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Embryology

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Embryology

  1. 1. What is embryology? The study of developmental events that occur during the prenatal stage . The branch of biology and medicine concerned with the study of embryos and their development.
  2. 2. Embryonic period vs. Fetal period  Embryonic – first 8 weeks Development of the three primary germ layers give rise to all structures and Basic body plan takes shape  Fetal period – remaining 30 weeks. Structures and organs continue to grow and develop.  8 weeks
  3. 3. Developmental History of a Human
  4. 4. Stages of Development 1. Fertilization 2. Cleavage 3. Gastrulation Embryogenesis 4.Organogenesis 5.Maturation
  5. 5. Fertilization  The process of fusion or union of the spermatozoon with the mature ovum is known as conception / fertilizaiton/impregnantation.  Which produced the fertilized single mono- nucleated cell called the zygote.
  6. 6. Embryogenesis: The formation and development of an embryo. 1. Cleavage is a series of rapid mitotic divisions (without cell growth)
  7. 7. 2. Gastrulation : is a phase early in the embryonic development of most animals/human being, during which the single-layered blastula is reorganized into a trilaminar ("three-layered") structure known as the gastrula. These three germ layers are known as the ectoderm, mesoderm, and endoderm. 3. Organogenesis: The production and development of the organs of an animal or plant.
  8. 8. How fertilization occurs????  Following ovulation, the ovum, which is about 0.15 mm in diameter is picked up by the tubal fimbriae and is moved along by the cilia and by peristaltic movement of the tube.  At the time the cervix under the influence of estrogen, secretes a flow of alkaline mucus that deposited in the vagina, only thousands capacitated spermatozoa enter the uterine tube while 300-500 reach the ovum, and remainder are destroyed by the acid medium of the vagina.  It takes about 1 hour for sperm to reach the site.
  9. 9.  The sperm release the enzyme, Hylluronidase which allows penetration of the zona pellucida and the cell membrane surrounding the ovum.  Many sperm are needed for this to take place but only one will enter the ovum.  After this the membrane is sealed to prevent entry of any further sperm and the nucluei of the two cell fuse.
  10. 10.  The sperm and ovum contribute half the complement of chromosomes to make a total of 46.  The sperm and ovum is known as the male and female gametes and the fertilized ovum as the zygote
  11. 11. . Fertilization
  12. 12. Normal site for Conception  The most common site of conception is the ampullary part (Ampulla ) of the fallopian tube which is the widest part located closed to the ovary
  13. 13. Usual Time for Conception  Neither sperm nor ovum can survive longer than 2-3 days and the fertilization is most likely to occur when intercourse takes place not more than 48 hours before and 24 hours after ovulation.  So the conception will take place about 14 days before the next period is due.  The sex of the new individual at the time of conception is determined by sex chromosomes.
  14. 14.  Every human cell contains 46 chromosomes, which are made up of 44 autosome chromosome and 2 sex chromosomes.  The sex chromosome are X and Y .  Woman have no Y chromosome and male has Y chromosome (male 44+X+Y) (female 44+X+X).   Therefor e, sex of child is always determined by father
  15. 15. Morula 1. After fertilization, the Zygote divides into 2 cells (blastomere) (mitosis division)in about 30 hours after fertilization. 2. The blastomeres continue to divide by binary division through 4, 8, and 16 cell stage until a cluster of cells is formed– Morula, resemblibg a mulberry
  16. 16.  The morula after spending about 3 days(72 hours) in the uterine tube enters the uterine cavity through the narrow uterine ostium (1mm) on the 4th day .
  17. 17. Morula Morula
  18. 18. Morula 72 hours post- fertilization entering uterus
  19. 19. 4. Blastocyst  Morula, once entering the uterine cavity, floats freely(next 2 days) and is covered by endometrial fluid and mucus.  This fluid is absorbed through the canaliculi of the zona pellucida and Morula begins to accumulate fluid and forms a cavity between its cells.  Once cavity appears, it is now called a blastocyst.
