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Flashcards in Stages Of Life Deck (64):
1

Stages of human development

1. Period of the Ovum/Proliferative Period
2. Period of the Embryo/Embryonic Period
3. Period of the Fetus/Fetal Period

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• Fertilization to 2 weeks
➢ Zygote à Morula à Blastocyst
• Implementation of blastocyst
• Formation of embryonic disc
➢ Implantation is on the uterus

Period of the ovum

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3 weeks to 8 weeks
• Duifferent types of tissue develop, organizing to form organ systems
• The heart begins to form and beat
• The face and oral structures develop

Embryonic stage

4

8th week – the embryo takes on a more human appearance
• Increase in body weight and size

Fetal stage

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female gametocyte or germ cell

Oocyte

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male gametocyte

Spermatozoa

➢ Genetic material – head
➢ Mitochondria – neck

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site of fertilization

Ampulla of the Fallopian tube –

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glycoprotein coat and seminal plasm protein are removed from the spermatozoon head

Capacitation process

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hyaluronidase and acrosin (trypsin-like substance) are released to penetrate the oocyte barrier

Acrosomal reaction

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– a cap-like structure at the anterior end of the spermatozoon that produces enzyme aiding in egg penetration

Acrosomes

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During fertilization the spermatozoon must penetrate

1. Corona radiata
2. Zona pellucida
3. Oocyte cell membrane

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As soon as the spermatozoa has entered the oocyte
a. the oocyte finishes the 2nd meiotic division
➔ Female pronucleus (haploid) (23 chromosomes)
b. Zona pellucida becomes impenetrable
c. The head of the sperm separates from the tail (male pronucleus)

Proliferative period

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Results in fertilization

• Restoration of diploid number of chromosomes both pronuclei have replicated their DNA paternal and maternal chromosomes intermingle
• Determination of the chromosomal sex
➢ The sperm determines chromosomal sex
➢ X is heavier; slower
➢ Life span of sperm is 48 hours
• Initiation of cleavage giving rise to 2-cell stage (blastomeres)

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– any of the cell formed by cleavage of a fertilized egg

Blastomere

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– the spherical embryonic mass of blastomeres

Morula

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– a sphere of cells (trophoblast) enclosing an inner mass of cells and fluid-filled cavity (blastocoel)

Blastocyst

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– extraembryonic tissue
Embryoblast à embryo
➢ Inner cell mass

Trophoblast

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cavity lined with ectoderm develops

Amniotic cavity

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forms the root of the yolk sac

Endoderm

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2 cell layers contact in the center to form an area of ectoderm and endoderm forming the

embryonic disc

21

• Produces human chorionic gonadotropin (NCG)
• Pregnancy test

Syncytiotrophoblast

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➢ Shared placenta
➢ 1 sperm, 1 egg
➢ Same chromosomes

• Identical (monozygotic)

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➢ Separate placentas

Fraternal (dizygotic)

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Slight enlargement of ectodermal and endodermal cells at the cranial end à axis of the embryo is established
• Firm union between the layers

Prochordal plate

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The primordia of all major external and internal structures are established during this period
• By the end of the 8th week
➢ All major organ systems have begun to develop with minimum function

Embryonic period

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Important events in embryonic period

• Appearance of the primary streak
• Gastrulation – differentitayion of 3 germ layers from which all tissues and organs develop
• Formation of notochord
• Beginning of morphogenesis – development of body form (most significant)

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differentitayion of 3 germ layers from which all tissues and organs develop

Gastrulation

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development of body form (most significant)

Beginning of morphogenesis

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• Localized thickening of the epiblast (top) at the caudal end of the embryonic disc
➢ Epiblast – ectodermal cells
➢ Start at caudal end
➢ Elongate in a cranial direction
• Has a narrow primitive groove with slightly bulging folds on each side
Gastrulation

Primitive streak

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Bilaminar embryonic disc

• Epiblast
• Hypoblast

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Thilaminar embryonic disc

• Ectoderm
• Mesoderm
• Endoderm

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• Invagination of the epiblastic cells from the primitive streak give rise to mesenchymal cells
• Mesenchymal cell produced from the primitive streak organize into the third germ layer, the intraembryonic mesoderm

Formation of mesoderm

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Derivatives of ectoderm

• Central nervous system
• Peripheral nervous system
• Sensory epithelium of the ear, nose, eye
• Epidermis hair, nails,
• Subcutaneous glands, mammary glands, pituitary gland, enamel of teeth

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Derivatives of mesoderm

• Muscle, cartilage, bone, subcutaneous tissue of the skin
• Vascular system
• Urogenital system except bladder
• Spleen, suprarenal glands

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Derivatives of endoderm

• Gastrointestinal tract
• Epithelial lining of the respiratory tract
• Parenchyma of the tonsil, thyroid, parathyroid, thymus, liver, pancreas
• Epithelial lining of urinary bladder and urethra
• Epithelial lining of tympanic cavity and the eustachian tube

