SEM_14_y_15_Bone Formation Process Quiz
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Questions and Answers

What type of muscle tissues are striated (skeletal) muscles, smooth muscles, and cardiac muscles?

  • Voluntary, involuntary, and cardiac
  • Striated, smooth, and cardiac (correct)
  • Skeletal, cardiac, and involuntary
  • Cardiac, smooth, and skeletal
  • From which type of mesoderm do the skeletal muscles of the trunk derive?

  • Branchial mesenchyme
  • Somites
  • Cranial paraxial mesoderm
  • Paraxial mesoderm (correct)
  • Where do most of the craniofacial muscles originate from?

  • Somites
  • Cranial paraxial mesoderm
  • Branchial mesenchyme (correct)
  • Myotomes
  • What is the process by which mesenchymal cells of the myotomes differentiate into muscular progenitors called myoblasts?

    <p>Myogenesis</p> Signup and view all the answers

    What type of bone formation involves the formation of bones directly from mesenchyme?

    <p>Intramembranous ossification</p> Signup and view all the answers

    Which process involves differentiation of mesenchyme into chondroblasts and the formation of a cartilaginous model?

    <p>Endochondral ossification</p> Signup and view all the answers

    Primary and secondary ossification occurs in which types of bones?

    <p>Long bones</p> Signup and view all the answers

    What remains in the joints as articular cartilage and growth plates?

    <p>Cartilaginous templates</p> Signup and view all the answers

    Where are growth plates located in relation to the diaphysis and epiphysis?

    <p>Between the diaphysis and epiphysis</p> Signup and view all the answers

    What is the fibrous layer surrounding bones called?

    <p>Periosteum</p> Signup and view all the answers

    Where does endochondral ossification take place in long bones?

    <p>Diaphysis and epiphysis</p> Signup and view all the answers

    What type of bone formation forms flat bones directly from mesenchyme?

    <p>Intramembranous Ossification</p> Signup and view all the answers

    Which of the following involves formation of a cartilage model which is gradually replaced by bone?

    <p>Endochondral Ossification</p> Signup and view all the answers

    The differentiation of mesenchyme into chondroblasts and formation of a cartilaginous matrix is part of which process?

    <p>Endochondral Ossification</p> Signup and view all the answers

    What allows the bone to grow in length?

    <p>Growth plates</p> Signup and view all the answers

    Where does primary and secondary ossification occur?

    <p>Long bones</p> Signup and view all the answers

    During muscle development, what creates myosin and actin resulting in the striated muscle appearance?

    <p>Myogenic cells</p> Signup and view all the answers

    What is the population of quiescent myoblasts that remain near muscle fibers, ready to differentiate and form new muscle fibers when needed?

    <p>Myosatellite cells</p> Signup and view all the answers

    From which part of the body do the dorsal (epaxial) muscles derive during myogenesis?

    <p>Epimere</p> Signup and view all the answers

    Which type of muscle forms from the visceral splanchnic mesoderm surrounding the heart tube?

    <p>Cardiac muscle</p> Signup and view all the answers

    What process can result in intramembranous ossification and endochondral ossification during muscle development?

    <p>Myogenesis</p> Signup and view all the answers

    What type of muscle is associated with thoracic and abdominal viscera?

    <p>Smooth muscle</p> Signup and view all the answers

    What is the origin of the ventral (hypaxial) muscles during myogenesis?

    <p>Hypomere</p> Signup and view all the answers

    Which cells form new muscle fibers when needed?

    <p>Myosatellite cells</p> Signup and view all the answers

    What is the process of bone development involving mesenchyme from different sources?

    <p>Ossification process</p> Signup and view all the answers

    What is the specific origin of the cardiac muscle cells called cardiomyocytes?

    <p>Heart tube</p> Signup and view all the answers

    From which mesoderm does smooth muscle associated with thoracic and abdominal viscera derive?

    <p>Visceral splanchnic mesoderm</p> Signup and view all the answers

    Which structure induces the mesoderm to continue growing into a limb?

