Dental embryology: Third Week of Development, Slides of Dental Anatomy

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Third Week of

Development: Trilaminar

Germ Disc

Maya Kiladze, PhD

General Embryology - Detailed Animation On Gastrulation.mp

Gastrulation (the third week of development), begins with the appearance of the primitive streak, which has at its cephalic end the primitive node. Through the node and streak, epiblast cells move inward (invaginate) to form new cell layers, endoderm and mesoderm. Cells that remain in the epiblast form ectoderm. Hence, epiblast forms all three germ layers in the embryo, ectoderm, mesoderm, and endoderm, and these layers form all of the tissues and organs in a cephalocaudal direction as gastrulation continues. A. Dorsal side of the germ disc from a 16-day embryo indicating the movement of surface epiblast cells (solid black lines) through the primitive streak and node and the subsequent migration of cells between the hypoblast and epiblast (broken lines). B. Cross section through the cranial region of the streak at 15 days with invagination of epiblast cells. The first cells to move inward displace the hypoblast to create the definitive endoderm. Once definitive endoderm is established, inwardly moving epiblast forms mesoderm.

Prenotochordal cells invaginating in the primitive pit move forward until they reach the prechordal plate. They intercalate in the endoderm as the notochordal plate. Then, the plate detaches from the endoderm, and a solid cord, the notochord, is formed. It is a midline axis, which will serve as the basis of the axial skeleton. A. Sagittal section through a 17-day embryo. The most cranial portion of the definitive notochord has formed, while prenotochordal cells caudal to this region are intercalated into the endoderm as the notochordal plate. B. Schematic cross section through the region of the notochordal plate. C. Schematic view showing the definitive notochord.

In the meantime, the trophoblast progresses rapidly. Primary villi obtain a mesenchymal core ( secondary villi ) in which small capillaries arise ( tertiary villi ). When these villous capillaries make contact with capillaries in the chorionic plate and connecting stalk, the villous system is ready to supply the embryo with its nutrients and oxygen. Embryo and the trophoblast at the end of the third week. A 13-day-old implantation

Development of a villus.

Third to Eighth Weeks: The

Embryonic Period

During embryonic period, ectoderm, mesoderm, and endoderm give rise to tissues and organ systems. The ectoderm gives rise to the organs and structures that maintain contact with the outside world:

• (^) central and peripheral nervous system ;
• (^) sensory epithelium of ear, nose, and eye;
• (^) skin , including hair and nails; and
• pituitary, mammary, and sweat glands and enamel of the teeth. The mesoderm give rise to mesenchyme of the head and organize into somites in occipital and caudal segments. Somites give rise to the supporting tissues of the body:
• (^) myotome (muscle tissue),
• (^) sclerotome (cartilage and bone), and
• dermatome (dermis of the skin). Mesoderm also gives rise to
• (^) vascular system,
• (^) urogenital system : kidneys, gonads, and their ducts (but not the bladder)
• (^) spleen and cortex of the suprarenal glands. The endoderm provides the epithelial lining of
• (^) gastrointestinal tract ,
• (^) respiratory tract ,
• (^) urinary bladder,
• (^) tympanic cavity, and
• (^) auditory tube It also forms the parenchyma of the thyroid , parathyroids , liver , and pancreas. General Embryology - Detailed Animation On Neurulation.mp

As a result of formation of organ systems and rapid growth of the central nervous system, the flat embryonic disc begins to lengthen and to form head and tail regions (folds) that cause the embryo to curve into the fetal position. The embryo also forms two lateral body wall folds that grow ventrally and close the ventral body wall. As a result of this folding, the amnion is pulled ventrally and the embryo lies within the amniotic cavity. Connection with the yolk sac and placenta is maintained through the vitelline duct and umbilical cord, respectively. Sagittal midline sections of embryos at various stages of development with cephalocaudal folding and its effect on position of the endoderm-lined cavity. A. 17 days. B. 22 days. C. 24 days. D. 28 days. Arrows , head and tail folds. General Embryology - Detailed Animation On Embryonic Folding.mp

The Gut Tube and the

Body Cavities

At the end of the third week, the neural tube is elevating and closing dorsally, while the gut tube is rolling and closing ventrally. Mesoderm holds the tubes together and the lateral plate mesoderm splits to form a visceral (splanchnic) layer associated with the gut and a parietal (somatic) layer that, together with overlying ectoderm, forms the lateral body wall folds. The space between the visceral and parietal layers of lateral plate mesoderm is the primitive body cavity. Transverse sections through embryos at various stages of closure of the gut tube and ventral body wall. A. At 19 days. B. At 20 days, the lateral plate is divided into somatic and visceral mesoderm layers that line the primitive body cavity. C. By 21 days, the primitive body cavity is still in open communication with the extraembryonic cavity. D. By 24 days, the lateral body wall folds are approaching each other in the midline. E. At the end of the fourth week, visceral mesoderm layers are continuous with parietal layers as a double- layered dorsal mesentery.

