In mammals, pregnancy is the period of reproduction during which a female carries one or more live offspring from implantation in the uterus through gestation. It begins when a fertilized zygote implants in the female's uterus, and ends once it leaves the uterus.
A zygote is a eukaryotic cell formed by a fertilization event between two gametes. The zygote's genome is a combination of the DNA in each gamete, and contains all of the genetic information of a new individual organism.
An embryo is an initial stage of development of a multicellular organism. In organisms that reproduce sexually, embryonic development is the part of the life cycle that begins just after fertilization of the female egg cell by the male sperm cell. The resulting fusion of these two cells produces a single-celled zygote that undergoes many cell divisions that produce cells known as blastomeres. The blastomeres are arranged as a solid ball that when reaching a certain size, called a morula, takes in fluid to create a cavity called a blastocoel. The structure is then termed a blastula, or a blastocyst in mammals.
Blastulation is the stage in early animal embryonic development that produces the blastula. The blastula is a hollow sphere of cells known as blastomeres surrounding an inner fluid-filled cavity called the blastocoel. Embryonic development begins with a sperm fertilizing an egg cell to become a zygote, which undergoes many cleavages to develop into a ball of cells called a morula. Only when the blastocoel is formed does the early embryo become a blastula. The blastula precedes the formation of the gastrula in which the germ layers of the embryo form.
A morula is an early-stage embryo consisting of a solid ball of cells called blastomeres, contained within the zona pellucida. The blastomeres are the daughter cells of the zygote, and in the human by day 4 after fertilisation the blastomeres number from 16–32 and the ball of cells is called the morula.
During fertilization, a sperm must first fuse with the plasma membrane and then penetrate the female egg cell to fertilize it. Fusing to the egg cell usually causes little problem, whereas penetrating through the egg's hard shell or extracellular matrix can be more difficult. Therefore, sperm cells go through a process known as the acrosome reaction, which is the reaction that occurs in the acrosome of the sperm as it approaches the egg. The acrosome is a cap-like structure over the anterior half of the sperm's head.
The blastocyst is a structure formed in the early development of mammals. It possesses an inner cell mass (ICM) which subsequently forms the embryo. The outer layer of the blastocyst is called the trophectoderm that consists of cells called trophoblasts. This layer surrounds the inner cell mass and a fluid-filled cavity known as the blastocoel. The trophoblast gives rise to the chorion and amnion that surround the embryo. The placenta derives from the embryonic chorion and the underlying uterine tissue of the mother. The name "blastocyst" arises from the Greek βλαστός blastos and κύστις kystis. In other animals this is a structure consisting of an undifferentiated ball of cells and is called a blastula.
Preimplantation genetic diagnosis is the genetic profiling of embryos prior to implantation, and sometimes even of oocytes prior to fertilization. PGD is considered in a similar fashion to prenatal diagnosis. When used to screen for a specific genetic disease, its main advantage is that it avoids selective abortion, as the method makes it highly likely that the baby will be free of the disease under consideration. PGD thus is an adjunct to assisted reproductive technology, and requires in vitro fertilization (IVF) to obtain oocytes or embryos for evaluation. Embryos are generally obtained through blastomere or blastocyst biopsy. The latter technique has proved to be less deleterious for the embryo, therefore it is advisable to perform the biopsy around day 5 or 6 of development.
A germ layer is a primary layer of cells that forms during embryonic development. The three germ layers in vertebrates are particularly pronounced; however, all eumetazoans produce two or three primary germ layers. Some animals, like cnidarians, produce two germ layers making them diploblastic. Other animals such as bilaterians produce a third layer between these two layers, making them triploblastic. Germ layers eventually give rise to all of an animal’s tissues and organs through the process of organogenesis.
The zona pellucida is a specialized extracellular matrix that surrounds the plasma membrane of mammalian oocytes. It is a vital constitutive part of the oocyte. The zona pellucida first appears in unilaminar primary oocytes. It is secreted by both the oocyte and the ovarian follicles. The zona pellucida is surrounded by the corona radiata. The corona is composed of cells that care for the egg when it is emitted from the ovary.
