Stages Of Life Flashcards by Lixette Anne Padilla

Stages of human development

Period of the Ovum/Proliferative Period
Period of the Embryo/Embryonic Period
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

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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

Corona radiata
Zona pellucida
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

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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

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)

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

Gastrulation

development of body form (most significant)

Beginning of morphogenesis

• 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

Bilaminar embryonic disc

* Epiblast | * Hypoblast

Thilaminar embryonic disc

* Ectoderm * Mesoderm * Endoderm

* 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

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

Derivatives of mesoderm

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

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

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

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 ```

* 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

• 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

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

* 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

Failure to migrate neural crest cells results to

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

* 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

Primary brain vesicles

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

* 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

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

Lateral folding of embryo

``` 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

• 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

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

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

Branchial clefts and arches

ectodermal depression

Branchial clefts or pharyngeal grooves –

– tissue bulges outward | ➢ Brachial arches/pharyngeal pouches inside

Brachial or pharyngeal arches

First branchial arch

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

Second branchial arch

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

ms of third and fourth branchial arch

serves as the throat

1. First pouch (pharyngeal)

➢ Middle ear ➢ Eardrum ➢ Eustachian tube

2. Second pouch (pharyngeal)

➢ Palatine tonsils

3. Third Pouch (pharyngeal)

➢ Inferior parathyroid glands | ➢ Thymus

4. Fourth Pouch (pharyngeal)

➢ Superior parathyroid gland

5. Fifth Pouch (pharyngeal)

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

``` 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

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

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

Thyroglossal duct

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

Foramen cecum