Artlabeling Activity Steps of the Preembryonic Period Fertilization Through Implantation

28.2 Embryonic Evolution

Learning Objectives

Past the end of this section, yous will be able to:

• Distinguish the stages of embryonic development that occur before implantation
• Draw the process of implantation
• List and describe four embryonic membranes
• Explicate gastrulation
• Describe how the placenta is formed and identify its functions
• Explain how an embryo transforms from a flat disc of cells into a iii-dimensional shape resembling a human
• Summarize the process of organogenesis

Throughout this affiliate, we will express embryonic and fetal ages in terms of weeks from fertilization, normally chosen formulation. The menstruation of time required for full evolution of a fetus in utero is referred to as gestation (gestare = "to conduct" or "to bear"). It can be subdivided into distinct gestational periods. The starting time 2 weeks of prenatal development are referred to as the pre-embryonic stage. A developing human is referred to every bit an embryo during weeks 3–8, and a fetus from the ninth week of gestation until birth. In this section, nosotros'll comprehend the pre-embryonic and embryonic stages of development, which are characterized by cell partitioning, migration, and differentiation. By the terminate of the embryonic menses, all of the organ systems are structured in rudimentary form, although the organs themselves are either nonfunctional or only semi-functional.

Pre-implantation Embryonic Development

Following fertilization, the zygote and its associated membranes, together referred to every bit the conceptus, continue to be projected toward the uterus by peristalsis and chirapsia cilia. During its journey to the uterus, the zygote undergoes five or six rapid mitotic jail cell divisions. Although each cleavage results in more cells, it does non increase the full volume of the conceptus (Figure 28.two.i). Each daughter cell produced past cleavage is called a blastomere (blastos = "germ," in the sense of a seed or sprout).

Approximately 3 days after fertilization, a 16-cell conceptus reaches the uterus. The cells that had been loosely grouped are now compacted and wait more like a solid mass. The proper 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 effectually a fluid-filled cavity, the blastocoel. At this developmental phase, the conceptus is referred to every bit a blastocyst. Within this structure, a grouping 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 commutation between mother and the developing offspring).

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

Effigy 28.ii.one – Pre-Embryonic Cleavages: Pre-embryonic cleavages make use of the abundant cytoplasm of the conceptus as the cells quickly divide without changing the total volume.

Equally 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.

External Website

View this time-lapse movie of a conceptus starting at day 3. What is the first structure y'all encounter? At what point in the movie does the blastocoel first appear? What event occurs at the end of the movie?

Implantation

At the terminate of the first week, the blastocyst comes in contact with the uterine wall and adheres to information technology, embedding itself in the uterine lining via the trophoblast cells. Thus begins the process of implantation, which signals the stop of the pre-embryonic phase of development (Effigy 28.2.2). Implantation tin be accompanied by modest haemorrhage. The blastocyst typically implants in the fundus of the uterus or on the posterior wall. Nonetheless, if the endometrium is not fully developed and ready to receive the blastocyst, the blastocyst will detach and find a better spot. A significant pct (l–75 percent) of blastocysts fail to implant; when this occurs, the blastocyst is shed with the endometrium during menses. The high charge per unit of implantation failure is one reason why pregnancy typically requires several ovulation cycles to attain.

Figure 28.2.2 – 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.

When implantation succeeds and the blastocyst adheres to the endometrium, the superficial cells of the trophoblast fuse with each other, forming the syncytiotrophoblast, a multinucleated trunk that digests endometrial cells to firmly secure the blastocyst to the uterine wall. In response, the uterine mucosa rebuilds itself and envelops the blastocyst (Effigy 28.2.3). The trophoblast secretes human chorionic gonadotropin (hCG), a hormone that directs the corpus luteum to survive, enlarge, and go along producing progesterone and estrogen to suppress menses. These functions of hCG are necessary for creating an environment suitable for the developing embryo. As a outcome 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. Only a few days after implantation, the trophoblast has secreted enough hCG for an at-home urine pregnancy exam to requite a positive result.

Figure 28.2.three – Implantation: During implantation, the trophoblast cells of the blastocyst adhere to the endometrium and digest endometrial cells until it is attached securely.

About of the time an embryo implants within the trunk of the uterus in a location that can support growth and development. However, in one to 2 percent of cases, the embryo implants either outside the uterus (an ectopic pregnancy) or in a region of uterus that tin create complications for the pregnancy. If the embryo implants in the inferior portion of the uterus, the placenta can potentially grow over the opening of the neck, a condition call placenta previa.

