Extraembryonal Organs

Fusion of Oocyte and Sperm cell membranes

● Cortical and zona reactions: release of cortical oocyte granules, which contain lysosomal enzymes, results in oocyte membrane to become impermeable to other spermatozoa, and the zona pellucida alters its structure and composition to prevent sperm binding and penetration.

● Resumption of the second meiotic division: oocyte finishes its second meiotic division after the entry of the spermatozoon. One of the daughter cells, is known as the second polar body (no cytoplasm), the other one is the definitive oocyte. Its chromosomes arrange themselves in a vesicular nucleus, known as female pronucleus.

● Metabolic activation of the egg: The activating factor is probably carried by the spermatozoon.

● The spermatozoon moves forward until it lies close to the female pronucleus. Its nucleus becomes swollen and forms the male pronucleus, the tail detaches and degenerates.

● Morphologically, the male and female pronuclei are indistinguishable, and eventually they come in close contact and lose their nuclear envelopes. During growth of male and female pronuclei (both haploid), each pronucleus must replicate its DNA.

Main Results of Fertilization:

● Restoration of the diploid number of chromosomes, half from the father and half from the mother. Hence, the zygote contains a new combination of chromosomes different from both parents.

● Determination of the sex of the new individual. An X- carrying sperm produces a female (XX) embryo, and a Y- carrying sperm produces a male (XY) embryo.

● Initiation of cleavage

● Cleavage : once the zygote has reached the 2- cell stage, it undergoes a series of mitotic divisions, increasing the number of cells. These cells, which become smaller with each cleavage division, are known as blastomeres. Until the 8-cell stage, they form a loosely arranged clump. After the third cleavage, blastomeres maximize their compact ball of cells held together by tight junctions.

● Compaction: segregates inner cells, which communicate extensively by gap junctions, from outer cells. Approximately 3 days after fertilization, cells of the compacted embryo divide again to form a 16-cell morula.

● Inner cells of the morula constitute the inner cell mass, and surrounding cells compose the outer cell mass. Inner cell mass gives rise to tissues of the embryo proper, and the outer cell mass forms the trophoblast- later placenta.

Blastocyst Formation● When morula enters the uterine cavity, fluid begins to penetrate through the zona

pellucida into the interecellular spaces of the inner cell mass

● Gradually the intercellular spaces become confluent, and finally a single cavity, the blastocele.

● At this time, the embryo is a blastocyst.

● Cells of the inner cell mass, now called embryoblast, are at one pole, and those of the outer cell mass, are called trophoblast, flatten and form the epithelial wall of the blastocyst. Zona pellucida has disappeared, allowing implantation to begin.

● Trophoblastic cells over the embryoblast pole begin to penetrate between the epithelial cells of the uterine mucosa about the 6th day.

● Attachment and invasion of the trophoblast involves integrins, expressed by the trophoblast, the extracellular matrix molecules laminin and fibronectin.

● Integrin receptors for laminin promote attachment, while those of fibronectin stimulate migration. These molecules also interact along signal transduction pathways to regulate trophoblast differentiation so that implantation is the result of mutual trophoblastic and endometrial action.

● By the end of the first week of development, the human zygote has passed through the morula and blastocyst stages and has begun implantation in the uterine mucosa.

Implantation

● Uterus at the time of implantation is in the secretory phase, and the blastocyst implants in the endometrium along the anterior or posterior wall.

● Decidual reaction: cells of the endometrium, become polyhedral and loaded with glycogen and lipids; intercellular spaces are filled with extravasate, and the tissue is edematous.

2nd Week of Development● Day 8: Blastocyst is partially embedded in the endometrial stroma. In

the area over the embryoblast, the trophoblast has differentiated into 2 layers: 1. cytotrophoblast: inner layer of mononucleated cells

2. syncytiotrophoblast : outer multinucleated zone without distinct cell boundaries, mitotic figures are found in the cytotrophoblast but not in the syncytiotrophoblast. Thus, cells in the cytotrophoblast divide and migrate into the syncytiotrophoblast, where they fuse and loose their individual cell membranes.

● Cells of the inner cell mass or embryoblast also differentiate into 2 layers: a) a layer of small cuboidal cells adjacent to the blastocyst cavity- hypoblast layer.

b) a layer of high columnar cells adjacent to the amniotic cavity , the epiblast layer. Together, the layers form a flat disc. A small cavity appears within the epiblast. The cavity enlarges to become the amniotic cavity. Epiblast cells adjacent to the cytotrophoblast are called amnioblasts; together with the rest of the epiblast, they line the amniotic cavity.

