Embryology Review Notes PDF

Important Syndromes Due to Numerical Chromosomal Abnormalities: Trisomy of Autosomes ◦ Trisomy 21 (Down Syndrome) ◦ Trisomy 18 (Edwards Syndrome) ◦ Trisomy 13 (Patau Syndrome) Trisomy of Sex Chromosomes ◦ Klinefelter Syndrome (4...

Important Syndromes Due to Numerical Chromosomal Abnormalities: Trisomy of Autosomes ◦ Trisomy 21 (Down Syndrome) ◦ Trisomy 18 (Edwards Syndrome) ◦ Trisomy 13 (Patau Syndrome) Trisomy of Sex Chromosomes ◦ Klinefelter Syndrome (47, XXY) ◦ Triple X Syndrome (47, XXX) Monosomy ◦ Turner Syndrome (45, XO) 4o Klinefelter Syndrome: A Syndrome Due to Numerical Chromosomal Abnormalities Overview ◦ Klinefelter syndrome is a genetic condition caused by an extra X chromosome (47, XXY). ◦ It is often not diagnosed until adulthood. Chromosomal Complement ◦ Cells contain 47 chromosomes with a sex chromosomal pattern of XXY. Clinical Features ◦ Found only in males. ◦ Key features include: ▪ Sterility ▪ Testicular atrophy ▪ Hyalinization of seminiferous tubules ▪ Longer limbs ▪ Gynecomastia Detection ◦ Typically identi ed through amniocentesis. Triple X Syndrome: A Syndrome Due to Numerical Chromosomal Abnormalities Overview ◦ Caused by an extra copy of the X chromosome (47, XXX), occurring in oocytes or sperm. fi  ◦ Affects approximately 1 in 1,000 females. Symptoms and Clinical Features ◦ Many affected individuals experience no symptoms or only mild symptoms. ◦ Common features include: ▪ Infantile characteristics ▪ Scanty menses ▪ Some degree of mental retardation ▪ Problems with speech and self-esteem ◦ Physical features are often mild, leading to frequent underdiagnosis. Monosomy: A Syndrome Due to Numerical Chromosomal Abnormalities Turner Syndrome (45, X) ◦ The only monosomy compatible with life. ◦ Despite this, 98% of fetuses with Turner syndrome are spontaneously aborted. Clinical Features ◦ Survivors are unmistakably female in appearance. ◦ Key characteristics: ▪ Absence of ovaries ▪ Short stature ▪ Webbed neck ▪ Lymphedema of the extremities ▪ Skeletal deformities ▪ Broad chest with widely spaced nipples Note ◦ Associated with signi cant prenatal and physical abnormalities. Structural Chromosome Abnormalities Overview ◦ Structural abnormalities occur when a piece of a chromosome is lost due to a break. ◦ Infants with partial deletions exhibit abnormalities, such as in Cri-du-chat syndrome. Cri-du-chat Syndrome ◦ Cause: Partial deletion of the short arm of chromosome 5. ◦ Clinical Features: ▪ Cat-like cry ▪ Microcephaly ▪ Mental retardation ▪ Congenital heart disease fi  Microdeletion Syndromes Overview ◦ Caused by the deletion of a few contiguous genes. ◦ The effects depend on whether the deletion occurs on the maternal or paternal chromosome. Angelman Syndrome ◦ Cause: Microdeletion on the maternal chromosome 15. ◦ Clinical Features: ▪ Mental retardation ▪ Inability to speak ▪ Poor motor development ▪ Prone to unprovoked and prolonged periods of laughter Prader-Willi Syndrome ◦ Cause: Microdeletion on the paternal chromosome 15. ◦ Clinical Features: ▪ Hypotonia ▪ Obesity ▪ Mental retardation ▪ Hypogonadism ▪ Cryptorchidism Fragile X Syndrome Cause ◦ Mutation in the FMR1 gene (Fragile X Mental Retardation 1). ◦ Normally, the FMR1 gene produces a protein essential for proper brain growth. ◦ The mutation results in the production of too little or no protein. Clinical Features ◦ Mental retardation ◦ Large ears ◦ Prominent jaw ◦ Pale blue irises Affected Individuals ◦ Both boys and girls can be affected. ◦ Boys, with only one X chromosome, are more severely impacted by the mutation. Primordial Germ Cells (PGCs) De nition and Function fi  ◦ Primordial germ cells are the precursors to sperm and eggs. ◦ They are essential for generating new organisms capable of creating endless new