Human Foetal Development

a) a) Explain the embryogenesis and developmental stages from zygote from zygote through cleavage, formation of morula, zygote from zygote through cleavage, formation of morula, blastocyst and gastrulablastocyst and gastrula

b) Explain organogenesis from germ layers to the formation b) Explain organogenesis from germ layers to the formation of organs of organs

c) Describe the extraembryonic development and functions of each extraembryonic membrane and placenta

d) State the foetal growth development: 1st, 2nd and 3rd trimester ( human gestation can be divided into three rimester ( human gestation can be divided into three trimesters of about three months each ) trimesters of about three months each )

Human Foetal DevelopmentHuman Foetal Development::

CHAPTER : REPRODUCTION AND DEVELOPMENT

EARLY DEVELOPMENT IN HUMANSEARLY DEVELOPMENT IN HUMANS

• After fertilization / conception, development After fertilization / conception, development begins in the oviductbegins in the oviduct as as the embryo is moved toward the uterusthe embryo is moved toward the uterus

• Early developmental stages:Early developmental stages:

i.i. CleavageCleavage

-- a succession of rapid cell divisionsa succession of rapid cell divisions

-- divides zygote into smaller cells called divides zygote into smaller cells called blastomeresblastomeres

-- 11stst division complete ~ 36 hours after fertilization division complete ~ 36 hours after fertilization


……EARLY DEVELOPMENT IN HUMANSEARLY DEVELOPMENT IN HUMANS

1 zygote forms

the two-celled stage

the four-celled stage

a morula

( solid ball of 16-32 cells )



-- ~ day 7, ~ day 7, blastocystblastocyst is formed ( with over 100 cells ) is formed ( with over 100 cells )- within it is a fluid-filled cavity called within it is a fluid-filled cavity called blastocoelblastocoel

-- at one end is a cluster of cells ( at one end is a cluster of cells ( inner cell massinner cell mass ) )

-- outer layer of cells is the outer layer of cells is the trophoblasttrophoblast

-- blastocyst reaches the uterus & begins to implant blastocyst reaches the uterus & begins to implant


……EARLY DEVELOPMENT IN HUMANSEARLY DEVELOPMENT IN HUMANS- trophoblast secretes enzymes to trophoblast secretes enzymes to

penetrate the endometriumpenetrate the endometrium

- it forms the it forms the chorionchorion which continues which continues to expand into the endometriumto expand into the endometrium

- inner cell mass forms 2 layers of inner cell mass forms 2 layers of cells ( cells ( epiblastepiblast & & hypoblast hypoblast ))

- epiblast will develop into the epiblast will develop into the embryo embryo proper & amnionproper & amnion

-- hypoblast will form the hypoblast will form the yolk sacyolk sac



ii. ii. GastrulationGastrulation

-- inward movementinward movement of epiblast forms the embryo proper with 3 germ of epiblast forms the embryo proper with 3 germ layers ( layers ( ectoderm, mesoderm & endoderm ectoderm, mesoderm & endoderm ))

-- development of development of 44 extraembryonic membranesextraembryonic membranes ( chorion, amnion, ( chorion, amnion, yolk sac & allantois ) occursyolk sac & allantois ) occurs



iii. iii. OrganogenesisOrganogenesis - ‘‘formation of organs’formation of organs’

-- begins with formation of neural tube, notochord & somitesbegins with formation of neural tube, notochord & somites- hollow hollow neural tubeneural tube originates from dorsal ectoderm & it will form originates from dorsal ectoderm & it will form

the CNSthe CNS

- notochordnotochord ( longitudinal, flexible rod ) forms from dorsal mesoderm ( longitudinal, flexible rod ) forms from dorsal mesoderm & it provides skeletal support & it provides skeletal support

-- somitessomites forms from mesoderm & they will form the vertebrae and forms from mesoderm & they will form the vertebrae and musclesmuscles



-- by end of organogenesis, regions of the by end of organogenesis, regions of the 3 germ layers3 germ layers develop into develop into rudiments of major organsrudiments of major organs

-- eg:eg: ectoderm ectoderm →→ epidermis of skin; nervous system; epidermis of skin; nervous system; sensory organssensory organs

mesodermmesoderm →→ skeletal system; muscular system; skeletal system; muscular system; kidney; heartkidney; heart

endodermendoderm →→ epithelial lining of intestines; epithelial lining of intestines; organs relating to the digestive systemorgans relating to the digestive system


