Ch 3 Embryology

endoderm

"inner skin" forms the innermost lining of the inner tube (epithelial lining) (epithelial tissue) derivatives: -inner epithelial lining of the gut tube, respiratory tubes, digestive organs, and the urinary bladder -also gives rise to sectretory cells of the glands: thyroid, thymus, parathyroid glands, liver, and pancreas

mesoderm

"middle skin" forms muscle, bone, and connective tissue (mesenchyme tissue- any embryonic tissue with star-shaped cells that do not attach to one another) -different parts: notochord, somites, intermediate mesoderm, somatic and splanchnic mesoderm

ectoderm

"outside skin" forms the outer layer of the skin, brain, and the spinal cord (epithelial tissue) Derivatives: -becomes brain, spinal cord, and skin -early epidermis produces the hair, fingernails, toenails, sweat glands, and oil glands -neural crest cells give rise to sensory nerve cells -neural crest breaks up mesenchyme tissue which wanders throughout body, these wandering neural crest produce such varies structures as the pigmented producing cells in the skin (melanocytes) and the bones of the face

sclerotome

(hard piece) its cells migrate medially, gather around the notochord and the neural tube and produce the vertebra and ribs

myotome

(muscle piece) grow ventrally until it extends the entire dorsal to ventral height , becomes segmented trunk musculature of the body wall, additionally the ventral parts grow into the limb buds and form the muscles of the limbs

dermatome

(skin piece) its cells migrate externally until they lie directly deep to the ectoderm where they form the dermis of the skin of the dorsal body region

fetal period

-a time of maturation and rapid growth -cells are differentiating during the first half of the fetal period -normal births occur 38 weeks after conception -premature birth is one that occurs before 38 weeks

neurulation

-as the notochord starts to develop it signals the overlying ectoderm to start forming the spinal cord and brain -the ectoderm in the dorsal midline thickens into a neural plate and starts to fold as a neural groove which deepens until a hollow neural tube is pinched off into the body (happens at the end of week 3 in region that will become the neck) -complete closure happens by the end of week 4, the cranial part of the neural tube becomes the brain and the rest becomes the spinal cord

intermediate mesoderm

-begins as a continuous strip of tissue just lateral to the paraxial mesoderm -influenced by the segmentation of the somites, the intermediate mesoderm divides into spherical segments in a cranial-caudal sequence -each segment attaches to a somite -forms kidneys and gonads -lies in the same relative location as the adult kidneys outside the peritoneal cavityscle

lateral plate

-begins as one layer but soon splits into two, a wedge is formed between these two sheets (coelom- will become the serous cavities of the ventral body cavity) -the two resulting divisions of the lateral plate are the somatic mesoderm (next to the ectoderm) and the splanchnic mesoderm (next to the endoderm)

9-12 weeks 6cm

-brain enlarges -trunk and limbs elongate, palate (roof of mouth) begins to fuse at the midline -heartbeat can be detected externally -lungs begin to develop fetus inhales and exhales amniotic fluid -intestines move into the abdomen -male and female genitalia distinctive

morula

-cleavage generates cluster of 12-16 cells happens about 72 hours after fertilization -on day 4 the late morula consists of about 60 cells and enters uterus -the morula hollows out and fills with fluid which gathers into a central cavity, new structure is called blastocyst

13-16 weeks 11cm

-differentiation of the dermis and hypodermic -hard palate is fused -myelin forms around nerve cells -glands develop in the GI tract -kidneys attain typical structure -body begins to outgrow head

folding

-embryo takes on a cylindrical shape when its sides fold medially and it lifts off the yolk sac and protrudes into the amniotic cavity, at the same time the head and tail regions fold under -the embryonic disc bulges upward, its lateral folding is caused by the fast growth of the somites, the folding at the head and tail is caused by expansion of the brain and lengthening of the spinal cord -as a result the embryo acquires a tadpole shape by day 24 -embryo encloses a tubular part of the yolk sac (primitive gut) this tube is lined by endoderm -the embryo remains attached to the yolk sac below by a duct located at the future naval, this duct becomes incorporated into the umbilical cord

30-38 weeks 47cm

-fat accumulates in hypodermis, skin thickens -immune system develops

amniotic sac

-formed by an extension of the epiblast -the outer membrane is called the amnion and the internal amniotic sac cavity is filled with amniotic fluid -the fluid buffers in developing embryo and fetus from physical shock until time of birth (water breaking)

yolk sac

-formed by an extension of the hypoblast -holds a very small amount of yolk which is not a major source of nutrients for embryo -important because the digestive tube forms from part of it and the tissue around the yolk sac gives rise to the earliest blood cells and vessels