generations through germ cells. Development ◦ PGCs mature to form male and female gametes. Gametogenesis: From Germ Cell Origin to Zygote Formation Fertilization ◦ A sperm fertilizes the ovum in the fallopian tube, forming a zygote. Zygote Development ◦ The zygote travels through the fallopian tube, developing into a blastocyst. ◦ The blastocyst contains a group of cells called the inner cell mass. Blastocyst Implantation ◦ The blastocyst implants into the endometrium of the uterus. ◦ The inner cell mass begins dividing and forms a two-layered disc. Formation of Layers ◦ The upper layer is called the epiblast. ◦ The lower layer is called the hypoblast. Development of Primordial Germ Cells Migration and Programming ◦ During the 2nd week of development, some cells of the epiblast are programmed to become primordial germ cells (PGCs). ◦ These cells migrate from the epiblast to the yolk sac, where they wait for activation signals. Activation and Migration ◦ During the 5th week, PGCs are activated and migrate from the yolk sac to the genital ridge. Proliferation in the Genital Ridge ◦ In the genital ridge, PGCs divide to increase their number. ◦ All cells at this stage are diploid, containing 44 autosomes and XY chromosomes. Formation of Gonads ◦ The somatic cells in the genital ridge, along with the primordial germ cells, contribute to the formation of the testes or ovaries. Development of Gametes  ◦ The PGCs reside in the testes or ovaries and develop into: ▪ Spermatogonia (in males) ▪ Ova (in females) ◦ Both gametes have 46 chromosomes and are diploid at this stage. Oogenesis De nition ◦ Oogenesis is the process by which oogonia differentiate into mature oocytes. Timeline ◦ The maturation of oocytes begins before birth. Differentiation ◦ After primordial germ cells (PGCs) arrive in the gonad, they differentiate into oogonia. Mitotic Divisions ◦ In the ovary, oogonia undergo several mitotic divisions. ◦ The majority of oogonia continue dividing by mitosis. Arrest in Meiosis ◦ Some oogonia arrest their cell division in prophase of meiosis I, becoming primary oocytes. Oogenesis: Further Development Rapid Increase and Degeneration ◦ In the months following their formation, oogonia increase rapidly in number. ◦ This is followed by signi cant cell death. By the Seventh Month ◦ Most oogonia have degenerated, except for a few located near the ovarian surface. Surviving Primary Oocytes ◦ All surviving primary oocytes have entered prophase of meiosis I. ◦ Most are individually surrounded by a layer of at follicular epithelial cells. Primordial Follicle Formation ◦ A primary oocyte, along with its surrounding at epithelial cells, is called a primordial follicle. Oogenesis: Primary Oocytes Prophase of Meiosis I fi fi fl fl  ◦ Near the time of birth, all primary oocytes have entered prophase of meiosis I. Number of Primary Oocytes ◦ At birth, the total number of primary oocytes is estimated to be between 600,000 and 800,000. Arrested State ◦ Primary oocytes remain arrested in prophase I until puberty. ◦ This arrested state is maintained by oocyte maturation inhibitor (OMI), a small peptide secreted by follicular cells. Decline During Childhood ◦ During childhood, most oocytes become atretic. ◦ By the beginning of puberty, only approximately 40,000 oocytes remain, and fewer than 500 will be ovulated. Oogenesis: Maturation of Oocytes at Puberty Initiation at Puberty ◦ Maturation of oocytes resumes at puberty. Follicle Selection and Development ◦ Each month, 15 to 20 follicles from the oocyte pool are selected for maturation. ◦ Some of these follicles degenerate, while others accumulate uid, entering the antral (vesicular) stage. Mature Vesicular Follicles ◦ Just before ovulation, the follicles swell signi cantly and are called mature vesicular