2nd Week● Day 9: The blastocyst is more deeply embedded in the endometrium,

and the penetration defect in the surface epithelium is closed by a fibrin coagulum.

● Trophoblast shows progress in development, particularly at the embryonic pole, where vacuoles appear in the syncytium.

● When the vacuoles fuse, they form large lacunae, and this phase of trophoblast development is known as the lacunar stage.

● At the abembryonic pole, flattened cells, originating from hypoblast form a thin membrane, the exocoelomic membrane, that lines the inner surface of the cytotrophoblast.

● This membrane, together with the hypoblast, forms the lining of the exocoelomic cavity, or primitive yolk sac.

2nd Week● Days 11, 12: the blastocyst is completely embedded in the

endometrial stroma. The trophoblast is characterized by lacunar spaces in the syncytium that form an intercommunicating network.

● The trophoblast still consists mainly of cytotrophoblastic cells.

● Cells of syncytiotrophoblast penetrate deeper into the stroma and erode the endothelial lining of the maternal capillaries. These capillaries, which are congested and dilated, are known as sinusoids.

● The syncytial lacunae become continuous with the sinusoids and maternal blood enters the lacunar system- uteroplacental circulation is established.

● In the meantime, a new population of cells appears between the inner surface of the cytotrophoblast and the outer surface of the exocoelomic cavity. They form a loose connective tissue, the extraembryonic mesoderm, which eventually fills all the space between the trophoblast externally and the amnion and exocoelomic membrane internally.

2nd week

● Large cavitites develop in the extraembryonic mesoderm, and when these become confluent, they form a new space known as the extraembryonic coelom, or chorionic cavity. This space surrounds the primitive yolk sac and amniotic cavity except where the germ disc is connected to the trophoblast by the connecting stalk.

● This space surrounds the primitive yolk sac and amniotic cavity, except where the germ disc is connected to the trophoblast by the connecting stalk.

● The extraembryonic mesoderm lining the cytotrophoblast and amnion is called extraembryonic somatopleuric mesoderm, the lining covering the yolk sac is called extraembryonic splachnopleuric mesoderm.

Day 13th

● The surface defect in the endometrium has usually healed.

● Trophoblast is characterized by villous structures. Cells of the cytotrophoblast proliferate locally and penetrate into the syncytiotrophoblast, forming cellular columns surrounded by syncytium- primary villi.

● In the meantime, the hypoblast produces additional cells that migrate along the inside of the exocoelomic membrane. These cells proliferate and gradually form a new cavity within the exocoelomic cavity- secondary yolk sac, which is much smaller than the original exocoelomic cavity (primitive yolk sac).

● During its formation, large portions of the exocoelomic cavity are pinched off. These portions are represented by exocoelomic cysts, which are often found in the extraembryonic coelom or chorionic cavity.

● The extraembryonic coelom expands and forms a large cavity, the chorionic cavity.

● Extraembryonic mesoderm lining the inside of the cytotrophoblast is then known as chorionic plate.

● The only place where extraembryonic mesoderm transverses the chorionic cavity is in the connecting stalk- which becomes the umbilical cord.

Amnion

● Fluid-filled, membranous amniotic sac that surrounds the embryo and fetus.

● As the amnion enlarges, it gradually obliterates the chorionic cavity and forms the epithelial covering of the umbilical cord.

● Amniotic fluid: plays a major role in fetal growth. Initially, it is secreted by amniotic cells, but most of the fluid is derived from maternal tissue (interstitial fluid), by diffusion across the amniochorionic membrane from the decidua parietalis.

● Later there is diffusion of fluid through the chorionic plate from blood in the intervillous space of the placenta.

● Before keratinization of the skin occurs, a tissue fluid from the fetus to the amniotic cavity is through the skin, thus, amniotic fluid is similar to fetal tissue fluid.

● Fluid is also secreted by the fetal respiratory tract and enters the amniotic cavity.

● Beginning on the 11th week, the fetus contributes to the amniotic fluid by expelling urine into the amniotic cavity.

Placenta- Structure and Function

● The placenta is a transitory composite structure with both fetal and maternal components.

● Disc shaped organ: 15-25 cm in diameter, and 2-3 cm thick, it weighs 400-600 g at term.

● Maternal side: it has 15-20 lobules, or cotyledons.

● Functions: physiologic exchanges between mother and developing embryo or fetus, such as exchange of gases, electrolytes, and metabolites, between maternal and fetal blood. Fetal waste products are excreted into the maternal blood. Maternal antibodies are transmitted to the fetus, and the placenta produces several hormones including estrogens, progesterone, and hCG.