Disorders of the…

Development of the Embryo
In the vast majority of ectopic pregnancies, the embryo does non complete its journey to the uterus and implants in the uterine tube, referred to as a tubal pregnancy. However, at that place are too ovarian ectopic pregnancies (in which the egg never left the ovary) and abdominal ectopic pregnancies (in which an egg was "lost" to the intestinal cavity during the transfer from ovary to uterine tube, or in which an embryo from a tubal pregnancy re-implanted in the abdomen). In one case 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.

Tubal pregnancies tin 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 1 half of tubal pregnancies resolve spontaneously. Implantation in a uterine tube causes bleeding, which appears to stimulate smooth musculus 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'south development tin can be arrested by the administration of the cytotoxic drug methotrexate, which inhibits the metabolism of folic acrid. If diagnosis is late and the uterine tube is already ruptured, surgical repair is essential.

Even if the embryo has successfully found its way to the uterus, it does not e'er implant in an optimal location (the fundus or the posterior wall of the uterus). Placenta previa tin can result if an embryo implants close to the internal os of the uterus (the internal opening of the cervix). Equally the fetus grows, the placenta can partially or completely comprehend the opening of the cervix (Figure 28.ii.four). Although it occurs in but 0.5 percentage of pregnancies, placenta previa is the leading cause of antepartum hemorrhage (profuse vaginal haemorrhage after week 24 of pregnancy but prior to childbirth).

Figure 28.two.4 – Placenta Previa: An embryo that implants too close to the opening of the neck can lead to placenta previa, a condition in which the placenta partially or completely covers the cervix.

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 extra-embryonic membranes needed to support and protect the growing embryo: the amnion, the yolk sac, the allantois, and the chorion.

At the kickoff of the second week, the cells of the inner cell mass grade into a two-layered disc of embryonic cells, and a space—the amniotic cavity—opens up between information technology and the trophoblast (Figure 28.two.five). Cells from the upper layer of the disc (the epiblast) extend effectually 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 on in development, amniotic fluid consists virtually entirely of a filtrate of maternal plasma, simply as the kidneys of the fetus begin to function at approximately the eighth week, they add together urine to the volume of amniotic fluid. Floating inside the amniotic fluid, the embryo—and after, the fetus—is protected from trauma and rapid temperature changes. Information technology can move freely within the fluid and tin can fix for swallowing and breathing out of the uterus.

Figure 28.2.five – Development of the Embryonic Disc: Germination of the embryonic disc leaves spaces on either side that develop into the amniotic cavity and the yolk sac.

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 captivated from the trophoblast and also provides archaic claret circulation to the developing embryo for the second and third week of evolution. When the placenta takes over nourishing the embryo at approximately week four, the yolk sac has been greatly reduced in size and its main function is to serve every bit the source of blood cells and germ cells (cells that will give rise to gametes). During calendar week three, 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 construction of the umbilical cord.

The last of the extra-embryonic membranes is the chorion, which is the one membrane that surrounds all others. The development of the chorion will be discussed in more item shortly, as information technology relates to the growth and development of the placenta.

Embryogenesis

Every bit the third week of development begins, the two-layered disc of cells becomes a three-layered disc through the process of gastrulation, during which the cells transition from totipotency to multipotency. The embryo, which takes the shape of an oval-shaped disc, forms an indentation chosen the primitive streak forth the dorsal surface of the epiblast. A node at the caudal or "tail" end of the archaic streak emits growth factors that direct cells to multiply and drift. Cells migrate toward and through the primitive streak and and then move laterally to create ii new layers of cells. The first layer is the endoderm, a sheet of cells that displaces the hypoblast and lies adjacent to the yolk sac. The 2d layer of cells fills in as the centre layer, or mesoderm. The cells of the epiblast that remain (not having migrated through the primitive streak) become the ectoderm (Figure 28.ii.6).

Figure 28.ii.6 – Germ Layers: Formation of the three chief germ layers occurs during the commencement two weeks of development. The embryo at this stage is merely a few millimeters in length.

Each of these germ layers volition develop into specific structures in the embryo. Whereas the ectoderm and endoderm grade tightly connected epithelial sheets, the mesodermal cells are less organized and exist every bit a loosely continued cell community. The ectoderm gives ascent to cell lineages that differentiate to get the central and peripheral nervous systems, sensory organs, epidermis, hair, and nails. Mesodermal cells ultimately become the skeleton, muscles, connective tissue, eye, blood vessels, and kidneys. The endoderm goes on to grade the epithelial lining of the gastrointestinal tract, liver, and pancreas, as well equally the lungs (Figure 28.two.vii).

