∞ generated and posted on 2016.04.25 ∞

Early embryonic development involves cell division, basic morphological changes to embryos, and the beginnings of cellular differentiation.

Development begins with fusion of sperm and egg (ovum) in the act of fertilization, which is then followed by cleavage, morula formation, blastocyst formation, gastrulation, implantation, further embryonic development and so on through birth (parturition) and then the raising of the baby that, early on, in no small part involves feeding, which for mammals involves lactation and for humans we refer to as breastfeeding.

This page contains the following terms: Zygote, Cleavage, Morula, Blastula, Blastocyst, Implantation, Parturition, Mammary gland, Lactation

The above video provides an overview of sex, fertilization, and contraception; discussion of fertilization begins at 3:06; discussion of contraception begins at 6:58.

The above video takes us from fertilization through afterbirth.


Diploid product of gamete fusion.
Zygote formation is the product of fertilization, with the zygote in turn representing the first cell of the embryo, from which all other cells during development are derived over the course of subsequent mitotic cell division (the latter initially is referred to as cleavage). Zygote formation occurs during ovum travel down the fallopian tube, towards but not yet into the uterus.

The process of zygote formation is separate from implantation, and in fact it is not even the zygote that ultimately implants into the uterine lining but instead an early embryonic form called a blastocyst. The conversion of zygote to embryo involves a combination of what is known as cleavage as well as subsequent cellular differentiation and further mitotic cell division.

Links to terms of possible interest: Blastocyst, Cell division, Cellular differentiation, Chromosomes, Cleavage, Cytoplasm, Diploid, Development, Egg, Embryo, Endosymbionts, Fallopian tube, Fertilization, Gamete, Haploid, Implantation, Mitotic, Ovum, Uterine lining, Uterus, Sperm, Spermatozoa, Syngamy, Zygote

The above video literally doesn't "say" much, but it's got great animation.


Mitotic division during early animal embryonic development.
Cleavage is notably associated with a lack of associated growth in embryo volume but instead involves an ongoing reduction in cell size. One can view this process as consisting of a strong bias towards mitosis and subsequent cytokinesis without an intervening interphase during which cytoplasmic volume increases.

The product of cleavage is a solid ball of cells known as a morula. The cells making up the morula along with embryonic stages leading up to the morula are known as blastomeres.

Links to terms of possible interest: Blastomeres, Cleavage, Cytokinesis, Cytoplasmic, Embryo, Embryonic development, Mitotic division, Morula

The above video shows a zygote displaying the initial stages of cleavage.


Embryonic product of cleavage that consists of a solid ball of cells.
The morula is the product of rapid mitotic cell division (cleavage) with little cellular differentiation or indeed cellular growth. The sphere that is the zygote is partitioned essentially into a sphere that is the morula, with retention of the original shape and approximately original volume.

Note that the presence of yolk in eggs can complicate this story, but human eggs lack yolk so we don't have to worry about those complications. The name morula actually comes from the Latin name for mulberry given the resemblance between the 16-cell stage morula and a mulberry.

Links to terms of possible interest: Animal, Cell division, Cellular differentiation, Cleavage, Development, Eggs, Embryonic, Mitotic, Morula, Yolk, Zygote

The above video starts with a nice image of a zygote and takes us through the blastocyst stage, with the morula stage well documented in the middle.


Hollow ball of cells that forms from the morula during early embryonic development.
The cells that make up the blastula are known as blastomeres while the hollow portion of this hollow ball is known as the blastocoele. The blastula forms from the morula, and in fact forms literally via the development of the blastocoele.

It is also the blastula that undergoes gastrulation, giving rise to the gastrula. Thus, fertilizationzygotecleavagemorulablastula (a.k.a., blastocyst) → gastrulationgastrula.

Note that the blastula lacks germ layers (i.e., endoderm, ectoderm, and mesoderm) as these do not form until gastrulation. For more on the blastula, particular as present in mammals, see blastocyst (below).

Links to terms of possible interest: Amnion, Amniotes, Blastocoele, Blastocyst, Blastomeres, Blastula, Cleavage, Development, Early embryonic development, Ectoderm, Embryonic development, Endoderm, Extraembryonic membranes Fertilization, Gastrula, Gastrulation, Mammals, Mesoderm, Morula, Zygote

The above video provides an animation of cleavage, morula formation, blastula formation, gastrulation, and the extraembryonic membranes seen in animals such as ourselves that are known as amniotes (so named for those extraembryonic membranes, particularly the amnion); caution, not the greatest sound quality.


Hollow ball of cells that forms from the morula during the early embryonic development of mammals.
Blastocyst is simply the name given to the blastula stage of mammals. They consist, in humans, of a few hundred cells and, again in humans, are formed a few days into embryonic development.

It is with the formation of the blastocyst that the first cellular differentiation comes to take place in earnest in the developing embryo, most notably in terms of what cells will give rise to the later-stage embryo versus those cells that will give rise to the placenta. It is the blastocyst specifically that undergoes implantation.

Links to terms of possible interest: Blastocoel cavity, Blastocyst, Cellular differentiation , Early embryonic development, Embryo, Embryonic development, Human blastocyst, Implantation, Inner cell mass, Morula, Placenta, Uterine lining, Zona pellucida

The above video walks us through blastocyst formation in animation but not narration.


