Dr. Akram M. Asbeutah, PhD Department of Radiologic Sciences Faculty of Allied Health Sciences

1. Prenatal Growth, Embryonic & Fetal Periods, & Twinning Dr. Akram M. Asbeutah, PhD Department of Radiologic Sciences Faculty of Allied Health Sciences Kuwait University

2. Embryology • Embryology – study of the origin and development of single individual • Prenatal period • Embryonic period – first 8 weeks • Fetal period – remaining 30 weeks

3. Week 4-8 Embryonic Period • Limb buds form • Embryo first looks recognizably human (week 8) • Head is disproportionately large • All major organs are in place

4. Week 4-8 Embryonic Period • All major external and internal structures are established • By the end of this period, the main organ systems have begun to develop; however, the function of most of them is minimal except for the cardiovascular system • As the tissues and organs form, the shape of the embryo changes, and by the 8th week, it has a distinctly human appearance • Exposure of embryos to teratogens during this period may cause major congenital anomalies

5. Phases of Embryonic development • Growth: It involves cell division and the elaboration of cell products • Morphogenesis:it is an elaborate process during which many complex interaction occur in an orderly sequence • Differentiation:It is maturation of physiologic processes. Completion of differentiation results in the formation of tissues and organs that are capable of performing specialized functions

6. Folding of the Embryo • A significant event in the establishment of body form is folding of the flat trilaminar embryonic disc into a somewhat cylindrical embryo • Folding occurs in both the median (longitudinal) and lateral (horizontal) planes results from rapid growth of the embryo • Folding at the cranial and caudal ends and sides of the embryo occurs simultaneously

7. Folding of the embryo in the median (Longitudinal) plane • Folding of the ends of the embryo ventrally produces head and tail folds that results in the cranial and caudal regions moving ventrally as the embryo elongates cranially and caudally

8. Folding of the embryo in the median plane-Head Fold • By the beginning of the 4th week, the neural fold in the cranial region have thickened to form the primordium of the brain • The developing brain projects dorsally into the amniotic cavity. Later, the developing forebrain grows cranially beyond the oropharyngeal membrane and overhangs the developing heart. Concomitantly, the septum transversum, primodial heart, pericardial coelom, and oropharyngeal membrane move onto the ventral surface of the embryo • During folding, part of the endoderm of the yolk sac is incorporated into the embryo as the foregut (primodium of pharynx, esophagus, …etc.). The foregut lies between the brain and the heart, and the oropharyngeal membrane separates the foregut from the stomodeum • After folding, the septum transversum lies caudal to the heart where it subsequently develops into the central tendon of the diaphgram • Head fold also affects the arrangement of the embryonic coelom (body cavities)

9. Folding of the embryo in the median plane-Tail Fold • It results primarily from growth of the distal part of the neural tube (primodium of the spinal cord) • As the embryo grows, the caudal eminence projects over the cloacal membrane (Future site of anus) • During folding, part of the endodermal germ layer is incorporated into the embryo as the hindgut (primodium of descending colon) • The terminal part of the hindgut soon dilates to form the cloaca (promodium of urinary bladder and rectum) • Before folding, the primitive streak lies cranial to the cloacal membrane and after folding it lies caudal to it • The connecting stalk (primodium of umbilical cord) is now attached to the ventral surfave of the embryo, and the allantois (yolk sac diverticulum) is partially incorporated into the embryo

10. Folding of the embryo in the median (Longitudinal) plane Figure 3.7a, b

11. Folding of the embryo in the median (Longitudinal) plane Figure 3.7c, d

12. Folding of the embryo in the lateral (horizontal) plane • It produces Right and Left Lateral Folds • Lateral folding is produced by the rapidly growing spinal cord and somites • The primodia of the ventrolateral wall fold toward the median plane, rolling the edges of the embryonic disc ventrally and forming a roughly cylindrical embryo • As the abdominal wall form, part of the endoderm germ layer is incorporated into the embryo as the midgut (primodium of the small intestine) • Omphaloenetric duct connectinh the midgut to the yolk sac • Umbilical cord form from the connecting stalk • Amniotic cavity expands and obliterates most of the extrembryonic coelom and the amnion form the epithelial covering of the umbilical cord

13. Folding of the embryo in the lateral (horizontal) plane • Folding of embryo laterally and at the head and tail • Embryonic disc bulges; growing faster than yolk sac • “Tadpole shape” by day 24 after conception • Primitive gut – encloses tubular part of the yolk sac • Site of future digestive tube and respiratory structures

14. Folding of the embryo in the lateral (horizontal) plane Figure 3.8

15. Germ Layers Derivatives • Derivatives of the germ layers • Ectoderm forms • Brain, spinal cord, and epidermis • Endoderm forms • Inner epithelial lining of the gut tube • Respiratory tubes, digestive organs, and urinary bladder • Notochord – gives rise to nucleus pulposus within intervertebral discs

