胚胎学总论
第20章胚胎学总论
——人体胚胎发生CHAPTER 20 GENENRAL EMBRYOLOGY
——The Embryonic Development of Human Body
KYEY POINTS
●Gametogenesis
●Fertilization
●Development in pre-embryonic period
●Development in embryonic period
●Development in fetal period
●Fetal membrane and placenta
●Twins, multiple birth and conjoined twins
一、配子发生和受精
GAMETOGENESIS AND FERTILIZATION
OUTLINE:The fertilization is a process by which the male and female gametes unite to give rise to the zygote. Before fertilization both male and female germ cells undergo a lot of changes involving the chromosome as well as the cytoplasm. The mature process of the germ cells is known as gametogenesis. The gametogenesis is chiefly accomplished by two specialized divisions, known as meiotic or maturation divisions, by means of which the number of chromosome is reduced by half, from 46 to 23. The male germ cell, initially large and round, loses almost all of its cytoplasm and develops a head, neck and tail (a tadpole-like structure). The female germ cell, on the contrary, increases in the amount of cytoplasm. Fertilization is an important biological process by which the zygote restores the normal diploid number of chromosome, determines the genetic sex of the new individual, and initiates cleavage.
(一)配子发生Gametogenesis
（二）受精Fertilization
OVULATION
The process by means of which the oocyte with its cumulus oophorus cells is discharged from the ovary is known as ovulation. In the meantime the first meiotic division is complete, and the secondary oocyte starts its second meiotic division. During ovulation some woman have slight pain called middle pain, because it normally occurs near the middle of the menstrual cycle. Ovulation is also generally accompanied by a rise in basal temperature, which can be monitored to aid in determining when release of the oocyte occurs. Some women fail to ovulate because of a low concentration of gonadotropins. In these cases administration of an agent to stimulate gonadotropin release and hence ovulation can be employed. Although such drugs are effective, they often produce multiple ovulation, so that the risk of multiple births is 10 times as high in these women as in the general pregnancies.
二、胚前期的发育
THE DEVELOPMENT OF EMBRYO IN PREEMBRYONIC PERIOD OUTLINE: The period from fertilized ovum to the end of the eighth week is known as the preembryonic period. During this period the fertilized ovum undergoes a series of mitosis known as cleavage to produce more and more blastomeres. On the third day after fertilization the blastomeres increase to approximately 16. Thus the morula forms. At the end of the third day or the beginning of the fourth day the morula enters the cavity of the uterus and becomes blastocyst. By the sixth day the blastocyst begins to penetrates uterine mucosa, and completely embedded in the endometrial stroma by the 11th to 12th day. This process is called implantation. In the meantime the uterine endometrium becomes deciduas, and the trophoblast differentiates into cytotrophoblast and syncytiotrophoblast. The inner cell mass of the blastocyst also differentiates into two layers, the epiblast and hypoblast. These two layers form the bilaminar germ disc. At the same time a small cavity appears within the epiblast, and soon enlarges to form the amniotic cavity. The epiblast cells adjacent to cytotrophoblast become amnioblast. Both the amnioblast (wall) and the epiblast (floor)
envelop the amniotic cavity to constitute amnion. In the meantime a layer of flattened cells deriving from hypoblast form a thin membrane known as exocoelomic membrane or Heuser’s membrane. Both the exocoelomic membrane (wall) and the hypoblast (roof) envelop the original blastocyst cavity to form primary yolk sac. By the 11th to 12th day some cells derived from Heuser’s membrane or cytotrophoblast or epiblast (the exact origin if these cells is unknown yet) fill the space between the trophoblast and the Heuser’s membrane to form the extraembryonic mesoderm. Several small spaces appear within the extraembryonic mesoderm and then fuse into one cavity called extraembryonic coelom. As the extraembryonic coelom appears the extraembryonic mesoderm is separated into two layers, the extraembryonic somatopleuric mesoderm and the extraembryonic splanchnopleuric mesoderm. At this time the germ disc is located in the extraembryonic coelom and connected to the trophoblast by a bundle of extraembryonic mesoderm tissue known as connecting stalk or body stalk. By the end of the second week the hypoblast produces additional cells which migrate along the inside of the extraembryonic membrane and gradually form the secondary yolk sac. Thus the primary yolk sac is pushed away and disintegrates into extracoelomic vesicles.