follicles or Graa an follicles. Stages of Maturation ◦ The antral stage is the longest phase of maturation. ◦ The mature vesicular stage lasts approximately 37 hours prior to ovulation. Oogenesis: Preparation for Ovulation Entry into Meiosis II ◦ The oocyte enters meiosis II but arrests at metaphase, approximately 3 hours before ovulation. Completion of Meiosis II ◦ Meiosis II is completed only if the oocyte is fertilized. ◦ If fertilization does not occur, the oocyte degenerates approximately 24 hours after ovulation. Outcome of Oogenesis fi fi fl  ◦ Upon completion of oogenesis (meiosis II): ▪ One viable mature oocyte is produced. ▪ Three polar bodies are formed. Continuation to Menopause ◦ Oogenesis continues until menopause, marking the permanent cessation of the menstrual cycle. Comparison of Oogenesis and Spermatogenesis Oogenesis Primary Oocyte (2n) ◦ Begins meiosis I. ◦ Once per month, a primary oocyte divides asymmetrically to form: ▪ Secondary Oocyte (n) ▪ First Polar Body (n) Secondary Oocyte (Oocyte II) ◦ Enters meiosis II but arrests at metaphase until fertilization. Fertilization ◦ If fertilized: ▪ Meiosis II completes, producing: ▪ Ootid → Ovum (n) ▪ Second Polar Body (n) ▪ The rst polar body may divide to form two additional polar bodies. Outcome ◦ One viable ovum (n). ◦ Two to three polar bodies (n), which degenerate. Spermatogenesis Primary Spermatocyte (2n) ◦ Undergoes meiosis I, forming: ▪ Two Secondary Spermatocytes (n) Secondary Spermatocytes ◦ Undergo meiosis II, producing: ▪ Four Spermatids (n) Spermatids ◦ Mature into functional sperm (n) through spermiogenesis. End Products fi  Oogenesis: One ovum and polar bodies. Spermatogenesis: Four functional sperm cells. Fertilization The sperm (n) fertilizes the ovum (n), forming a zygote (2n), which begins development into an embryo. Ovulation: Ovarian Cycle and Preparation Initiation of Cycles at Puberty ◦ At puberty, females begin to experience regular monthly cycles, known as sexual cycles. ◦ These cycles are controlled by Gonadotropin-Releasing Hormone (GnRH), released from the hypothalamus. Hormonal Control ◦ GnRH stimulates the anterior pituitary gland to secrete: ▪ Follicle-Stimulating Hormone (FSH) ▪ Luteinizing Hormone (LH) ◦ FSH and LH regulate and control the cyclic changes in the ovary. Follicular Growth ◦ At the beginning of each ovarian cycle, 15 to 20 primary-stage (preantral) follicles are stimulated to grow under the in uence of FSH. Zygote Formation Fusion of Pronuclei ◦ The de nitive oocyte contains the female pronucleus, which has 23 chromosomes. ◦ The sperm's tail detaches, and its nucleus enters the cytoplasm of the oocyte as the male pronucleus, also carrying 23 chromosomes. ◦ The male and female pronuclei: ▪ Are morphologically indistinguishable. ▪ Move close to each other, unite, and restore the diploid number of chromosomes. ◦ This union results in the formation of a zygote. End Results of Fertilization ◦ Restoration of the diploid number of chromosomes. ◦ Determination of the sex of the new individual. ◦ Initiation of cleavage and division of the zygote. Cleavage and Blastocyst Formation fi fl  Cleavage and Morula Formation ◦ The zygote divides into two cells, and cleavage continues until a morula is formed. ◦ The morula consists of 16 cells. Differentiation of the Morula ◦ The cells of the morula differentiate into: ▪ Outer cell mass ▪ Inner cell mass Formation of the Blastocyst ◦ Intercellular spaces form within the morula, eventually uniting to create a single cavity called the blastocele. ◦ At this stage, the embryo is called a blastocyst. Entry into the Uterine Cavity ◦ The blastocyst enters the uterine cavity but remains surrounded by the zona pellucida, which prevents implantation. Hatching of the Blastocyst ◦ The blastocyst digests the zona pellucida using trophoblastic enzymes and emerges in a process called hatching. Structure of a Mature Blastocyst ◦ A mature blastocyst consists of: ▪ Inner cell mass (embryoblast) ▪ Outer cell mass (trophoblast) ▪ Blastocele Preparation for Implantation ◦ The outer trophoblast of the blastocyst comes into direct contact with the super cial cells of the endometrium, enabling implantation. Stages of Human Embryo Development Leading to Blastocyst Hatching 1. Zygote Formation ◦ The fertilized ovum begins its journey of development. 