  20. 20. Fertilization and the Events of the First 6 Days of Development
  21. 21. Blastocyst  The zona pellucida becomes stretched, thinned and gradually disappear soon prior to implantation.  The cell of the outer cell mass forms the wall of the blastocyst and is known as trophoblast.  The inner cell mass is concerned with the development of the embryo.
  22. 22. Two Distinct Cell Types 1. Trophoblasts – will form the invading placenta 2. Inner cell mass – will form the embryo Trophoblasts
  23. 23. Blastocyst Trophoblast Inner cell mass Placenta Chorion Fetus Amnion umbilical cord
  24. 24. hCG is produced  hCG is human chorionic gonadotropin hormone  It is produced by the trophoblasts starting on day 6  hCG causes endometrium of uterus to grow and proliferate  hCG prevents the menstrual cycle from occuring  This is why a female misses her periods when she is pregnant
  25. 25. Implantation (Week 2)
  26. 26.  Implantation occurs about 6th day after fertilization (approx the 20th day of a regular menstrual cycle).  As the zona pellucida surrounding the blastocyst disappear, there is increased adhesiveness of the trophoblastic cells to the endometrium.  The trophoblastic cells invade the stromal cells lying between the secretary endometrial glands by histolytic action of the blastocyst bringing about deeper penetration in the decidua.
  27. 27.  The syncytial cells ( multinucleate cell) ultimately penetrate the endothelial coat of the maternal decidual capillaries establishing communication with the syncytial lacunar system  Thus maternal blood circulates through the syncytial lacunae providing nourishment to the blastocyst. Maternal blood vassels
  28. 28.  Further penetration is arrested by the maternal immunological blocking factor. The point of entry of the blastocyst into the decidual is sealed by a fibrin coat and later by epithelium  The process of implantation is completed by the 10th or 11th day of fertilization.
  29. 29.  This type of deeper penetration of the human blastocyst is called interstitial implantation and the blastocyst is covered on all sides by the endometrium (decidual).  Occasionally, there may be increased blood flow into the lacunar space and cause bleeding. It is called implantation bleeding and This corresponds approx to 13th day after fertilization (at about the expected day of period).  This may produce confusion in determination of the EDD
  30. 30.  The outer cell mass of the blastocyst form the trophoblast layer.  The inner cell mass on the 8th day differentiates into bilaminar germ disc consisting of a dorsal ectodermal layer of the tall columnar cells and a ventral endodermal layer of flat polyhydral cells.  The bilaminar germ disc is connected to the trophoblast by a mesenchymal connecting stalk called the body stalk which later forms the umblical cord.
  31. 31. Formation of Amniotic cavity and Yolk sac  Two cavities appear on either side of the bilaminar embryonic disc.  The dorsal cavity between the ectoderm and trophoblastic layers lined by mesenchyme(connective tissue) is called the amniotic cavity.  The cavity on the ventral aspect lined by the primitive mesenchyme on the outside and the endodermal layer of the germ cell disc on the inside is called the yolk sac
  32. 32. Week 2  Inner cell mass divides into epiblast and hypoblast  2 fluid filled sacs  Amniotic sac from epiblast within which the embryo and later the fetus develop until birth  Yolk sac from hypoblast which is one of the structures through which the mother supplies nutrients to the early embryo  Bilaminar embryonic disc: area of contact (gives rise to the whole body)
  33. 33.  Bilaminar to trilaminar disc  Three primary “germ” layers: all body tissues develop from these  Ectoderm  Endoderm  Mesoderm Week 3
  34. 34. Formation of the 3 “germ” layers  Primitive streak (groove) on dorsal surface of epiblast  Grastrulation: invagination of epiblast cells  Days 14-15: they replace hypoblast becoming endoderm  Day 16: mesoderm (a new third layer) formed in between  Epiblast cells remaining on surface: ectoderm
  35. 35. Major derivatives of the embryonic germ layers
  36. 36. Trophoblast  Small projection begins to appear all over the surface of the blastocyst, becoming most prolific at the area of contact. These trophoblastic cells differentiate into 3 layer.  1. The outer syncytiotrophoblast (syncytium): it erodes the walls of the blood vessels of the decidua, making the nutrient in the maternal blood accessible to the developing fetus.
