Welcome to The Visible Embryo

Home- - -History-- -Bibliography- -Pregnancy Timeline- --Prescription Drugs in Pregnancy- -- Pregnancy Calculator- --Female Reproductive System- News Alerts -Contact

Welcome to The Visible Embryo, a comprehensive educational resource on human development from conception to birth.

The Visible Embryo provides visual references for changes in fetal development throughout pregnancy and can be navigated via fetal development or maternal changes.

The National Institutes of Child Health and Human Development awarded Phase I and Phase II Small Business Innovative Research Grants to develop The Visible Embryo in 1993 as a first generation internet teaching tool consolidating human embryology teaching for first year medical students.

Today, The Visible Embryo is linked to over 600 educational institutions and is viewed by more than 1 million visitors each month. The field of early embryology has grown to include the identification of the stem cell as not only critical to organogenesis in the embryo, but equally critical to organ function and repair in the adult human.

The identification and understanding of genetic malfunction, inflammatory responses, and the progression in chronic disease, begins with a grounding in primary cellular and systemic functions manifested in the study of the early embryo.

WHO International Clinical Trials Registry Platform
The World Health Organization (WHO) has created a new Web site to help researchers, doctors and patients obtain reliable information on high-quality clinical trials. Now you can go to one website and search all registers to identify clinical trial research underway around the world!




Pregnancy Timeline

Prescription Drug Effects on Pregnancy

Pregnancy Calculator

Female Reproductive System

Contact The Visible Embryo

News Archive
Disclaimer: The Visible Embryo web site is provided for your general information only. The information contained on this site should not be treated as a substitute for medical, legal or other professional advice. Neither is The Visible Embryo responsible or liable for the contents of any websites of third parties which are listed on this site.
Content protected under a Creative Commons License.

No dirivative works may be made or used for commercial purposes.

Return To Top Of Page
Pregnancy Timeline by SemestersFemale Reproductive SystemFertilizationThe Appearance of SomitesFirst TrimesterSecond TrimesterThird TrimesterFetal liver is producing blood cellsHead may position into pelvisBrain convolutions beginFull TermWhite fat begins to be madeWhite fat begins to be madeHead may position into pelvisImmune system beginningImmune system beginningPeriod of rapid brain growthBrain convolutions beginLungs begin to produce surfactantSensory brain waves begin to activateSensory brain waves begin to activateInner Ear Bones HardenBone marrow starts making blood cellsBone marrow starts making blood cellsBrown fat surrounds lymphatic systemFetal sexual organs visibleFinger and toe prints appearFinger and toe prints appearHeartbeat can be detectedHeartbeat can be detectedBasic Brain Structure in PlaceThe Appearance of SomitesFirst Detectable Brain WavesA Four Chambered HeartBeginning Cerebral HemispheresEnd of Embryonic PeriodEnd of Embryonic PeriodFirst Thin Layer of Skin AppearsThird TrimesterDevelopmental Timeline
Click weeks 0 - 40 and follow fetal growth
Search artcles published since 2007

December 7, 2012--------News Archive Return to: News Alerts

Immature mammary glands from a female (left) and male (right) mouse after the nerve
cells (yellow) have retracted from the male tissue. Mammary gland cells labeled in blue.

WHO Child Growth Charts


His and hers: Male Hormones Control Differences in Mammary Gland Nerve Growth

Mechanism identified in mice could explain other sex-specific variations in nerve networks

Johns Hopkins scientists have found a surprising mechanism that gives male sex hormones like testosterone control over the gender-specific absence or presence of mammary gland nerves that sense the amount of milk available in breast milk ducts.

In a Dec. 7 report on their discovery in Science, they say the hormones do the job by altering the availability of a nerve growth factor, called BDNF for short.

The most obvious differences between males and females
involve the presence or absence of physical structures.

Below the surface, however, these structures
are penetrated by nerves, which also are
present in a sex-specific way.

