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Questions and Answers
What are the three major stages of prenatal development?
What is gametogenesis primarily responsible for?
During which weeks does the pre-embryonic period occur?
Which process describes the formation of a blastocyst?
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What structure is formed when the bilaminar embryonic disc develops?
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Which of the following is NOT an extraembryonic membrane formed during week two?
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What happens during the process of gastrulation?
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Which of the following germ layers gives rise to the nervous system?
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During which weeks is the embryo most susceptible to teratogenic effects?
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What is the result of meiosis in oogenesis?
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What does the term 'haploid' refer to in the context of gametes?
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What occurs at the centromere during meiosis?
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Which term describes the earliest stage of human development?
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What is formed from the mesoderm during embryonic development?
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What structure does the paraxial mesoderm primarily develop into?
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What is the primary function of intermediate mesoderm?
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Which layer of mesoderm is responsible for forming the intraembryonic coelom?
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What does the endoderm primarily give rise to?
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What is the primary outcome of craniocaudal folding during embryonic development?
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Which of the following defines transverse folding in embryonic development?
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During which weeks of prenatal development is the embryo most susceptible to teratogens?
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What structure is formed from the somatic layer of lateral plate mesoderm?
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Which membranes are formed where ectoderm fuses directly with endoderm?
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What is the primary role of teratogens in embryonic development?
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How many somites typically develop from days 26 to 32?
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From which germ layer do the primary derivatives arise?
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What is established as a signaling hub that initiates neurulation?
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What initiates the release of gonadotropins from the pituitary gland during the ovarian cycle?
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What structure does the follicle transform into after ovulation?
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Which of the following occurs during the cleavage process following fertilization?
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What is the primary outcome of the bilaminar disc formation during implantation?
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What is the role of hCG during the early stages of pregnancy?
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Which of the following terms describes the transformation of the embryoblast during implantation?
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What primarily occurs during the gastrulation process in embryonic development?
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What is formed from the mesoderm during embryonic development?
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Which embryonic structure is responsible for the future formation of the central nervous system?
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What represents the main anatomical organization established during gastrulation?
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What happens to the neural folds during neurulation?
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What are the two layers that differentiate in the trophoblast during implantation?
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What primarily occurs with the lacunar network in the developing embryo?