16. Germ Layers Derivatives • Mesoderm – forms • Muscle • Bone • Dermis • Connective tissues (all) • Mesoderm differentiates further and is more complex than the other two layers

17. Germ Layers Derivatives • Mesoderm (continued) • Somites divides into • Sclerotome • Dermatome • Myotome • Intermediate mesoderm forms • Kidneys and gonads

18. Germ Layers Derivatives • Mesoderm (continued) • Splanchnic mesoderm • Forms musculature, connective tissues, and serosa of the digestive and respiratory structures • Forms heart and most blood vessels • Somatic mesoderm – forms • Dermis of skin • Bones • Ligaments

19. Germ Layers Derivatives Figure 3.10

20. Germ Layers Derivatives Figure 3.9a–d

21. The embryo is 0.25“ in length almost straight and has 4-12 somites that produce conspicuous surface elevations Neural tube is formed opposite the somites and it is widely opened at the rostral and caudal neuropores By 24 days, the first two pharyngeal arches are visible. The first (mandibular arch) and the second (hyoid arch) are distinct By 26 days, three pairs of pharyngeal arches are visible and the rostral neuropore is closed Highlights of the 4th-8th weeks- Fourth Week Day 22 Day 24

22. The forebrain produces a prominent elevation of the head, and folding of the embryo has given the embryo a C-shaped curvature By day 27, the upper limb buds are recognizable as a small swelling on the ventrolateal body walls and the otic pits (primodia of the internal ears), are also visible Ectodermal thickening (lens placodes) indicating the future lenses of the eyes are visible on the sides of the head By the end of the 4th week, the fourth pair of the pharyngeal arches and the lower limb buds are visible and a long tail-like caudal eminence is a characteristic feature. The caudal neuropore is usually closed Highlights of the 4th-8th weeks- Fourth Week Day 26 Day 28

23. Enlargement of the head due to rapid growth of the brain and facial prominences The rapidly growing 2nd pharyngeal arch overgrows the 3rd and 4th arches, forming a lateral ectodermal depression on each side (cervical sinus) Mesonephric ridges indicate the site of the mesonephric kidneys, which are interim excretory organs in human Highlights of the 4th-8th weeks- Fifth Week

24. Highlights of the 4th-8th weeks- Sixth Week • Embryos shows reflex response to touch • The upper limbs begins to show regional differentiation as the elbows and large handplatesdevelop. Digital rays begin to develop in the handplates • Spontaneous movement such as twitching of the trunk and limbs. Development of the lower limbs occurs 5 days later than that of the upper limbs • Auricular hillocksdevelop around the pharyngeal groove between the first two pharyngeal arches (this groove become the external auditory meatus) • The head is more larger than the trunk and is bent over the heart prominence • Umbilical herniation is a normal event at the time because the abdominal cavity is too small to accommodate the rapidly growing intestine

25. Highlights of the 4th-8th weeks- Seventh Week • Notches appear between the digital rays in the handplates indicating the future digits • Omphaloenteric duct appears • Ossification of the bones of the upper limb start

26. Highlights of the 4th-8th weeks- Eighth Week • The embryo is about 1 ½" in length • Notches are now clearly visible between digits of the feet • Caudal eminence is still present but stubby • The scalp vascular plexus appears • All regions of the limbs are apparent, the digits have lengthened and are completely separated • Purposeful limb movements and ossification begins in the femur • All evidence of caudal eminence disappear • Hands and feet approach each other ventrally • The embryo has distinct human characteristic

27. Last normal menstrual period (LNMP) Estimated time of fertilization Ultrasound measurement of the chorionic sac and embryo Examination of external characteristics of the embryo Estimation of the embryonic age

28. The Fetal Period • A time of maturation and rapid growth • Cells are differentiating during the first half of the fetal period • Normal births occur 38 weeks after conception • Premature birth is one that occurs before 38 weeks

29. Developmental Events of the Fetal Period Table 3.1 (1 of 3)

30. Developmental Events of the Fetal Period Table 3.1 (2 of 3)

31. Developmental Events of the Fetal Period Table 3.1 (3 of 3)

32. Highlights of the Fetal Period 9th-38th weeks

33. 10 Weeks • The embryo, is about 1 to 1¼ inches long (the head is about half the length) and weighs less than ½ ounce • The beginnings of all key body parts are present, but they are not completed • Structures that will form eyes, ears, arms and legs can be seen • Muscles and skeleton are developing and the nervous system becomes more responsive

34. 12 Weeks

35. 14 Weeks • The fetus is about 3½ inches long and weighs about 1½ ounces • The fetus begins to swallow, the kidneys make urine, and blood begins to form in the bone marrow • Joints and muscles allow full body movement • There are eyelids and the nose is developing a bridge • External genitals are developing

