Development and Inheritance

How many chromosomes are contained within a human zygote?

46 chromosomes

Extraembryonic membranes and placenta formation (5 steps)

5 steps 1: week 2. Migration of mesoderm around the inner surface of the cellular trophoblast forms the chorion. Mesodermal migration around the outside of the amniotic cavity, between the ectodermal cells and the trophoblast, forms the amnion. Mesodermal migration around the endodermal pouch creates the yolk sac. 2: Week 3. The embryonic disc bulges into the amniotic cavity at the head fold. the allantois, and endodermal extension surrounded mesoderm, extends toward the trophoblast. 3: Week 4. The embryo now has a head fold and a tail fold. Constriction of the connections between the embryo and the surrounding trophoblast narrows the yolk stalk and body stalk. 4: Week 5. The developing embryo and extraembryonic membranes bulge into the uterine cavity. The trophoblast pushing out into the uterine cavity remains covered by endometrium but no longer participates in nutrient absorption and embryo support. The embryo moves away from the placenta, and the body stalk and yolk stalk fuse to form an umbilical stalk. 5. Week 10. The amnion has expanded greatly, filling the uterine cavity. The fetus is connected to the placenta by an elongated umbilical cord that contains a portion of the allantois, blood vessels, and the remnants of the yolk stalk.

Placenta

A complex organ that permits exchange between maternal and embryonic blood.

Inner cell mass

A mass of cells clustered at one end of the blastocyst. These cells are exposed only to the blastocoele and insulated from contact with the outside uterine environment by the trophoblast. In time, this mass will form the embryo.

Cleavage

A process or series of cell divisions that subdivides the cytoplasm of the zygote to produce ever-increasing number of smaller and smaller daughter cells. This begins immediately after fertilization. During this, the zygote becomes a pre-embryo, which develops into a multicellular complex known as a blastocyst. Cells produced from cleavage are called blastomeres.

Fertilization and oocyte activation (2)

Acrosomal enzymes from multiple sperm create gaps in the corona radiata. A single sperm then makes contact with the oocyte membrane, and membrane fusion occurs, triggering oocyte activation and the completion of meiosis.

How do the nucleus and cytoplasm communicate or interact with one another?

Activity in the nucleus varies in response to chemical messages that arrive from the surrounding cytoplasm. In turn, ongoing nuclear activity alters conditions within the cytoplasm by directing the synthesis of specific proteins.

Explain the blood supply separation between the fetus and mother. Start by discussing the villi and then blood supply.

As the chronic villi enlarge, more maternal blood vessels are eroded. Maternal blood now moves more slowly through complex lacunae lined by the syncytial trophoblast. Chorionic blood vessels pass close by, and gases and nutrients diffuse between the embryonic and maternal circulations across the layers of the trophoblast. Recall that fetal hemoglobin has a higher affinity for oxygen than does maternal hemoglobin, enabling fetal hemoglobin to strip oxygen from maternal hemoglobin. Maternal blood then reenters the venous system of the mother through the broken walls of small uterine veins. No mixing of maternal and fetal blood takes place, because layers of trophoblast always separate the two

Gastrulation

At the primitive streak, the migrating cells leave the surface and move between the two existing layers. This movement creates three distinct embryonic layers: (1) the ectoderm, (2) endoderm, and (3) mesoderm. The migration process is called gastrulation.

Germ Layers

At the primitive streak, the migrating cells leave the surface and move between the two existing layers. This movement creates three distinct embryonic layers: (1) the ectoderm, consisting of superficial cells that did not migrate into the interior of the blastodisc; (2) endoderm, consisting of the cells that face the yolk sac; and (3) mesoderm, consisting of the poorly organized layer of migrating cells between the ectoderm and endoderm. Collectively, these three embryonic layers are called germ layers, and the migration process is called gastrulation.

Induction

Chemical interplay among developing cells. As development proceeds, some of the cells release chemical substances, including RNA molecules, polypeptides, and small proteins that affect the differentiation of other embryonic cells

Blastomeres

Daughter cells from the original zygote that have undergone a process of cleavage.

Cleavage and blastocyst formation (6 days)

Day 0: first cleavage division. the first division produces a pre-embryo consisting of two identical cells (blastomeres) The first division is complete roughly 30 hours after fertilization. Divisions continue about every ten hours thereafter. Day 3: After three days of cleaving, the pre-embryo is a solid ball of cells resembling a blackberry. This stage is called morula. Day 4: the murola typically reaches the uterus. Day 5-6: The blastomeres form a blastocyst, a hollow ball with an inner cavity known as the blastocoele. The blastomeres are no longer identical in size or shape. The outer layer of cells of the blastocyst are called the trophoblast. The inner mass of cells is called the inner cell mass.

Formation of the Amniotic Cavity (days 10-12)

Day 10-11: Yolk sac formation. Inner mass separates from the trophoblast creating a fluid-filled chamber called the amniotic cavity. While cells from the superficial layer of the inner cell mass migrate around the amniotic cavity, forming the amnion, cells from the deeper layer migrate around the outer edges of the blastocoele. This is the first stem in the formation of the yolk sac, a second extra-embryonic membrane. For about the next two weeks, the yolk sac is the primary nutrient source for the inner cell mass; it absorbs and distributes nutrients released into the blastocoele by the trophoblast. Day 12: Gastrulation. By day 12, superficial cells of the blastodisc have begun to migrate toward a central line known as the primitive streak. At the primitive streak, the migrating cells leave the surface and move between the two existing layers. This movement creates three distinct embryonic layers: (1) the ectoderm, consisting of superficial cells that did not migrate into the interior of the blastodisc; (2) endoderm, consisting of the cells that face the yolk sac; and (3) mesoderm, consisting of the poorly organized layer of migrating cells between the ectoderm and endoderm. Collectively, these three embryonic layers are called germ layers, and the migration process is called gastrulation. Gastrulation produces an oval, three-layered sheet known as the embryonic disc. This disc will form the body of the embryo, whereas all other cells of the blastocyst will be a part of the extra-embryonic membranes.

