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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.
The uterus or womb is the main hormone-responsive, secondary sex organ of the female reproductive system in humans, and most other mammals. The uterus is the organ that accommodates the developing embryo through prenatal development until childbirth.
The placenta is a temporary embryonic and later fetal organ that begins developing from the blastocyst shortly after implantation. It plays critical roles in facilitating nutrient, gas and waste exchange between the physically separate maternal and fetal circulations, and is an important endocrine organ producing hormones that regulate both maternal and fetal physiology during pregnancy. The placenta connects to the fetus via the umbilical cord, and on the opposite aspect to the maternal uterus in a species-dependent manner. In humans, a thin layer of maternal decidual (endometrial) tissue comes away with the placenta when it is expelled from the uterus following birth. Placentas are a defining characteristic of placental mammals, but are also found in marsupials and some non-mammals with varying levels of development.
Birth is the act or process of bearing or bringing forth offspring, also referred to in technical contexts as parturition. In mammals, the process is initiated by hormones which cause the muscular walls of the uterus to contract, expelling the fetus at a developmental stage when it is ready to feed and breathe.
Development of the human body is the process of growth to maturity. The process begins with fertilization, where an egg released from the ovary of a female is penetrated by a sperm cell from a male. The resulting zygote develops through mitosis and cell differentiation, and the resulting embryo then implants in the uterus, where the embryo continues development through a fetal stage until birth. Further growth and development continues after birth, and includes both physical and psychological development, influenced by genetic, hormonal, environmental and other factors. This continues throughout life: through childhood and adolescence into adulthood.
The amniotic sac, also called the bag of waters or the membranes, is the sac in which the embryo and later fetus develops in amniotes. It is a thin but tough transparent pair of membranes that hold a developing embryo until shortly before birth. The inner of these membranes, the amnion, encloses the amniotic cavity, containing the amniotic fluid and the embryo. The outer membrane, the chorion, contains the amnion and is part of the placenta. On the outer side, the amniotic sac is connected to the yolk sac, the allantois, and via the umbilical cord, the placenta.
The amnion is a membrane that closely covers the human and various other embryos when first formed. It fills with amniotic fluid, which causes the amnion to expand and become the amniotic sac that provides a protective environment for the developing embryo. The amnion, along with the chorion, the yolk sac and the allantois protect the embryo. In birds, reptiles and monotremes, the protective sac is enclosed in a shell. In marsupials and placental mammals, it is enclosed in a uterus.
Blastulation is the stage in early animal embryonic development that produces the blastula. In mammalian development the blastula develops into the blastocyst with a differentiated inner cell mass and an outer trophectoderm. 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.
The blastocyst is a structure formed in the early embryonic development of mammals. It possesses an inner cell mass (ICM) also known as the embryoblast which subsequently forms the embryo, and an outer layer of trophoblast cells called the trophectoderm. This layer surrounds the inner cell mass and a fluid-filled cavity known as the blastocoel. In the late blastocyst the trophectoderm is known as the trophoblast. The trophoblast gives rise to the chorion and amnion, the two fetal membranes 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.
The blastocoel, also spelled blastocoele and blastocele, and also called cleavage cavity, or segmentation cavity is a fluid-filled or yolk-filled cavity that forms in the blastula during very early embryonic development. At this stage in mammals the blastula develops into the blastocyst containing an inner cell mass, and outer trophectoderm.
The trophoblast is the outer layer of cells of the blastocyst. Trophoblasts are present four days after fertilization in humans. They provide nutrients to the embryo and develop into a large part of the placenta. They form during the first stage of pregnancy and are the first cells to differentiate from the fertilized egg to become extraembryonic structures that do not directly contribute to the embryo. After gastrulation, the trophoblast is contiguous with the ectoderm of the embryo and is referred to as the trophectoderm. After the first differentiation, the cells in the human embryo lose their totipotency and are no longer totipotent stem cells because they cannot form a trophoblast. They are now pluripotent stem cells.
