streak begins to produce mesenchymal cells, it is known as a trilaminar embryo.


 * Amnion and amniotic cavity
 * This starts as a small space in the embryoblast. Concurrently, the bilaminar embryonic disc, consisting of epiblast and hypoblast, forms.  The epiblast forms the floor of the amniotic cavity and is continuous with the amnion. The roof is made of amnion (i.e. amnioblasts).


 * Chorion (chorionic membrane) and chorionic cavity
 * At the end of the second week, there is the appearance of primary chorionic villi. Proliferation of cytotrophoblastic cells produces cellular extensions that grow into the syncytiotrophoblast.  The cellular exchanges form primary chorionic villi, the first stage in the development of the chorionic villi of the placenta.  The extraembryonic mesoderm increases and isolated extraembryonic coelomic spaces appear with it.  These spaces rapidly fuse to form the extraembryonic coelom.  The extraembryonic coelom splits the extraembryonic mesoderm into two layers: the extraembryonic somatic mesoderm and the extraembryonic splanchnic mesoderm.  The EEM somatic mesoderm and the two layers of trophoblast form the chorion, which forms the walls of the chorionic sac, within which the embryo and its amniotic and yolk sacs are suspended by the connecting stalk.  The extraembryonic coelom is now called the chorionic cavity.


 * Trophoblast
 * See earlier definition


 * Cytotrophoblast and Syncytiotrophoblast
 * The cytotrophoblast is a mononucleated layer of cells that is mitotically active and forms new cells that migrate into the increasing mass of syncytiotrophoblast, where they fuse and lose their cell membranes. The syncytiotrophoblast is a rapidly expanding, multinucleated mass in which no cell boundaries are discernable.


 * Interstitial implantation
 * When the blastocyst lies within the substance of the endometrium. This also implies the involves the decidual cell reaction.


 * Extraembryonic mesoderm and the connecting stalk
 * See above for extraembryonic mesoderm. The connecting stalk attaches to the chorion and the embryo and its amniotic and yolk sacs are suspended by it.


 * Lacunae and Lacunar Network
 * Lacunae appear in the syncytiotrophoblast. These are filled with a mixture of maternal blood from ruptured endometrial capillaries and cellular debris from eroded uterine glands.


 * Chorionic Villi (primary, secondary and tertiary)
 * Shortly after primary chorionic villi (made of syncytiotrophoblast and cytotrophoblasts) appear at the end of the second week, they begin to branch.  Early in the third week, mesenchymn grows into the primary villi, forming a core of mesenchymal tissue.  At this stage they are second chorionic villi and cover the entire surface of the chorionic sac.  Some mesenchymal cells in the villi soon differentiate into capillaries and blood cells.  They are then called tertiary chorionic villi.


 * Maternal sinusoids
 * These are thin-walled terminal vessels that are larger than ordinary capillaries. They are the endometrial capillaries around the implanted embryo that become congested and dilated.


 * Intervillous space
 * It contains maternal blood derived from the lacunae that developed in the syncytiotrophoblast during the second week of development. This large blood-filled space results from the coalescence and enlargement of the lacunar networks.  The intervillous space of the placenta is divided into compartments by the placental septa; however, there is free communication between the compartments because the septa do no reach the chorionic plate.


 * Cytotrophoblastic shell
 * Cytotrophoblastic cells of the chorionic villi proliferate and extend through the synctiotrophoblast to form a cytotrophoblastic shell which gradually surrounds the chorionic sac and attaches it to the endometrium. Villi that attach to the maternal tissues through the cytotrophoblastic shell are stem villi.  The villi that grow from the sides of the stem villi are branch villi.


 * Anchoring and Stem villi: see above


 * Decidua Basalis, Decidua Capsularis, Decidua Parietalis
 * The decidua basalis is the part of the decidua deep to the conceptus that forms the maternal part of the placenta. The decidua capsularis is the superficial part of the decidua overlying the conceptus.  The decidua parietalis is all the remaining parts of the decidua.


 * Amniochorionic membrane
 * The amniotic sac enlarges faster than the chorionic sac. As a result, the amnion and smooth chorion soon fuse to form the amniochorionic membrane.  This composite membrane fuses with the decidua capsularis and, after disappearance of this capsularis part of the decidua, adheres to the decidua parietalis.  It is the amniochorionic membrane that ruptures during labor.


 * Smooth Chorion
 * The villi associated with the decidua capsularis are compressed, reducing the blood supply to them. The villi soon degenerate producing a relatively avascular area, which is the smooth chorion.


 * Villous Chorion
 * As the villi disappear, those associated with the decidua basalis rapidly increase in number, branch profusely, and enlarge. This bushy part of the chorionic sac is the villous chorion.