2. Early Cleavage Stages ◦ The zygote divides into 2 cells, then 4 cells, and continues dividing to form a morula (16 cells). 3. Morula Formation ◦ The morula differentiates into an inner cell mass and outer cell mass as it continues to develop. fi  4. Blastocyst Formation ◦ A cavity called the blastocele forms within the morula, resulting in the formation of a blastocyst. ◦ The blastocyst consists of: ▪ Inner cell mass (embryoblast) ▪ Outer cell mass (trophoblast) ▪ Blastocele 5. Hatching of the Blastocyst ◦ The blastocyst digests the zona pellucida using trophoblastic enzymes. ◦ It emerges from the zona pellucida in a process known as hatching, allowing implantation to occur. 6. Ready for Implantation ◦ After hatching, the outer trophoblast is in direct contact with the endometrium, enabling implantation and the next stage of development. Blastocyst Formation: Key Poles Embryonic Pole ◦ The side of the blastocyst where the inner cell mass is attached. Abembryonic Pole ◦ The side of the blastocyst opposite to the inner cell mass. End of the 1st Week of Development Key Structures ◦ The embryo consists of: ▪ Inner Cell Mass (Embryoblast): ▪ Forms the embryo proper. ▪ Outer Cell Mass (Trophoblast): ▪ Contributes to the formation of the placenta. Further Development of the Embryoblast Differentiation of the Embryoblast ◦ The embryoblast differentiates into: ▪ Dorsal Epiblast Layer (columnar cells) ▪ Ventral Hypoblast Layer (cuboidal cells) Formation of the Amniotic Cavity ◦ Within the epiblast, clefts begin to form. ◦ These clefts eventually coalesce to create the amniotic cavity.  ◦ The amniotic cavity will later ll with amniotic uid. Further Development of the Embryoblast Amnioblasts ◦ Epiblast cells adjacent to the cytotrophoblast are referred to as amnioblasts. Formation of the Exocoelomic Membrane ◦ Hypoblast cells migrate and line the blastocyst cavity, forming the exocoelomic membrane. Exocoelomic Cavity (Primary Yolk Sac) ◦ The blastocyst cavity is now called the exocoelomic cavity or primary yolk sac. Bilaminar Disc Formation ◦ The epiblast and hypoblast together form a at structure called the bilaminar disc. Week 2 Summary: Key Developmental Events Embryoblast Development Epiblast: ◦ Forms the amniotic cavity through cavitation. ◦ Contains amniotic cells. Hypoblast: ◦ Thickens at one end to form the anterior visceral endoderm (AVE). ◦ Contributes to lining the primitive yolk sac, which later forms the secondary yolk sac (yolk sac). Blastocyst Formation The blastocyst cavity develops and differentiates further. Trophoblast Development Cytotrophoblast: ◦ Forms the structural core of primary chorionic villi. Syncytiotrophoblast: ◦ Synthesizes hCG. ◦ Creates lacunae that connect into lacunar networks. ◦ Invades endometrial sinusoids, allowing maternal blood ow into lacunae. ◦ Establishes primitive uteroplacental circulation. Extraembryonic Mesoderm Formation fi fl fl fl  Forms from cells surrounding the trophoblast and blastocyst cavity. Cavitation: ◦ Results in the formation of the chorionic cavity. Differentiates into: ◦ Somatic Extraembryonic Mesoderm ◦ Splanchnic Extraembryonic Mesoderm This outlines the major events and structures formed during the second week of development. Clinical