    <p>Apical ridge</p> Signup and view all the answers

    What differentiates into specific bones and muscles in the developing limb?

    <p>Proliferating mesodermal masses</p> Signup and view all the answers

    What is responsible for causing the interdigital zones to degenerate in the developing limbs?

    <p>Apoptosis</p> Signup and view all the answers

    In which pattern does the developing limbs undergo regional differentiation?

    <p>Proximal-distal, medial-lateral, and dorsal-ventral</p> Signup and view all the answers

    What are the three stances based on how mammals' limbs touch the ground?

    <p>Plantigrade, digitigrade, and unguligrade</p> Signup and view all the answers

    Which type of animals stand or walk on their digits and retain the original embryonic pattern of five digital fingers?

    <p>Digitigrade animals</p> Signup and view all the answers

    What is a congenital defect characterized by the presence of extra digits?

    <p>Polydactyly</p> Signup and view all the answers

    'Achondroplasia' is a form of dwarfism caused by what?

    <p>Premature ossification of growth cartilages</p> Signup and view all the answers

    'Arthrogryposis' is a condition resulting in what?

    <p>Malformed joints</p> Signup and view all the answers

    'Brachydactyly' is characterized by what?

    <p>'Brachydactyly' results in stumpy digits</p> Signup and view all the answers

    What are the two types of ungulates mentioned in the text?

    <p>Odd-toed ungulates and even-toed ungulates</p> Signup and view all the answers

    What is responsible for creating morphogenic gradients for each zone of the developing limb?

    <p>Three specific signaling centers</p> Signup and view all the answers

    How does bone growth in length occur?

    <p>Via the epiphyseal plate, the cartilage between primary and secondary ossification centres</p> Signup and view all the answers

    What is responsible for the growth in diameter of bones?

    <p>Bone deposition beneath the periosteum</p> Signup and view all the answers

    Which process is responsible for joint development?

    <p>Mesenchyme differentiation</p> Signup and view all the answers

    What is required for proper synovial joint development?

    <p>Nerve-driven muscle activity and joint movement</p> Signup and view all the answers

    How do vertebrae initially develop?

    <p>Formation of vertebral arch from mesenchyme</p> Signup and view all the answers

    What is the origin of the intervertebral disc regions?

    <p>Mesenchyme between newly formed adjacent vertebrae</p> Signup and view all the answers

    How do ribs develop?

    <p>As part of the cartilage model for each vertebra</p> Signup and view all the answers

    What is responsible for the formation of sternebrae?

    <p>Paired hyaline cartilage models that later fuse</p> Signup and view all the answers

    What initiates limb development?

    <p>Localised proliferation and condensation of parietal mesenchyme covered by ectoderm</p> Signup and view all the answers

    What encloses the neural tube during vertebrae development?

    <p>Vertebral arch formed from mesenchyme cells around the notochord</p> Signup and view all the answers

    What differentiates into fibrous, cartilaginous, or synovial joints?

    <p>Mesenchyme between bones forming an interzone region</p> Signup and view all the answers

    What is needed for proper synovial joint development to prevent ankylosis?

    <p>Nerve-driven muscle activity and joint movement</p> Signup and view all the answers

    What are the two components that assemble the bones of the head or skull?

    <p>The neurocranium and the splanchnocranium</p> Signup and view all the answers

    From which part of the body does the visceral skeleton of the face differentiate?

    <p>First pharyngeal arches</p> Signup and view all the answers

    How do the individual bones of the face develop?

    <p>By intramembranous ossification from the branchial ectomesenchyme</p> Signup and view all the answers

    From which pharyngeal arch does the nerve supply sensation to the anterior 2/3 of the tongue?

    <p>1st pharyngeal arch</p> Signup and view all the answers

    Which ectodermal structure gives rise to the individual teeth in the process of tooth development?

    <p>Dental papilla</p> Signup and view all the answers

    Which type of mesoderm does the cardiac muscle cells called cardiomyocytes originate from?

