Chapter 18 Development

Fertilization, Cleavage, and Implantation

FIGURE 18.2 Early stages in the reproductive process

Fertilization Sperm and egg unite: 6-24 hours after intercourse, sperm survives days in female reprod. tract One sperm fertilizes one egg Occurs in upper oviduct Acrosome: contains digestive enzymes Sperm digests thru follicle cells and zona pellucida Sperm membrane fuses with egg membrane Zona pellucida made impermeable to other sperm

FIGURE 18.3a Fertilization. (a) Diagram showing the processes that occur before and during fertilization.

FIGURE 18.A Intracytoplasmic sperm injection. An egg, held in place with a blunt-nosed pipette (right), is injected with a single sperm.

How many sperm normally fertilize a single egg? 1 2 4 More than a million

Early Development Cleavage involves: cell division without cell growth, up to about day four Function is to restore cell size to normal (i.e. smaller than the egg cell) Produces a “morula” – a solid ball of cells

FIGURE 18.8 Early stages of development in cross section

Blastocyst Inner cell mass cells form embryo proper Source of embryonic stem cells Outer (trophoblast) cells form placenta Secrete hCG (human chorionic gonadotropin) after implantation Extends life of corpus luteum Basis of pregnancy test

Implantation FIGURE 18.5 Implantation. About 6 days after fertilization, the blastocyst attaches to the endometrium of the uterus and begins to digest its way inward.

Implantation

The cells of the blastocyst that produce the embryo proper are called: Zona cells Trophoblast cells Inner cell mass Amnion

Gastrulation Cells migrate to produce a three layered embryo Ectoderm: outermost layer Forms nervous system, skin, etc. Mesoderm: middle layer Muscle, bones, blood, gonads, etc. Endoderm: innermost layer Viscera, gut, etc.

Neurulation Forms Neural Tube The neural tube forms the CNS (brain & spinal cord) Inductive interactions between tissues Chemical signals influence development of nearby tissue Ex: Neural tube induced by notochord

FIGURE 18.9 Formation of the central nervous system from ectoderm. Dorsal views of a human embryo (left) and corresponding cross-sectional views or parts of such views (right) are shown at 4 days during development of the brain and spinal cord.

FIGURE 18.9 part 1 Formation of the central nervous system from ectoderm. Dorsal views of a human embryo (left) and corresponding cross-sectional views or parts of such views (right) are shown at 4 days during development of the brain and spinal cord.

FIGURE 18.9 part 2 Formation of the central nervous system from ectoderm. Dorsal views of a human embryo (left) and corresponding cross-sectional views or parts of such views (right) are shown at 4 days during development of the brain and spinal cord.

FIGURE 18.9 part 3 Formation of the central nervous system from ectoderm. Dorsal views of a human embryo (left) and corresponding cross-sectional views or parts of such views (right) are shown at 4 days during development of the brain and spinal cord.

Somites: form bone & muscle FIGURE 18.9 part 4 Formation of the central nervous system from ectoderm. Dorsal views of a human embryo (left) and corresponding cross-sectional views or parts of such views (right) are shown at 4 days during development of the brain and spinal cord. Somites: form bone & muscle

The neural tube develops into: The digestive tube Reproductive tract Brain and spinal cord

Extraembryonic Membranes and Placenta Amnion: lines amniotic cavity, filled with amniotic fluid Chorion: makes human chorionic gonadotropin (hCG)

FIGURE 18.6 Extraembryonic membranes. The amnion, yolk sac, chorion, and allantois begin to form during the second to third week after fertilization.

Placenta Functions Nutrient exchange with mother Umbilical cord connects fetus to placenta Filters nutrients, waste, and provides antibodies for fetus No mixing of mother and fetal circulations Endocrine organ: produces hormones hCG Estrogen and Progesterone

FIGURE 18.7 The placenta is formed from the chorion of the embryo and the endometrium of the mother. (a) The placenta begins to form at about 2 weeks (shortly after implantation) and is fully developed by about 12 weeks. (b) The internal structure of the placenta.

FIGURE 18.7b The placenta is formed from the chorion of the embryo and the endometrium of the mother. (b) The internal structure of the placenta.

The following are functions of the placenta Nutrient exchange with the mother Produces hormones required for pregnancy

FIGURE 18.13c The developing human. (c) An embryo about 7 weeks old.

FIGURE 18.13d The developing human. (d) A fetus about 4 months old.

FIGURE 18.13e The developing human. (e) A fetus about 5 months old.

During the first stage of labor, the cervix opens (dilates) and thins out (effaces) to allow the baby to move into the birth canal. In figures A and B, the cervix is tightly closed. In figure C, the cervix is 60 percent effaced and 1 to 2 cm dilated. In figure D, the cervix is 90 percent effaced and 4 to 5 cm dilated. The cervix must be 100 percent effaced and 10 centimeters dilated before a vaginal delivery.

Factors That Contribute to Labor Estrogen: Progesterone ratio High P promotes uterine quiescence (early pregnancy) High E promotes uterine contractility (late pregnancy) Oxytocin promotes uterine contractions via neuroendocrine reflex

Figure: 10-10 Title: Oxytocin and childbirth. Caption: The steps by which oxytocin stimulates uterine contractions during childbirth.

Lactation During Pregnancy: After Childbirth: High E and P during pregnancy promote breast development but inhibit milk prodn. After Childbirth: E & P levels plummet Prolactin promotes milk synthesis Oxytocin induces milk ejection via neuroendocrine reflex

Figure: 10-11 Title: Oxytocin and milk ejection. Caption: The steps by which oxytocin stimulates milk ejection (letdown) from the mammary glands.

Environmental Disruptions During the Embryonic Period Cause Major Birth Defects

Lower limbs FIGURE 18.16 Critical periods in development

Spina bifida

Spina bifida baby