（一）卵裂和胚泡形成Cleavage and Formation of Blastocyst
（二）植入Implantation
IVF-ET AND CONTRACEPTIVE METHOD
In vitro fertilization(IVF) and embryo transfer(ET) in human being is a frequent practice conducted by laboratories throughout the world. Follicle growth in the ovary is stimulated by administration of gonadotropins. Oocytes are recovered by laparoscopy from ovarian follicles with an aspirator just prior to ovulation. When the oocyte is in late stages of the first meiotic division. The egg is placed in a simple culture medium, and sperm are added immediately. Fertilized ova are monitored to the six-cell stage and then placed in the uterus to develop to term.
A disadvantage of IVF-ET is its low success rate, since only 20% of fertilized ova
implant and develop to term. Therefore, to increase chances of a successful pregnancy, four of five ova are collected, fertilized, and placed in the uterus. This approach sometimes leads to multiple births.
Barrier techniques of contraception mainly include the male condom which fits over the penis and the female condom which line the vagina, and other barriers placed in the vagina include the diaphragm, the cervical cap, and the contraceptive sponge. The contraceptive pill is a combination of estrogen and progesterone analogue progestin, which together inhibit ovulation but permit menstruation. Both hormone act at the level of FSH and LH, preventing their release from the pituitary. The pills are taken for 21 days and then stopped to allow menstruation, after which the cycle is repeated. The intrauterine device (IUD) is placed in the uterine cavity. Its mechanism for preventing pregnancy is not clear but may entail direct effects on sperm and oocyte or inhibition of pre-implantation stages of development. The vasectomy and tubule ligation are effective means of contraception, and both procedures are reversible, although not in every case.
（三）蜕膜和初级绒毛的形成Formation of Decidua and Primary Villus
1.蜕膜的形成(Formation of decidua)
2.初级绒毛的形成(Formation of primary villus)
（四）二胚层胚盘及相关结构的发生Development of Bilaminar Germ Disc and Relative Structures
1. 二胚层胚盘的发生(Development of bilaminar germ disc)
2. 羊膜囊和初级卵黄囊的形成(Formation of amnion and primary yolk sac)
3. 胚外体腔和次级卵黄囊的形成(Formation of extraembryonic coelom and secondary yolk sac)
三、胚期的发育
THE DEVELOPMENT OF EMBRYO IN EMBRYONIC PERIOD OUTLINE: The period from the beginning of the third week to the end of the eighth week is called embryonic period. The most characteristic events occurring during this period are formation of three germ layers and their differentiation. First of all the primitive streak and the primitive node originate from proliferating of epiblast. The
cells of the epiblast continue to proliferate and invaginate through primitive streak and node to form endoderm and mesoderm, and then the epiblast becomes ectoderm. Hence the epiblast gives rise to all three germ layers. After appearance of three germ layers, each of them gives rise to its own tissue and organs. As a result the major features of the body form are established. The ectoderm differentiates chiefly into the organs and structures that maintain contact with the outside world, such as peripheral nervous system; the sensory epithelium of ear, nose and eye; the epidermis, hair, nail, sweat gland and mammary gland; pituitary and enamel of teeth. The mesoderm differentiates firstly into paraxial mesoderm, intermediate mesoderm and lateral mesoderm. And then the paraxial mesoderm forms somitomeres which give rise to the mesenchyme of head and organize into somites. The somites differentiate into myotome, sclerotome and dermatome. Subsequently they differentiate into supporting tissue, such as muscle tissue, cartilage and bone, dermis and subcutaneous tissue. The endoderm provides the epithelial lining and glandular epithelium of gastrointestinal tract, respiratory tract and urinary bladder. The endoderm also gives rise to the parenchyma of the thyroid gland, parathyroid gland, liver and pancreas. The epithelial lining of tympanic cavity and auditory tube also originate from endoderm.