Considerations: Human Chorionic Gonadotropin (hCG) Overview ◦ hCG is a glycoprotein produced by the syncytiotrophoblast. Function ◦ Stimulates the production of progesterone by the corpus luteum, supporting pregnancy. Detection ◦ Can be assayed in: ▪ Maternal blood: Detectable by day 8. ▪ Maternal urine: Detectable by day 10. ◦ Forms the basis of pregnancy testing and remains detectable throughout pregnancy. Clinical Implications ◦ Low hCG Levels: ▪ May predict a spontaneous abortion. ▪ May indicate an ectopic pregnancy. ◦ High hCG Levels: ▪ May suggest a multiple pregnancy. ▪ Could indicate a hydatidiform mole. ▪ May be a sign of gestational trophoblastic neoplasia. Gestational Trophoblastic Disease Hydatidiform Mole (Molar Pregnancy) Description: ◦ Benign enlargement of chorionic villi (trophoblast). ◦ Marked by grapelike vesicles in the uterus. ◦ Absence of an embryo.  Clinical Marker: ◦ Associated with high hCG levels. Gestational Trophoblastic Neoplasia (Choriocarcinoma) Description: ◦ Malignant tumor of the trophoblast. ◦ May occur following: ▪ Normal or ectopic pregnancy. ▪ Abortion. ▪ Hydatidiform mole. Metastasis: ◦ May spread to the liver and other organs. Prognosis: ◦ Generally poor. Oncofetal Antigens De nition ◦ Cell surface antigens (proteins) that are normally present during fetal development. ◦ For unknown reasons, they re-express themselves in human malignant cells. Clinical Relevance ◦ Used as tumor markers for: ▪ Diagnosis ▪ Treatment prognosis Examples of Oncofetal Antigens ◦ Alpha-Fetoprotein (AFP): ▪ Produced by hepatocellular carcinoma and some germ cell tumors. ◦ Carcinoembryonic Antigen (CEA): ▪ Elevated in individuals with colon cancer. ◦ Beta 2 Microglobulin: ▪ Associated with multiple myeloma. Gastrulation: Formation of the Trilaminar Embryonic Disc De nition ◦ Gastrulation is the process by which the bilaminar disc is converted into a trilaminar embryonic disccomposed of: fi fi  ▪ Ectoderm ▪ Mesoderm ▪ Endoderm Initiation of Gastrulation ◦ Begins with the formation of the primitive streak in the epiblast. Epiblast Differentiation ◦ The epiblast differentiates into: ▪ Endoderm ▪ Mesoderm ◦ After differentiation, the remaining epiblast is renamed ectoderm. Formation of the Primitive Streak De nition ◦ The primitive streak is a linear band of thickened epiblast formed by the proliferation and migration of epiblast cells toward the midline. Cellular Regulation ◦ Cell migration, invagination, and speci cation in the primitive streak are regulated by FGF8, synthesized by the streak cells themselves. Primitive Streak Development ◦ As the primitive streak elongates, its cranial end develops into the primitive node. ◦ A groove appears in the primitive streak and primitive nodes, changing their names to: ▪ Primitive groove ▪ Primitive pit Signi cance ◦ The formation of the primitive groove marks the beginning of gastrulation. Functions and Fate of the Primitive Streak Functions of the Primitive Streak 1. Determination of the Site of Gastrulation ◦ Marks the location where gastrulation occurs. 2. Initiation of Germ Layer Formation ◦ Plays a key role in forming the ectoderm, mesoderm, and endoderm. 3. De ning Major Body Axes ◦ Establishes the cranial-caudal, dorsal-ventral, and left-right axes of the embryo. Fate of the Primitive Streak fi fi fi fi  1. Normal Regression ◦ The primitive streak gradually diminishes in size. ◦ By the end of the 4th week, it becomes an insigni cant structure in the sacrococcygeal region and disappears. 