First Week of Development

Fertilization

Fertilization occurs when a sperm and an egg have fused together to form a zygote, which begins to divide as it moves towards the uterus.

Learning Objectives

Describe human fertilization

Key Takeaways

Key Points

  • Fertilization is commonly known as conception. Once the fertilized gamete (ovum) implants itself in the uterine lining, pregnancy begins.
  • The fusion of male and female gametes ( sperm and ovum, respectively) usually occurs following the act of sexual intercourse. However, artificial insemination and in vitro fertilization have made achieving pregnancy possible without engaging in sexual intercourse.
  • The process of fertilization occurs in several steps and the interruption of any of them can lead to failure.
  • Prior to fertilization, sperm undergo a process of capacitation in response to conditions in the female reproductive tract, which include increases in motility and destabilization of the cell membrane that allows the head of the sperm to penetrate the egg.

Key Terms

  • fertilization: The act of fecundating or impregnating animal or vegetable gametes.
  • capacitation: A step spermatozoa undergo in the female reproductive track that renders them capable of fertilizing an oocyte.
  • implantation: The embedding of the fertilized ovum into the uterine wall.
  • Nondisjunction: Nondisjunction is the failure of homologous chromosomes or sister chromatids to separate properly during cell division.
  • zygote: A fertilized egg cell.

If pregnancy is considered to begin at the point of implantation, the process leading to pregnancy occurs earlier as the result of the female gamete, or oocyte, merging with the male gamete, or spermatozoon. In medicine, this process is referred to as fertilization; in lay terms, it is more commonly known as conception.

After the point of fertilization the fused product of the female and male gamete is referred to as a zygote or fertilized egg. For species that undergo internal fertilization, such as humans, the fusion of male and female gametes usually occurs following the act of sexual intercourse.

However, the advent of artificial insemination and in vitro fertilization have made achieving pregnancy possible without engaging in sexual intercourse. This approach may be undertaken as a voluntary choice or due to infertility.

This is a diagram of human fertilization. It shows the sperm and ovum uniting through fertilization. This union creates a zygote that develops over the course of 8 to 9 days that will implant itself in the uterine wall, where it will reside over the course of 9 months.

Human fertilization: The sperm and ovum unite through fertilization, creating a zygote that (over the course of 8–9 days) will implant in the uterine wall, where it will reside over the course of 9 months.

The process of fertilization occurs in several steps and the interruption of any of them can lead to failure. At the beginning of the process, the sperm undergoes a series of changes, as freshly ejaculated sperm is unable or poorly able to fertilize.

The sperm must undergo capacitation in the female’s reproductive tract over several hours, which increases its motility and destabilizes its membrane. By destabilizing the membrane, the sperm prepares for the acrosome reaction, the enzymatic penetration of the egg’s tough membrane, the zona pellucida. The sperm and the egg cell (which has been released from one of the female’s two ovaries) unite in one of the two fallopian tubes.

The fertilized egg, known as a zygote, then moves toward the uterus, a journey that can take up to a week to complete until implantation occurs. Through fertilization, the egg is activated to begin its developmental process (progressing through meiosis II), and the haploid nuclei of the two gametes come together to form the genome of a new diploid organism.

Nondisjunction during the completion of meiosis or problems with early cell division in the zygote to blastula stages can lead to problems with implantation and pregnancy failure.

Cleavage of the Zygote

The process of cleavage is the step of embryogenesis where the zygote divides to produce a cluster of cells known as the morula.

Learning Objectives

Describe cleavage of the zygote

Key Takeaways

Key Points

  • Following fertilization a series of rapid cell divisions occur that decrease the cells’ size with each subsequent division—this eventually produces a morula. The different cells derived from cleavage, up to the blastula stage, are called blastomeres.
  • For species such as humans, there is little yolk in eggs, and the divisions are relatively symmetrical, or holoblastic.
  • For other species, such as birds and reptiles, the presence of yolk dictates uneven meroblastic divisions that produce cells of uneven size and distribution.
  • The end of cleavage is known as the midblastula transition and coincides with the onset of zygotic transcription.
  • The cells of the morula are at first closely aggregated, but quickly become arranged into an outer or peripheral layer, the trophoblast, which does not contribute to the formation of the embryo proper, and an inner cell mass from which the embryo develops.

Key Terms

  • cleavage: In embryology, this is the division of cells in the early embryo.
  • trophoblast: The membrane of cells that forms the wall of a blastocyst during early pregnancy and provides nutrients to the embryo, and later develops into part of the placenta.
  • zygote: A fertilized egg cell.
  • morula: A spherical mass of blastomeres that forms following the splitting of a zygote; it becomes the blastula.