    <p>Lateral plate mesoderm</p> Signup and view all the answers

    Where does the invagination of surface epithelial cells occur during the typical formation of exocrine glands like salivary glands?

    <p>Underlying mesenchymal cells</p> Signup and view all the answers

    Which embryonic structure forms the nose, nasal cavity, and primary palate?

    <p>Nasal processes</p> Signup and view all the answers

    What congenital abnormality involves the failure of the palate to close along the midline?

    <p>Cleft palate</p> Signup and view all the answers

    From which branchial arch does the rostral two-thirds of the tongue originate?

    <p>First branchial arch</p> Signup and view all the answers

    Which of the following embryonic structures contributes significantly to the development of the head in mammals?

    <p>Cranial somites</p> Signup and view all the answers

    What is the origin of the flat bones that ossify to form the roof of the cranium in mammalian embryos?

    <p>Ectomesenchyme migrated from branchial arches</p> Signup and view all the answers

    What is the function of the splanchnocranium in mammalian embryos?

    <p>Formation of the visceral skeleton of the face</p> Signup and view all the answers

    From which embryonic structure does the visceral skeleton of the face differentiate in mammalian embryos?

    <p>First pharyngeal arches</p> Signup and view all the answers

    Which nerve supplies sensation to the anterior 2/3 of the tongue?

    <p>Glossopharyngeal nerve (IX)</p> Signup and view all the answers

    From which pharyngeal arch do the muscles of the tongue originate?

    <p>First pharyngeal arch</p> Signup and view all the answers

    What gives rise to the individual teeth during tooth development?

    <p>Dental papilla</p> Signup and view all the answers

    Where does the invagination of surface epithelial cells occur during the typical formation of exocrine glands like salivary glands?

    <p>Mesenchymal cells</p> Signup and view all the answers

    What is responsible for forming the outer layer of the tooth or the crown during tooth development?

    <p>Ameloblasts</p> Signup and view all the answers

    Where do the parotid, zygomatic, and labial and buccal accessory salivary glands derive from?

    <p>Ectoderm</p> Signup and view all the answers

    What differentiates into odontoblasts during tooth development?

    <p>Dental papilla</p> Signup and view all the answers

    In which type of gland development do surface epithelial cells undergo localised proliferation?

    <p>Exocrine gland development</p> Signup and view all the answers

    Which nerve innervates the muscles of the tongue?

    <p>Hypoglossal nerve (XII)</p> Signup and view all the answers

    From which pharyngeal arch does the posterior 1/3 of the tongue (root) derive?

    <p>Third pharyngeal arch</p> Signup and view all the answers

    What process results in the formation of sternebrae?

    <p>Chondrogenesis</p> Signup and view all the answers

    Which part of the skull forms the facial skeleton?

    <p>Viscerocranium</p> Signup and view all the answers

    What structure forms as a result of the separation of the oral and nasal cavities?

    <p>Secondary palate</p> Signup and view all the answers

    What congenital abnormality involves the failure of the palate to close along the midline?

    <p>Cleft palate</p> Signup and view all the answers

    From which branchial arch does the rostral two-thirds of the tongue originate?

    <p>First branchial arch</p> Signup and view all the answers

    Which type of congenital abnormality involves a failure of the palate to close along the midline, affecting various parts of the palate?

    <p>Cleft palate</p> Signup and view all the answers

    What does the nasal cavity develop from?

    <p>Frontonasal prominence</p> Signup and view all the answers

    What separates the oral and nasal cavities?

    <p>Primary palate</p> Signup and view all the answers

    What are congenital abnormalities of the palate that include cleft palate and hare lip known as?

    <p>Cheiloschisis</p> Signup and view all the answers

    Which embryonic structure gives rise to the nose, nasal cavity, and primary palate in mammalian embryos?

    <p>Frontonasal prominence</p> Signup and view all the answers

    What is the condition characterized by the failure of the palate to close along the midline?

