619 8/25 GLA Embryology and Development

Prenatal Development vs Pregancy

pregnancy: 40 weeks

prenatal development: 38 weeks (fertilization to birth)

Last Menstrual Period

date two weeks before fertilization that most women use to know if they are pregnant

Gestation

the 40-week pregnancy time span in reference to the fetus

Developmental Age

the actual developmental age of the fetus

Gestational Age

the age of an embryo or fetus timed from the first date of the lmp

Divisions of Prenatal Development

weeks 1-2

weeks 3-8

weeks 9-38

Weeks 1-2 (Simple)

fertilization, implantation in endometrium of uterus, formation of two layers, development of primitive streak

Weeks 3-8 Embryonic Period (Simple)

formation of three layers, beginning of development of all orgnas and body parts (aka organogenesis)

Weeks 9-38 Fetal Period (Simple)

growth and maturation of organs and body parts

Overall Impact of Tertogens: Week 1-2

all or none

Teratogens

substances that can harm the fetus

Overall Impact of Tertogens: Week 3-8 Embryonic Period

major morphologic anomalies

Overall Impact of Tertogens: Week 9-38 Fetal Period

physiologic and minor morphologic anomalies

What is the most sensitive time for the development of all organs and body parts?

embryonic period (3-8)

Overall Fetal Development Chart

Week 1: Fertilization and Implantation

egg and sperm encounter each other in the distal oviduct/uterine tube

sperm release enzymes that create a hole in the egg covering (zona pellucida) so one sperm enters the egg

diploid zygote is formed by haploid sperm/egg

mitosis begins

morula develops, then blastocyst develops from morula

while the morula and blastocyst form, they are moved through the oviduct into the uterus

in the uterus, the blastocyst implants in the endometrium during the last days of week 1

Distal Oviduct/Uterine Tube

region where egg contacts multiple sperm

Zona Pellucida

covering of the egg that is degraded by sperm enzymes to allow fertilization

Fertilization Diagram

Morula

solid ball of cells created when mitosis happens after diploid zygote formation

Blastocyst

formed when a fluid-filled cavity develops in the morula

2 Regions of Blastocyst

trophoblast

inner cell mass

Trophoblast

circle of peripheral cells of the blastocyst

Trophoblast Develops into

placenta and fetal membranes, aka extraembryonic tissues

Inner Cell Mass

inner ball of cells at one pole of the blastocyst

Inner Cell Mass Develops Into

embryo

Blastocyst Implantation

while the blastocyst is developing from the morula, it is traveling through the oviduct to the uterus where it is then implanted into the endometrium during the last days of week 1

Week 1 Fertilization and Implantation Diagram

Week 2: Bilaminar Disc

inner cell mass of blastocyst organizes itself into the two layers of the bilaminar disc

layers of bilaminar disc develop cavities

primitive streak develops

Bilaminar Embryonic Disc

formed during week 2 from the inner cell mass

2 Layers of Bilaminar Disc

epiblast

hypoblast

Bilaminar Disc Formation Diagram

Epiblast

dorsal layer of bilaminar disc that forms the entire embryo

Hypoblast

the ventral layer of the bilaminar disc that forms the lining of the yolk sac but does not form any parts of the embryo

eventually disappears

2 Cavities that Develop from Bilaminar Disc

amniotic cavity

yolk sac

Amniotic Cavity

develops dorsally in the epiblast during week 2

Yolk Sac

develops dorsally in the hypoblast during week 2

Primitive Streak

thickening that arises in the caudal midline of the epiblast at the end of week two

Primitive Node

cranial end of the primitive streak (near the midpoint of the embryonic disc), the organizer and inducer for a variety of embryonic events through release of signaling molecules

What is the primitve streak the source of?

cells that are involved in gastrulation in week 3

Teratogen in Weeks 1 & 2

the conceptus either dies from injury or lives without an effects from the agent, called all-or-none effecta

Conceptus Susceptibilityin Weeks 1 & 2

conceptus is not very susceptible to teratogens in week 1 and 2

Week 3: Gastrulation

bilaminar disc becomes trilaminar (3 layers)

forms ectoderm, mesoderm, endoderm

organogenesis can begin

somites develop as soon as the three germ layers have formed in week 3

3 Layers of Trilaminar Disc Formed During Gastrulation (from dorsal to ventral)

ectoderm

mesoderm

endoderm

Trilaminar Disc Diagram

Ectoderm is Associated with

amniotic cavity

Endoderm is Associated with

yolk sac

Somites

42-44 paired blocks of mesoderm, next to the midline, that arise in a craniocaudal sequence

form the body segments, visible in dermatomes

Somites Formation Diagram

Dermatomes Fetus vs Adult Diagram

Somites Development Order

sequentially in the cranial to caudal direction

occurs from proximal to distal

Week 4: Folding

portions of the flat trilaminar embryonic disc fold

the cranial and caudal end folds and lateral sides each fold ventrally to create a 3D trunk

four limbs begin to develop from the trunk as well as gastrointestinal, respiratory, genitourinary, and cardio

neurulation ocurs

Folding of Week 4 Diagram

When does the heart start to beat?