"We now think we have a broader understanding of how sex-specific nerves reach their proper target in a given sex, say in mammary milk ducts in females, but disappear in the other sex," says David Ginty, Ph.D., a Howard Hughes investigator and professor of neuroscience in the Institute for Basic Biomedical Sciences at the Johns Hopkins School of Medicine.

For their experiments with sex-specific neural wiring, Yin Liu, a student in Ginty's laboratory, studied nerves in mice that monitor the fullness of milk ducts in females. If the milk supply is low, the nerves are believed to report this to the brain to stimulate milk production, Ginty says. Early in embryonic development, there are no differences between the mammary glands of males and females and this milk-monitoring set of nerves is present in both. Later in development, the nerves are lost in males.

In one experiment, to figure out how the nerves
find their way to the immature mammary glands
of both sexes during early development,
the researchers analyzed the gland cells for the
presence of four proteins known to encourage
nerve growth. They found only one that was
there in significant quantities — BDNF — and it
was present at similar levels in both sexes.

Ginty says BDNF is known to bind to a protein, known as TrkB, found on the surface of nerve cells. This binding event triggers a series of messages within the nerve cell, telling it to grow towards the source of the signal. When the researchers looked for TrkB in developing mice, they found it, as expected, on the surfaces of nerve cells that grow into the immature glands of both sexes. "So early development could be explained quite simply," says Ginty. "The cells of the early mammary glands released the signaling molecule BDNF, which was detected by TrkB on the nerve cells, which made them grow toward the mammary glands. What remained a mystery was why these nerve cells are lost a few hours later in males."

In a subsequent experiment, using molecular tests, the scientists ruled out the possibility that nerve cells in male pre-mammary glands were receiving a "suicide signal" and dying off. They reasoned that if the nerve cells weren't dying, they must be retracting, and went hunting for what signal was telling them to do so.

Since sex hormones play many different roles in determining sex-specific differences, the researchers monitored the effects of adding male sex hormones to females, and the effects of blocking male sex hormones in males. They found that the female pattern of nerve growth was the "default" and that male sex hormones would cause withdrawal of the nerves from the glands of either sex.

"At this point, we knew that BDNF is found at comparable levels in the glands of both sexes, that TrkB is found at comparable levels on the nerve cells of both sexes, and that male sex hormones were still somehow creating a difference in the system," says Ginty.

To figure out just how the hormones caused the nerve growth differences, they searched for the BDNF receptor protein TrkB in the immature gland tissue (instead of in the nerve cells). They found it — but only in males.

It turns out that, in addition to regular TrkB made by nerve cells, a shorter version of the protein, dubbed TrkB.T1, also exists. From their experiments, Ginty and his team concluded that as the male embryos got older, male sex hormones, produced by the testes, commanded non-nerve cells in the immature gland tissue to produce TrkB.T1. Though TrkB.T1 can still bind BDNF, once it does so, both proteins are taken inside the cell and recycled, essentially removing BDNF — and its nerve-growth-promoting signals — from early mammary gland tissue in males.

"It's as if testosterone sounds the horn for retreat,
so that without BDNF present, the nerve endings
that had already reached the male mammary glands
pull away. We believe this is the first study to show
sex hormones regulating nerve growth and retraction
by affecting the availability of BDNF.

It will be interesting to see if similar mechanisms
create other sex-specific differences in neural wiring,
including those that affect general behavior."

David Ginty, Ph.D.

Other authors of the report include Yin Liu, Michael Rutlin and Siyi Huang of the Johns Hopkins University School of Medicine; Colleen Barrick and Lino Tessarollo of the National Cancer Institute; Fan Wang of Duke University Medical Center; and Kevin Jones of the University of Colorado.

This work was supported by grants from the National Institute of Stroke and Neurological Disorders (NS34814), the National Eye Institute (EY014998), the National Institute of Dental and Craniofacial Research (DE019440) and the Howard Hughes Medical Institute.

Original article: http://www.eurekalert.org/pub_releases/2012-12/jhm-hah120412.php