A pregnant lioness.

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. Reproduction is the Biological process by which new individual Organisms are produced Female (♀ is the Sex of an Organism, or a part of an organism which produces ova (egg cells Implantation is an event that occurs early in pregnancy in which the Embryo adheres to the wall of Uterus. Gestation is the carrying of an Embryo or Fetus inside a Female Viviparous Animal. It begins when a fertilized zygote implants in the female's uterus; and ends once it leaves the uterus. For soil improvement see Fertilization (soil. For other meanings see Zygote (disambiguation. A zygote (from Greek ζυγωτός zugōtos "joined" or "yoked"

Fertilization and implantation

A male and female copulate, the male inseminating the female. Male (♂ refers to the sex of an organism or part of an organism which produces small mobile Gametes called spermatozoa. Female (♀ is the Sex of an Organism, or a part of an organism which produces ova (egg cells Sexual intercourse, in its biological sense is the act in which the male reproductive organ (in humans and other higher animals enters the female reproductive tract Insemination is the introduction of sperm into the female Uterus of a Mammal or the Oviduct of an Oviparous (egg-laying animal during The spermatozoan fertilizes an ovum or various ova in the uterus or fallopian tubes, and this results in one or multiple zygotes. A spermatozoon or spermatozoan ( pl spermatozoa) from the Ancient Greek σπέρμα (seed and ζῷον (living being and more commonly known For soil improvement see Fertilization (soil. An ovum (plural ova) is a Haploid Female reproductive cell or Gamete. The uterus (from the Latin word for womb) is the major Female reproductive organ of most Mammals including Humans One end the The Fallopian tubes, also known as oviducts, uterine tubes, and salpinges ( singular salpinx) are two very fine tubes lined with ciliated For other meanings see Zygote (disambiguation. A zygote (from Greek ζυγωτός zugōtos "joined" or "yoked" Sometimes, a zygote can be created by humans outside of the animal's body in the artificial process of in-vitro fertilization. In vitro fertilisation ( IVF) is a process by which After fertilization, the newly formed zygote then begins to divide through mitosis, forming an embryo, which implants in the female's endometrium. Mitosis is the process in which a Eukaryotic cell separates the Chromosomes in its Cell nucleus, into two identical sets in two daughter nuclei An embryo (from Greek:, plural, lit "that which grows" from en- "in" + bryein "to swell be full" is a multicellular The endometrium is the inner membrane of the Mammalian Uterus. At this time, the embryo usually consists of 16 cells.

Development

After implantation

A blastocele is a small cavity on the center of the embryo, and the developing embryonary cells will grow around it. A blastocoel(e or blastocele (also called blastocyst cavity, cleavage cavity or segmentation cavity) is the Fluid -filled central Then, a flat layer cell forms on the exterior of this cavity, and the zona pellucida, the blastocyst's barrier, remains the same size as before. The zona pellucida (or zona striata in older texts is a Glycoprotein membrane surrounding the Plasma membrane of an Oocyte. Cells grow increasingly smaller to fit in. This new structure with a cavity in the center and the developing cells around it is known as a blastocyst. The blastocyst is the structure formed in early Embryogenesis, after the formation of the Blastocoel, but before Implantation.

The presence of the blastocyst means that two types of cells are forming, an inner-cell mass growing on the interior of the blastocele and cells growing on the exterior of it. In 24 to 48 hours, the zona pellucida breaches. The cells on the exterior of the blastocyst begin excreting an enzyme which erodes epithelial uterine lining and creates a site for implantation. In biology and medicine epithelium is a tissue composed of cells that line the cavities and surfaces of structures throughout the body

A pregnant woman at 26 weeks. Human beings, humans or man (Origin 1590–1600 L homō man OL hemō the earthly one (see Humus Human Pregnancy is the most studied of all mammalian pregnancies. Pregnancy ( Latin graviditas) is the carrying of one or more offspring known as a Fetus or Embryo, inside the Uterus of a Female

Placental circulation system

The cells surrounding the blastocyst now destroy cells in the uterine lining, forming small pools of blood, which in turn stimulate the production of capillaries. This is the first stage in the growth of the placenta. The placenta is an Ephemeral organ present in placental Vertebrates, such as Eutherial Mammals and Sharks during Gestation The inner cell mass of the blastocyst divides rapidly, forming two layers. The top layer becomes the embryo, and cells from there occupy the amniotic cavity. At the same time, the bottom layer forms a small sac. (If the cells begin developing in an abnormal position, an ectopic gestation may also occur at this point. )

Several days later, chorionic villi in the forming placenta anchor the implantation site to the uterus. Chorionic villi are villi that sprout from the Chorion in order to give a maximum area of contact with the maternal blood A system of blood and blood vessels now develops at the point of the newly forming placenta, growing near the implantation site. The blood vessels are part of the Circulatory system and function to transport Blood throughout the body The small sac inside the blastocyst begins producing red blood cells. Red blood cells are the most common type of Blood cell and the Vertebrate body's principal means of delivering Oxygen to the body tissues via the Blood For the next 24 hours, connective tissue develops between the developing placenta and the growing embryo. Connective tissue is one of the four types of tissue in traditional classifications (the others being epithelial, Muscle, and Nervous tissue) An embryo (from Greek:, plural, lit "that which grows" from en- "in" + bryein "to swell be full" is a multicellular This later develops into the umbilical cord. In placental Mammals the umbilical cord (also called the birth cord or funiculus umbilicalis) is the connecting cord from the developing Embryo