 * Placental Septum
 * As the chorionic villi invade the decidua basalis, decidual tissue is eroded to enlarge the intervillous space. This erosion produces several wedge-shaped areas of decidua, called placenatal septa, that project toward the chorionic plate.


 * Placental Cotyledon
 * The placental septa divide the fetal part of the placenta into irregular convex areas called contyledons.


 * Chorionic Plate
 * This is the part of the chorionic wall related to the placenta.


 * Fate of the Yolk Sac
 * After ten weeks the small yolk sac lies in the chorionic cavity between the amniotic and chorionic sacs.  It atrophies as pregnancy advances, eventually becoming very small.  In very unusual cases, the yolk sac persists throughout pregnancy and appears under the amnion as a small structure on the fetal surface of the placenta near the attachment of the umbilical cord.  Persistence of the yolk sac is of no significance.  The yolk stalk usually detaches from the midgut loop by the end of the sixth week.  In about 2% of adults, the proximal intra-abdominal part of the yolk stalk persists as a Meckel diverticulum.


 * Contents and layers of the umbilical cord
 * The umbilical cord usually has two arteries and one vein that are surrounded by mucoid connective tissue.

Placental Exchange Mechanisms
March 29th: Lecture 3

1. Identify all the major ingredients of a newborn baby (the general classes of compounds that cross the placenta)

Glucose, AA, Vitamins, minerals, fat and water.

''2. Identify the mechanisms by which different substances cross the placenta, especially glucose, amino acids and fats. '' Oxygen, CO2, anesthetic gases, ethyl alcohol and many drugs cross via diffusion. D-glucose is transported via facilitated diffusion using the Glut-1 transporter, i.e. facilitated diffusion. Amino acids, calcium ion, phosphate ion, Vit B-12 are all transported via active transport. IgG is transported via clatherin coated pits, this is vesicular transport. Finally, water crosses due to osmotic and hydrostatic pressures.

3. Discuss the concept of placental permeability and indicate why some animal studies of placental permeabilities of drugs may no apply to humans.

In humans, the maternal placenta is separated from the fetal placenta by two cell layers: the syncytial trophoblast and the capillary endothelium. Some substances are able to diffuse rapidly across the barrier because they can cross lipid bilayers easily. Other substances must find water “pores” through which to cross to the synctium. The number (and size) of pores is species dependent. This is why animal studies are not necessarily applicable to humans. We have the most porous placenta in town!

4. Compare the transport of oxygen and CO2 across the placenta.

There is a blood-blood interface allowing the diffusion of O2 and CO2 in the placenta. In terms of O2, it is delivered to the placenta as a product of maternal placental blood flow times [O2] and is delivered to the fetal tissue as fetal umbilical blood flow times [O2]ven. The partial pressure of oxygen equilibrates across the placental capillary as blood travels through the villus. Nevertheless, the umbilical vein oxygen pressures are lower than for intervillous space or even uterine vein blood. This is because the oxygen consumption of the placenta is significant and there is a perfusion-perfusion mismatch (shunt). The oxygen stores of maternal hemoglobin are large and the transplacental O2 flux is much larger than would be predicted by the dissolved oxygen concentration at the same PO2 and flow rate. The binding of oxygen to hemoglobin enhances O2 transfer across the placenta as though the flow had been increased by some 10 times because oxygen delivery is enhanced by that much. Also, fetal hemoglobin is left shifted. Fetal tissues must live at a much lower oxygen tension than adult tissues.

CO2, produced by the fetus, must be excreted into maternal blood. Therefore, CO2 tensions are, as expected, higher in fetal arterial blood than in maternal blood. CO2 is very lipid soluble (20x more than O2). The placenta is a very efficient exchanger for CO2 excretion. Even so, there remains a CO2 gradient between the maternal and fetal veins due to CO2 production by the placenta and a Q/Q mismatch within the placenta.

5. Compare the concept of “flow limited” transport with “diffusion limited” transport.

Lipid soluble compounds “dissolve” through membranes and have the entire 10-15M2 surface area of the placenta available for transport. For these molecules, fetal capillary concentrations completely equilibrate with maternal blood concentrations in a single pas through the placenta. They cross the placental barrier so rapidly that the rate at which they are delivered to the fetus depends primarily on the rate that they arrive at the barrier. These compounds are thus said to be “flow dependent” because their transfer rate is in proportion to blood flow. Examples include oxygen, carbon dioxide, anesthetic gases, ethyl alcohol, and many drugs.

Lipid insoluble compounds cross the placental barrier slowly and do not equilibrate during passage. Their rate of transplacental flux is determined by the permeability of the placental barrier that is unique for each compound. Their rate of transfer is said to be “diffusion limited.”