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In which stage of prenatal development does the formation of the amnionic cavity occur?
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Study Notes
Stages of Prenatal Development
- Three stages: Pre-embryonic, Embryonic, Fetal
- Most visible development occurs during the embryonic period
Gametogenesis
- Definition: Germ cells transform into gametes
- Meiosis: Chromosome number halves (46 diploid → 23 haploid)
-
Gametes: Sex cells capable of reproduction
- Male: Sperms/Spermatozoa
- Female: Oocytes
Spermatogenesis
- Spermatogonia: Diploid germ cells transforming into primary spermatocytes during puberty
- Meiosis I: Two haploid secondary spermatocytes
- Meiosis II: Four haploid spermatids
- Spermiogenesis: Spermatids elongate and mature into sperms
Oogenesis
- Oogonia: Diploid cells surrounded by a follicular layer
- Meiosis I: Begins before birth, arrests in prophase, and completes monthly beginning at puberty, forming one secondary oocyte and one polar body
- Meiosis II: Begins at ovulation, arrests at metaphase, and completes upon sperm penetration, forming one fertilized oocyte and one polar body
Ovarian Cycle
- Begins at puberty, terminates at menopause
-
Hypothalamus: Stimulates the pituitary gland to release gonadotropins
- FSH: Stimulates follicular maturation
- LH: Stimulates estrogen production and ovulation
- Ovulation: Follicle becomes corpus luteum, secreting estrogen and progesterone, thickening the endometrium of the uterus
Pre-embryonic Period
- Week 1: Fertilization, cleavage, blastocyst formation, implantation
- Week 2: Bilaminar disc forms, lacunae form, extraembryonic mesoderm develops, hCG secretion begins
Week 1
- Fertilization: Union of genetic material, beginning when a capacitated sperm penetrates a secondary oocyte
- Cleavage: Repeated mitotic divisions of the zygote, increasing the number of blastomeres, forming a morula
- Blastocyst Formation: Development of a fluid-filled cavity within the morula, pushing the inner-cell mass to one side, forming the inner cell mass/embryoblast and the trophoblast
- Implantation: Blastocyst embeds in the uterine endometrium, the trophoblast differentiates into two layers: cytotrophoblast and syncytiotrophoblast
Week 2
- Bilaminar Embryonic Disc: Embryoblast differentiates into a bilaminar disc (epiblast and hypoblast)
- Amnionic Cavity: Forms within the epiblast, amnioblasts surround the amniotic cavity forming the amnion
- Primary Umbilical Vesicle: Cells continuous with the hypoblast line the cytotrophoblast forming the primary umbilical vesicle
- Lacunar Network: Syncytiotrophoblast erodes uterine blood vessels forming lacunae, allowing diffusion of nutrients and gases
- Extraembryonic Mesoderm: Cells migrate between the exocoelomic membrane and cytotrophoblast forming spaces, combining to form the future chorionic cavity
- Prechordal Plate: Localized thickening of the hypoblast, an organizing center for head structures
Human Chorionic Gonadotropin (hCG)
- Produced by the syncytiotrophoblast at the end of Week 2
- Maintains the corpus luteum of the ovary, responsible for progesterone and estrogen production
Early Embryonic Period
- Week 3: Gastrulation, neurulation, intraembryonic mesoderm differentiation
- Week 4-8: Organogenesis
Gastrulation
- Bilaminar disc → trilaminar disc (ectoderm, endoderm, mesoderm)
- Establishment of body plan: Cranial-caudal axis, left-right sides
- Begins with the appearance of the primitive streak
Primitive Streak
- Epiblast cells migrate to the median plane
- Primitive node forms, marking the cranial side of the primitive streak
- Invagination of cells creates the primitive groove
Trilaminar Disc
- Ectoderm: Epithelial tissues derived from the epiblast
- Endoderm: Epithelial tissues derived from the hypoblast
- Mesoderm: Mesenchymal or epithelial tissues derived from cells migrating between the ectoderm and endoderm
Notochord Formation
- Migrating mesenchymal cells form the notochord process
- Notochord process binds to the underlying endoderm
- Fused layers degenerate leaving the notochord plate
- Notochord plate folds inward to form the notochord
Neurulation
- Notochord signaling induces the neural plate
- Neural Plate: Thickening of the overlying ectoderm (neuroectoderm)
- Neural Groove: Neural plate invaginates, forming the neural groove and neural folds
- Neural Tube: Neural folds meet in the midline, forming the neural tube
Closure of the Neural Tube
- Begins in the cervical region and occurs bidirectionally
- Complete by the end of Week 4
- Incomplete closure can result in anencephaly or spina bifida
Neural Crest Cells
- Undergo epithelial-to-mesenchymal transition