Cellular differentiation

Following this, a narrow line of cells appears on the surface on the embryo. Its growth makes the embryo undergo gastrulation, in which the three primary tissue layers of the fetus, the ectoderm, mesoderm, and endoderm, develop. Gastrulation is a phase early in the development of animal Embryos during which the morphology of the embryo is dramatically restructured by Cell migration. A germ layer is a collection of cells formed during animal Embryogenesis. The narrow line of cells begin to form the endoderm and mesoderm. The ectoderm begins to grow rapidly as a result of chemicals being produced by the mesoderm. These three layers give rise to all the various types of tissue in the body.

The endoderm later forms the lining of the tongue, digestive tract, lungs, bladder and several glands. The tongue is the large bundle of Skeletal muscles on the floor of the Mouth that manipulates Food for chewing and swallowing (deglutition lung is the essential Respiration organ in air-breathing Animals including most Tetrapods a few Fish and a few Snails The most primitive In Anatomy, the urinary bladder is a hollow muscular, and distensible (or elastic organ that sits on the Pelvic floor in Mammals It is the A gland is an organ in an animal's body that synthesizes a substance for release such as Hormones or Breast milk, often into the Bloodstream The mesoderm forms muscle, bone and lymph tissue, as well as the interior of the lungs, heart, reproductive and excretory systems. Muscle (from Latin musculus, diminutive of mus "mouse" is contractile tissue of the body and is derived from the Bones are rigid organs that form part of the Endoskeleton of Vertebrates They function to move support and protect the various organs of the body produce The lymphatic system in Vertebrates is a network of conduits that carry a clear fluid called Lymph. The heart is a muscular organ in all Vertebrates responsible for pumping Blood through the Blood vessels by repeated rhythmic The reproductive system is a system of organs within an Organism which work together for the purpose of Reproduction. The excretory system is an organ system that performs the function of Excretion, the bodily process of discharging nitrogeneous wastes It also gives rise to the spleen, and produces blood cells. The spleen is an organ found in all Vertebrate animals In humans the spleen is located in the abdomen of the body where it functions in the destruction of redundant Red The ectoderm forms the skin, nails, hair, cornea, lining of the internal and external ear, nose, sinuses, mouth, anus, teeth, pituitary gland, mammary glands, eyes and all parts of the nervous system. The skin is the outer covering of living tissue of an animal (or plant A nail is a horn -like structure at the end of an animal's Finger or Toe. Hair is a keratinised protein filament that grows through the epidermis from follicles deep within the Dermis. The cornea is the transparent front part of the Eye that covers the iris, Pupil, and Anterior chamber. The ear is the sense organ that detects Sounds The Vertebrate ear shows a common biology from Fish to Humans with variations Anatomically a nose is a protuberance in Vertebrates that houses the Nostrils or nares which admit and expel air for respiration in conjunction with the Paranasal sinuses are air-filled spaces communicating with the nasal cavity within the bones of the Skull and face The mouth, buccal cavity, or oral cavity is the first portion of the Alimentary canal that receives food and begins digestion by mechanically breaking up The anus is an opening at the opposite end of an Animal 's Digestive tract from the Mouth. The pituitary gland, or hypophysis, is an Endocrine gland about the size of a Pea. Mammary glands are the organs that in Mammals produce Milk for the sustenance of the young Eyes are organs that detect Light, and send signals along the Optic nerve to the visual areas of the brain The nervous system is a Network of specialized cells that communicate information about an animal's surroundings and itself

Approximately 18 days after fertilization, the embryo has divided to form much of the tissue it will need. It is shaped like a pear, where the head region is larger than the tail. The embryo's nervous system is one of the first organic systems to grow. It begins growing in a concave area known as the neural groove. Between the Neural folds is a shallow median groove the neural groove.

The blood system continues to grow networks which allow the blood to flow around the embryo. Blood cells are already being produced and are flowing through these developing networks. Secondary blood vessels also begin to develop around the placenta, to supply it with more nutrients. Blood cells begin to form on the sac in the center of the embryo, as well as cells which begin to differentiate into blood vessels. In Developmental biology, cellular differentiation is the process by which a less specialized cell becomes a more specialized Cell type. Endocardial cells begin to form the myocardium. The endocardium is the innermost layer of tissue that lines the chambers of the heart Myocardium is the muscular tissue of the Heart. Relationship to other layers The other tissues of the heart are the Endocardium

At about 24 days past fertilization, there is a primitive S-shaped tubule heart which begins beating. The flow of fluids throughout the embryo begin at this stage.

References

See also

• Pregnancy in humans
• Nesting instinct