As for lipid soluble compounds, Fick’s law describes their transplacental flux: J = D ~ A/h ~ (Cm – Cf). Where the flux (J) is determined by the product of D (coefficient of free diffusion), the surface area (A)/thickness (or pore height, h), and the mean concentration difference between maternal and fetal capillaries. The placental permeability is defined as the flux to concentration gradient ratio. Therefore: Permeability = J/deltaC. DeltaC is the mean maternal-fetal capillary concentration difference. The greater the permeability for a substance, the greater will be transplacental flux for a given concentration gradient.

Bottom line is for the least lipid soluble substances, the permeability predicts measurable transfer up to some 10,000 Da. These substances includes most drugs, short chain polypeptides, small proteins and sugars.  6. Discuss the importance of non-diffusional transpot—which substances are involved?

Non-diffusional transport encompasses: active transport, vesicular transport, and water transport. See earlier objective for substances. Active transport and vesicular transport allow a greater number of particles to enter the placenta than would simply by diffusion alone.  7. Identify the mechanism by which water gets across the human placenta.

More water enters the fetus during gestation than any other substance. Water moves across the placenta under osmotic and hydrostatic pressures (i.e. the Starling forces).  8. Discuss the role of the placenta in predicting disease in offspring in their later lives.

Yeah, we know.

Oogenesis, Gemet Transport, and Fertilization
""3/30/10; 8am""

Definitions:
Oogonium: oocyte stem cell Vs. Oocyte: the cell committed to becoming a female gamete

Zona reaction: Secretory event where the secretion of cortical granules during egg activation destroys receptors on the oocyte plasma membrane that recognize sperm. The enzymes also modify the zona pellucida glycoproteins to eliminate receptivity and change its physical characteristics.

Sperm capacitation: change in mammalian sperm that occurs after exposure to female genital tract making the sperm competent to undergo the acrosome reaction; this change is necessary for penetration of the cumulus matrix and for fertilization

Centrosome: The primary microtubule organizing center (MTOC) of animal cells, that divides prior to cell division. Each daughter MTOC acts as one pole of the spindle apparatus. The centrosome usually contains a pair of centrioles

Learning Objectives:
""1. To understand meiosis relates to oocyte maturation.""

""2. Discuss the various mechanisms for getting egg and sperm together in the reproductive tract.""

""3. Outline the major steps in fertilization.""

Study Questions: (page 28)
""1. When does meiosis begin in female germ cells? When does it end?""

Meiosis for all oocytes begins in the prenatal ovary. The oogonium commits to being a gamete by entering into Prophase I in the prenatal ovary.

""2. How does meiosis differ from mitosis?"

Meiosis: The resulting cells contain a haploid number of chromosomes. Meiosis consists of two successive divisions: separation of homologous pairs and then separation of sister chromatids. Recombination occurs in prophase I.

Mitosis: The resulting cells contain a diploid number of chromosomes. Mitosis consists of one division: separation of sister chromatids. There is no recombination.

""3. What is the time scale of meiotic maturation in the human egg?""

Prophase I begins in the prenatal ovary. When an oocyte is selected for ovulation, it completes the first meiotic division near the time of ovulation. After the first meiotic division is completed, the ovary enters the second meiotic division immediately. (There is no interphase period between the end of the first meiotic division and the start of the next.) The oocyte arrests in Metaphase II. When the sperm fertilizes the oocyte, the oocyte completes the second division of meiosis.

Time stuff: Meiotic prophase can persist for over 40 years. The fertilizable lifespan of the oocyte is 12-20 hours

Study Questions: (page 32)
"1. What is the significance of the cumulus oophorus? Discuss the advantages to the egg being surrounded by cumulus cells."

The cumulus oophorus are the cells that surround the oocyte during ovulation. The oocyte-cumulus cell complex is sticky because of the extracellular matrix produced by the cumulus cells. This allows the fimbria to stick to the oocyte-cumulus cell complex and transport the egg to the fallopian tube.

""2. What are the most likely mechanisms of egg transport to the site of fertilization?"

The fimbria are ciliated, which beat towards the oviductal ostium, leading to the tubal lumen. The fimbria sweep over the surface of the ovary through the action of smooth muscle within the oviductal mesentery. The ovary also moves under the fimbria through the action of the ovarian ligament. The oocyte-cumulus cell complex is sticky, so when it comes into contact with the fimbrial surface, it is moved through the ostium of the fallopian tube and to the ampulla via ciliary action. Further transport to the site of fertilization (the ampullo-isthmic junction) is managed by ciliary activity and smooth muscle contraction of the oviductal wall.

""3. Sperm transport in the female tract is due to a combination of active and passive forces. At which point(s) is/are sperm motility thought to be important, if not essential, for continued sperm transport?""

