Understanding Embryogenesis and Development - AP Biology

Developing mammalian embryos exhibit rotational cleavage. In this type of cleavage, the first cell division occurs on meridionally. The next cell division in the two resulting daughter cells occurs meridionally in one and equatorially in the other.

In the developing embryo, transcription initiates at the end of cleavage, during a stage called the midblastula transition.

The blastula is defined as beginning following the cleavage stages. Following seven cleavages 128 cells have formed, and will later begin to form the differentiated layers of endoderm, mesoderm and ectoderm before going into the gastrula stage.

During gastrulation, the ectoderm begins to fold in on itself following the primitive streak. These neural folds form the basis of the neural tube, later developing into the adult nervous system.

Somites go on to form skeletal muscle, vertebrae, and the dermis.

The corpus luteum is a structure in the ovary that secretes progesterone, and is preserved during pregnancy with the help of human chorionic gonadotropin (HCG). If fertilization does not take place, the corpus luteum will degenerate into the corpus albicans. As a result, corpus albicans formation only takes place if the woman is not pregnant.

Blastocyst implantation is considered the first event of pregnancy, and the placenta will later form in order to secrete hormones crucial for embryonic development.

The ectoderm is generally credited with the formation of the skin, teeth, nervous system, and sense organs. The lining of the digestive tract is formed from the endoderm.

The fusion of haploid gametes (sperm and egg) produces a zygote. The addition of the chromosomes from sperm and egg restores the diploid state of the cell and gives rise to a diploid organism.

An embryo refers to the developing organism after the first cellular division, following formation of the zygote. A blastocyst forms during the first stages of embryogenesis, and is defined by cellular replication without cellular growth.

The first stage after fertilization is the single-celled zygote, which quickly begins to divide. These divisions, however, do not increase the overall size of the embryo and are known as cleavage divisions. As the mass of cells divides, it is considered a morula.

Eventually, the morula develops an inner cell mass and associated trophoblast. At this stage, it is considered a blastula or blastocyst. It is during this stage that implantation occurs. After implantation, the cell develops the three germ layers through the process of gastrulation. Once the endoderm, mesoderm, and ectoderm have been established, the embryo is considered a gastrula.

Mitosis is the process by which diploid cells duplicate and divide to ensure all cells have the same number of chromosomes and genetic material. Meiosis is the process by which haploid gametes are produced by the primary sex organs.

The term “zygote” is sued to describe a fertilized egg. The zygote is diploid and formed by the fusion of the sperm and egg gametes. The developing organism is called the zygote until it is composed of 16 cells during cleavage, when it is called the “morula.”

Developing embryos exhibit an asymmetry; the zygote can be described as having two hemispheres. These two poles are called the animal and vegetal poles. The vegetal pole contains more yolk and will become the endoderm. The animal pole contains less yolk and will become the ectoderm and mesoderm.

Holoblastic cleavage is the type of cleavage found in developing embryos with a low yolk concentration. This allows for the complete cleavage that forms planes that extend through the entirety of the zygote. Different cleavage patterns in holoblastic cleavage include bilateral, radial, rotational, and spiral. Animals whose zygotes undergo holoblastic cleavage include placental mammals, amphibians, and annelids.

Meroblastic cleavage is partial cleavage in zygotes with a high concentration of yolk. In developing embryos that undergo this process, cell divisions do not penetrate the yolk. Two types of meroblastic cleavage are discoidal and superficial cleavage. This process occurs in insects, some fish, and some types of birds.

Determinate cleavage is a process found in most protostomes, in which the developmental fate of cells is determined early on in the developmental process. In other words, blastomeres do not have the capacity to develop into any cell type.

Indeterminate cleavage takes place in most deuterostomes. It is a type of cleavage in which embryonic cells can be separated and successfully develop into different organisms.

The trophoblast is the outer layer of cells that make up the blastocyst. These cells provide nutrients to the developing embryo, and eventually develop into the placenta. During the later stages of cleavage, the trophoblast becomes distinguished from the inner layer of cells, which will develop into the embryo. There is a layer of fluid between the trophoblast and inner cell mass.

In a human egg cell the yolk—ooplasm—located at the cell center in an area containing the nucleus and nucleolus. The ooplasm is composed of fatty granules that support embryo development through the early stages of embryogenesis. In human oocytes, there is a relatively small amount of ooplasm compared to the embryos of other species.

During fertilization and throughout the process of cleavage, the developing embryo is located in the fallopian tube. Towards the end of cleavage, around day 5, the developing embryo enters the uterus, where it will implant in the uterine wall after blastulation.