17-20 weeks 16cm

-hair follicles and sebaceous and sweat glands form, body is covered with vernix cases and lanugo (silklike hair) covers the skin -mother can feel fetal movements -brain grows rapidly

8 weeks (end of embryonic period) 3cm

-head is nearly as large as the body -nose, ears, and eyes are recognizably human and all major divisions of the brain are formed -limbs are formed -heart has been pumping since week 4 -all major organ systems are present in rudimentary form

bilaminar embryonic disc

-made up of epiblast and hypoblast, about 9 days after fertilization the inner cell mass has divided into these two sheets of cells -extensions of these cell sheets form two fluid-filled sacs resembling two ballons touching this will give rise to the whole body

neural crest

-originate from ectodermal cells on the lateral ridges (neural folds) of the neural plate and they come to lie external to the closed neural tube -forms the sensory nerve cells and other important structures

21-30 weeks 38cm

-periods of sleep and wakeness -fingernails and toenails are complete -lungs complete development -bone marrow becomes only site of blood cell formation -testes descend to scrotum in month 7

basic body plan

-skin: dermis and epidermis -outer body wall: trunk muscles, ribs, vertebrae -body cavity and digestive tube: inner tube -kidney and gonads: deep to body wall -limbs

Weeks 5-8: Second Month of Embryonic Development

-start of the second month the embryo is only .5cm long -around day 28 limb buds appear (upper earlier than lower) -body looks less tadpole like and starts to become recognizably human -limbs grow from buds to form fingers and toes -head enlarges and occupies almost half the volume of the entire body -eyes, ears, and nose appear -tail disappear after week 8 -all major organs are in place after month 2

notochord

-the epiblast cells that move through the primitive node (swelling on one end of the primitive streak) migrate straight anteriorly these mesodermal cells along with a few endoderm cells form a rod called the notochord -defines the body axis (midline that separates the body left and right), it extends the length of the body and is the site of the future vertebral column -appears on day 16 and by day 18 it reaches the future head region -gives rise to springy cores of the discs between the vertebrae (nucleus pulposus)

somites

-the mesoderm closest to the notochord begins as paraxial mesoderm, starting cranially the paraxial mesoderm divides into a series of blocks (somites) -visible in surface view as a row of subectodermal bulges on each side of the back -first body segments and about 40 pairs develop by the end of week 4 and divides into 3 parts: sclerotome, dermatome, and myotome

match each developmental stage with their date 1. blastocyst 2. implantation occurs 3. first few somites appear 4. flat embryo becomes cylindrical 5. embryo/fetus boundary 6. birth 7. primitive streak occurs

1. 4-7 days 2. 6-13 days 3. about 21 days 4. about 21-24 days 5. 2nd of week 8 6. 38 weeks 7. 15 days

Week 2: The Two-Layered Embryo

1. bilaminar embryonic disc 2. amniotic sac 3. yolk sac

indicate whether each of the following relates to a) cleavage or b) events of primitive streak 1. period when morula forms 2. period when notochord forms 3. period when the three embryonic germ layers appear 4. period when individual cells become markedly smaller

1. cleavage 2. primitive streak 3. primitive streak 4. cleavage

gastrulation

1. during week 3 the embryo grows from a two layered disc to a three layered disc and forms 3 primary germ layers- ectoderm, mesoderm, and endoderm 2. the germ layers begin to form on days 14&15 when a raised groove (primitive streak) appears on the back of the epiblast, epiblast cells migrate inward at this streak 3. on days 14-15 the first cells that migrate through the primitive streak displace the cells of the underlying hypoblast to form the endoderm 4. on day 16 the ingressing epiblast cells form a new layer between epiblast and endoderm called mesoderm 5. the epiblast cells that remain on the embryos dorsal surface make up the ectoderm

Week 4: Body Takes Shape

1. folding 2. derivatives

match these embryonic structures to with their adult derivatives 1. kidney 2. peritoneal cavity 3. pancreas secretory cells 4. parietal serosa 5. nucleus pulposus 6. visceral serosa 7. hair and epidermis 8. brain 9. ribs

1. intermediate mesoderm 2. scleretome 3. coelom 4. somatic mesoderm 5. notochord 6. splanchnic mesoderm 7. ectoderm 8. neural tube 9. sclerotome

Week 3: The 3 Layered embryo

1. primitive streak 2. gastrulation 3. three germ layers 4. notochord 5. neurulation 6. neural crest 7. induction 8. mesoderm differentiate