    <p>Cleft palate</p> Signup and view all the answers

    From which embryonic structure does the tongue derive its rostral two-thirds?

    <p>First branchial arch</p> Signup and view all the answers

    From which embryonic structures does the visceral skeleton of the face differentiate in mammalian embryos?

    <p>Pharyngeal arches and mesenchyme</p> Signup and view all the answers

    Where does primary and secondary ossification occur?

    <p>Long and flat bones</p> Signup and view all the answers

    What gives rise to the individual teeth during tooth development?

    <p>Ectomesenchyme from branchial arches</p> Signup and view all the answers

    Where does the nerve supply sensation to the anterior 2/3 of the tongue originate from?

    <p>First pharyngeal arch</p> Signup and view all the answers

    What is responsible for the formation of the outer layer of the tooth or the crown during tooth development?

    <p>Predentin</p> Signup and view all the answers

    Which embryonic structure does the visceral skeleton of the face differentiate from in mammalian embryos?

    <p>Ectoderm of the first pharyngeal arch</p> Signup and view all the answers

    From which part of the body do the dorsal (epaxial) muscles derive during myogenesis?

    <p>Occipital somites</p> Signup and view all the answers

    During the first period of development, the primordial genital structures are identical in both sexes. This period is referred to as the:

    <p>Indifferent stage</p> Signup and view all the answers

    In mammals, genetic sex is established at fertilization with XY indicating male and XX denoting female. This is part of which stage in sex differentiation?

    <p>Genetic stage</p> Signup and view all the answers

    The development of the sex-specific organs of the genital system is preceded by the formation of a set of temporary undifferentiated structures in the embryo. This period is part of which stage in sex differentiation?

    <p>Indifferent stage</p> Signup and view all the answers

    Sex differentiation is a complex process that includes distinct sequential stages such as genetic, gonadal, hormonal, phenotypic, and psychological. Which stage involves the physical manifestation of the individual's genetic sex?

    <p>Phenotypic stage</p> Signup and view all the answers

    In which scenario does the genital tubercle keep growing to generate an elongated phallus or penis?

    <p>Under the influence of testosterone (male scenario)</p> Signup and view all the answers

    What does the genital tubercle involute and become in the absence of testosterone?

    <p>The clitoris</p> Signup and view all the answers

    What do the urogenital folds elongate and overgrow in the absence of testosterone?

    <p>The labia minora</p> Signup and view all the answers

    What happens to the genital swellings in most domestic mammals in the absence of testosterone?

    <p>They become flat and disappear</p> Signup and view all the answers

    What forms as a result of the separation of the oral and nasal cavities?

    <p>The palate</p> Signup and view all the answers

    In which scenario do the urogenital folds beneath the genital tubercle begin to fuse in the midline to form the penile urethra?

    <p>Under the influence of testosterone (male scenario)</p> Signup and view all the answers

    What changes in size as it forms the glans of the clitoris?

    <p>The genital tubercle</p> Signup and view all the answers

    What changes to form a urogenital groove and later closes to form the penile urethra under the influence of testosterone?

    <p>Urogenital folds</p> Signup and view all the answers

    Which hormone is responsible for the differentiation of the Wolffian ducts into the epididymides, vas deferens, and seminal vesicles?

    <p>Testosterone</p> Signup and view all the answers

    What is the main cause of cryptorchidism in dogs?

    <p>Genetic, epigenetic, and environmental factors</p> Signup and view all the answers

    What structure secretes Müllerian inhibiting hormone?

    <p>Fetal Sertoli cells</p> Signup and view all the answers

    What type of disorder is cryptorchidism in dogs?

    <p>Disorder of sexual development</p> Signup and view all the answers

    Where are cryptorchid testicles more prone to problems such as torsion and cancer?

    <p>Abdominal cavity</p> Signup and view all the answers

    What is responsible for causing the mesonephric ducts to thrive in male sexual development?

    <p>Testosterone</p> Signup and view all the answers

    Where should testicular descent be completed before birth in some species?