Figure 28.2.7 – Fates of Germ Layers in Embryo: Following gastrulation of the embryo in the third week, embryonic cells of the ectoderm, mesoderm, and endoderm begin to migrate and differentiate into the cell lineages that volition give ascent to mature organs and organ systems in the infant.

Evolution of the Placenta

During the first several weeks of development, the cells of the endometrium—referred to every bit decidual cells—nourish the nascent embryo. During prenatal weeks four–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 mother to the fetus through the single umbilical vein. The umbilical cord is surrounded by the amnion, and the spaces inside the cord around the blood vessels are filled with Wharton'south jelly, a mucous connective tissue.

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-similar structures called chorionic villi that couch 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 make full the villi and differentiate into blood vessels, including the three umbilical blood vessels that connect the embryo to the developing placenta (Effigy 28.2.8).

Figure 28.2.eight – Cross-Section of the Placenta: In the placenta, maternal and fetal blood components are conducted through the surface of the chorionic villi, just maternal and fetal bloodstreams never mix direct.

The placenta develops throughout the embryonic flow and during the first several weeks of the fetal period; placentation is complete by weeks 14–16. Equally a fully adult organ, the placenta provides diet and excretion, respiration, and endocrine function (Table 28.1 and Figure 28.2.ix). 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 movement beyond the placenta by elementary diffusion. Oxygen, carbon dioxide, and any other lipid-soluble substances take this road. Other substances move across by facilitated improvidence. This includes h2o-soluble glucose. The fetus has a high demand for amino acids and fe, and those substances are moved across the placenta by agile send.

Maternal and fetal blood does not commingle because blood cells cannot motion beyond the placenta. This separation prevents the female parent'south cytotoxic T cells from reaching and subsequently destroying the fetus, which bears "non-self" antigens. Further, information technology ensures the fetal red blood cells practise not enter the mother'southward circulation and trigger antibody evolution (if they carry "non-self" antigens)—at least until the last stages of pregnancy or nativity. This is the reason that, fifty-fifty in the absence of preventive treatment, an Rh⁻ mother doesn't develop antibodies that could crusade hemolytic disease in her first Rh⁺ fetus.

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

Functions of the Placenta (Table 28.i)
Nutrition and digestion	Respiration	Endocrine function
Mediates diffusion of maternal glucose, amino acids, fatty acids, vitamins, and minerals Stores nutrients during early pregnancy to adapt increased fetal demand afterward in pregnancy Excretes and filters fetal nitrogenous wastes into maternal blood	Mediates maternal-to-fetal oxygen transport and fetal-to-maternal carbon dioxide send	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 claret and vice versa

Effigy 28.2.nine – Placenta: This mail service-expulsion placenta and umbilical cord (white) are viewed from the fetal side.

Organogenesis

Following gastrulation, rudiments of the central nervous organization develop from the ectoderm in the process of neurulation (Effigy 28.2.10). Specialized neuroectodermal tissues along the length of the embryo thicken into the neural plate. During the quaternary week, tissues on either side of the plate fold upward into a neural fold. The ii folds converge to form the neural tube. The tube lies atop a rod-shaped, mesoderm-derived notochord, which eventually becomes the nucleus pulposus of intervertebral discs. Block-similar structures called somites form on either side of the tube, eventually differentiating into the axial skeleton, skeletal musculus, and dermis. During the fourth and fifth weeks, the anterior neural tube dilates and subdivides to form vesicles that will go the brain structures.

Folate, i of the B vitamins, is important to the healthy development of the neural tube. A deficiency of maternal folate in the first weeks of pregnancy can result in neural tube defects, including spina bifida—a nascency defect in which spinal tissue protrudes through the newborn's vertebral column, which has failed to completely close. A more astringent neural tube defect is anencephaly, a fractional or complete absenteeism of brain tissue.

Figure 28.2.10 – Neurulation: The embryonic process of neurulation establishes the rudiments of the future central nervous system and skeleton.

The embryo, which begins as a flat canvas of cells, begins to acquire a cylindrical shape through the process of embryonic folding (Figure 28.two.11). The embryo folds laterally and once again at either end, forming a C-shape with distinct head and tail ends. The embryo envelops a portion of the yolk sac, which protrudes with the umbilical cord from what will become the belly. The folding essentially creates a tube, called the primitive gut, that is lined by the endoderm. The amniotic sac, which was sitting on top of the flat embryo, envelops the embryo equally it folds.