Cell division with no significant growth that produces a cluster of cells that is the same size as the original zygote, is called cleavage. At least four initial cell divisions occur, resulting in a dense ball of at least sixteen cells called the morula.

The different cells derived from cleavage up to the blastula stage are called blastomeres. Depending mostly on the amount of yolk in the egg, the cleavage can be holoblastic (total) or meroblastic (partial).

This is a drawing of cell cleavage. The cell stays the same size in all 5 drawings, but the zygotes inside the cell increase in number.

Cell cleavage: Early development is characterized by cleavage of the zygote, which refers to cell divisions that are not associated with significant growth of the embryo.

Holoblastic cleavage occurs in animals with little yolk in their eggs. These species, such as humans and other mammals, receive nourishment as embryos from the mother via the placenta or milk after birth.

On the other hand, meroblastic cleavage occurs in animals whose eggs have more yolk, such as birds and oviparous reptiles (although some viviparous reptiles also exist). Since cleavage is impeded by the vegetal pole, there is a very uneven distribution and size of cells. Cells are more numerous and smaller at the animal pole of the zygote than at the vegetal pole.

In holoblastic eggs, the first cleavage always occurs along the vegetal–animal axis of the egg, and the second cleavage is perpendicular to the first. From here, the spatial arrangement of blastomeres can follow various patterns, due to different planes of cleavage in various organisms.The end of cleavage is known as the midblastula transition and coincides with the onset of zygotic transcription.

In amniotes, the cells of the morula are at first closely aggregated. However, they quickly become arranged into an outer or peripheral layer, the trophoblast, and an inner cell mass. The trophoblast does not contribute to the formation of the embryo proper; the embryo develops from the inner cell mass.

Fluid collects between the trophoblast and the greater part of the inner cell mass, and thus the morula, is converted into the blastodermic vesicle (also called the blastocyst or blastula). The inner cell mass remains in contact with the trophoblast at one pole of the ovum. This is named the embryonic pole, since it indicates the location where the future embryo will develop.

In the case of monozygotic twins (derived from one zygote), a zygote divides into two separate cells (embryos) at the first cleavage division. Monozygotic twins can also develop from two inner cell masses.

A rare occurrence is the division of a single inner cells mass giving rise to twins. However, if one inner cell mass divides incompletely, the result is conjoined twins. Dizygotic twins is the development of two embryos from two different zygotes.

Blastocyst Formation

The blastocyst forms early in embryonic development and has two layers that form the embryo and placenta.

Learning Objectives

Outline the process of blastocyst formation

Key Takeaways

Key Points

  • The human blastocyst possesses an inner cell mass (ICM), or embryoblast, which subsequently forms the embryo, and an outer layer of cells, or trophoblast, which later forms the placenta.
  • The trophoblast surrounds the inner cell mass and a fluid-filled, blastocyst cavity known as the blastocoele or the blastocystic cavity. The trophoblast combines with the maternal endometrium to form the placenta in eutherian mammals.
  • Before gastrulation, the cells of the trophoblast become differentiated into two strata: the ectoderm of the chorion plays a role in the development of the placenta, and the endoderm differentiates and quickly assumes the form of a small sac, called the yolk sac.
  • The embryoblast is the source of embryonic stem cells and gives rise to all later structures of the adult organism.
  • The floor of the amniotic cavity is formed by the embryonic disk, which is composed of a layer of prismatic cells, and the embryonic ectoderm, which is derived from the inner cell mass and lies in opposition to the endoderm.

Key Terms

  • embryonic disk: The floor of the amniotic cavity is formed by the embryonic disk (or disc) that is composed of a layer of prismatic cells called the embryonic ectoderm. It is the part of the inner cell mass from which the embryo is developed.
  • blastocyst: An early form in the development of an embryo that consists of a spherical layer of cells filled with fluid.
  • gastrulation: The stage of embryonic development when a gastrula is formed from the blastula by the inward migration of cells.
  • eutherian: Refers to all species of which the female gives birth to live young that receive prenatal nourishment via the placenta.

In humans, the blastocyst is formed approximatelyy five days after fertilization. This stage is preceded by the morula. The morula is a solid ball of about 16 undifferentiated, spherical cells. As cell division continues in the morula, the blastomeres change their shape and tightly align themselves against each other. This is called compaction and is likely mediated by cell surface adhesion glycoproteins.

The blastocyst possesses an inner cell mass (ICM), or embryoblast, which subsequently forms the embryo, and an outer layer of cells, or trophoblast, which later forms the placenta. The trophoblast surrounds the inner cell mass and a fluid-filled, blastocyst cavity known as the blastocoele or the blastocystic cavity.