Placenta- Maternal Component

● During implantation, endometrial connective tissue goes through profound changes. The fibroblasts of the lamina propria become enlarged and round and exhibit the characteristics of protein synthesizing cells- Decidua cells (epitheloid cells) , and the whole endometrium is called decidua.

● 3 parts of the decidua: 1. Decidua Basalis: situated between embryo and myometrium.

2. Decidua Capsularis: Between the embryo and the lumen of the uterus.

3. Decidua Parietalis: remainder of the decidua.

● In the late pregnancy, as the embryo is growing there is fusion between decidua capsularis and decidua parietalis.

Placenta- Fetal Component

● It is formed by the chorionic sac surrounding the embryo, consists of the chorionic plate and its branching chorionic villi that extend from the chorion like branches of a tree.

● The tips of the villi are attached to the decidua and , by 6 weeks, branches are formed with free tips, which create a villous spongework.

● Intervillous spaces contain maternal blood. Many chorionic villi end freely; others fuse with the decidua as anchoring elements (stem villi)

● The villi provide a large surface area in contact with maternal blood for nutrient exchange.

● Fetal and maternal blood are close to each other but follow independent courses and do not mix, being separated by placental barrier.

Placenta- Histology● Chorionic villi are the fundamental units of the placenta. Each villous is formed from

2 epithelial cell layers derived from the trophoblast of the embryo, which are closely associated with extraembryonic connective tissue.

● An inner single layer of cytotrophoblasts, consists of cuboidal epithelial cells with light staining cytoplasm and distinct cell boundaries.

● They give rise to a continuous superficial layer of larger syncytiotrophoblasts, which stain darker, and have ill-defined cell boundaries.

● Trophoblast cell layers initially form proliferating villous cords that invade the endometrium and destroy the walls of coiled arterioles and venules of the endometrial stroma.

● Extravasated maternal blood creates irregular intervillous spaces in eroded decidual tissue, circulates in these spaces, and bathes the chorionic villi. The core of each villus consists of loose mesenchymal connective tissue containing fetal capillaries, fibroblasts, and isolated smooth muscle cells.

● Macrophages (Hofbauer cells) are also present in the villi.

● In the second half of pregancy, cytotrophoblasts gradually disappear, and a thin layer of multinucleated syncytiotrophoblasts remains on villi surfaces.

Chorionic Villi

● Primary Chorionic Villi: They are small and only consist of an inner single layer of cuboidal epithelium (cytotrophoblasts), and an outer layer of syncytiotrophoblasts.

● Secondary Chorionic Villi: Villi increase in size, and branch, and the extraembryonic mesoderm grows into them.

● Tertiary Chorionic Villi: Branches of umbillical vessels grow into them and they become vascularized.

● Chorion laeve: the chorion that is in contact with the decidua capsularis which undergoes atrophy, so that at the 4th month of pregnancy there are no villi left.

Placental Barrier● It separates fetal and maternal blood, and it is the site of fetal-maternal exchange.

● It consists of: 1. Continuous endothelium of the fetal capillary and its basal lamina

2. Extraembryonic mesoderm

3. A layer of cytotrophoblasts and its adjacent basal lamina.

4. A layer of syncytiotrophoblasts exposed to maternal blood.

● Syncytiotrophoblasts: Undertake passive and facilitated diffusion, active transport, receptor-mediated endocytosis of immunoglobulins, and exocytosis for secretion.

● Ultrastructure: - Apical microvilli that amplify the surface area (absorptive epithelia).

- Their cytoplasm contains, mitochondria, lysosomes, secretory vesicles, sER, rER (metabolically active, and secretory)

-sER, lipid droplets, Golgi- Steroid producing cells for synthesis and release of progesterone and estrogens.

-rER and ribososomes in protein synthesis and secretion of hormones such as hCG and placental lactogen.

● Syncytiotrophoblasts arise from cytotrophoblasts, which have a normal complement of organelles. Their plasma membranes interdigitate with those neighbouring cytotrophoblasts and possess desmosomes at lateral margins of adjacent cells and at surfaces in contact with syncytiotrophoblasts.

Placental Circulation

● Maternal Circulation: The uterine endometrium undergoes “decicualization”. Spiral arteries in decidua are remodelled so that they become less convoluted and their diameter is increased. The increased diameter and straighter flow path act to increase maternal blood flow to the placenta.