- Migrate away from the neural folds, forming ganglia, Schwann cells, pharyngeal arches and their derivatives, leptomeninges, melanocytes, odontoblasts, suprarenal medulla
Ectoderm Summary
- Derived from the epiblast
- Neuroectoderm: Thickened region over the notochord that becomes the neural tube and neural crest
- Surface Ectoderm: Becomes the epidermis
Intraembryonic Mesoderm
- Germ layer between the ectoderm and endoderm
- Derived from the epiblast
- Differentiates into three parallel columns:
- Paraxial Mesoderm: Forms somites
- Intermediate Mesoderm: Forms kidneys & gonads
- Lateral Plate Mesoderm: Forms the heart, blood vessels, and lining of body cavities
Paraxial Mesoderm
- Forms paired columns flanking the notochord
- Columns condense and segment into somites around day 22, when the embryo is about 2.5 mm
- Development of 38-39 somites occurs craniocaudally between days 26 and 32
- Somites give rise to:
- Sclerotome: vertebrae, ribs, posterior skull
- Myotome: skeletal muscles of limbs and trunk
- Dermatome: skin over the back and limbs
Intermediate Mesoderm
- Continuous laterally with the paraxial mesoderm
- Precursor to components of the urinary and reproductive systems
- Forms:
- Kidneys and ureters
- Gonads, reproductive ducts, glands
- Uterus
Lateral Plate Mesoderm
- Extends laterally and connects with extraembryonic mesoderm
- Covers the umbilical vesicle and amnion
- Divides into two layers:
- Somatic (parietal) mesoderm
- Splanchnic (visceral) mesoderm
- Transverse folding fuses the right and left sides in the midline
- Intraembryonic coelom forms (future thoracic and abdominopelvic cavities)
Mesoderm Summary
- Derived from the epiblast
- Paraxial mesoderm gives rise to somites
- Intermediate mesoderm forms urogenital structures
- Lateral plate mesoderm splits into somatic and splanchnic layers
Endoderm
- Germ layer surrounding the umbilical vesicle
- Derived from the epiblast
- Forms the lining of the gut tube
- Most internal organs develop from the gut tube
- Divided into three sections:
- Foregut, connected to the oropharyngeal membrane
- Midgut, connected to the secondary umbilical vesicle
- Hindgut, connected to the cloacal membrane
Oropharyngeal & Cloacal Membranes
- Locations where ectoderm directly fuses with endoderm
- Oropharyngeal membrane is at the cranial end:
- Future site of the mouth
- Connects to the foregut
- Cloacal membrane is at the caudal end:
- Future site of the anal and urogenital openings
- Connects to the hindgut
Week 4 Embryonic Development
- Characterized by embryonic folding:
- Neural tube closure
- Conversion of elongated disc into curved cylinder driven by differential growth
Embryonic Folding
- Results in the formation of a curved cylinder from the elongating disc
- Driven by differential growth along the longitudinal axis and lateral parts of the disc
- Craniocaudal and transverse folding occur concurrently
Craniocaudal Folding
- Initiated by rapid growth of the neural tube
- Oropharyngeal and cloacal membranes migrate to the ventral surface
- The endoderm of the umbilical vesicle is internalized, establishing the foregut, midgut, and hindgut
Transverse Folding
- Driven by rapid growth of the somites
- Bilateral edges of the conceptus fuse at the midline
- Splanchnic mesoderm and endoderm fusion forms the gut tube
- Somatic mesoderm and surface ectoderm fusion forms the body wall, except at the vitelline duct
Weeks 3 & 4 Summary
- Gastrulation establishes body planes and generates the three germ layers
- Ectoderm, mesoderm, endoderm
- Notochord acts as a signaling center involved in neurulation
- Neurulation establishes the nervous system and neural crest cells
- Intraembryonic mesoderm differentiates into three paired columns:
- Paraxial, intermediate, lateral plate
- Endoderm forms the lining of the gut tube
- Oropharyngeal and cloacal membranes are formed
- The conceptus transitions from a disc to a tube via folding:
- Craniocaudal folding internalizes thoracoabdominal structures
- Transverse folding forms the gut tube, body wall, and surrounds the conceptus with the amnionic cavity
Teratogens
- Most infant deaths result from birth defects caused by genetic factors, environmental factors, or a combination
- Teratogens are substances, organisms, or physical agents that disrupt normal development
- Consequences of teratogen exposure:
- Slowed growth
- Loss of function
- Malformations
- Death
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Description
This quiz covers the stages of prenatal development, including the pre-embryonic, embryonic, and fetal stages. It also delves into gametogenesis, detailing the processes of spermatogenesis and oogenesis. Test your understanding of these fundamental biological concepts.