    <p>Humans, pigs, ruminants, and cats</p> Signup and view all the answers

    What is the consequence of bilateral cryptorchidism in dogs?

    <p>Sterility</p> Signup and view all the answers

    How can cryptorchid testicles be identified?

    <p>By trans-rectal or trans-abdominal ultrasonography</p> Signup and view all the answers

    What is the purpose of neutering in dogs with cryptorchidism?

    <p>To prevent torsion and cancer in retained testicles</p> Signup and view all the answers

    Which gene is activated in genetically male (XY) embryos, leading to the differentiation of the indifferent gonad into a testis?

    <p>SRY gene</p> Signup and view all the answers

    What is the primary hormone responsible for the regression of the Müllerian ducts in male embryos?

    <p>MIS (Müllerian-inhibiting substance)</p> Signup and view all the answers

    What triggers the development of male genital ducts and sex-specific external genitalia during embryonic development?

    <p>Testosterone</p> Signup and view all the answers

    What is the first discernible event in the development of the testes during embryonic development?

    <p>Appearance of primordial Sertoli cells</p> Signup and view all the answers

    Which cells produce androgen hormone (testosterone) under the influence of the seminiferous cords?

    <p>Leydig cells</p> Signup and view all the answers

    Where do primordial germ cells differentiate and migrate to induce further gonadal development?

    <p>Gonadal ridge</p> Signup and view all the answers

    What is the main hormone produced by fetal Sertoli cells that induces regression of the Müllerian ducts?

    <p>MIS (Müllerian-inhibiting substance)</p> Signup and view all the answers

    What is the term for the condition characterized by the failure of the palate to close along the midline?

    <p>Cleft palate</p> Signup and view all the answers

    "Brachydactyly" is characterized by what?

    <p>Shortened digits or toes</p> Signup and view all the answers

    What does SRY gene activation lead to in genetically male (XY) embryos?

    <p>Differentiation of indifferent gonad into a testis</p> Signup and view all the answers

    What is responsible for inducing regression of Mullerian ducts in male embryos?

    <p>MIS (Mullerian-inhibiting substance)</p> Signup and view all the answers

    When does differentiation of primordial germ cells occur?

    <p>Gastrula stage</p> Signup and view all the answers

    Which cells differentiate from primitive interstitial cells in the testis, producing steroids?

    <p>Leydig cells</p> Signup and view all the answers

    What happens to the entire population of oogonia at birth in mammalian ovaries?

    <p>They enter the first meiotic division</p> Signup and view all the answers

    What determines the reproductive life of mammals at birth?

    <p>Number of primordial follicles</p> Signup and view all the answers

    What forms the uterine tubes in female genital development?

    <p>Paramesonephric ducts</p> Signup and view all the answers

    What remains in the joints as articular cartilage and growth plates?

    <p>Primordial germinal cells</p> Signup and view all the answers

    What initiates limb development?

    <p>Testicular hormones</p> Signup and view all the answers

    Where are growth plates located in relation to the diaphysis and epiphysis?

    <p>At the junction of diaphysis and epiphysis</p> Signup and view all the answers

    What separates the oral and nasal cavities during development?

    <p>Paramesonephric duct formation</p> Signup and view all the answers

    What differentiates into odontoblasts during tooth development?

    <p>Dental papilla</p> Signup and view all the answers

    What gives rise to the individual teeth during tooth development?

    <p>Dental lamina and dental sac</p> Signup and view all the answers

    What is responsible for causing the interdigital zones to degenerate in the developing limbs?

    <p>Mesenchyme differentiation into chondroblasts</p> Signup and view all the answers

    During the development of male external genitalia, what does the urogenital groove form under the influence of testosterone?

    <p>The penile urethra</p> Signup and view all the answers

    In the female scenario, what do the genital swellings become?

    <p>The labia majora</p> Signup and view all the answers

    What structure forms the glans of the penis under the influence of testosterone?