Sperm motility is very important for the sperm to passage through the cervical mucus, from the cervical canal to the uterine lumen. The sperm reaches the internal os the cervix and is distributed throughout the uterus via random contractions of the myometrium. Sperm motility is very important for the sperm to enter the oviducts from the uterus through the utero-tubal junction (a sphincter-like opening into each oviduct). Sperm is stored in the lower isthmus for some hours and become quiescent, possibly to store energy and undergoing a phase of capacitation. Sperm motility is not important for the final ascent of the sperm to the site of fertilization; coordinated isthmic smooth muscle contraction is the most important factor in this process.

"4. Why is midcycle cervical mucus more easily penetrated by sperm than luteal phase mucus?"

Cervical mucus is synthesized and secreted by the epithelium of cervical mucosa, which lines the endocervical canal and the endocervical crypts. In the periovulatory period, estrogen influences this secretion to become profuse, watery, and maximally receptive to sperm. At other times, the secretion is scant, viscous, and resists sperm penetration.

"5. What is the primary factor causing the change in the cervical mucus after ovulation?"

Increased level of estrogen.

"6. Describe the contractile behavior of the oviductal isthmus in the periovulatory period as opposed to the luteal phase"

Tubal musculature is sensitive to estrogen and progesterone and behaves differently as the concentrations of these hormones change with the cycle. The changes are based on alterations in the density and sensitivity of alpha-adrenergic receptors in the smooth muscle. Before ovulation, the contractions are random segmental contractions. In the periovulatory period, the isthmus’s contractions are coordinated adovarian peristaltic waves of contraction, sweeping the isthmic contents upward. During the luteal phase, the contractions are segmental, which cause the lamenal contents (containing the embryo) to swish back and forth.

"7. What are the most critical aspects of embryo transport through the oviduct and into the uterus?"

The entry of the embryo into the uterus has to coincide with maximal receptivity of the endometrium for implantation. Perhaps the luteal contractions and the utero-tubal junction help prevent the premature entry of the embryo into the uterine lumen. Prolonged retention of embryos within the oviduct may result in ectopic pregnancies.

Study Questions: (page 39)
"1. Describe the two discrete secretory events that occur during fertilization. What do they accomplish and how are they controlled?"

One secretory event is the acrosome reaction. The acrosome reaction is the secretion of the acrosomal vesicle that contains acrosomal enzymes that destroy the sperm’s receptivity to the zona and digest a path through the zona matrix.

The second secretory event is the secretion of phospholipase C-zeta from the sperm, which mediates egg activation.

The next secretory event is the secretion of cortical granules during egg activation, which destroys receptors on the oocyte plasma membrane that recognize sperm. The enzymes also modify the zona pellucida glycoproteins to eliminate receptivity and change its physical characteristics. These events comprise the zona reaction.

Another secretory event is that of a specific hatching enzyme. This enzyme is secreted by the embryo after 5-6 days of developing within the altered zona matrix. The enzyme allows the embryo to hatch and implant into the endometrium.

""2. What are the putative roles of the zona pellucida during and after fertilization?"

Sperm-zona binding induces the acrosome reaction when a capacitated sperm binds to it. As the sperm is penetrating the egg, the sperm relies upon another zona protein for binding to keep the sperm in close contact with the zona matrix. Once through the zona, the sperm head fuses with the oocyte plasma membrane. Then, egg activation occurs leading to the zona reaction. The zona reaction destroys the sperm receptors on the egg to prevent polyspermy, and the zona reaction produces a sterile microenvironment in which the early embryo will develop. Furthermore, the zona shell prevents any immunological reaction to the presence of the embryo prior to implantation.

"3. What is meant by “egg activation” and how does it come about?"

Egg activation occurs when the sperm-egg fusion occurs and the depolarization of the oocyte plasma membrane results in ion fluxes within the egg. Egg activation is mediated by phospholipase C-zeta, which is released from the incorporating sperm into the oocyte cortex. Phospholipase C-zeta interacts with PIP2 and produces IP3. IP3 binds receptors on the ER, releasing free calcium, which activates enzyme cascades that alter oocyte metabolism and induce the secretion of thousands of cortical granules.

"4. Discuss the importance of specific sperm components on early pre-implantation development."

When the sperm incorporates itself into the egg, it brings along with it a centrosome and its DNA. The centrosome is necessary to assemble microtubule-based structures such as the aster and the mitotic spindle. The maternal centrosome is terminally differentiated and not capable of microtubule nucleation, so the male centrosome is needed for syngamy to occur, which is the completion of fertilization. The first cleavage division of the zygote requires a mitotic spindle, whose organization is dependent on the male centrosome as well.

"5. Why is the timing of embryo transport in the oviduct important?"

The timing of embryo transport in the oviduct is important because if the embryo is retained in the oviduct for too long, then an ectopic pregnancy can occur. The entry of the embryo into the uterine lumen should occur prior to hatching to prevent premature implantation.