Effigy 28.two.11 – Embryonic Folding: Embryonic folding converts a flat canvass of cells into a hollow, tube-similar construction.

Within the first eight weeks of gestation, a developing embryo establishes the rudimentary structures of all of its organs and tissues from the ectoderm, mesoderm, and endoderm. This process is called organogenesis.

Like the fundamental nervous system, the heart also begins its development in the embryo as a tube-similar structure, continued via capillaries to the chorionic villi. Cells of the primitive tube-shaped heart are capable of electric conduction and contraction. The heart begins chirapsia in the beginning of the quaternary week, although it does non actually pump embryonic blood until a week later, when the oversized liver has begun producing red blood cells. (This is a temporary responsibility of the embryonic liver that the os marrow will assume during fetal development.) During weeks 4–five, the eye pits class, limb buds become credible, and the rudiments of the pulmonary organization are formed.

During the sixth calendar week, uncontrolled fetal limb movements begin to occur. The gastrointestinal organisation develops too rapidly for the embryonic abdomen to adjust it, and the intestines temporarily loop into the umbilical cord. Paddle-shaped easily and feet develop fingers and toes by the procedure of apoptosis (programmed cell death), which causes the tissues between the fingers to disintegrate. By week 7, the facial construction is more complex and includes nostrils, outer ears, and lenses (Figure 28.2.12). Past the eighth week, the head is nearly as big as the residue of the embryo's body, and all major brain structures are in place. The external genitalia are apparent, but at this indicate, male and female embryos are duplicate. Os begins to replace cartilage in the embryonic skeleton through the procedure of ossification. By the end of the embryonic menstruation, the embryo is approximately three cm (one.2 in) from crown to rump and weighs approximately 8 thousand (0.25 oz).

Figure 28.2.12 – Embryo at 7 Weeks: An embryo at the end of 7 weeks of development is only ten mm in length, merely its developing eyes, limb buds, and tail are already visible. (This embryo was derived from an ectopic pregnancy.) (credit: Ed Uthman)

External Website

Apply this interactive tool to view the process of embryogenesis from the perspective of the conceptus (left panel), as well as fetal evolution viewed from a maternal cantankerous-section (right console). Can y'all place when neurulation occurs in the embryo?

Chapter Review

As the zygote travels toward the uterus, it undergoes numerous cleavages in which the number of cells doubles (blastomeres). Upon reaching the uterus, the conceptus has become a tightly packed sphere of cells called the morula, which and then forms into a blastocyst consisting of an inner prison cell mass inside a fluid-filled cavity surrounded by trophoblasts. The blastocyst implants in the uterine wall, the trophoblasts fuse to form a syncytiotrophoblast, and the conceptus is enveloped by the endometrium. Four embryonic membranes form to support the growing embryo: the amnion, the yolk sac, the allantois, and the chorion. The chorionic villi of the chorion extend into the endometrium to form the fetal portion of the placenta. The placenta supplies the growing embryo with oxygen and nutrients; it also removes carbon dioxide and other metabolic wastes.

Following implantation, embryonic cells undergo gastrulation, in which they differentiate and separate into an embryonic disc and establish 3 primary germ layers (the endoderm, mesoderm, and ectoderm). Through the procedure of embryonic folding, the fetus begins to accept shape. Neurulation starts the process of the development of structures of the central nervous organization and organogenesis establishes the bones plan for all organ systems.

Interactive Link Questions

View this time-lapse movie of a conceptus starting at day 3. What is the get-go structure you see? At what point in the motion picture does the blastocoel first appear? What event occurs at the end of the movie?

The offset construction shown is the morula. The blastocoel appears at approximately 20 seconds. The movie ends with the hatching of the conceptus.

Use this interactive tool to view the procedure of embryogenesis from the perspective of the conceptus (left console), every bit well equally fetal evolution viewed from a maternal cross-section (right panel). Can you place when neurulation occurs in the embryo?

Neurulation starts in week 4.

Review Questions

Critical Thinking Questions

1. Approximately 3 weeks afterward her last menstrual menstruation, a sexually active woman experiences a brief episode of abdominopelvic cramping and minor bleeding. What might be the caption?

ii. The Food and Nutrition Board of the Institute of Medicine recommends that all women who might become pregnant consume at least 400 µg/day of folate from supplements or fortified foods. Why?