2. Clinical Signi cance ◦ If remnants of the primitive streak persist, they can lead to the formation of a sacrococcygeal teratoma, the most common tumor in newborns. Clinical Correlates: Tumors and Birth Defects Associated with Gastrulation Tumors Associated with Gastrulation Sacrococcygeal Teratomas: ◦ Caused by remnants of the primitive streak persisting in the sacrococcygeal region. ◦ These clusters of pluripotent cells proliferate to form tumors. ◦ Tumors commonly contain tissues derived from all three germ layers (ectoderm, mesoderm, and endoderm). Birth Defects Associated with Laterality Situs Inversus: ◦ A condition where the positioning of all organs is reversed, forming a mirror image arrangement. ◦ Example: Cardiac inversions, where the two ventricles of the heart are reversed. Clinical Correlates: Teratogenesis Associated with Gastrulation Sensitivity of the Third Week ◦ The third week of development, when gastrulation begins, is a highly sensitive stage for teratogenic insult. ◦ Fate maps for various organ systems can be damaged by teratogens during this period. Alcohol-Induced Teratogenesis ◦ In animal models, high doses of alcohol during gastrulation can kill cells in the anterior midline of the germ disc. ◦ This results in a midline de ciency in craniofacial structures, leading to a condition known as holoprosencephaly. Characteristics of Holoprosencephaly ◦ Small forebrain ◦ Merged lateral ventricles forming a single ventricle fi fi fi  ◦ Hypotelorism: Eyes are abnormally close together Development of Primordial Germ Cells (LO-2) Migration During the 2nd Week ◦ Some cells of the epiblast, programmed to become primordial germ cells (PGCs), migrate to the yolk sacand remain there awaiting signals for activation. Activation During the 5th Week ◦ PGCs are activated and migrate from the yolk sac to the genital ridge. Division in the Genital Ridge ◦ In the genital ridge, PGCs divide to increase in number. ◦ All germ cells at this stage are diploid (44 autosomes and XY chromosomes). Formation of Testis or Ovary ◦ The somatic cells of the genital ridge, along with the primordial germ cells, contribute to the formation of the testis or ovary. Final Differentiation ◦ PGCs reside in the male or female gametes (testis or ovary) and differentiate into: ▪ Spermatogonia in males ▪ Ova in females ◦ Both have 46 chromosomes and diploid DNA. Neurulation: Neural Tube Formation (LO-1) De nition ◦ Neurulation is the process by which the neural plate forms the neural tube. Initial Changes ◦ The neural plate and body axis lengthen. ◦ Lateral edges of the neural plate elevate, forming neural folds. ◦ The depressed midregion of the neural plate forms the neural groove. Fusion of Neural Folds ◦ The neural folds gradually approach each other and fuse at the midline, forming the neural tube. ◦ Fusion begins in the cervical region ( fth somite) and proceeds both cranially and caudally. Communication with the Amniotic Cavity ◦ The cephalic (cranial) and caudal ends of the neural tube communicate with the amniotic cavity via: ▪ Anterior (cranial) neuropore fi fi  ▪ Posterior (caudal) neuropore Neurulation: Completion and Induction of the Neural Tube (LO-1) Communication with the Amniotic Cavity ◦ The neural tube communicates with the amniotic cavity via: ▪ Anterior (cranial) neuropore ▪ Posterior (caudal) neuropore Closure of Neuropores ◦ Cranial neuropore: Closes around day 25 (18- to 20-somite stage). ◦ Posterior neuropore: Closes around day 28 (25-somite stage). Completion of Neurulation ◦ Neurulation concludes with the formation of the central nervous system as a closed tubular structure: ▪ Narrow caudal portion: Forms the spinal cord. ▪ Broader cephalic portion: Develops into the brain vesicles. Induction of the Neural Plate ◦ Fibroblast Growth Factor (FGF) signaling is upregulated. ◦ Bone Morphogenic Protein (BMP4) activity is inhibited, leading to the induction of the neural plate. Neural Tube Defects (LO) De nition ◦ Neural tube defects occur when the neural tube fails to close during development. Types of Neural Tube