Development

Differentiation and development Gradual modification of physical and physiological characteristics Differentiation The creation of different types of cells

Stages of development Prenatal development Embryological Changes occurring the first two months after fertilization Fetal Begins at the start of the ninth week and continues until birth Postnatal development Commences at birth and continues to maturity

Fertilization (conception) Occurs in the uterine tubes Within a day of ovulation Spermatozoa cannot fertilize an ovum until after capacitation

Fertilization

The Oocyte at Ovulation Oocyte is in meiosis II Surrounded by the corona radiate Spermatozoa release hyaluronidase and acrosin Enzymes required to penetrate corona radiate Single spermatozoan contacts oocyte, fertilization begins Oocyte activation -Oocyte completes meiosis II -Functionally mature ovum -Female pronucleus and male pronucleus fuse (amphimixis) -Polyspermy prevented by membrane depolarization and cortical reaction

Embryonic and Fetal Periods Gestation periods Three trimesters First trimester Second trimester Third trimester

The First Trimester Cleavage Zygote becomes a preembryo then a blastocyst Implantation Blastocyst burrows into uterine endometrium Placentation Blood vessels form around blastocyst and placenta develops Embryogenesis Formation of a viable embryo

Cleavage and Blastocyst Formation

Cleavage and blastocyst formation A series of cell divisions that subdivides the cytoplasm of the zygote outer layer of cells- Trophoblast Inner cell mass – cluster of cells at one end of blastocyst

Implantation Occurs about 7 days after fertilization Trophoblast enlarges and spreads Maternal blood flows through open lacunae Gastrulation Embryonic disc composed of germ layers Endoderm Mesoderm Ectoderm

Stages in Implantation

The Inner Cell Mass and Gastrulation

Germ layers Gastrulation By day 12 surface cells move toward the primitive streak A third germ layer forms The three germ layers are: Ectoderm – superficial cells that did not migrate Endoderm – cells facing the blastocoele Mesoderm – migrating cells between ectoderm and endoderm

Extraembryonic Membranes Four extraembryonic membranes: Yolk sac Amnion Allantois Chorion

Extraembryonic Membranes and Placenta Formation

Extraembryonic Membranes and Placenta Formation

Extraembryonic Membranes and Placenta Formation

Embryo Anatomy Yolk sac Important site of blood cell formation Amnion Encloses fluid that surrounds and cushions developing embryo Allantois Eventually becomes bladder Chorion

A Three-Dimensional View of Placental Structure

Placentation Chorionic villi extend into maternal tissue Forms intricate branching network for maternal blood Umbilical cord connects fetus to placenta

Hormones of the placenta Trophoblast secretes hormones to maintain pregnancy HCG Estrogens Progesterone Placental prolactin relaxin

Second and Third Trimesters Second trimester Organ systems increase in complexity Third trimester Many organ systems become fully functional Fetus undergoes largest weight change At end of gestation fetus and uterus push maternal organs out of position

The Second and Third Trimesters