DEVELOPMENT OF EXTRAEMBRYONIC MEMBRANESDEVELOPMENT OF EXTRAEMBRYONIC MEMBRANES

• Tissue layers Tissue layers formed outside the embryoformed outside the embryo proper proper• SupportSupport further embryonic development further embryonic development• 4: chorion, amnion, yolk sac & allantois4: chorion, amnion, yolk sac & allantois

i. i. ChorionChorion • Develops from trophoblastDevelops from trophoblast• Surrounds the embryo & other extraembyonic membranesSurrounds the embryo & other extraembyonic membranes• Outer layer forms Outer layer forms chorionic villichorionic villi where O where O22 & nutrients are acquired & nutrients are acquired

for the foetusfor the foetus• Will Will form the foetal portion of placentaform the foetal portion of placenta


……DEVELOPMENT OF EXTRAEMBRYONIC MEMBRANESDEVELOPMENT OF EXTRAEMBRYONIC MEMBRANES

• PlacentaPlacenta::-- combination of chorion ( the foetal portion ) & endometrium combination of chorion ( the foetal portion ) & endometrium

tissues ( the maternal portion )tissues ( the maternal portion )

-- contains foetal & maternal blood vesselscontains foetal & maternal blood vessels

-- transports nutrients, respiratory gases & transports nutrients, respiratory gases & metabolic metabolic wasteswastes between foetus & mother ( blood from embryo travels to between foetus & mother ( blood from embryo travels to placenta via the placenta via the umbilical cord umbilical cord ))

-- also also secretes estrogens, progesterone, secretes estrogens, progesterone, HCGHCG ( human ( human chorionic gonadotropin )chorionic gonadotropin ), , HCSHCS ( human chorionic somatotropin ) ( human chorionic somatotropin ) & & prostaglandinprostaglandin



ii. ii. AmnionAmnion • Forms from epiblastForms from epiblast• Encloses the embryo in a fluid-filled amniotic cavityEncloses the embryo in a fluid-filled amniotic cavity• Protects embryoProtects embryo from drying out & cushions it against mechanical from drying out & cushions it against mechanical

shocksshocks

iii. iii. Yolk sacYolk sac • Forms from hypoblastForms from hypoblast• Encloses another fluid-filled cavity ( no yolk )Encloses another fluid-filled cavity ( no yolk )• In humans, it is smallIn humans, it is small• Yolk sac membrane is site of Yolk sac membrane is site of early formation of blood cellsearly formation of blood cells



iv. iv. AllantoisAllantois • Develops as an outpocketing of

embryo’s gut

• Incorporated into the umbilical cord

• Forms blood vessels that transport respiratory gases, nutrients & wastes between embryo / foetus and placenta


THE FOETAL GROWTH DEVELOPMENTTHE FOETAL GROWTH DEVELOPMENT

• PregnancyPregnancy / / gestationgestation is the condition of carrying new developing is the condition of carrying new developing individuals in the uterusindividuals in the uterus

• Averages 38 weeks ( from conception ) or 40 weeks ( from the start Averages 38 weeks ( from conception ) or 40 weeks ( from the start of last menstrual cycle )of last menstrual cycle )

• Divided into Divided into 3 trimesters3 trimesters ( 3 months each ) ( 3 months each )


11stst trimester trimester

• Zygote divides 24 hours after fertilizationZygote divides 24 hours after fertilization• Blastocyst formed ~ day 7Blastocyst formed ~ day 7

• Blastocyst implants into endometrium ~ day 12Blastocyst implants into endometrium ~ day 12• Embryo obtains nutrients from endometrium for first 2 – 4 weeks Embryo obtains nutrients from endometrium for first 2 – 4 weeks

( while placenta is developing ) ( while placenta is developing )

• Organogenesis occursOrganogenesis occurs in 1 in 1stst trimester ( during which embryo is most trimester ( during which embryo is most sensitive to radiation & drugs )sensitive to radiation & drugs )

• Early organogenesis: neural tube, notochordEarly organogenesis: neural tube, notochord• Later: heart, liver, brain, eyes, pharyngeal grooves, limb buds & tailLater: heart, liver, brain, eyes, pharyngeal grooves, limb buds & tail

……THE FOETAL GROWTH DEVELOPMENTTHE FOETAL GROWTH DEVELOPMENT


• By 8By 8thth week, rudiments of all major organs are present week, rudiments of all major organs are present• Embryo is now a Embryo is now a foetusfoetus ( ~ 5cm by end of 1 ( ~ 5cm by end of 1stst trimester ) trimester )

……11stst trimester trimester



22ndnd trimester trimester

• Foetus grows (~ 30cm)Foetus grows (~ 30cm)• Pregnancy becomes obviousPregnancy becomes obvious• Very activeVery active ( mother may feel movement ) ( mother may feel movement )



33rdrd trimester trimester

• Rapid growthRapid growth of foetus ( ~ 3.5kg, 50cm ) of foetus ( ~ 3.5kg, 50cm )• Foetal Foetal activity may decreaseactivity may decrease• Mother’s abdominal organs become compressed & displacedMother’s abdominal organs become compressed & displaced


Human Foetal Development

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