Defects ◦ Anencephaly: ▪ Results from failure of neural tube closure in the cranial region. ◦ Spina Bi da: ▪ Occurs when closure fails anywhere from the cervical region caudally. ▪ Most commonly affects the lumbosacral region. Prevention ◦ Folic acid (a type of vitamin B) taken before and during pregnancy can prevent most neural tube defects. Placenta De nition ◦ The placenta is a temporary organ formed in the uterus during pregnancy. fi fi fi  Function ◦ Attaches to the uterine wall. ◦ Provides nutrients and oxygen to the baby through the umbilical cord. Structure ◦ The placenta has two main parts: 1. Maternal Part: Decidua basalis 2. Fetal Part: Chorion frondosum Maternal Part of the Placenta: Decidua De nition ◦ The uterine endometrium after implantation is referred to as the decidua. ◦ The decidua is shed or sloughed off after childbirth. Types of Decidua ◦ Decidua Basalis ▪ The portion of the decidua where the placenta forms. ◦ Decidua Capsularis ▪ The part of the decidua that separates the embryo from the uterine lumen. ◦ Decidua Parietalis (Vera) ▪ The part that lines the rest of the uterine cavity. Fetal Part of the Placenta: Chorion Frondosum Formation of Chorionic Villi ◦ The functional unit of the placenta is called the villi. ◦ Initially, villi form as offshoots from the entire surface of the trophoblast (chorion). Differentiation of Villi ◦ Chorion Laeve: ▪ Villi associated with the decidua capsularis degenerate. ▪ The chorion in this region becomes smooth and is called the chorion laeve. ◦ Chorion Frondosum: ▪ Villi associated with the decidua basalis proliferate to form the placenta. ▪ This part of the chorion is called the chorion frondosum. Twinning: Dizygotic (Fraternal) Twins (LO-8) De nition ◦ Dizygotic twins result from the fertilization of two different secondary oocytes by two different sperms. fi fi  Incidence ◦ Approximately 90% of twins are dizygotic. ◦ The incidence increases with maternal age. Genetic Characteristics ◦ The two zygotes have entirely different genetic constitutions. ◦ They may or may not be of the same sex. Development ◦ Each zygote implants individually in the uterus. ◦ Typically, each twin develops: ▪ Its own placenta ▪ Its own amnion ▪ Its own chorionic sac Twinning: Monozygotic (Identical) Twins De nition ◦ Monozygotic twins arise from the fertilization of one secondary oocyte by one sperm. Formation ◦ The resulting zygote splits at different stages of development to form two embryos. Genetic Characteristics ◦ Monozygotic twins are genetically identical. Splitting of the Zygote: Stages and Outcomes Splitting at the Two-Cell Stage Characteristics: ◦ Two placentas ◦ Two chorionic sacs ◦ Two amniotic sacs Incidence: ◦ Occurs in 35% of cases. ◦ Development resembles dizygotic twins. Splitting at the Early Blastocyst Stage Characteristics: ◦ One placenta fi  ◦ One chorionic sac ◦ Two amniotic sacs Incidence: ◦ Occurs in 65% of cases. Splitting at the Bilaminar Germ Disc Stage Characteristics: ◦ Incomplete division of the inner cell mass. ◦ One placenta ◦ One chorionic sac ◦ One amniotic sac Outcome: ◦ The fetuses remain united, leading to conjoined twins. Vanishing Twins De nition ◦ A vanishing twin refers to the death of one fetus during the rst or early second trimester. ◦ The fetus may undergo resorption or become compressed and mummi ed, forming a fetus papyraceus. Timing ◦ Typically occurs in the rst trimester or early second trimester. Cause ◦ Often associated with Twin Transfusion Syndrome, where unequal blood supply affects one twin's viability. Outcome ◦ The deceased twin is either resorbed or compressed into a at, mummi ed structure. Twin Transfusion Syndrome De nition ◦ A condition that occurs in monozygotic twins sharing a common placenta. Cause ◦ Vascular placental anastomoses: ▪ Connecting blood vessels within the shared placenta allow blood to pass from one twin to the other. ▪ This results in one twin receiving more blood than the other. fi fi fi fi fl fi fi  Outcome ◦ Unequal blood supply leads to disparities in growth and development between the twins. Striated Skeletal Muscles Migration and Formation ◦ Cells from the VLL region migrate into the parietal layer of the lateral plate mesoderm to form: ▪ Infrahyoid muscles (neck muscles) ▪ Abdominal wall muscles: ▪ Rectus abdominis ▪ Internal and external oblique ▪ Transversus abdominis ▪ Limb muscles (abaxial or hypaxial muscles) Remaining Myotome Cells ◦ Cells that remain in the myotome form: ▪ Back muscles ▪ Shoulder girdle muscles ▪ Intercostal muscles (primaxial or epaxial muscles) Epaxial Muscles in Humans: ◦ Erector spinae ◦ Transversospinales ◦ Splenius ◦ Suboccipital muscles Innervation ◦ Each myotome receives innervation from spinal nerves derived from the same segment as the muscle cells. Developmental Origin of Muscles from Abaxial and Primaxial Precursors Cervical Region Primaxial: ◦ Scalenes ◦ Geniohyoid ◦ Prevertebral Abaxial: ◦ Infrahyoid  Thoracoabdominal Region Primaxial: ◦ Intercostals Abaxial: ◦ Pectoralis major and minor ◦ External oblique ◦ Internal oblique ◦ Transversus abdominis ◦ Sternalis ◦ Rectus abdominis ◦ Pelvic diaphragm Upper Limb Primaxial: ◦ Rhomboids ◦ Levator scapulae ◦ Latissimus dorsi Abaxial: ◦ Distal limb muscles Lower Limb Abaxial: ◦ All lower limb muscles (The precise origin of muscles in the pelvic and lower limb regions is mostly abaxial in origin.) Limb Defects Amelia ◦ Complete absence of one or more extremities. Meromelia ◦ Partial absence of one or more extremities. Micromelia ◦ All segments of the extremities are present but abnormally short. Brachydactyly ◦ Digits are shortened. Syndactyly ◦ Two or more ngers or toes are fused. fi  Polydactyly ◦ Presence of extra ngers or toes. Polydactyly De nition ◦ Polydactyly is a condition where a baby is born with one or more extra ngers. Cause ◦ Results from abnormal duplication of the Zone of Polarizing Activity (ZPA). Limb Defects: Cleft Hand and Foot (LO-10) De nition ◦ Cleft hand and foot involve an abnormal cleft between the second and fourth metacarpal/metatarsal bones and the surrounding soft tissues. Characteristics ◦ The third metacarpal and phalangeal bones are almost always absent. ◦ The thumb and index nger may be fused. ◦ The fourth and fth ngers may also be fused. Limb Defects: Arthrogryposis De nition ◦ Arthrogryposis refers to congenital joint contractures, usually involving more than one joint. Causes ◦ Neurological Defects: ▪ Motor horn cell de ciency ▪ Meningomyelocele ◦ Muscular Abnormalities: ▪ Myopathies ▪ Muscle agenesis ◦ Joint and Contiguous Tissue Problems: ▪ Synostosis ▪ Abnormal development Associated Condition ◦ Clubfoot may result from arthrogryposis. Limb Defects: Amniotic Bands fi fi fi fi fi fi fi fi fi  De nition ◦ Amniotic bands can cause ring constrictions and amputations of limbs or digits. Cause ◦ The exact origin is unclear, but they may result from: ▪ Adhesions between the amnion and affected fetal structures. ▪ Tears in the amnion that detach and surround parts of the fetus. Clinical Correlates: Preeclampsia De nition ◦ Preeclampsia is a condition characterized by maternal hypertension and proteinuria caused by reduced organ perfusion. ◦ It occurs in approximately 5% of pregnancies. Onset and Timing ◦ Begins suddenly between 20 weeks’ gestation and term. Potential Outcomes ◦ Fetal growth retardation ◦ Fetal death ◦ Maternal death Progression to Eclampsia ◦ Preeclampsia may progress to eclampsia, which is characterized by seizures. Signi cance ◦ It is a leading cause of maternal mortality in the United States. fi fi fi