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 Blastocoel Many cells (solid ball) Cross section of blastula BLASTULA (hollow ball)

Embryonic Development 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

Embryonic Development 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

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

Embryonic Development 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.

Embryonic Development Organs start to form after gastrulation Embryonic tissue layers begin to differentiate into specific tissues and organ systems

Tissues and organs take shape in a developing embryo as a result of 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) Cell suicide Dead cell engulfed and digested by adjacent cell programmed cell death (apoptosis)

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 mesoderm Adjacent 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 birth Human gestation is 266 days (38 weeks or 9 months) Mouse gestation is 1 month Elephant gestation is 22 months

HUMAN DEVELOPMENT Human development begins with fertilization in the oviduct Cleavage starts Fertilization of ovum Ovary Oviduct Blastocyst (implanted Ovulation Endometrium Uterus

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 Inner cell mass Cavity Trophoblast

HUMAN DEVELOPMENT The trophoblast secretes enzymes to enable the blastocyst to implant in the uterine wall ENDOMETRIUM Blood vessel (maternal Future embryo Multiplying cells of trophoblast Future yolk sac Trophoblast UTERINE CAVITY

HUMAN DEVELOPMENT Gastrulation occurs and organs develop from the ectoderm, endoderm, and mesoderm Amnion Mesoderm cells Chorion Yolk sac

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) Chorion Amnion Allantois Yolk sac

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 Mother’s blood vessels Placenta Yolk sac Amniotic cavity Amnion Embryo Chorion Chorionic villi

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

Human development from conception to birth is divided into three trimesters 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

Human fetal development 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

Human fetal development 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

Human development from conception to birth is divided into three trimesters Third trimester Growth and preparation for birth

Childbirth is hormonally induced and occurs in three stages Hormonal changes induce birth Labor is controlled by a positive feedback mechanism Estrogen 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

positive feedback Birth

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

Three stages of labor 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

Three stages of labor 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 delivered Decreasing 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 Hormone therapy can increase sperm or egg production Surgery can correct blocked oviducts Assisted reproductive technology In vitro fertilization (IVF) Surrogate motherhood