（一）三胚层的发生Development of Trilaminar Germ Disc
（二）脊索和尿囊的发生Development of Notochord and Allantois
1. 脊索的发生(Development of notochord)
2. 尿囊的发生(Development of allantois)
（三）绒毛膜的形成和演变Formation and Evolution of Chorion
（四）三胚层的分化Differentiation of Three Germ Discs
1. 外胚层的分化(Differentiation of ectoderm)
2. 中胚层的分化(Differentiation of mesoderm)
3. 内胚层的分化(Differentiation of endoderm)
（五）胚期胚胎外形的变化Changes of Embryonic Form in Embryonic Period
BIRTH DEFECTS ASSOCIATED WITH THE EMBRYONIC PERIOD Most major organs and structures are differentiated during the embryonic period. This period is critical for normal development and therefore is called the period of organogenesis. Stem cell populations are establishing each of the organ primordia, and these processes are sensitive to insult from genetic and environmental influences. Thus this period is when most gross structural birth defects are induced. Unfortunately, the mother may not realize she is pregnant during this critical time, especially during the third and fourth weeks, which are particularly vulnerable. Consequently, the mother may not avoid harmful influences, such as cigarette smoking and alcohol.
四、胎期的发育和胚胎龄的计算
THE DEVELOPMENT OF EMBRYO IN FETAL PERIOD AND THE
CALCULATION OF EMBRYONIC AGE
OUTLINE:The period from the beginning of the ninth week to the end of intrauterine life is known as the fetal period. During this period the characteristics of the development are chiefly maturation of tissues and organs and the rapid growth of the body. During the third, fourth and fifth month the fetal growth in length is particularly striking, approximately increasing in length 5cm per month, whereas the increasing in weight is most striking during the last 2 month of gestation, approximately increasing in weight 700g per month. A striking feature in growth of fetus during this period is the relative slowdown in growth of the head compared with the rest of the body. By the fifth month fetal movement can be clearly recognized by mother.
In general, the duration of pregnancy for a full-term fetus is considered to be 280 days after onset of the last menstruation, or more accurately, 266 days after fertilization. The embryonic or fetal age can be calculated by means of measuring the length of embryo or observing the outer feature of embryo.
（一）胎期的发育Development of Embryo in Fetal Period
（二）胚胎龄的推算Calculation of Embryonic Age
五、胎膜和胎盘
FETAL MEMBRANE AND PLACENTA
OUTLINE: Fetal membrane chiefly includes chorion, yolk sac, amnion, allantois and umbilical cord. All of them originate from blastocyst, having the same origin with embryo. But they don’t participate in the formation of embryo, as the fetus is delivered they will be separated from fetus.
The placenta has two components, the fetal portion formed by the chorion frondosum and the mother portion formed by deciduas basalis. After birth the placenta is discoid. On the maternal surface of the placenta there are 15 to 30 cotyledons covered by a thin layer of deciduas basalis, and the grooves between them are formed by decidual septa. The fetal surface of the placenta is covered by the amniotic membrane. On this surface attachment of the umbilical cord is usually eccentric and occasionally marginal, and a number of large blood vessels converge toward the umbilical cord. The main function of the placenta are exchange of metabolic and gaseous products between maternal and fetal blood streams, and production of hormones.
（一）绒毛膜Chorion
（二）卵黄囊Yolk Sac
（三）尿囊Allantois
（四）羊膜囊Amnion
（五）脐带Umbilical Cord
ABNORMALITIES OF UMBILICAL CORD AND AMNION Normally there are two arteries and one vein in the umbilical cord. In one of two hundred newborns, however, only one artery is present, and these babies have approximately a 20% chance of having cardiac and other vascular defects. The missing artery either fails to form(agenesis) or degenerates early in development. Occasionally, tears in the amnion result in amniotic bands that may encircle part of the fetus, particularly the limbs and digits. Amputation, ring constrictions, and other abnormalities, including craniofacial deformations, may result. Origin of the bands is probably from infection or toxic insults that involve either the fetus, fetal membranes, or both. Then bands form from the amnion, like scar tissue, constricting fetal structures.