In developmental biology, animal embryonic development, also known as animal embryogenesis, is the developmental stage of an animal embryo. Embryonic development starts with the fertilization of an egg cell (ovum) by a sperm cell, (spermatozoon). Once fertilized, the ovum becomes a single diploid cell known as a zygote. The zygote undergoes mitotic divisions with no significant growth and cellular differentiation, leading to development of a multicellular embryo after passing through an organizational checkpoint during mid-embryogenesis. In mammals, the term refers chiefly to the early stages of prenatal development, whereas the terms fetus and fetal development describe later stages.
Prenatal development includes the development of the embryo and of the fetus 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.
Placentation refers to the formation, type and structure, or arrangement of the placenta. The function of placentation is to transfer nutrients, respiratory gases, and water from maternal tissue to a growing embryo, and in some instances to remove waste from the embryo. Placentation is best known in live-bearing mammals (theria), but also occurs in some fish, reptiles, amphibians, a diversity of invertebrates, and flowering plants. In vertebrates, placentas have evolved more than 100 times independently, with the majority of these instances occurring in squamate reptiles.
In embryology, Carnegie stages are a standardized system of 23 stages used to provide a unified developmental chronology of the vertebrate embryo.
Uterine glands or endometrial glands are tubular glands, lined by ciliated columnar epithelium, found in the functional layer of the endometrium that lines the uterus. Their appearance varies during the menstrual cycle. During the proliferative phase, uterine glands appear long due to estrogen secretion by the ovaries. During the secretory phase, the uterine glands become very coiled with wide lumens and produce a glycogen-rich secretion known as histiotroph or uterine milk. This change corresponds with an increase in blood flow to spiral arteries due to increased progesterone secretion from the corpus luteum. During the pre-menstrual phase, progesterone secretion decreases as the corpus luteum degenerates, which results in decreased blood flow to the spiral arteries. The functional layer of the uterus containing the glands becomes necrotic, and eventually sloughs off during the menstrual phase of the cycle.
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 fetal membranes are the four extraembryonic membranes, associated with the developing embryo, and fetus in humans and other mammals.. They are the amnion, chorion, allantois, and yolk sac. The amnion and the chorion are the chorioamniotic membranes that make up the amniotic sac which surrounds and protects the embryo. The fetal membranes are four of six accessory organs developed by the conceptus that are not part of the embryo itself, the other two are the placenta, and the umbilical cord.
Hormonal regulation occurs at every stage of development. A milieu of hormones simultaneously affects development of the fetus during embryogenesis and the mother, including human chorionic gonadotropin (hCG) and progesterone (P4).
Most mammals are viviparous, giving birth to live young. However, the five species of monotreme, the platypuses and the echidnas, lay eggs. The monotremes have a sex determination system different from that of most other mammals. In particular, the sex chromosomes of a platypus are more like those of a chicken than those of a therian mammal.
References
- ↑ Gilbert SF (2000). "Early Mammalian Development". Developmental Biology (6th ed.). Sunderland (MA): Sinauer Associates.
- ↑ Koch L (2014-12-18). "Development: Cell fate decisions in mammalian embryogenesis". Nature Reviews Genetics. 16: 5. doi: 10.1038/nrg3882 .
- ↑ Mercola M, Stiles CD (March 1988). "Growth factor superfamilies and mammalian embryogenesis". Development. 102 (3): 451–60. doi:10.1242/dev.102.3.451. PMID 3053123.
- ↑ "Mammalian Development". Developmental Biology Interactive. Georgia Institute of Technology. Retrieved 2019-03-15.
- ↑ Findlay JK, Gear ML, Illingworth PJ, Junk SM, Kay G, Mackerras AH, Pope A, Rothenfluh HS, Wilton L (April 2007). "Human embryo: a biological definition". Human Reproduction. 22 (4): 905–11. doi: 10.1093/humrep/del467 . PMID 17178746.
- ↑ Yoshinaga K (August 2013). "A sequence of events in the uterus prior to implantation in the mouse". Journal of Assisted Reproduction and Genetics. 30 (8): 1017–22. doi:10.1007/s10815-013-0093-z. PMC 3790116 . PMID 24052329.