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Formation of notochord

• Arising from the primitive node, mesenchymal cells from the notochordal process, which extends cranially
• Rod of cells between the embryonic endoderm and ectoderm
➢ Start: primitive node
• Notochordal process extends from the primitive node to the prochordal plate

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Function of notochord

1. Patterning
➢ Left and right sides, symmetry
➢ Patterning of the neural tube
2. Structural
➢ Main axial skeletal element of the early embryo
➢ Notochord à mesoderm

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• Major event that begins at the end of the 3rd week
• Formation of the neural plate
• Formation of the neural tube
• Neural plate appears as a thickening of the embryonic ectoderm, cranial to the primitive node

Neurulation

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• Neural plate is induced to form by the developing notochord
• A longitudinal neural groove develops in the neural plate, which is flanked by the neural folds
• Infolding of the neural folds from the neural tube which is the primordia of the CNS
➢ Neural crest cells

Neurulation

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Neuroectodermal cells at the lateral margins of the neural plate migrate dorsolaterally to form a neural crest between the neural tube and the surface ectoderm

Formation of neural crest cell

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• Differentiates to form most of the CT of the head
• Proper migration of the neural crest cells is essential for the development of face and teeth

Neural crest cells

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Failure to migrate neural crest cells results to

Teacher Collins Syndrome
➢ Downward slanting eyes
➢ Small lower jaw
➢ Malformed or absent ears
➢ Prominent nose

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• Masses of mesoderm that lie on either side of the neural tube
• Give rise to the most axial skeleton and musculature, as well as the dermis of the skin

Somites

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Primary brain vesicles

• Prosencephalon (forebrain)
➢ Cerebrum
➢ Thalamus
• Mesencephalon (midbrain)
➢ Brain stem
• Rhombencephalon (hindbrain)
➢ Cerebellum
➢ Pons
➢ Medulla oblongata

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• 4th embryonic week
• Brought about by differential growth
• Growth in the midline > growth in the peripheral areas
• Head fold leads to formation of stomodeum (primitive mouth)
• End of 3rd week: the formation of the stomodeum marks the start of facial development

Folding of embryo

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With lateral folding, the amniotic cavity encompasses the embryo and the ectoderm forms the surface epithelium

Lateral folding of embryo

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With the appearance of body folds, the flattened trilaminar embryo begins to exhibit the basic tubular configuration of the adult organism
• Giving rise to:
➢ Foregut
➢ Midgut
➢ Hindgut

Formation of the digestive system

48

• Combined ectoderm and endoderm
➢ No mesoderm
• Located in the region where palatine tonsils will later appear
• Separates the primitive mouth from the foregut
• During the 4th week, the buccopharyngeal membrane ruptures creating a comm…between

Buccopharyngeal/oropharyngeal membrane

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Invagination of the stomodeal ectoderm in the roof of the primitive mouth in front of the buccopharyngeal membrane
• Deepens toward the developing brain develops into the anterior lobe of the hypophysis or pituitary gland

Rathke’s pouch

➢ Anterio – Rathke’s Pouch
➢ Posterior - brain

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Below the stomodeum, bars of tissue will bend around the sides of the pharynx

Branchial clefts and arches

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ectodermal depression

Branchial clefts or pharyngeal grooves –

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– tissue bulges outward
➢ Brachial arches/pharyngeal pouches inside

Brachial or pharyngeal arches

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First branchial arch

➢ mandibular arch
➢ will later form the mandible, maxilla, the associated muscles, nerves and blood supply

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Second branchial arch

➢ hyoid arch
➢ forms the facial muscles, vessels, and hyoid bone

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ms of third and fourth branchial arch

serves as the throat

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1. First pouch (pharyngeal)

➢ Middle ear
➢ Eardrum
➢ Eustachian tube

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2. Second pouch (pharyngeal)

➢ Palatine tonsils

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3. Third Pouch (pharyngeal)

➢ Inferior parathyroid glands
➢ Thymus

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4. Fourth Pouch (pharyngeal)

➢ Superior parathyroid gland

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5. Fifth Pouch (pharyngeal)

➢ Ultimobrachial body (gives rise to calcitonin producing parafollicular cells)

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begins to develop from the anterior wall f the primitive throat
➢ Lateral lingual swellings
• Anterior 2/3
➢ 2 lateral lingual swellings
➢ Tuberculum impar
• Posterior 1/3
➢ Copula (2nd arch)
➢ Hypobrachial eminence (3rd and 4th arches)

Tongue

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Begins to develop from a depression found between the tuberculum impar of His and copula
• Migrates downwards and eventually lie in the lower front neck region

Thyroid gland

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Connects the migrating thyroid gland to the tongue by a narrow canal
• Later becomes solid then disappears

Thyroglossal duct

64

• The small depression that is left in the center of the tongue, which is remnant of the thyroglossal duct

Foramen cecum