Growth and Development of the Embryo
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Transcript of Growth and Development of the Embryo
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Growth and Development of the Embryo/Fetus
The act of fertilization begins the first of three stages of development in the
human being:
Fertilized Ovum
Morula
Blastocyte
(inner cell mass)
Ectoderm Mesoderm Entoderm
(outermost layer) (middle layer) (innermost layer)
Epidermis, cutaneous glands
Sweat glands
Nails, hair
Nervous system, including
cranial and spinal nerves
Sense organs- neuroepithelium
Optic lens, epithelium of
portions of eye
Sensory epithelium of ear
Pituitary gland
Adrenal medulla
Upper pharynx and nasal
passages
Nasopharynx and mouth-
epithelium
Teeth-enamel
Salivary glandsMammary glands
Urethra
Lower portion of anal canal-
lining
Dermis
Skeleton
Cartilage
Muscles (all types)
Connective tissue
Adrenal cortex
Pleura, pericardium,
peritoneum
Teeth-dentine (all but enamel)
HeartSpleen
Blood, bone marrow
Blood and lymphatic vessels
Kidneys, ureters
Gonads, uterus
Lining of auditory canal,tympanic membrane
Thyroid gland
Pharynx, tonsils
Trachea, bronchi. lungs
Lining of larynx, trachea,
air passages,-alveoli
Gastrointestinal tract
Pancreas, liver, gallbladder
Urinary bladder
Lining of alimentary tract
(except that arising from
ectoderm)
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PRECONCEPTION
Length (approximate : Crown rump 0.8 mm weight : not known
Crown heel 0.5 mm
Appearance internal development
Fertilization of ovum in distal third of fallopian tube no organ differentiation
Zygote divided (mitotic division)
Morula
Early and late blastocyst
Implantation begins
Length (approximate : Crown rump 0.8 mm weight : not known
Crown heel 0.5 mm
Appearance internal development
Blostocyte superficially than completely buried in flat embryonic disk
endoperidium ectoderm and entoderm
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Primitive placental circulation
Length (approximate): Crown-rump 1.5-3mm Weight: not known
Crown-heel 1.5-3 mm
Appearance
Primitive streak develops in posterior midline of
embryo
Streak thickens: embryonic disk elongates
Neural groove closes partially (Later becomes
central nervous system)
Head and tail folds form
Internal Development
Trilaminar embryo: mesoderm develops
between ectoderm and entoderm
Thyroid begins to develop
Heart tubes begin to fuse and to circulate early
blood cells formed in the yolk sac Lung buds
present
Somites, thickened groups of mesodermal cells
form in pairs alongside the neural folds (later
becomes skeleton and muscles of the skeleton,
etc.)
Four weeks
Length (approximate): Crown-rump 3-4 mm
Crown-heel 3-4 mm
Appearance Head is at right angle to body; the
embryo is C shaped because the neural tube
grows faster than ventral surface
Head is 1/3 of body length
Primordia of eye and ear present
Primitive jaws formed (as branchial arches)
Limb buds present
Tail prominent
Yolk sac diminishing in size
Weight: 0.4 gm (approximate)
Internal Development
Initial stages of most organs have begun to
develop
Vitelline duct present
Heart begins to beat
Aortic arches and major veins developed
Right and left primary bronchi form
Neural folds fusing: anterior end forms
brain; posterior end forms spinal cord (if
anterior end does not close, anencephaly or
absence of cranial vault results; if posterior
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end does not close, meningocele, see p. 516,
results)
Esophagotracheal septum divides trachea
and esophagus (if esophagotracheal septum
does not develop correctly,
tracheoesophageal fistula. see p. 448,results)
Early beginning of peritoneal and pleural
cavities
All 42 somites present
Five week
Length (approximate): Crown-rump 7-8 mm
Crown-heel 7-8 mm
Appearance
Primitive umbilical cord developing from body
stalk
Head much larger in proportion to trunk (1/2
size of fetus)
Lens pits and optic cups formed and nasal pitsforming
Primitive mouth
Facial features move closer together
Hand plates (paddle-shaped)
Wrist and elbow develop
Leg buds (paddle-shaped)
Weight: 1 gm (approximate)
Internal Development
Brain differentiated into 5 areas
10 pairs of cranial nerves forming Division
of cardiac atria occurring Beginning of
primitive kidneys
Length (approximate): Crown-rump 10-13 mm
Crown-heel 10-13 mm
Weight: 1.5 gm (approximate)
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Appearance
Upper lip formed (cleft lip and palate
developed between 5 to weeks, see page 497)
Upper and lower jaw recognizable
Oral and nasal cavities confluentPalate developing buds forming
Formation evident
