Stunning crop art across rice fields in Japan.
(Source: nikolawashere, via lord-kitschener)
Top: Uterine lining at 5 1/2 months, displaying thin maternal separation from fetus, and high level of placental implantation
Center: Relation of placenta to uterus at 5 weeks and 8.5 months
Bottom: Major arteries and veins of the placentaDid you know that the placenta is a temporary organ that’s actually created by the fetus, and not the woman?
The human female is a curious creature; like our close great ape cousins, but unlike almost all other mammals, they build up a thick barrier in the uterine wall, to protect against any potential embryo that might implant itself. When there’s no embryo implantation, the thickened wall is shed, in the process known as menstruation.
The thing is, most mammals don’t menstruate. They go into heat, and occasionally shed uterine lining (if the uterus is scratched, or an egg tries to implant but fails, for example), but there’s no regular cycle of bloody discharge relating to breeding. This is because other mammals go through triggered decidualization (developing a uterine lining only when a fertilized egg begins to implant itself), while the great apes (and a couple other convergently evolved families, including bats) experience spontaneous decidualization, where they develop a thick uterine lining during every ovulation, before an egg can even attempt to implant itself.
Why the different linings? Well, it turns out that there are three types of mammal placentas (remember, placentas are developed by the embryo/fetus, not the mother):
- Epitheliochordal, which is completely superficial, and does not connect in any significant way to the mother’s body. The endometrial epithelium, connective tissue, and uterine epithelium are all preserved and undisturbed in the mother. The fetus is separated from the mother by three layers of tissue. Nutrients and waste are delivered and eliminated through diffusion, rather than direct connection. This group includes equids, swine, and ruminants.
- Endotheliochordal, which is slightly more invasive to the mother, only preserves the uterine epithelium. Nutrients and waste are not exchanged through direct connection to the mother, but the placenta only leaves one layer of tissue between it and the mother. This group includes cats and dogs.
- Hemochorial is the most invasive form of placenta in the animal kingdom. The embryo directly hooks itself up to the host (mother’s) blood flow, and leaves no tissue layers between the female and the placenta. This allows much more efficient nutrient transfer to the embryo or fetus, but is also potentially the most harmful to the female since the embryo attaches itself so securely to the uterine wall. The female must develop preemptive measures (a thickened uterine lining) to protect herself from a life-form that is literally driven to take all of the nutrients it needs to develop, and which has adapted to connect itself directly to the host. This group includes elephant shrews, most bats, and most primates.
Interested in more about the science behind reproduction and how amazingly efficient the human embryo is at sucking its host clean, just to obtain its needed resources for development?
PZ Meyers at Pharyngula has an understandable explanation of the article I referenced for this post.
There is also a great site by R. Bowen about the pathophysiology of the reproductive system.
An American Text-Book of Obstetrics for Practitioners and Students. Edited by Richard C. Norris, 1895.
Background
“We describe the genome of the western painted turtle, Chrysemys picta bellii, one of the most widespread, abundant and well-studied turtles. We place the genome into a comparative evolutionary context, and focus on genomic features associated with tooth loss, immune function, longevity, sex differentiation and determination, and the species? physiological capacities to withstand extreme anoxia and tissue freezing.
Results
Our phylogenetic analyses confirm that turtles are the sister group to living archosaurs, and demonstrate an extraordinarily slow rate of sequence evolution in the painted turtle. The ability of the painted turtle to withstand complete anoxia and partial freezing appears to be associated with common vertebrate gene networks, and we identify candidate genes for future functional analyses. Tooth loss shares a common pattern of pseudogenization and degradation of tooth-specific genes with birds, although the rate of accumulation of mutations is much slower in the painted turtle. Genes associated with sex differentiation generally reflect phylogeny rather than convergence in sex determination functionality. Among gene families that demonstrate exceptional expansions or show signatures of strong natural selection, immune function and musculoskeletal patterning genes are consistently overrepresented.
Conclusions
Our comparative genomic analyses indicate that common vertebrate regulatory networks, some of which have analogs in human diseases, are often involved in the western painted turtle?s extraordinary physiological capacities. As these regulatory pathways are analyzed at the functional level, the painted turtle may offer important insights into the management of a number of human health disorders.”
(Source: erinkilkenny, via scientificillustration)
you know, I’m starting to think that Saint Sebastian doesn’t really mind being used as a pincushion
(via lord-kitschener)