（六）胎盘Placenta
1. 胎盘的形态结构(Form and structure of placenta)
2. 胎盘的血液循环和胎盘膜(Blood circulation of placenta and placenta barrier)
3. 胎盘的生理功能(Physiological functions of placenta)
PRENATAL DIAGNOSIS
Several approaches for assessing growth and development of the fetus in uterus are used. In combination of these technique are designed to detect malformations, chromosomal abnormalities, and overall growth of fetus. The least intrusive of these procedures is ultrasonography, which employs ultrasound to produce images of the placenta and offspring. Ultrasonic scans can determine size and position of the placenta and fetus, multiple births and malformations such as neural tube defects, cardiac and abdominal wall defects. Amniocentesis is also a frequently used approach which entails withdrawing amniotic fluid. A needle is inserted through the mother’s abdominal wall and uterus into the amniotic cavity. Approximately 20 to 30ml of fluid are withdrawn. This procedure is usually not performed prior to the 14th week of gestation. The fluid may be analyzed for α-fetoprotein which is present in high concentrations in the amniotic fluid of offspring with open neural tube defects and abdominal malformations. The fetal cells in the amniotic fluid can be grown in culture and analyzed for chromosomal abnormalities. Another technique for prenatal diagnosis is chorionic villus sampling which entails obtaining a small piece of chorionic willus tissue. This tissue contains numerous rapidly dividing fetal cells that are available for immediate analysis of chromosomal and biochemical defects. This procedure can be done early in pregnancy(8 weeks) and offer immediate analysis without waiting for cell cultures.
六、双胎、多胎和连体双胎
TWINS, MULTIPLE BIRTHS,CONJOINED TWINS
Outline: There are two types of twins, the dizygotic twins and the monozygotic twins.
Approximately two thirds of twins are dizygotic twins, and their incidence of 7 to 11 per 1000 births increases with maternal age. They result from simultaneous shedding of two oocyte and fertilization by different spermatozoa. Since the two zygotes have different genetic constitutions, the twins have no more resemblance than any other brothers or sisters. They may or may not be of different sex. The zygote implant individually in the uterus, and usually each develops its own placenta, amnion and chorion. Sometimes the two placentas fuse, and the two amnions also fuse. The monozygotic twins develop from a single fertilized ovum, having an incidence of 3 to 4 per 1000, resulting from splitting of zygote and various stages of development. The time of separation may occur at two-cell stage, or at the early blastocyst, or at the formation stage of the primitive streak.
The multiple births are rare, but in recent years have occurred more frequently in mothers given gonadotropins for ovulatory failure.
The conjoined twins result from incomplete separation of the monozygotic twins, having one amnion, one chorion, and one placenta.
（一）双胎Twins
（二）多胎Multiple Births
（三）连体双胎Conjoined Twins
TWIN DEFECTS
Twin pregnancies have a high incidence of perinatal mortality and morbidity and a tendency toward preterm delivery. Approximately 12% of premature infants are twins, and twins are usually small at birth. Low birth weight and prematurity place infants of twin pregnancies at great risk, and approximately 10 to 20% of them die, compared with only 2% of infants from single pregnancies.
The incidence of twinning may be much higher, since twins are conceived more often than they are born. Many twins die before birth, and some studies indicate that only 29% of women pregnant with twins actually give birth to two infants. The term vanishing twin refers to the death of one fetus. This disappearance, which occurs in the first trimester or early second trimester, may result from resorption or formation of a fetus papyraceus.
QUESTIONS FOR REVIEW
1.How many periods is the human embryonic developmental process divided? Point
out their names and durations.
2.How does the fertilization develop? Describe the time, position, process and
significance of the fertilization.
3.How does the gamete develop?
4.Describe the definition of cleavage and the microstructure of blastocyst.
5.Describe the definition, time and process of implantation.
6.What’s the germ disc? Describe the developmental process of the bilaminar germ
disc and trilaminar germ disc.
7.Describe the differentiation of the three germ disc briefly.
8.What’s the fetal membranes? How many types of fetal membranes are there?
Describe the development and evolution of each fetal membrane.
9.Describe the formation, structure and function of the placenta.