Parietal mesoderm will form the parietal layer of serous membranes lining the peritoneal, pleural, and pericardial cavities. The visceral mesoderm will form the visceral layer of serous membranes covering the lungs, heart, and abdominal organs. These layers are continuous at the root of each organ. Visceral and parietal layers are continuous with each other as the dorsal mesentery (extends from foregut to hindgut). Ventral mesentery exist only in the region of esophagus, stomach and the upper portion of duodenum.

SEROUS MEMBRANES

Mesenteries provide a pathway for vessels, nerves, and lymphatics to the organs.

The septum transversum is a thick plate of mesoderm occupying the space

between the primitive thoracic and abdominal cavities.

DIAPHRAGM AND THORACIC CAVITY

B. Portion of an embryo at 5 weeks with parts of the body wall and septum transversum removed to show the pericardioperitoneal canals. Growth of the lung buds Into the pericardioperitoneal canals. Note the pleuropericardial folds.

A. Transformation of the pericardioperitoneal canals into pleural cavities and formation of the pleuropericardial membranes. Mesoderm of the body wall forms the definitive wall of the thorax and pleuropericardial membranes which are extensions of the pleuropericardial folds. B. After fusion of pleuropericardial folds with each other and with the root of lungs, the thoracic cavity is divided into the definitive pericardial cavity and two pleural cavities.

DIAPHRAGM AND THORACIC CAVITY

DIAPHRAGM AND THORACIC CAVITY

The diaphragm divides the body cavity into the thoracic and peritoneal cavities. It develops from four components:

• (^) the septum transversum , which forms the central tendon of the diaphragm;
• (^) the two pleuroperitoneal membranes ;
• (^) muscular components from somites at cervical segments three to five ; and
• (^) the mesentery of the esophagus. Development of the diaphragm. A. Pleuroperitoneal folds appear at the beginning of the fifth week. B. Pleuroperitoneal folds fuse with the septum transversum and mesentery of the esophagus in the seventh week, separating the thoracic cavity from the abdominal cavity. C. Transverse section at the fourth month of development. Formation - Body Cavities & Diaphragm.mp

Third Month to Birth: The

Fetus and Placenta

• (^) The period from the beginning of the ninth week to birth is fetal period. It is characterized by maturation of tissues and organs and rapid growth of the body. The length of the fetus is indicated as the crown-rump length (CRL) (sitting height) or as the crown-heel length (CHL) , the measurement from the vertex of the skull to the heel (standing height) in centimeters.
• (^) Growth in length is striking during the third, fourth, and fifth months ( 5 cm per month), while an increase in weight is most striking during the last 2 months of gestation ( 700 gr per month).
• (^) In general, the length of pregnancy is considered to be 280 days, or 40 weeks after the onset of the last normal menstrual period (LNMP) or, more accurately, 266 days or 38 weeks after fertilization. At the beginning of the third month, the head constitutes approximately half of the CRL. By the beginning of the fifth month, the size of the head is about one third of the CHL, and at birth, it is approximately one quarter of the CHL. Hence, over time, growth of the body accelerates but that of the head slows down.

A 9-week fetus. Note the large head size compared with that of the rest of the body. The yolk sac and long vitelline duct are visible in the chorionic cavity. Note the umbilical cord and herniation of intestinal loops. One side of the chorion has many villi (chorion frondosum), while the other side is almost smooth (chorion laeve).

During the third month, the face becomes human looking. The eyes, initially directed laterally, move to the ventral aspect of the face, and the ears come to lie close to their definitive position at the side of the head. The limbs reach their relative length in comparison with the rest of the body, although the lower limbs are still shorter and less well developed than the upper extremities. Primary ossification centers are present in the long bones and skull. Also by the 12th week, external genitalia develop to such a degree that the sex can be determined by ultrasound. A 12-week fetus in utero. Movements begin at this time but are usually not felt by the mother. An 11-week fetus. The umbilical cord shows a swelling at its base, caused by herniated intestinal loops. The skull of this fetus lacks the normal smooth contours. Fingers and toes are well developed.