    <p>The genital tubercle</p> Signup and view all the answers

    Which part of the female external genitalia becomes the opening of the vulva in the absence of testosterone?

    <p>The urogenital orifice</p> Signup and view all the answers

    Under the influence of testosterone, what do the urogenital folds elongate and overgrow to become in males?

    <p>The labia minora</p> Signup and view all the answers

    What happens to the genital swellings in most domestic mammals in the female scenario?

    <p>They become flat and disappear</p> Signup and view all the answers

    In females, what do the unfused urogenital folds beneath the genital tubercle form?

    <p>The inner labia minora</p> Signup and view all the answers

    What forms as a result of the separation of the oral and nasal cavities during development?

    <p>The palate</p> Signup and view all the answers

    What forms as a result of midline fusion in males that initially remains separate in females?

    <p>The scrotum</p> Signup and view all the answers

    Study Notes

    • The epiphyseal plate, the cartilage between primary and secondary ossification centres, allows bone growth in length.

    • The growth in diameter of bones occurs by bone deposition beneath the periosteum.

    • Joint development: mesenchyme between bones forms an interzone region, which develops into fibrous, cartilaginous, or synovial joints depending on the future joint type.

    • Synovial joint formation: mesenchyme in the interzone cavitates to form the synovial cavity, which is then surrounded by the synovial membrane and synovial folds.

    • Joint development: interzone mesenchyme forms intra-articular ligaments and the perichondral tissue surrounding the interzone forms the joint capsule, which gives rise to ligaments.

    • Proper synovial joint development requires nerve-driven muscle activity and joint movement to prevent ankylosis.

    • Vertebrae development begins with mesenchyme cells around the notochord, which differentiate into hyaline cartilage models that ossify through endochondral ossification.

    • Vertebrae development: sclerotomes split and differentiate into cranial and caudal regions to form a cartilage model of one vertebra by the combination of the caudal half of one sclerotome with the cephalic part of the adjacent sclerotome.

    • Vertebrae development: mesenchyme forms the vertebral arch, which encloses the neural tube, and the transverse and costal processes, which eventually form the ribs.

    • Intervertebral disc regions develop between newly formed adjacent vertebrae, with the mesenchyme forming the annulus fibrosus and the persisting notochord as the nucleus pulposus.

    • Ribs develop as part of the cartilage model for each vertebra, but the costal processes separate and the cartilage model ossifies except for the anterior portion, which remains as costal cartilage.

    • Sternum forms as paired hyaline cartilage models on either side of the anterior midline, which later fuse and undergo endochondral ossification to form sternebrae.

    • Limbs develop as limb buds, which are produced by localised proliferation and condensation of parietal mesenchyme covered by the ectoderm.

    • The skull consists of two parts: the neurocranium (protective cranial vault) and the viscerocranium (facial skeleton).

    • The nasal cavity develops from the frontonasal prominence, which expands and divides into the frontal bone and the nasal processes.

    • The medial and lateral nasal processes form the nose, nasal cavity, and primary palate, and the growth of the nasal placodes leads to the formation of the nasal pits and primitive nasal cavity.

    • The palate forms as the nasal and oral cavities are separated by the development of the primary palate and the subsequent growth of the secondary palate.

    • The palate, including the fusion of the primary and secondary palate, leads to the separation of the oral and nasal cavities.

    • Congenital abnormalities of the palate include cleft palate and cheiloschisis (hare lip).

    • Cleft palate is a failure of the palate to close along the midline and can be inherited or caused by exposure to a teratogen.

    • Clefts can affect the lip, alveolar process, entire extension of the primary palate, or soft and hard palate.

    • The oral cavity develops as a consequence of the formation of the upper and lower jaw.

    • The tongue develops from several swellings on the floor of the pharynx, with the rostral two-thirds originating from the first branchial arch and the caudal third from the second branchial arch.

    • The skull consists of two parts: the neurocranium (protective cranial vault) and the viscerocranium (facial skeleton).