Implantation (days 7-9)

Day 7: The blastocyst "hatches" from the zona pellucida and the trophoblast attaches to the uterine wall. Trophoblast cells divide rapidly, making the trophoblast several cells thick. Day 8: cellular membranes of trophoblast cells near the endometrial wall disappear, creating a layer of cytoplasm with multiple nuclei. This is called the synctial trophoblast. The synctial trophoblast erodes a path through the uterine epithelium by secreting an enzyme. Day 9: Implantation proceeds, and the synctial trophoblast continues to enlarge and spread into the surrounding endometrium. The erosion of uterine glands releases nutrients that are absorbed by the synctial trophoblast. They diffuse into the inner mass. The nutrients provides energy for the early stages of embryo formation. Fingerlike villi begin to form and extend away from the trophoblast. Day 10: By day 10 the blastocyst has lost contact with the uterine cavity. Further development occurs entirely within the functional zone of the endometrium

Embryonic development

Development that includes the events during the first two months after fertilization.

Fetal development

Development that includes the events from the start of the ninth week until birth.

Hatching

During blastocyst formation, enzymes released by the trophoblast erode an opening in the zona pellucida, which is then shed in this process.

Amnion

Ectodermal cells spread over the inner surface of the amniotic cavity, soon followed by mesodermal cells. Amniotic fluid is produced, which cushions the developing embryo.

Acrosomal Enzymes

Enzymes on the head of the sperm that disrupt the corona radiata, the layer of follicle cells that surround the oocyte.

Amphimixis

Fusion of male and female pronucleus which creates a Zygote.

Amniotic cavity

Inner mass separates from the trophoblast creating this fluid-filled chamber.

Amphimixis occurs and cleavage begins (4)

Metaphase of first cleavage division. The male pronucleus migrates toward the center of the cell, and spindle fibers form. The male and female pronuclei then fuse in a process called amphimixis.

Fate of the mesoderm

Muscle, bone, cartilage, blood

Fate of the ectoderm

Nervous system, skin

Oocyte at ovulation

Ovulation releases a secondary oocyte and the first polar body; both are surrounded by corona radiata. The oocyte is suspended in metaphase of meiosis II.

Allantois

The allantois begins as an outpocket of the endoderm near the base of the yolk sac. The free endodermal tip then grows toward the wall of the blastocyst, surrounded by a mass of mesodermal cells. The base of the allantois eventually gives rise to the urinary bladder.

Cleavage begins (5)

The cell is now a zygote that contains the normal diploid complement of 46 chromosomes, and fertilization is complete. This is the "moment" of conception. The first cleavage division nears completion about 30 hours after fertilization.

Embryogenesis

The formation of a viable embryo. This process establishes the foundation for all major organ systems.

Differentiation

The formation of different types of cells that occurs when a single cell becomes trillions of organized cells in an organism. This involves changes in the genetic activity of some cells but not others.

Development

The gradual modification of anatomical structures and physiological characteristics during the period from fertilization to maturity.

Spindle formation and cleavage preparation (3)

The male pronucleus develops, and spindle fibers appear in preparation for the first cleavage division.

Chorion

The mesoderm associated with the allantois spreads around the entire blastocyst, separating the cellular trophoblast from the blastocoele. The appearance of blood vessels in the chorion is the first step in the creation of a functional placenta. The the third week of development, the mesoderm extends along the core of each trophoblastic villus, forming chorionic villi in contact with maternal tissue and blood vessels. These villi continue to enlarge and branch forming the placenta, the exchange platform between mother and fetus for nutrients, oxygen, and wastes.

Trophoblast

The outer layer of cells of the blastocyst. These cells provide nutrients to the developing embryo.

First trimester

The period of embryonic and early fetal development. During this time, the beginnings of all the major organ systems appear.

Pronucleus Formation Begins (2)

The sperm is absorbed into the cytoplasm, and the female pronucleus develops.

Prenatal development

The term for development covering both embryonic and fetal periods. Everything from conception to birth.

Inheritance or hereditary

The transfer of genetically determined characteristics from generation to generation.

Yolk Sac

The yolk sac begins as a layer of cells spread out around the outer edges of the blastocoele to form a complete couch. It is the primary nutrient source for early embryonic development, and becomes an important site for blood cell formation.

Implantation

This begins when the blastocyst attaches to the endometrium of the uterus. The process continues as the blastocyst invades maternal tissues.

Placentation

This occurs as blood vessels form around the periphery of the blastocyst, and as the placenta develops.

Third trimester

This period is characterized by rapid fetal growth and deposition of adipose tissue. Early in the third trimester, most of the fetus's major organ systems become fully functional.

Second trimester

This period is dominated by the development of organs and organ systems. By the end of this phase body shape is distinctively human.

Location of fertilization

This typically takes place near the junction between the ampulla and isthmus of the uterine tube. It generally occurs within a day after ovulation.

Gestation

Time spend in prenatal development. We usually divide the gestation period into three trimesters, each lasting three months.

Fate of the endoderm

Urinary bladder, GI tract

Fertilization or conception

When the male and female gametes fuse. The fusion of two haploid gametes, each containing 23 chromosomes, producing a zygote that contains 46 chromosomes, the normal diploid number for a human somatic cell.

Extraembryonic membranes

Yolk sac Amnion Allantois Chorion These membranes support embryonic and fetal development, but few traces of their existence remain in adults.

Genetics

the study of the mechanisms responsible for inheritance is called genetics.