week 4

Neurulation

occurs during week 4 to give rise to the neurological system

Weeks 5-8: Organ Systems

organs and body parts arise and begin to mature

by the end of this period the exterior of the body is nearly complete and all organ systems have begun to form

Why do teratogens in weeks 3-8 cause major atomic anomalies?

because all organs and body parts are just beginning to form, so an injury would have a profound impact on that structure

Weeks 9-38: Fetal Period Development

each body part and organ continues to mature and grow, and by week 38 all organs and body parts are present and mature enough to allow for life outside of the uterus

Why do teratogens have a minor effect on weeks 9-38?

in these later weeks, each body organ has already been formed

8 Families of Genes Important for Human Development

homeobox

hox

sox

pax

wnt

hedgehog

transforming growth factor b

fibroblast growth factor (fgf)

Homeobox

family of genes important for human development, encode transcription factors and are involved in body positioning and patterning

Hox

family of genes important for human development, encode transcription factors and are involved in reproductive and nervous system development

Pax

family of genes important for human development, encode transcription factors

Wnt

family of genes important for human development, encode signaling proteins and are involved in cell fate and patterning

Hedgehog

family of genes important for human development, includes the sonic the hedgehog (SHH) gene which encodes a signaling protein that has roles in cell growth, cell specialization, and the normal shaping/patterning of the body

Transforming Growth Factor b

family of genes important for human development, encode signaling proteins and are involved in cell mitosis, differentiation, motility, adhesion, and apoptosis

includes bone morphogenic protein for skeletal and organ development

Fibroblast Growth Factor

family of genes important for human development, encode signaling proteins and are involved in mitosis, cell growth, shaping, and other functions

Induction

the process by which one group of cells, the inducing tissue, directs the development of another group of cells, the responding tissue

Division of Somities

4 occipital

8 cervical

12 thoracic

5 lumbar

5 sacral

2-3 coccygeal

Where do the somites develop?

either side of the neural tube and notochord

3 Parts of Somite

dermatome

myotome

sclerotome

Dermatome

part of somite, deep dermis (deep to the ectoderm) of the body

Myotome

part of somite, muscle of the body wall and limbs

Sclerotome

part of somite, vertebrae and axial bones

Somite Layers Diagram

Origin of the Typical Spinal Nerve

neurons in the spinal cord anterior horn send motor axons to skeletal muscles

sensory axons from the posterior root ganglia extend laterally towards dermatomes and medially to reach the posterior horn of the spinal cord

the motor and sensory axons tether together as the spinal nerve

Spinal Nerve Diagram

What innervates the myotome?

anterior horn axons

What innervates the dermatome and posterior horn?

posterior root ganglion axons

Sclerotome to Vertebra

they split in half as fissure develop so axons can pass, then remnants fuse with their neighbors to become vertebrae

Origin of Spinal Nerve Diagram

Vertebral Development

intervertebral discs and the intervertebral foramina will remain at the site of the fissures

annulus fibrosus develops from the sclerotome (one dermatome separates, top of one connects to bottom of other)

nucleus pulposus is the only mature remnant of the notochord

Sclerotome Mesenchyme

surrounds the neural tube and notochord, forming a loose model of the vertebra and ribs

Chondrification Centers

form in the sclerotome and start to replace it with cartilage in the 6th week

When does bone being replaxing cartilage in the vertebra?

week 7

Vertebral Ossification

occurs in two steps, primary ossification during development and secondary ossification during adolscence

Primary Ossification Centers of Vertebral Ossification

form in the vertebra body, pedicles, and lamina

the bone replaces the mesenchymal cartilage expanding outward from the primary ossification centers, so in the intermediary stage the vertebra are both bone and cartilage

Secondary Ossification Centers in Vertebral Ossification

in the transverse and spinous processes (and annular epiphyes) fuse to the rest of the vertebra

Spina Bifida/Dysraphism

occurs when ossification of the spinous processes of the last part of the vertebra fails, or when the spinous processes do no form at all

How is severe spina bifida detected?

in utero by the presence of a-fetoprotein in materal blood

5 Types of Spina Bifida

occulta

meningocele

meningomyelocele

myeloschesis

rachischesis

Spina Bifida Occulta

failure of the neural arch (spinous process) to ossify

the resulting defect is not large and is sometimes marked by a tuft of dense hair or a small depression in the lower back

Meningocele

occurs when meninges and CSF herniate through the hole left by the failed neural arch

Meningomyelocele

occurs when meningies, CSF, spinal cord and/or roots herniate through the hold left by the failed neural arch

Myeloschesis

failure of the neural tube to fold properly, the neural arch cannot form

Rachischesis

typically occurs when multiple neighboring segments are affected by myeloschesis

Abnormalities in Hox Gene Expression

may cause lumbarization of the S1 vertebrae or sacralization of the L5 vertebrae

Ascent of the Spinal Cord

embryo (3 months): spinal cord runs full length of vertebral column

end of 5th month: conus medullaris ascends to the S1 level

newborn: conus is now at the L3 level