Growth and Development of the Embryo

7/27/2019 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)


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


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)


Brain differentiated into 5 areas

10 pairs of cranial nerves forming Division

of cardiac atria occurring Beginning of

primitive kidneys


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


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



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



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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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)


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)


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


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


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)


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


Crown-rump 27cm (10% inches)

Crown-heel 35cm (14 inches)

Weight: 1200 gm (2.5 Ib) (approximate)


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


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)


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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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)


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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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)


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

Growth and Development of the Embryo

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