embryonic development

Embryonic Development

Development: series of orderly, precise steps that transform a zygote into a multicellular embryo ~early stage of development of multicellular organism

Includes:

1. cell division 2. cell growth 3. cell differentiation

~changing of unspecialized embryonic cells into specialized cells, tissues,& organs

Early Embryonic Development

Cleavage is the first major phase of embryonic development It is the rapid succession

of cell divisions (Mitotic) It creates a multicellular

embryo from the zygote NO growth Stages:

1. Morula~solid ball of cells

2.Blastula~ single layer of cells surrounding a fluid-filled cavity called the blastocoel

ZYGOTE 2 cells

4 cells

8 cells

Many cells(solid ball)

BLASTULA(hollow ball)

Cross sectionof blastula

Blastocoel


Gastrulation is the second major phase of embryonic development

The cells at one end of the blastula move inward,

It adds more cells to the embryo

It sorts all cells into three distinct cell layers

The embryo is transformed from the blastula into the gastrula


The three layers produced in gastrulation

Ectoderm, the outer layer

Endoderm, an embryonic digestive tract

Mesoderm, which partly fills the space between the ectoderm and endoderm


The tissues and organs of a tadpole emerge from cells of the ectoderm, mesoderm, and endoderm


ectoderm cells~ eventually they develop into the skin and nervous tissue of the animal

endoderm cells ~develop into the lining of the animal’s digestive tract and into organs associated with digestion.

mesoderm cells ~develop into the muscles, circulatory system, excretory system, and, in some animals, the respiratory system.


Organs start to form after gastrulationEmbryonic tissue layers begin to

differentiate into specific tissues and organ systems

Changes in cell shape, cell migration, and programmed cell death give form to the developing

animal

Tissues and organs take shape in a developing embryo as a result of cell shape changes cell migration

Ectoderm

Development

programmed cell death (apoptosis)

Cellsuicide

Dead cellengulfed anddigested byadjacentcell

Embryonic induction initiates organ formation

Induction is the mechanism by which one group of cells influences the development of tissues and organs from ectoderm, endoderm, and mesodermAdjacent cells and cell layers use chemical

signals to influence differentiation Chemical signals turn on a set of genes

whose expression makes the receiving cells differentiate into a specific tissue

Pattern formation organizes the animal body

Pattern formation is the emergence of a body form with structures in their correct relative positions It involves the response of genes to spatial

variations of chemicals in the embryo

HUMAN DEVELOPMENT

The embryo and placenta take shape during the first month of pregnancy

Gestation is pregnancy It begins at conception and continues until

birthHuman gestation is 266 days

(38 weeks or 9 months)Mouse gestation is 1 monthElephant gestation is 22 months

HUMAN DEVELOPMENT

Human development begins with fertilization in the oviduct

Ovary

Ovulation

Fertilizationof ovum

Cleavage starts

Blastocyst(implanted

Endometrium

Uterus

Oviduct

HUMAN DEVELOPMENT

Cleavage produces a blastocyst

A blastocyst is a fluid-filled cavity The inner cells of

the blastocyst form the baby

The outer cells form the embryo trophoblast

ENDOMETRIUM

ENDOMETRIUM

Cavity

Trophoblast

Inner cell mass

HUMAN DEVELOPMENT

The trophoblast secretes enzymes to enable the blastocyst to implant in the uterine wall

ENDOMETRIUM

UTERINE CAVITY

Trophoblast

Multiplying cellsof trophoblast

Blood vessel(maternal

Futureyolk sac

Future embryo

HUMAN DEVELOPMENT

Gastrulation occurs and organs develop from the ectoderm, endoderm, and mesoderm

Yolk sac

Mesodermcells

Amnion

Chorion

HUMAN DEVELOPMENT

Meanwhile, the four embryonic membranes develop Amnion~fluid filled sac

for protection Chorion ~ will form the

embryo’s part of the placenta

Yolk sac ~ produces first blood cells &germ cells

Allantois ~ will form the umbilical cord (ropelike structure that attaches embryo to uterus)

Yolk sac

Chorion

Amnion

Allantois

HUMAN DEVELOPMENT

The embryo floats in the fluid-filled amniotic cavity,

The placenta’s chorionic villi absorb food and oxygen from the mother’s blood

Placenta

Amnioticcavity

Chorion

Chorionicvilli

Yolk sac

Mother’s bloodvessels

Embryo

Amnion

Placenta

Food & gases diffuse across blood vessels

HUMAN DEVELOPMENT

Embryonic development of essential organs occur in early pregnancy

The embryo may encounter risks from faults in its genes & from mother’s exposure to environmental factors

The placenta allows for a variety of substances to pass from mother to fetus Protective antibodies German measles virus HIV Drugs (prescription and nonprescription) Alcohol Chemicals in tobacco smoke

Human development from conception to birth is divided into three trimesters

First trimester First three months The most rapid changes occur during the first trimester

7 weeks 10 weeks

4 weeks


Second trimester Increase in size of fetus General refinement of human features

12 weeks

Human fetal development

The fetus just spends much of the 2nd & 3rd trimesters just growing

…and doing various flip-turns & kicks inside amniotic fluid

Week 20


24 weeks (6 months; 2nd trimester)

fetus is covered with fine, downy hair called lanugo. Its skin is protected by a waxy material called vernix


30 weeks (7.5 months)

umbilical cord

Getting crowded in there!!

32 weeks (8 months)

The fetus sleeps 90-95% of the day & sometimes experiences REM sleep, an indication of dreaming


Third trimester Growth and

preparation for birth

Childbirth is hormonally induced and occurs in three stages

Hormonal changes induce birthLabor is controlled by a positive feedback

mechanismEstrogen released from the ovaries

increases the sensitivity of the uterus to oxytocin

Positive feedback Mechanism

Oxytocin is a powerful stimulant for the smooth muscles of the uterus Oxytocin also

stimulates the placenta to make prostoglandins that stimulate the uterine muscles to contract even more

Uterine contractions stimulate the release of more and more oxytocin and prostoglandins

Birthpositive feedback

Three stages of labor

1.Dilation of the cervix is the first stage

-Cervix reaches full dilation at 10cm Longest stage of

labor (6-12 hours or longer)

Dilation of the cervix


Expulsion is the second stage Period from full dilation of

the cervix to delivery of the infant

Uterine contractions occur every 2-3 minutes

Mother feels urge to push down with her abdominal muscles

Infant is forced down and out of uterus and vagina within a period of 20 minutes


The delivery of the placenta is the final stage of labor Usually occurs

within 15 minutes after the birth of the baby

The end of the journey!

And you think 9 months of Biology is hard!

Childbirth is hormonally induced and occurs in three stages

Hormones continue to be important after the baby and placenta are deliveredDecreasing progesterone and estrogen

levels allow the uterus to return to its pre-pregnancy state

Oxytocin and prolactin stimulate milk secretion

Reproductive technology increases our reproductive options

Reproductive technology Hormone therapy can

increase sperm or egg production

Surgery can correct blocked oviducts

Assisted reproductive technology In vitro fertilization

(IVF) Surrogate motherhood

embryonic development

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