Prenatal development includes the development of the embryo and of the foetus during a viviparous animal's gestation. Prenatal development starts with fertilization, in the germinal stage of embryonic development, and continues in fetal development until birth.
"Cytotrophoblast" is the name given to both the inner layer of the trophoblast or the cells that live there. It is interior to the syncytiotrophoblast and external to the wall of the blastocyst in a developing embryo.
The corona radiata is the innermost layer of the cells of the cumulus oophorus and is directly adjacent to the zona pellucida, the inner protective glycoprotein layer of the ovum. Cumulus oophorus are the cells surrounding corona radiata, and are the cells between corona radiata and follicular antrum. Its main purpose in many animals is to supply vital proteins to the cell. It is formed by follicle cells adhering to the oocyte before it leaves the ovarian follicle, and originates from the squamous granulosa cells present at the primordial stage of follicular development. The corona radiata is formed when the granulosa cells enlarge and become cuboidal, which occurs during the transition from the primordial to primary stage. These cuboidal granulosa cells, also known as the granulosa radiata, form more layers throughout the maturation process, and remain attached to the zona pellucida after the ovulation of the Graafian follicle. For fertilization to occur, sperm cells rely on hyaluronidase to disperse the corona radiata from the zona pellucida of the secondary (ovulated) oocyte, thus permitting entry into the perivitelline space and allowing contact between the sperm cell and the nucleus of the oocyte.
In embryology, Carnegie stages are a standardized system of 23 stages used to provide a unified developmental chronology of the vertebrate embryo.
In female mammals implantation is the stage in embryonic development in which the blastocyst hatches as the embryo, and adheres to the wall of the uterus. Once this adhesion is successful, the female is considered to be pregnant and the embryo will receive oxygen and nutrients from the mother in order to grow.
The bilaminar embryonic disc, or bilaminar blastoderm refers to the two-layered structure of epiblast and hypoblast, evolved from the inner cell mass also known as the embryoblast. These two layers of cells lie between two cavities: the primitive yolk sac and the amniotic cavity.
Assisted zona hatching (AZH) is a procedure of assisted reproductive technology in which a small hole is made in the zona pellucida, using a micromanipulator, thereby facilitating zona hatching. Zona hatching is where the blastocyst gets rid of the surrounding zona pellucida to be able to implant in the uterus.
Human embryonic development, or human embryogenesis, is the development and formation of the human embryo. It is characterised by the processes of cell division and cellular differentiation of the embryo that occurs during the early stages of development. In biological terms, the development of the human body entails growth from a one-celled zygote to an adult human being. Fertilization occurs when the sperm cell successfully enters and fuses with an egg cell (ovum). The genetic material of the sperm and egg then combine to form the single cell zygote and the germinal stage of development commences. Embryonic development in the human, covers the first eight weeks of development; at the beginning of the ninth week the embryo is termed a fetus. Human embryology is the study of this development during the first eight weeks after fertilization. The normal period of gestation (pregnancy) is about nine months or 40 weeks.
The perivitelline space is the space between the zona pellucida and the cell membrane of an oocyte or fertilized ovum. In the slow block to polyspermy, the cortical granules released from the ovum are deposited in the perivitelline space. Polysaccharides released in the granules cause the space to swell, pushing the zona pellucida farther from the oocyte. The hydrolytic enzymes released by the granules cause the zona reaction, which removes the ZP3 ligands from the zona pellucida.
Repeated Implantation failure (RIF) is the failure of the embryo to implant onto the side of the uterus wall following IVF treatment. Regularly, this happens at 6–7 days after conception and involves the embedding of the growing embryo into the mothers uterus and a connection being formed. A successful implantation can be determined by using an ultrasound to view the sac which the baby grows in, inside the uterus.