Week 1: Zygote to blastocyst

1. zygote 2. 4-cell 3. morula 4. early blastocyst 5. late blastocyst

when it is 1.5 months old an average embryo is the size of a

a black bean

the epiblast forms

all 3 germ layers

cleavage

division sequence during fertilization where daughter cells are formed from zygote, provides a large number of cells needed as building blocks for the embryo

describe gastrulation, during which week of embryonic development does it occur

during gastrulation cells from the epiblast move inward and form three separate germ layers, this occurs during the third week of development

fertilization

each month one of a fertile woman's two ovaries releases an immature egg (oocyte) which is drawn into the fallopian tube and fertilized by a sperm, once fertilized (now called zygote) moves towards the uterus where is multiplies -occurs in the lateral third of the uterine tube

during which prenatal period is the basic body plan established

embryonic period

early blastocyst

floats freely inside cavity of the uterus, composed of two distinct cells: 1. inner cell mass- cluster of cells on one side of the blastocyst cavity (forms the embryo) 2. trophoblast- layer of cells that surrounds the cavity (forms the placenta)

gastrulation is the

formation of three germ layers by the epiblast at the primitive streak

what does the splanchnic lateral plate mesoderm form

forms the connective tissue, muscle tissue, visceral serosa of the respiratory and digestive tubes, and the heart and blood vessels

the fetal period is

from weeks 9-38, the time of rapid growth of body organs, a time of cell differentiation

splanchnic mesoderm

gives rise to the entire wall of digestive and respiratory tubes (except epithelial lining), it forms the musculature, connective tissues, and the slippery visceral serosae , also gives rise to the heart and blood vessels

does endoderm form the inner lining of the inner tube or outer tube

inner lining of the inner tube

which embryonic structures are segmented

intermediate mesoderm and endoderm

why is respiratory distress common in babies born in their sixth or seventh month of gestation

lungs are not fully developed until the end of the seventh month in utero

the notochord develops primarily from the

mesoderm

the primary germ layer that forms the trunk muscles, heart, and skeleton

mesoderm

Mesoderm differentiation

middle of week 3 the mesoderm lies lateral to the notochord on both sides of the body and extends cranially to caudally (head to tail) by the end of this week it has divided into 3 regions: somites and intermediate mesoderm are segmented and form the segmented structures of the outer tube, lateral place mesoderm is unsegmented an is associated with the developing inner tube organs

a birth defect in the heart is caused by a development error during which weeks of embryonic development

most organs including the heart are formed during the second month of embryonic development (weeks 5-8)

late blastocyst

on day 6 the blastocyst burrows into the wall of the uterus (implantation) and takes about a week to complete during this time the trophoblast layer erodes inward until the entire blastocyst is embedded in the uterine wall -if the inner cell mass splits into 2 during the early stages of cleavage which produces identical twins

what important events occurred at the primitive streak? notochord?

primitive streak- epiblast cells migrate toward this streak, on days 14&15 the first cells that migrate through here displace the cells of the underlying hypoblast to become the endoderm notochord- signals the ectoderm to start forming the spinal cord and brain (neurulation), gives rise to nucleus pulposus (springy cores in between each vertebral disc)

somatic mesoderm

produces the parietal serosa and the dermal layer of the ventral body region, its cells migrate into the forming limbs and produce bones , joints, and ligaments

most birth defects can be traced to disruption of the developmental events during this part of the prenatal period

second half of month 1 and and all of month 2

somites are evidence of

segmentation in the body

which type or types of mesoderm cluster into segments along the body axis

somites from the paraxial mesoderm and intermediate mesoderm

the limbs develop during the

start of the second month

induction

the ability of one group of cells to influence the developmental direction of other cells, disruption of these processes can result in development abnormalities -ex. the influence of the notochord on the formation of the neural tube

what is the function of the amniotic sac and the fluid it contains

the amniotic sac carry the fetus, the amniotic fluid buffers the developing embryo and fetus from physical shock until the time of birth , the rupture of the amniotic sac and release of its amniotic fluid nears the start of labor

which abdominal structures form from the inner tube

the digestive system (stomach, intestines, forms from inner tube), liver, pancreas, and gallbladder

after folding takes place which embryonic germ layer forms the outer covering of the embryo

the ectoderm

using directional terms describe the position of the kidneys in reference to the peritoneal cavity

the kidneys are located posterior (dorsal) to the peritoneal cavity

what structure induces the formation of the neural tube

the notochord

which part of somite forms the vertebrae and ribs

the sclerotome forms the vertebrae and ribs

embryology

the study of the origin and development of a single individual

prenatal period

time between conception and birth, divided into two stages: embryonic period (first 8 weeks) and the fetal period (remaining 30 weeks)

the outer layer of the blastocyst which attached to the uterine wall is the

trophoblast

it is currently possible to save some premature babies as early as

week 22