● Fetoplacental circulation: Deoxygenated fetal blood passes through umbilical arteries to the placenta. They branch into chorionic arteries, and then cotyledon arteries. In the villi there is extensive arterio-capillary venous system, bringing fetal and maternal blood in contact but not intermingling them.

1st vs 3rd Trimester

1. In early pregnancy, villi consist of both syncytiotrophoblast and cytotrophoblast, while in the 3rd trimester only of syncytiotrophoblast.

2. In 1st trimester, there are nucleated erythroblasts within the lumen of chorionic vessels, while in the 3rd trimester, there are mature RBCs.

3. Placental Barrier: In the late pregnancy, the barrier consists only from the syncytiotrophoblast and endothelium, with the Basement Membrane in between. Cytotrophoblast and extraembryonic mesoderm has regressed.

Fibrinoid

● It is the result of the immune reaction of maternal antibodies against the fetal part of the placenta.

Umbilical Cord

● It develops from and contains remnants from the yolk sac and allantois.

● It forms in the 5th week of embryonic development replacing the yolk sac.

Umbilical Cord● It connects the fetus to the placenta.

● It is on average 55 cm long.

● It contains 2 umbilical arteries and one umbilical vein coiled around each other in Wharton jelly ( a matrix of embryonic connective tissue that has mucous consistency).

● EC matrix has an interlacing network of delicate collagen fibers and is rich in hyaluronic acid and chondroitin sulfate. It contains stellate or fusiform cells that resemble mesenchymal cells but lacks other blood vessels, lymphatics and nerve fibers.

● A single layer of cuboidal epithelium, derived from the lining of amniotic cavity- amniotic epithelium- covers the cord. This epithelium is protective and secretes amniotic fluid.

● Umbilical arteries carry deoxygenated blood from the fetus to the chorion. Blood pressure is low in these vessels, so their tunica media is usually not as thick as that in typical adult arteries.

● Umbilical arteries lack the internal elastic lamina and have a double- layered coat of smooth muscle composed of interlacing network of cells.

● Umbilical vein delivers oxygenated blood to the fetus. It has a thick single layer of circular smooth muscle, and lacks valves.

Yolk sac ● Primary yolk sac derives from cells of the hypoblast, and its development to

secondary requires its attachment to the extraembryonic mesoderm.

● At 32 days, the yolk sac is large, and by 10 weeks the yolk sac has shrunk to pear-shaped remnant about 5 mm in diameter.

● By 20 weeks the yolk sac is very small, thereafter it's not visible.

● Significance: 1. Transfer of nutrients to the embryo during the 2nd and 3rd week of development when the uteroplacental circulation is being established.

2. Blood development first occurs in well vascularized extraembryonic mesoderm covering the wall of the yolk sac beginning in the 3rd week, and continues to form there until hemopoietic activity begins in the liver during the 6th week.

3. During the 4th week the endoderm of the yolk sac is incorporated into the embryo as the primitive gut. Its endoderm, derived from the epiblast, gives rise to the epithelium of the trachea, bronchi, lungs and digestive tract.

4. Primordial germ cells appear in the endodermal lining of the wall of the yolk sac in the 3rd week and subsequently migrate to the developing sex glands. They differentiate into the germ cells (spermatogonia in males, and oogonia in females).

Fate of Yolk Sac

● At 10 weeks the small yolk sac lies in the chorionic cavity between the amnion and the chorionic sac. It atrophies as pregnancy advances, eventually becoming very small. The yolk stalk usually detaches from the midgut loop by the end of 6th week.

● In about 2% of adults, the proximal intraabdominal part of the yolk stalk persists as an ileal diverticulum (Merkel diverticulum).

Allantois

● It appears on about day 16 as a small, sausage shaped diverticulum from the caudal wall of the yolk sac that extends into the connecting stalk.

● The allantois is a large sac-like structure in embryos of reptiles, birds, and some mammals, which has a respiratory function and/or acts as a reservoir for urine during embryonic life.

● It remains very small in human embryos because the placenta and amniotic sac take over its functions.

● It is involved in early blood formation in the human embryo and is associated with development of the urinary bladder.

● As the bladder enlarges, the allantois becomes the urachus, which is represented in adults by the median umbilical ligament. The blood vessels of the allantois become the umbilical arteries and veins.

Allantois- Egg

Fetal Membranes

● As the fetus grows there is obliteration of 2 cavities:

1. Chorionic cavity: During the 8th week there is fusion between the chorionic cavity and the amniotic cavity, creating the amniochorion.

2. Uterine cavity: Decidua capsularis fuses with decidua parietalis. Amniochorion fuses with the decidua.