    • The nasal cavity develops from the frontonasal prominence, which expands and divides into the frontal bone and the nasal processes.

    • The medial and lateral nasal processes form the nose, nasal cavity, and primary palate, and the growth of the nasal placodes leads to the formation of the nasal pits and primitive nasal cavity.

    • The palate forms as the nasal and oral cavities are separated by the development of the primary palate and the subsequent growth of the secondary palate.

    • The palate, including the fusion of the primary and secondary palate, leads to the separation of the oral and nasal cavities.

    • Congenital abnormalities of the palate include cleft palate and cheiloschisis (hare lip).

    • Cleft palate is a failure of the palate to close along the midline and can be inherited or caused by exposure to a teratogen.

    • Clefts can affect the lip, alveolar process, entire extension of the primary palate, or soft and hard palate.

    • The oral cavity develops as a consequence of the formation of the upper and lower jaw.

    • The tongue develops from several swellings on the floor of the pharynx, with the rostral two-thirds originating from the first branchial arch and the caudal third from the second branchial arch.

    • The skull consists of two parts: the neurocranium (protective cranial vault) and the viscerocranium (facial skeleton).

    • The nasal cavity develops from the frontonasal prominence, which expands and divides into the frontal bone and the nasal processes.

    • The medial and lateral nasal processes form the nose, nasal cavity, and primary palate, and the growth of the nasal placodes leads to the formation of the nasal pits and primitive nasal cavity.

    • The palate forms as the nasal and oral cavities are separated by the development of the primary palate and the subsequent growth of the secondary palate.

    • The palate, including the fusion of the primary and secondary palate, leads to the separation of the oral and nasal cavities.

    • Congenital abnormalities of the palate include cleft palate and cheiloschisis (hare lip).

    • Cleft palate is a failure of the palate to close along the midline and can be inherited or caused by exposure to a teratogen.

    • Clefts can affect the lip, alveolar process, entire extension of the primary palate, or soft and hard palate.

    • The oral cavity develops as a consequence of the formation of the upper and lower jaw.

    • The tongue develops from several swellings on the floor of the pharynx, with the rostral two-thirds originating from the first branchial arch and the caudal third from the second branchial arch.

    • In the testis, Sertoli cells proliferate, forming cordlike structures (seminiferous tubules) around primitive germ cells. Leydig cells differentiate from primitive interstitial cells, producing steroids.

    • In the absence of a Y chromosome (female embryos with two X chromosomes), the primordial gonadal cords stop growing and become disorganized. Ovarian differentiation begins.

    • In the central region of the ovary, the sex cords disappear, forming the ovarian medulla. In the cortical region, gonadal cords remain, forming primordial follicles.

    • Female primordial germinal cells differentiate into oogonia, undergoing mitotic activity. These, along with supporting cells, form primordial follicles.

    • The entire population of oogonia enters the first meiotic division but stops at the prophase stage.

    • At birth, mammalian ovaries contain a reserve of primordial follicles. The number of these follicles (follicular reserve) determines reproductive life.

    • During puberty, some follicles develop, secrete hormones, and either regress or ovulate.

    • Female genital development is the default pathway. The paramesonephric (Müllerian) ducts form the female genital tract without hormonal requirement.

    • The cranial part of the paramesonephric ducts forms the uterine tubes, the middle portion becomes the uterine horns, and the caudal portion becomes the uterus, cervix, and cranial vagina.

    • Length of the paramesonephric ducts that become fused varies among species. In humans and primates, the uterus has no horns. In rodents and rabbits, there is a double uterus. In monotremes and marsupials, there are two vaginas.

    • Remnants of the embryonic mesonephric ducts can be found in the vaginal vestibule in females (Gartner's duct).

    • In embryos, both sexes have primordia of male and female genital ducts (mesonephric and paramesonephric ducts).

    • Male genital development depends on testicular hormones. Female genital development is predetermined in the absence of testicular influence, but recent evidence suggests some level of female genital development is not completely passive.

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