The embryoblast is the source of embryonic stem cells and gives rise to all later structures of the adult organism. The trophoblast combines with the maternal endometrium to form the placenta in eutherian mammals.

This is a drawing of a blastocyst. The blastocyst possesses an inner cell mass from which the embryo will develop, and an outer layer of cells, called the trophoblast, which will eventually form the placenta. The endometrium, inner cell mass, trophoblast, and blastocyst cavity are identified on the drawing.

Blastocyst: The blastocyst possesses an inner cell mass from which the embryo will develop, and an outer layer of cells, called the trophoblast, which will eventually form the placenta.

Before gastrulation, the cells of the trophoblast become differentiated into two strata. The outer stratum forms a syncytium, which is a layer of protoplasm studded with nuclei that shows no evidence of subdivision into cells (termed the syncytiotrophoblast).

The inner layer, the cytotrophoblast or layer of Langhans, consists of well-defined cells. As already stated, the cells of the trophoblast do not contribute to the formation of the embryo proper; they form the ectoderm of the chorion and play an important part in the development of the placenta.

On the deep surface of the inner cell mass, a layer of flattened cells, called the endoderm, is differentiated and quickly assumes the form of a small sac, called the yolk sac. Spaces appear between the remaining cells of the mass and, by the enlargement and coalescence of these spaces, a cavity called the amniotic cavity is gradually developed.

The floor of this cavity is formed by the embryonic disk, which is composed of a layer of prismatic cells called the embryonic ectoderm. This layer is derived from the inner cell mass and lies in opposition to the endoderm.

Implantation

Implantation is the very early stage of pregnancy at which the embryo adheres to the wall of the uterus and begins to form the placenta.

Learning Objectives

Outline the process of implantation in pregnancy

Key Takeaways

Key Points

  • At this stage of prenatal development the embryo is a blastocyst. In humans, implantation of a fertilized ovum occurs between 6 to 12 days after ovulation.
  • In preparation for implantation, the blastocyst sheds its outside layer, the zona pellucida, and is replaced by a layer of underlying cells called the trophoblast. The trophoblast will give rise to the placenta after implantation.
  • During implantation, the trophoblast differentiates into two distinct layers: the inner cytotrophoblast, and the outer syncytiotrophoblast. The syncytiotrophoblast then implants the blastocyst into the endometrium by forming finger-like projections into the uterine wall called chorionic villi.

Key Terms

  • endometrium: The mucous membrane that lines the uterus in mammals and in which fertilized eggs are implanted.
  • trophoblast: The membrane of cells that forms the wall of a blastocyst during early pregnancy and provides nutrients to the embryo and later develops into part of the placenta.
  • human chorionic gonadotropin (hCG): In molecular biology, human chorionic gonadotropin (hCG) is a hormone produced during pregnancy that is made by the developing placenta after conception, and later by the placental component.
  • implantation: The embedding of the blastocyst to the uterine wall.

Implantation is the very early stage of pregnancy during which the embryo embeds into the wall of the uterus. At this stage of prenatal development, the embryo is a blastocyst.

It is by this adhesion that the fetus receives oxygen and nutrients from the mother to be able to grow. In humans, implantation of a blastocyst occurs between 6 to 12 days after ovulation.

In preparation for implantation, the blastocyst sheds its outside layer, the zona pellucida, which binds sperm during fertilization. The zona pellucida degenerates and decomposes, and is replaced by a layer of underlying cells called the trophoblast.

The trophoblast will give rise to the placenta after implantation. During implantation, the trophoblast differentiates into two distinct layers: the inner cytotrophoblast, and the outer syncytiotrophoblast.

This is a drawing of chorionic villi. The syncytiotrophoblasts are identified within the endometrium and are seen attaching to the uterine wall to form chorionic villi.

Chorionic villi: During implantation, extensions of the trophoblast, the syncytiotrophoblasts, embed within the endometrium and form chorionic villi.

The syncytiotrophoblast then implants the blastocyst into the endometrium of the uterus by forming finger-like projections into the uterine wall called chorionic villi. The chorionic villi grow outwards until they come into contact with the maternal blood supply.

The chorionic villi will be the border between maternal and fetal blood during the pregnancy, and the location of gas and nutrient exchange between the fetus and the mother. The creation of chorionic villi is assisted by hydrolytic enzymes that erode the uterine epithelium.

The syncytiotrophoblast also produces human chorionic gonadotropin (hCG), a hormone that notifies the mother’s body that she is pregnant and prevents menstruation by sustaining the function of the progesterone-producing corpus luteum within the ovary.

Human chorionic gonadotropin is the hormone that is detected by pregnancy tests, as it is found in the maternal bloodstream and urine.