36. 16 Weeks • Fetus is 4 to 4.5 inches long & weighs about 6 to 7 ounces • Fetus is developing reflexes such as sucking and swallowing • Fetus may begin sucking his/her thumb • Tooth buds are developing • Sweat glands are forming on palms and soles • Fingers and toes are well defined • Sex is identifiable • Skin is bright pink, transparent and covered with soft, downy hair • Although recognizably human in appearance, the baby would not be able to survive outside the mother's body

37. 18 Weeks • The fetus is about 5½ inches long and weighs about 7 ounces • The skin is pink and transparent and the ears are clearly visible • All the body and facial features are now recognizable • The fetus can grasp and move its mouth • Nails begin to grow • The fetus has begun to kick and some women feel this movement

38. 20 Weeks • Fetus is 6 to 7 inches long & weighs about 1 pound • Hair begins to grow on his/her head • All organs and structures are formed • Skin is wrinkled and pink to reddish in color - thin and close to the blood vessels • Protective skin coating, (vernix) begins to develop • Respiratory movements occur - lungs have not developed enough to permit survival outside the uterus • By this time, mothers usually feel the fetus moving • At this time an ultrasound can often identify the sex of the fetus • Soft woolly hair called lanugo will cover its body (and some may remain until a week after birth when it is shed) • Mother begins to feel fetal movement • Internal organs are maturing • Eyebrows, eyelids and eyelashes appear

39. 22 Weeks • The fetus is about 7½-8 inches long weighs about one pound • It has fingerprints and some head and body hair • It may suck its thumb and is more active • The brain is growing very rapidly • The fetal heartbeat can be easily heard • The kidneys start to work • At 23 weeks, approximately 31% of babies born survive. Babies born at this age require intensive care and usually have lifelong disabilities and chronic health conditions

40. 24 Weeks • The fetus is about 8¼ inches long and weighs about 1¼ pounds • Bones of the ears harden making sound conduction possible. The fetus hears mother’s sounds such as breathing, heartbeat and voice • The first layers of fat are beginning to form • This is the beginning of substantial weight gain for the fetus • Lungs continue developing • At 25 weeks, approximately 68% of babies born survive. Babies born at this age require intensive care and usually have life-long disabilities and chronic health conditions

41. 26 Weeks • The fetus is about 9 inches long and weighs about 2 pounds • The fetus can respond to sound from both inside and outside the womb • Reflex movements continue to develop and body movements are stronger • Lungs continue to develop • The fetus now wakes and sleeps • The skin is slightly wrinkled • At 27 weeks, approximately 87% of babies born survive. Babies born at this age require intensive care and have an increased risk of developmental delays and chronic health conditions

42. 28 Weeks • The fetus is about 10 inches long and weighs about 2 pounds, 3 ounces • Mouth and lips show more sensitivity • The eyes are partially open and can perceive light • More than 90% of babies born at this age will survive. Some survivors have developmental delays and chronic health conditions

43. 30 Weeks • The fetus is about 10½ inches long and weighs about 3 pounds • The lungs that are capable of breathing air, although medical help may be needed • The fetus can open and close its eyes, suck its thumb, cry and respond to sound • The skin is smooth • Rhythmic breathing and body temperature are now controlled by the brain • Most babies born at this age will survive

44. 32 Weeks • The fetus is about 11 inches long and weighs about 3 pounds, 12 ounces • The connections between the nerve cells in the brain increase • Fetal development now centers on growth • Almost all babies born at this age will survive

45. 34 Weeks • The fetus is about 12 inches long and weighs about 4½ pounds • Ears begin to hold shape • Eyes open during alert times and close during sleep • Almost all babies born at this age will survive

46. 36 Weeks • The fetus is about 12 to 13 inches long and weighs about 5½ to 6 pounds • Scalp hair is silky and lies against the head • Muscle tone has developed and the fetus can turn and lift its head • Almost all babies born at this age will survive

47. 38 Weeks • The fetus is about 13½ to 14 inches long and weighs about 6½ pounds • Lungs are usually mature • The fetus can grasp firmly • The fetus turns toward light sources • Almost all babies born at this age will survive

48. 40 Weeks • The fetus is about 18 to 20 inches long and may weigh about 7½ pounds • At the time of birth, a baby has more than 70 reflex behaviors, which are automatic behaviors necessary for survival • The baby is full-term and ready to be born

49. Fraternal twins (dizygotic) independent release of 2 oocytes fertilized by 2 separate sperm as different as any 2 siblings Identical twins (monozygotic) 2 individuals that develop from a single fertilized ovum genetically identical & always the same sex if ovum does not completely separate, conjoined twins (share some body structures) Nature of Twins

50. How dizygotic twins develop from two zygotes?