CH 3 - Basic Embryology

conceptus is from

zygotę to birth

during the first week, the cells of the embryo cleave as

it travels through the oviduct to the uterus

conceptus - by the end of the week, a

blastocyst has formed

conceptus - during week 2, the embryo

implants into the uterus

embryology

study of the origin and development of a single individual

prenatal period - 2 stages

embryonic and fetal period

embryonic period duration

first 8 weeks postfertizilation

embryonic period major events

organs form from 3 primary germ layers; basic body plan emerges

fetal period duration

9-38 (or birth)

fetal period major events

organs grow in size and complexity

critical or danger period =

weeks 3-8

prior to week 3 development

terminates naturally (destruction of critical cell lines)

after week 8,

deformities are less severe

congenital defects

3% of all newborns exhibit a significant birth defect

• increases 6% by age 1 initially unrecognizable conditions becomes evident (multiple causes)

teratology =

study of birth defects and fetuses with congenital deformities

teratogen

an agent or factor which causes malformation of an embryo

• heavy metals, alcohol, pesticides, prescription drugs

fetal alcohol syndrome (FAS)

result of alcohol consumption during pregnancy

microcephaly

small head and jaw, growth of deficiencies

fetal alcohol spectrum disorders (FASD)

mental retardation, unusually shaped eyes and lips, heart defects, behavioral, learning and social problems

• occurs in one percent of births

thalidomide was used

as a sedative and to relived morning sickness

when thalidomide is taken during days 24-36 of pregnancy results in

severe limb defects

the embryonic period - week one is

zygote to blastocyst

fertilization occurs in

the lateral third of uterine tube

fertilization action

zygotę (fertilized oocyte) moves toward the uterus, cleavage occurs and daughter cells formed from zygote

the embryonic period - morula

solid cluster 12-16 cells

blastocyst

fluid-filled structure of 60 cells

blastocyst - inner cell mass (ICM) -

forms embryo

blastocyst - throphoblast

helps form placenta

the embryonic period - stages of first week

zygote (day1) → four-cell(day2) → morula (day3) →early blastocyst (day4)→ late blastocyst (implants at this stage) (day7)

the embryonic period - week 2

two-layered embryo

• bilaminar embryonic disc

the embryonic period week 2 - bilaminar embryonic disc

ICM divided into 2 sheets

• epiblast and they hypoblast

the embryonic period week 2 - amniotic sac

formed by an extension of epiblast

amniotic sac outer membrane forms

the amnion

amniotic sac inner membrane forms

the amniotic sac cavity (filled with amniotic fluid)

the embryonic period week 2 - yolk sac formed by

an extension of hypoblast

digestive tube forms from

tolk sac

tissues around yolk sac die rise to

earliest blood cells and blood vessels

the embryonic period week 2 - day 5

blastocyst floats in uterine cavity

the embryonic period week 2 - day 6

blastocyst adheres to uterine wall

the embryonic period week 2 - day 7

implantation begins as throphoblast invades into uterine wall

the embryonic period week 2 - day 9

implantation continues; ICM form bilinear disc

the embryonic period week 2 - day 11

implantation complete; amniotic sac and yolk sac form

the embryonic period week 3 =

the three-layered embryo

the embryonic period week 3 - gastrulation

a process of invagination of epiblast cells

• begins at the primitive streak

• forms the three primary germ layers

three primary germ laye

endoderm

formed from migrating cells that replace the hypoblast

mesoderm -

formed between epiblast and endoderm

ectoderm

formed from epiblast cells that stay on dorsal surface

primitive streak

raised groove on the dorsal surface of the epiblast

on day 16

the notochord forms

primitive node

a swelling at one end of primitive streak

notochord defines

body axis

• site of the future vertebral column

  • appears on day 16 and reaches head region by day 18

epiblast cells that migrate through the primitive node form the

notochord

epiblast cells that migrate through primitive streak form

the mesoderm layer

neurulation

ectoderm starts forming brain and spinal cord

• days 17-22

day 17

neural plate forms

day 17 - flat three-layered embryo has

completed gastrulation

day 17 - ectoderm overlying the notochord

thickens, forming the neural plate

day 20 =

neural plate folds

day 20 - neural folds form by

folding of the neural plate and then deepen, producing the neural groove

day 20 - neural fold cell migrate to form

the neural crest

• forms sensory nerve cells

day 22 =

neuralation - neural tube forms

day 22 - neural folds have closed,

forming the neural tube

material folic acid deficiency causes

neural tube defects

folding =

embryonic body is beginning to undercut

day 24/week 4

the body takes shape folding

day 24/week 4 - the sides of the embryo fold

medially and the head and tail fold under

• embryonic disc bulges; growing faster than yolk sac

day 24/week 4 - body shape

• tadpole shape by day 24 after conception

day 24/week 4 - primitive gut

encloses tubular part of the yolk sac

• site of future digestive tube and respiratory structures

epiblast → ectoderm

• epidermis, hair, nails, glands of skin

• brain and spinal cord

• neural crest and derivatives

epiblast → mesoderm

• notochord

• somite

• intermediate mesoderm (kidneys and serosa)

• lateral plate mesoderm

  • somatic and splanchnic

epiblast → endoderm

epithelial lining and glands of digestive and respiratory tracts

weeks 5-8

• limb buds form

• embryo first looks recognizably human

• head is disportionately large

• all major organs are in place

fetal period - weeks 9-38

• a time of maturation and rapid growth

• cell are differentiating during the first half of the fetal period

normal birth occurs

38 weeks after conception

premature birth is one that occurs

before 38 weeks

8 weeks after fertilization

• head is nearly as large as the body; nose, ears, and eye are recognizably human; all major division are formed; first brain waves

• limbs are formed; digits are initiated webbed

• heart has even pumping

• all major organs systems are present in rudimentary form

9-12 weeks after fertilization

• brain continue to enlarge; cervical and lumbar enlargements are apparent; retinas are present

• trunks and limbs elongagte

• fetus begins to move

• heartbeat detected externally

• lungs begins to devleop

• intestines move into the abdomen

• genitalia are distinctive; sex can be determined

13-16 weeks after fertilization (month 4)

• skin development continues

• torso elongates; bone formation begins in vertebrae; most bones are distinct

• myelin begins to form around nerves cells

• glands develop in the GI tract

• kidneys attain typical structures

17-20 weeks after fertilization (month 5)

• hair follicle and sebaceous and sebum form

• brown fat forms in the neck, chest, and groin

• mother can feel fetal movements

• brain grows rapidly

21-30 weeks after fertilization (months 6 and 7)

• period of substantial weight gain

• fingernails and toenails are complete

• distal limbs begin to ossify

• cerebrum grow

• lungs complete development

• bone marrow becomes only site of blood cell formation

• testes descend to scrotum

20-28 weeks after fertilization (month 8 and 9)

• fat accumulates in subcutaneous tissue; skin thickens

• surfactant production in the lungs increases

• immune system develops