  37. 37.  2. The inner cytotrophoblast: it’s a single layer cells which produces a hormone HCG. This is responsible for informing the corpus luteum that a pregnancy has begun and corpus luteum continues to produce estrogen and progesterone, which maintain the integrity of the decidua.  3. In the inner aspect: a layer of mesoderm or primitive mesenchyme, which consists of loose connective tissue.  There is similar tissue in the inner cell mass and the two are continuous at the point where they join in the body stalk. (fig 3.9 1st picture from book tuitui)
  38. 38.  The trophoblast is responsible for the formation of the placenta and fetal membrane and sub-serve the important functions of attachment of the fetus to maternal tissues, providing nutrition, oxygenation, and clearing of the fetal metabolic wastes and producing hormones, thus ensuring growth and development of the fetus.
  39. 39. Decidua  The endometrial lining of the uterus is called decidua during pregnancy and it is shed after delivery.  Progesterone from the corpus luteum and placenta maintain decidua during pregnancy. It is called the decidual reaction.
  40. 40. Well developed Decidua consists of 3 layers. 1. Superficial compact layer: consists of compact mass of decidual cells, gland, ducts and dilated capillaries. The greater part of the surface epithelium is either thinned out or lost. 2. Intermediate spongy layer (cavernous or functional layer): contains dilated uterine glands, decidual cells and blood vessesls. Placenta separation occurs in this layer.
  41. 41. 3. The basal layer: containing the basal portion of the glands and is opposed to the uterine muscle. Regeneration of the mucous coat occurs from this layer following parturition. After the implantation of the blastocyst into the compact layer of the decidua, the different portions of the decidua are renamed as:
  42. 42. 1. Decidua basalis or serotina: The portion of the decidua in contact with the base of the blastocyst. 2. Decidua capsularies or reflexa: The thin superficial compact layer covering the blastocyst. 3. Decidua vera or parietalis: the rest of the decidua lining the uterine cavity outside the site of implantation.
  43. 43.  The ovum bulges into the uterine cavity. The space between the decidua capsularies and decidua vera is called the decidual space.  It progressively diminishes as the ovum enlarges in pregnancy  Until at 16 weeks, the space is obliterate because of the fusion of the decidua capsularis with the decidua vera.  At term this fused layer is gradually attenuated and its constituent layers can not be identified.  The decidua basalis becomes the maternal portion of the placenta.
  44. 44. Functions of Decidua 1. To provide place for implantation of the fertilized ovum. 2. To provide nutrition to the growing ovum. 3. Provide a protective action. 4. Provides the basal plate of the placenta.
  45. 45. Important Events Following Fertilization  “O” hour - Fertilization  30 hours - 2 cell stage  40-50 hours - 4 cell stage  72 hours - 12 cell stage  96 hours - 16 cell stage morula enter the uterine cavity  5th day - blastocyst  6-7th day - zona pellucida disappear. Interstitial implantation occurs.
  46. 46.  9th day - lacunar period, endometrial vessels tapped  10-11th day - implantation completed  13th day - primary villi seen  16th day - secondary villi  21st day - tertiary villi  21st -22nd - fetal heart, fetal placental circulation started
  47. 47. The prenatal development of the fetus can be divided into 3 phases. 1. Ovular period or germinal period (It lasts from the stage of fertilization upto 2 weeks after ovulation) The embryonic period (first 8 weeks Development of the three primary germ layers give rise to all structures and Basic body plan takes shape ) Fetal period (remaining 30 weeks. Structures and organs continue to grow and develop)
  48. 48. Fetal Length  In the earlier weeks, it is expressed as the measurement from the vertex to the coccyx (crown rump length) while after mid pregnancy (20 weeks onwards), the measurement of the fetus is determined from the vertex to the heel (crown heel length).
  49. 49. Age of the Fetus  The length is more reliable criterion than the weight to calculate the age of the fetus.  Haase’s rule is employed in calculating the age of fetus from its length in centimeters, divided by 5.