Process early in animal development during which endodermal and ectodermal tissue as well as the archenteron form.
In gastrulation there is an infolding of the wall of the blastula to form the primitive gut, or archenteron. The tissue lining the archenteron is the endoderm whereas that remaining on the outside of the embryo is the ectoderm. The resulting structure is known as the gastrula, and it is in association with the gastrula that mesodermal tissue subsequently forms.

Links to terms of possible interest: Archenteron, Blastula, Blastocoel, Blastopore, Ectoderm, Ectodermal, Embryo, Endoderm, Gastrula, Gastrulation, Mesodermal, Zygote

Gastrulation as seen during development.


Blastocyst attachment to the uterine lining.
The close association with the endometrium that is established upon implantation provides a closer association between the embryo and the nutrient and oxygen supply provided by the uterus.

This attachment matures into what is known as the placenta, which is a much more sophisticated and elaborate association between the developing embryo and the uterus across which oxygen, nutrients, and wastes as well as antibodies diffuse.

Links to terms of possible interest: Blastocyst, Embryo, Endometrium, Gastrulation, Implantation, Nutrient, Organogenesis, Ovulation, Oxygen, Uterine lining, Uterus

The above video depicts development from ovulation through organogenesis and beyond. There is no sound, however.


The transition from antepartumto postpartum.
Parturition is live birthing, that is, contrasting offspring being laid within an egg. This is a potentially dangerous moment for an individual, and especially so in humans, the latter in no small part due to compromises stemming from upright bipedalism in combination with babies even at birth possessing relatively large heads.

The trick with parturition is to transition from a stable state, in utero, to a stable state ex utero, but first the baby must pass through an intervening much less stable state, the birthing process itself.

This birthing process, parturition, is substantially the result of oxytocin-driven uterine smooth muscle contractions in combination with cervical dilation. This represents a positive feedback process whereby the baby pressing against the cervix results in release of oxytocin (by the posterior pituitary gland), which stimulates uterine contraction. The latter causes further pressing on the cervix, which results in further oxytocin release, and so on until parturition is complete.

The terms antepartumand postpartum refer to the before versus following birth, respectively, and note that I am attempting in these descriptions to avoid emphasizing the baby versus the mother as co-participants in the birthing process.

Links to terms of possible interest: Antepartum, Birthing, Cervical dilation, Cervical effacement, Cervix, Dilation, Effacement, Labor, Normal human birth, Oxytocin, Parturition, Placenta, Positive feedback, Posterior pituitary gland, Postpartum, Uterine contraction, Uterus, Vagina

The above video is nicely narrated though does not have the greatest of visuals.

The above video nicely shows all of the twists and turns of a normal human birth.

Mammary gland

Milk producing organ associated with mammals.
Mammary glands mature in the course of pregnancy and begin producing milk soon after parturition (birth). Initially a special antibody-rich form of milk is produced, called colostrum, which serves as the newborn's first meal. Colostrum is followed, however, by the production of what more traditionally is thought of as milk. This milk in turn serves as a liquid meal for infants, one which within the stomach is converted to a solid form and therefore which is more slowly digested and assimilated by the baby than a purely liquid meal.

In humans the mammary glands are housed within the breasts and connect to outside of the body via ducts which terminate within the nipple, which in turn is located on the surface of the body within darker regions of skin that are known as areolae. In addition to ducts, the mature mammary gland consists of numerous lumen-containing alveoli within which milk is both produced and accumulates prior to ejection (a.k.a., letdown) in the course of lactation.

Links to terms of possible interest: alveoli, Antibody, Areola, Areolar gland, Birth, Breasts, Colostrum, Cooper's ligaments, Lactation, Lactiferous ducts, Lactiferous sinus, Letdown, Lobe, Lobule, Lymphatic vessels Mammals, Mammary glands, Milk, Nipple, Parturition, Pregnancy, Suspensory ligaments, Teat

The above video provides a basic introduction to the anatomy of the breast as well as the role of associated lymphatic vessels and nodes.

The above video provides a brief introduction to breast anatomy as well as the sites of formation of breast cancers.

The above video shows a partial dissection of a cow's teat.

The above video addresses the question, why do men have nipples?


Milk secretion by mammary glands.
Milk secretion in humans and the vast majority of mammals occurs from teats which the baby suckles. Suckling as well as other baby associated cues stimulate what is known as letdown or instead milk ejection, which is a reflex that involves a combination of release of the hormone oxytocin by the posterior pituitary gland and the oxytocin-stimulated contraction of mammary gland cells (myoepithelial cells) that surround the breast milk storage vessels known as alveoli. The result is an under-pressure release of milk through the nipple.

Suckling in addition to stimulating letdown also has the effect of at least potentially inhibiting ovulation and thus can serve as a natural form of birth control particularly for infants whose sole source of nourishment is supplied via lactation. Such an approach to birth control, however, is far from foolproof and regardless requires a substantial amount of breastfeeding to be reasonably effective. In addition, keep in mind that fertility in fact proceeds rather than follows menses, so it is very possible to ovulate while still breastfeeding but to be unaware of this postpartum return of fertility.

Links to terms of possible interest: Areola, Alveoli, Attachment, Birth control, Breast milk, Feeding positions, Fertility, Hormone, Infants, Lactation, Lactiferous ductsLetdown, Lactiferous sinuses, Lobules, Mammals, Mammary glands, Menses, Milk, Milk ejection, Nipple, Nourishment, Ovulation, Oxytocin, Posterior pituitary, Postpartum, Pregnancy, Reflex, Secretion, Suckling, Teats

The above video provides a nice overview from hormones to feeding positions.