An 18-week fetus connected to the placenta by its umbilical cord. The skin of the fetus is thin because of lack of subcutaneous fat. A 7-month fetus. This fetus would be able to survive. It has well-rounded contours as a result of deposition of subcutaneous fat. At the end of the first half of intrauterine life, the CRL is  15 cm, about half the total length of the newborn. The weight of the fetus by the end of the fifth month is still <500 g. The fetus is covered with fine hair, called lanugo hair ; eyebrows and head hair are visible. During the fifth month, movements of the fetus can be felt by the mother. By 6.5 to 7 months, the fetus has a CRL of about 25 cm and weighs approximately 1,100 g. If born at this time, the infant has a 90% chance of surviving. By the end of intrauterine life, the skin is covered by a whitish, fatty substance (vernix caseosa) from sebaceous glands. At the end of the ninth month, the skull has the largest circumference of all parts of the body. At the time of birth, the weight of a normal fetus is 3,000 to 3,400 g, its CRL is about 36 cm, and its CHL is about 50 cm. Sexual characteristics are pronounced.

PLACENTA

Functions of the placenta are: (1) exchange of gases; (2) exchange of nutrients and electrolytes; (3) transmission of maternal antibodies, providing the fetus with passive immunity; (4) production of hormones, such as progesterone, estrogen, and hCG (5) detoxification of some drugs. The fetal component of placenta is derived from trophoblast and extraembryonic mesoderm (the chorionic plate); the maternal component is derived from uterine endometrium (decidua). Maternal blood is delivered to the placenta by spiral arteries in the uterus. Erosion of these maternal vessels to release blood into intervillous spaces is accomplished by endovascular invasion by cytotrophoblast cells. These cells, released from the ends of villi, invade the terminal ends of spiral arteries, where they replace maternal endothelial cells in the vessels’ walls, creating hybrid vessels containing both fetal and maternal cells. Human embryo at the beginning of the second month of development

PLACENTA

A 6-week embryo. The amniotic sac and chorionic cavity are opened to expose the embryo, showing the bushy appearance of the trophoblast at the embryonic pole (chorion frondosum) in contrast to small villi at the abembryonic pole (chorion laeve). Note the connecting stalk and yolk sac with its extremely long vitelline duct. End of the second month End of the third month By the beginning of the fourth month, the placenta has two components: (1) a fetal portion, formed by the chorion frondosum and (2) a maternal portion, formed by the decidua basalis – functional layer of endometrium. The space between the chorionic and decidual plates is filled with intervillous lakes of maternal blood. Villous trees (fetal tissue) grow into the maternal blood lakes and are bathed in them. The fetal circulation is always separated from the maternal circulation by (1) a syncytial membrane (a chorion derivative) and (2) endothelial cells from fetal capillaries. Hence, the human placenta is of the hemochorial type.

PLACENTA

A 19-week fetus in its natural position in the uterus, showing the umbilical cord and placenta. The lumen of the uterus is obliterated. In the wall of the uterus is a large growth, a myofibroma. A 23-week fetus in the uterus. Portions of the wall of the uterus and the amnion are removed to show the fetus. In the background are placental vessels converging toward the umbilical cord. The umbilical cord is tightly wound around the abdomen, possibly causing abnormal fetal position in the uterus (breech position). At full term, the placenta is discoid with a diameter of 15 to 25 cm, is  3 cm thick, and weighs about 500 to 600 g. At birth, it is torn from the uterine wall and,  30 minutes after birth of the child, is expelled from the uterine cavity as the afterbirth. Intervillous lakes of the fully grown placenta contain  150 mL of maternal blood, which is renewed three or four times per minute. The villous area varies from 4 to 14 m 2 , facilitating exchange between mother and child.

AMNION AND UMBILICAL CORD

At the fifth week of development, the following structures pass through primitive umbilical ring (junction between amnion and embryonic ectoderm): (1) connecting stalk, with allantois and the umbilical two arteries and one vein; (2) the yolk stalk (vitelline duct) , accompanied by the vitelline vessels; and (3) the canal connecting the intraembryonic and extraembryonic cavities. At the end of the third month, the amnion has expanded so that it comes in contact with the chorion, obliterating the chorionic cavity. When the yolk sac, allantois and vitelline duct with its vessels are obliterated, all that remains in the cord are the umbilical vessels (two arteries and one vein) surrounded by Wharton’s jelly. A. A 5-week embryo. B. The primitive umbilical cord of a 10-week embryo.

AMNIOTIC FLUID

The amnion is a large sac with amniotic fluid in which the fetus is suspended by its umbilical cord. The fluid: (1) absorbs jolts, (2) allows for fetal movements, and (3) prevents adherence of the embryo to surrounding tissues. The fetus swallows amniotic fluid, which is absorbed through its gut and cleared by the placenta. The fetus adds urine to the amniotic fluid, but this is mostly water.

PARTURITION (BIRTH)

Signals initiating parturition (birth) are not clear, but preparation for labor usually begins between 34 and 38 weeks. Labor itself consists of three stages: (1) effacement (thinning and shortening) and dilatation of the cervix, (2) delivery of the fetus, and (3) delivery of the placenta and fetal membranes.