  50. 50. 1. First lunar month: Fertilization to 2weeks of embryonic growth  Implantation is complete.  Primary chorionic villi forming  Embryo develops into two cell layers (bilaminar embryonic disks)  Amniotic cavity appears.  Note: lunar month is a period that is measured based on the movement of the moon. It is roughly 28 days and it constitutes a month. In that context, pregnancy is 10.
  51. 51. 2. Second lunar month: 3 to 6 weeks of embryonic growth  At the end of 6 weeks of growth, the embryo is approximatey 1.2 cm long.  Arms and leg buds are visible; arm buds are more developed with finger ridges beginning to appear.  Rudiments of the eyes, ears and nose appear.  Primitive intestinal tract is developing.  Primitive cardiovascular system is functioning.  Neural tube, which forms the brain and spinal cord closes by the 4th week.
  52. 52. 3. Third lunar month: 7 to 10 weeks of growth  The middle of this period (8 weeks)marks the end of the embryonic period and beginning of the fetal period.  At the end of 10 weeks of growth, the fetus is 6.1 cm from crown to rump and weigh 14 gm.  Appearance of external genitalia.  By the middle of this month, all major organs, systems have formed.  The heart has formed four chambers (by 7th week)  The fetus assumes a human appearance  Bone ossification begins  Rudimentary kidney begins to secrete urine.
  53. 53. 4. Fourth lunar month: 11 to 14 weeks old fetus  At the end of 14 weeks of growth, the fetus is 12 cm crown to rump length and 110 gm.  Head erect; lower extremities well developed.  Hard palate and nasal septum have fused.  External genitalia of male and female can now be differentiated.  Eyelids are sealed
  54. 54. 5.Fifth lunar month: 15 to 18 weeks old fetus  At the end of 18 weeks of growth, the fetus is 16 cm crown rump length and 320 gm.  Ossification of fetal skeleton can be seen on X-ray.  Ears stand out from head.  Fetus makes sucking motions and swallows amniotic fluid.  Fetal movement may be felt by the mother (end of month)
  55. 55. 6. Sixth lunar month: 19-22 weeks old fetus  At the end of 22 weeks of growth, the fetus is 21 cm crown rump length and 630 gm.  Vernix caseosa covers the skin.  Head and body hair (lanugo) visible.  Skin is wrinkled and red.  Brown fat(adipose tissue), an important site of heat production, is present in neck and sternal area.
  56. 56. 7. Seventh lunar months: 23-26 weeks old fetus  At the end of 26 weeks of growth, the fetus is 25 cm crown to rump length and 1000 gm.  Fingernails present.  Lean body  Eyes partially open: eyelashes present.  Bronchioles are present: primitive alveoli (terminal sacs) are forming  Skin begins to thicken on hands and feet  Startle reflex present, grasp reflex is strong.
  57. 57. 8. Eighth lunar month: 27-30 weeks old fetus  At the end of 30 weeks of growth, the fetus is 28 cm crown rump length and 1,700 gm.  Eyes open  Ample hair on head: lanugo begins to fade  Skin slightly wrinkled  Toe nails presents  Testes in inguinal canal, begin descent to scrotal sac  Surfactant coat in much of the alveolar epithelium
  58. 58. 9. Ninth lunar months: 31-34 weeks of old fetus  At the end of 34 weeks of growth, the fetus is about 32 cm crown –rump length and 2,500 gm  Fingernails reach finger tips  Skin pink and smooth  Testes in scrotal sac
  59. 59. 10. Tenth lunar month: 35 to 38 weeks old fetus  End of 38 weeks of growth, fetus is about 36 cm crown –rump length and 3400 gm.  Ample of subcutaneous fat present  Lanugo is decreasing  Toe nails reach upto toe tips  Testes in scrotum  Vernix caseosa mainly on the back  Breasts are firm.
  60. 60. Weeks 3-4
  61. 61. Weeks 5-8  Embryo will develop all structures that an adult has by the end of week 8
  62. 62. Fetal Period 9-12 13-16 17-20 21-38

Editor's Notes

  • Invagination is the infolding of one part within another part of a structure, a folding that creates a pocket
  • attenuated
  • the length of the fetus in centimeters, divided by 5, is the duration of pregnancy in months, the age of the fetus.
  • ×