Finger rays present
Foot plate forming
Rail still present, but regression evident
Placenta begins to support maternal placental
circulation
Internal Development
Lung formation beginning due to tracheal
bifurcation
Liver forming red blood cells
Some intestines outside abdominal cavity;
abdomen too small to hold all intestines. (Ifthey remain outside the abdomen, the
neonate will have an omphalocele, a defect
in the abdominal wall, see page 455, or
gastroschisis, a defect of the bottom of the
umbilical stalk, see page 457)
Primitive skeletal shape beginning to form
Muscle and cartilage formation beginning
Length (approximate): Crown-rump 16-30 mm (3 cm)
Crown-heel 16-35 mm (3.5 cm)
Appearance
Presence of beginnings of all essential external
structures
Recognizable eyes, ears, nose, mouth
Eyelids forming of nose distinct
Digits formed
Arms longer and bent at elbows
Anal membrane perforatedExternal genitalia are sexless but have begun to
differentiate
Toe rays appear
Trunk elongating and straightening
Tail almost gone
Fetal movements begin
Umbi1ical cord organized
Weight: 2 gm (approximate)
Internal Development
Presence of beginnings of all essential
internal structures
Optic nerve formed
Pericardial and pleural cavities form
Valves and septa of heart, and inferior ven
cava form
Heart fairly well developed and beating 40
80 beats a minute (most cardiac anomaliesdeveloped by end of 7th week)
Abdominal and thoracic cavities are
separated by diaphragm (If the diaphragm
does not develop normally, a diaphragmati
hernia, see page 451, results)
Stomach well developed
Gastrointestinal tract rotates midgut
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Urethra and bladder are separating from
rectum
Urogenital membrane degenerating
Anal canal developed (if there is abnormal
development of the urorectal septum, an
imperforate anus, see page 457, results)Urine is first formed and excreted into
amniotic fluid
Sex glands beginning to differentiate into
ovaries or testes
Ossification of bones begins in occiput,
mandible, humerus (diaphysis)
Muscles continue to develop
Reflex arc ready to function
Sensory organ development progressing
Length (approximate):
Crown-rump 6 cm (2.5 inches)
Crown-heel 9 cm (3.5 inches)
Appearance
Face and mouth developing
Eyelids fuse
Palate fusingTooth enamel beginning to be formed
Beginning of nail growth
Male and female external genitalia, which
develop from analogous structures, still appear
fairly similar
Responds to tactile stimulation
Weight: 10 gm (approximate)
Internal Development
Basic division of brain
Erythropoiesis in liver but gradually
regresses
Intestines return to abdomen
Bladder sac forms; urine continues to formTestes forming testosterone
Bone marrow formation and functioning
begins
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Length (approximate):
Crown-rump 7-8 cm (3 inches)
Crown-heel 11.5 cm (4.5 inches)
Appearance
Skin delicate and pinkHuman-like in appearance, but head is still
large in proportion to body (1/3 of size of
fetus)
Fusion of palate complete
Respiratory motion is visible
Distinctive external genitalia present
Sucks thumb and begins to swallow
Lower trunk moves more freely
Weight: 19-30 gm (approximate) (1 oz)
Internal Development
Brain appearance roughly complete
Growth hormone produced in pituitary
glandErythropoiesis in spleen begins
Uterus and testes now specific; uterus is no
longer bicornuate (having 2 horns)
Ossification of upper cervical to lower
sacral arches and bodies
Length (approximate):Crown-rump 13.5 cm (5.25 inches) Crown-
heel 1 5-1 9 cm (6-7 .5 inches)
Appearance
Rapid growth-appears human
Hair begins to grow on scalp
Lanugo begins to grow on body (the amount
Weight 100- 200 gm (31/3-62/3OZ)
Internal Development
Cerebral lobes present; rapid growth of
neurons, (If rubella occurs during this
period, inhibition of neuronal growth could
occur, resulting in decreased intellectual
ability,)
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at birth indicates gestational age, see page
362)
Hard and soft palates differentiated Nipples
present
Fetus quite active: sucks thumb, upper lip
protrudes on stimulation, grasp reflex
Maternal 16-20 weeks: mother feels fetal
movements (quickening)
Myelinization beginning
Heart muscle well developed
16-20 weeks-heart tones of fetus can be
heard using fetoscope at 120-160 beats per
minute
30% nephrons in kidney matureBladder has adult form
Meconium in intestines, (Presence of
meconium in amniotic fluid indicates fetal
distress,)
Anal canal open
Vagina open
Bone ossification evident on radiography;
ischium is ossified Sense organs
differentiated
Length (approximate): Crown-rump 18.5 cm (71/4 inches)
