Mammalian pregnancy
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mammal Mammals () are a group of vertebrate animals constituting the class Mammalia (), characterized by the presence of mammary glands which in females produce milk for feeding (nursing) their young, a neocortex (a region of the brain), fur or ...
s, pregnancy is the period of
reproduction Reproduction (or procreation or breeding) is the biological process by which new individual organisms – "offspring" – are produced from their "parent" or parents. Reproduction is a fundamental feature of all known life; each individual or ...
during which a female carries one or more live offspring from implantation in the uterus through
gestation Gestation is the period of development during the carrying of an embryo, and later fetus, inside viviparous animals (the embryo develops within the parent). It is typical for mammals, but also occurs for some non-mammals. Mammals during pregna ...
. It begins when a fertilized zygote implants in the female's uterus, and ends once it leaves the uterus.


Fertilization and implantation

During
copulation Sexual intercourse (or coitus or copulation) is a sexual activity typically involving the insertion and thrusting of the penis into the vagina for sexual pleasure or reproduction.Sexual intercourse most commonly means penile–vaginal penetrat ...
, the male inseminates the female. The
spermatozoon A spermatozoon (; also spelled spermatozoön; ; ) is a motile sperm cell, or moving form of the haploid cell that is the male gamete. A spermatozoon joins an ovum to form a zygote. (A zygote is a single cell, with a complete set of chromosomes, t ...
fertilize Fertilisation or fertilization (see spelling differences), also known as generative fertilisation, syngamy and impregnation, is the fusion of gametes to give rise to a new individual organism or offspring and initiate its development. Proce ...
s an
ovum The egg cell, or ovum (plural ova), is the female reproductive cell, or gamete, in most anisogamous organisms (organisms that reproduce sexually with a larger, female gamete and a smaller, male one). The term is used when the female gamete is ...
or various ova in the uterus or
fallopian tube The fallopian tubes, also known as uterine tubes, oviducts or salpinges (singular salpinx), are paired tubes in the human female that stretch from the uterus to the ovaries. The fallopian tubes are part of the female reproductive system. In ot ...
s, and this results in one or multiple zygotes. Sometimes, a zygote can be created by humans outside of the animal's body in the artificial process of in-vitro fertilization. After fertilization, the newly formed zygote then begins to divide through
mitosis In cell biology, mitosis () is a part of the cell cycle in which replicated chromosomes are separated into two new nuclei. Cell division by mitosis gives rise to genetically identical cells in which the total number of chromosomes is mainta ...
, forming an
embryo 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 spe ...
, which implants in the female's
endometrium The endometrium is the inner epithelial layer, along with its mucous membrane, of the mammalian uterus. It has a basal layer and a functional layer: the basal layer contains stem cells which regenerate the functional layer. The functional laye ...
. At this time, the embryo usually consists of 50 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. 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. 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 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 t ...
. 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.


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 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 Chorionic villi are villi that sprout from the chorion to provide maximal contact area with maternal blood. They are an essential element in pregnancy from a histomorphologic perspective, and are, by definition, a product of conception. Branche ...
in the forming placenta anchor the implantation site to the uterus. A system of blood and blood vessels now develops at the point of the newly forming placenta, growing near the implantation site. The small sac inside the blastocyst begins producing red blood cells. For the next 24 hours,
connective tissue Connective tissue is one of the four primary types of animal tissue, along with epithelial tissue, muscle tissue, and nervous tissue. It develops from the mesenchyme derived from the mesoderm the middle embryonic germ layer. Connective tiss ...
develops between the developing placenta and the growing
embryo 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 spe ...
. This later develops into the umbilical cord.