Glossary

allantois

finger-like outpocketing of yolk sac forms the primitive excretory duct of the embryo; forerunner to the urinary bladder

amnion

transparent membranous sac that encloses the developing fetus and fills with amniotic fluid

amniotic cavity

cavity that opens up between the inner cell mass and the trophoblast; develops into amnion

blastocoel

fluid-filled cavity of the blastocyst

blastocyst

term for the conceptus at the developmental phase that consists of about 100 cells shaped into an inner jail cell mass that is blighted to go the embryo and an outer trophoblast that is fated to go the associated fetal membranes and placenta

blastomere

girl cell of a cleavage

chorion

membrane that develops from the syncytiotrophoblast, cytotrophoblast, and mesoderm; surrounds the embryo and forms the fetal portion of the placenta through the chorionic villi

chorionic membrane

precursor to the chorion; forms from extra-embryonic mesoderm cells

chorionic villi

projections of the chorionic membrane that burrow into the endometrium and develop into the placenta

cleavage

course of mitotic cell partitioning in which the jail cell divides but the full volume remains unchanged; this process serves to produce smaller and smaller cells

conceptus

pre-implantation stage of a fertilized egg and its associated membranes

ectoderm

primary germ layer that develops into the central and peripheral nervous systems, sensory organs, epidermis, hair, and nails

ectopic pregnancy

implantation of an embryo outside of the uterus

embryo

developing human during weeks three–8

embryonic folding

process by which an embryo develops from a flat disc of cells to a three-dimensional shape resembling a cylinder

endoderm

master germ layer that goes on to form the gastrointestinal tract, liver, pancreas, and lungs

epiblast

upper layer of cells of the embryonic disc that forms from the inner cell mass; gives rise to all three germ layers

fetus

developing human during the time from the end of the embryonic period (week ix) to birth

gastrulation

process of prison cell migration and differentiation into iii primary germ layers following cleavage and implantation

gestation

in human development, the period required for embryonic and fetal development in utero; pregnancy

human chorionic gonadotropin (hCG)

hormone that directs the corpus luteum to survive, enlarge, and continue producing progesterone and estrogen to suppress menses and secure an environment suitable for the developing embryo

hypoblast

lower layer of cells of the embryonic disc that extend into the blastocoel to form the yolk sac

implantation

process by which a blastocyst embeds itself in the uterine endometrium

inner cell mass

cluster of cells within the blastocyst that is fated to become the embryo

mesoderm

primary germ layer that becomes the skeleton, muscles, connective tissue, center, blood vessels, and kidneys

morula

tightly packed sphere of blastomeres that has reached the uterus merely has not even so implanted itself

neural plate

thickened layer of neuroepithelium that runs longitudinally forth the dorsal surface of an embryo and gives rise to nervous system tissue

neural fold

elevated edge of the neural groove

neural tube

precursor to structures of the cardinal nervous system, formed past the invagination and separation of neuroepithelium

neurulation

embryonic process that establishes the central nervous system

notochord

rod-shaped, mesoderm-derived structure that provides support for growing fetus

organogenesis

development of the rudimentary structures of all of an embryo'due south organs from the germ layers

placenta

organ that forms during pregnancy to nourish the developing fetus; also regulates waste and gas exchange between female parent and fetus

placenta previa

low placement of fetus within uterus causes placenta to partially or completely embrace the opening of the neck equally it grows

placentation

formation of the placenta; complete past weeks 14–xvi of pregnancy

primitive streak

indentation along the dorsal surface of the epiblast through which cells drift to grade the endoderm and mesoderm during gastrulation

somite

one of the paired, repeating blocks of tissue located on either side of the notochord in the early embryo

syncytiotrophoblast

superficial cells of the trophoblast that fuse to grade a multinucleated body that digests endometrial cells to firmly secure the blastocyst to the uterine wall

trophoblast

fluid-filled beat of squamous cells destined to go the chorionic villi, placenta, and associated fetal membranes

umbilical cord

connection between the developing conceptus and the placenta; carries deoxygenated claret and wastes from the fetus and returns nutrients and oxygen from the female parent

yolk sac

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

Solutions

Answers for Disquisitional Thinking Questions

1. The timing of this discomfort and bleeding suggests that information technology is probably caused past implantation of the blastocyst into the uterine wall.
2. Folate, one of the B vitamins, is of import for the healthy germination of the embryonic neural tube, which occurs in the first few weeks following conception—often before a woman even realizes she is pregnant. A folate-deficient environment increases the chance of a neural tube defect, such equally spina bidifa, in the newborn.

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Source: https://open.oregonstate.education/aandp/chapter/28-2-embryonic-development/

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