Crown-heel 22-25 cm (8,75-9,75
inches)
Appearance
Vernix caseosa forming and will be present at birth (see
page 362)
Eyebrows and eyelashes developing
Tooth enamel and dentin continue to form
Primitive respiratory rhythms
Lower abdomen from umbilicus to pubis increases in
length
Legs elongate to final proportions with body length
Sucking reflex increasingly apparent
Weight: 300-400 gm (10-13'/3 oz)(approxim
Internal Development
Sebaceous glands appear
Brain stem, and spinal cord myelinization
begins
Ossification of sternum
Bone marrow is more important In bloodformation
Storage of iron begins (premature infants
have less iron stored,
therefore, may have iron deficiency anemia
see page 649)
Brown fat forming
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Maternal
Stronger and more frequent fetal movement that mother
can feel
Length (approximate):
Crown-rump 23 cm (9 1/8 inches)Crown-heel 30 cm (12 inches)
Appearance
Vernix apparent
Skin wrinkled and red (blood is visible in capillaries)
Small amount of subcutaneous fat
Structure of eyes complete
Respiratory-like movements
Weight: 600-960 gm (1,25-2 pounds)
(approximate)
Internal Development
Beginning development of primordia of
permanent teeth
Alveolar ducts and sacs evident in lungs
Alveolar cells begin to make surfactant
Testes present at inguinal ring
Os pubis ossifies
IgG levels rise to maternal values to protect
fetus and neonate from diseases for which
the mother has immunitySurvival rate: few, but fetus is viable
Length (approximate):
Crown-rump 27cm (10% inches)
Crown-heel 35cm (14 inches)
Weight: 1200 gm (2.5 Ib) (approximate)
Internal Development
Cerebral fissure appears
Convolutions appear on brain Formation of
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Appearance
Subcutaneous fat minimal
Skin less wrinkled and red
Nails more obvious
Pupils respond to light
Movements not well sustained
glial cells and myelinization of brain. (If
mother is poorly nourished at this time or
during infancy, infant may have learning
difficulties and other neurologic problems)
Capillaries develop around alveoli
Surfactant forming on surface of alveoli;lungs capable of breathing air
Testes below inguinal ring
Bone marrow is major area in formation of
blood; liver decreasing in importance
Ossification of astragalus
Survival rate: 10% with optimal care
because gas exchange is possible
Length (approximate):
Crown-rump 31 cm (12 inches)Crown-heel 40 cm (16 inches)
Appearance
Lanugo starting to disappear
Skin smooth and pink
Subcutaneous fat increasing. (Length
increases proportionately less than weight)
Pupillary light refiex-30 weeks
Increased muscle tone in legs and trunk
Movements better sustained
Responds to sounds from outside worldMoro's reflex present-30 weeks
Delivery position is assumed
Weight: 1900 gm (4 Ib) (approximate)
Internal Development
Lecithin/sphingomyelin (LS) ratio = 1.2: 1
Testes descending to scrotum
Ossification of middle and 4th phalanges
Survival rate: 33% with optimal care
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Length (approximate):
Crown-rump 35 cm (13.5 inches)
Crown-heel 45 cm (17.5 inches)
Appearance
Skin pinkBody rounded: limbs flexed
Lanugo disappearing
Fuzzy hair growth
Lobes of ears soft little cartilage
Umbilicus near center of body
Scrotum small. few rugae
Few creases in soles of foot
spontaneous orientation to light
Firm grasp
Cries when hungry
May show greater or less activity because oftightness of uterine space
Weight: 2900, gm (6 Ib) (approximate)
Internal Development
Lecithin/sphingomyelin ratio 2: 1 or more
Iron storage increased in liverNew nephrons no longer form
Spinal cord ends at L3 level
Testes in inguinal canals
Distal ossification centers of femurs present
Survival rate: 70-80% with optimal care
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Length (approximate):
Crown-rump 40 cm (15.5 inches)
Crown-heel 50 cm (19.5-20 inches)
Appearance
Flexed limbsMuscle tone well developed
Skin pink. smooth, plump
Vernix caseosa covers skin
Lanugo remains on upper part of body and
shoulders
May have moderate-to-profuse silky hair
Cartilage present in nose and ears .
Nails extend over ends of fingers and toes
Creases cover soles
Strong sucking reflex
Weight: 3200-3400 gm (7-7.5 Ib)
(approximate)
Internal Development
Pulmonary branching 66% complete
Myelinization reaches level of hemispheresTestes in rougous scrotum
Labia majora well developed
Bones and skull continue to firm
Proximal tibial ossification centers present
Continuing to store minerals and fat
Absorption of maternai hormones
Ready to be born
Survival rate: over 95% with optimal care