Cellular differentiation

Following this, a narrow line of cells appears on the surface on the embryo. Its growth makes the embryo undergo
gastrulation Gastrulation is the stage in the early embryonic development of most animals, during which the blastula (a single-layered hollow sphere of cells), or in mammals the blastocyst is reorganized into a multilayered structure known as the gastrula. Be ...
, in which the three primary tissue layers of the fetus, the
ectoderm The ectoderm is one of the three primary germ layers formed in early embryonic development. It is the outermost layer, and is superficial to the mesoderm (the middle layer) and endoderm (the innermost layer). It emerges and originates from t ...
,
mesoderm The mesoderm is the middle layer of the three germ layers that develops during gastrulation in the very early development of the embryo of most animals. The outer layer is the ectoderm, and the inner layer is the endoderm.Langman's Medical E ...
, and
endoderm Endoderm is the innermost of the three primary germ layers in the very early embryo. The other two layers are the ectoderm (outside layer) and mesoderm (middle layer). Cells migrating inward along the archenteron form the inner layer of the gast ...
, develop. 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,
lung The lungs are the primary organs of the respiratory system in humans and most other animals, including some snails and a small number of fish. In mammals and most other vertebrates, two lungs are located near the backbone on either side of t ...
s, bladder and several
gland In animals, a gland is a group of cells in an animal's body that synthesizes substances (such as hormones) for release into the bloodstream (endocrine gland) or into cavities inside the body or its outer surface (exocrine gland). Structure De ...
s. The mesoderm forms
muscle Skeletal muscles (commonly referred to as muscles) are organs of the vertebrate muscular system and typically are attached by tendons to bones of a skeleton. The muscle cells of skeletal muscles are much longer than in the other types of muscl ...
, bone, and lymph tissue, as well as the interior of the lungs, heart, and
reproductive The reproductive system of an organism, also known as the genital system, is the biological system made up of all the anatomical organs involved in sexual reproduction. Many non-living substances such as fluids, hormones, and pheromones are als ...
and excretory systems. It also gives rise to the spleen, and produces blood cells. The ectoderm forms the skin, nails,
hair Hair is a protein filament that grows from follicles found in the dermis. Hair is one of the defining characteristics of mammals. The human body, apart from areas of glabrous skin, is covered in follicles which produce thick terminal and f ...
, cornea, lining of the internal and external ear, nose, sinuses,
mouth In animal anatomy, the mouth, also known as the oral cavity, or in Latin cavum oris, is the opening through which many animals take in food and issue vocal sounds. It is also the cavity lying at the upper end of the alimentary canal, bounded on ...
,
anus The anus (Latin, 'ring' or 'circle') is an opening at the opposite end of an animal's digestive tract from the mouth. Its function is to control the expulsion of feces, the residual semi-solid waste that remains after food digestion, which, d ...
, teeth, pituitary gland, mammary glands,
eye Eyes are organs of the visual system. They provide living organisms with vision, the ability to receive and process visual detail, as well as enabling several photo response functions that are independent of vision. Eyes detect light and conv ...
s, and all parts of the nervous system. 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. 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. Endocardial cells begin to form the
myocardium Cardiac muscle (also called heart muscle, myocardium, cardiomyocytes and cardiac myocytes) is one of three types of vertebrate muscle tissues, with the other two being skeletal muscle and smooth muscle. It is an involuntary, striated muscle that ...
. At about 24 days past fertilization, there is a primitive S-shaped tubule heart which begins beating. The flow of fluids throughout the embryo begins at this stage.


Gestation periods

For
mammal Mammals () are a group of vertebrate animals constituting the class Mammalia (), characterized by the presence of mammary glands which in females produce milk for feeding (nursing) their young, a neocortex (a region of the brain), fur or ...
s the
gestation Gestation is the period of development during the carrying of an embryo, and later fetus, inside viviparous animals (the embryo develops within the parent). It is typical for mammals, but also occurs for some non-mammals. Mammals during pregna ...
period is the time in which a fetus develops, beginning with fertilization and ending at birth. The duration of this period varies between species. For most species, the amount a fetus grows before birth determines the length of the gestation period. Smaller species normally have a shorter gestation period than larger animals. For example, a cat's gestation normally takes 58–65 days while an elephant's takes nearly 2 years (21 months). However, growth does not necessarily determine the length of gestation for all species, especially for those with a
breeding season Seasonal breeders are animal species that successfully mate only during certain times of the year. These times of year allow for the optimization of survival of young due to factors such as ambient temperature, food and water availability, and cha ...
. Species that use a breeding season usually give birth during a specific time of year when food is available. Various other factors can come into play in determining the duration of gestation. For humans, male fetuses normally gestate several days longer than females and multiple pregnancies gestate for a shorter period. Ethnicity in humans is also a factor that may lengthen or shorten gestation. In dogs there is a positive correlation between a longer gestation time and fewer members of the litter.


See also

* Nesting instinct * Non-human fetuses *
Pseudopregnancy In mammalian species, pseudopregnancy is a physical state whereby all the signs and symptoms of pregnancy are exhibited, with the exception of the presence of a fetus, creating a false pregnancy. The corpus luteum (the remains of an ovulated ovari ...


References

{{Authority control Fertility Mammalian pregnancy Theriogenology