An unborn female giraffe can have ovaries containing structures that look strikingly mature. Some follicles, the small structures in which egg cells develop, have already enlarged. Others have transformed into luteal tissue, the kind associated with hormone production in an adult ovary.
That observation presents a biological puzzle. Why should an animal that has not yet been born develop these tissues, and could they help support the pregnancy in which it is growing?
A 2013 study by S. Wilsher and colleagues investigated the ovaries alongside the placenta and other maternal and fetal tissues. It found hormone-related proteins in several locations, expanding the list of possible contributors to pregnancy. The evidence also suggested a placental signal that might help explain the ovarian changes. Establishing that signal's precise action, however, required more than the study could show.
Two pregnancies provided an unusual opportunity
The researchers obtained fresh reproductive tissues from two giraffes culled for management reasons. One carried a male fetus at an estimated eight months of gestation; the other carried a female fetus at about thirteen and a half months. Those ages were inferred from fetal size and published growth information, rather than known conception dates.
The team examined the placenta, maternal ovaries and fetal reproductive tissues, along with an adrenal gland from the later fetus. They prepared thin sections for conventional microscopy and used antibodies to locate selected proteins. They also measured pregnancy-related hormones in maternal and fetal blood and in fluids surrounding the fetuses.
These were two detailed examinations, not a growth series. The difference between the male and female fetus was also a difference in estimated age. Their tissues cannot establish a general sex difference in development, or show how one individual's organs changed through gestation.
A placenta with specialised points of contact
The giraffe placenta contained rows of attachment and exchange structures called placentomes. Each joined a fetal component with a corresponding projection of the mother's uterine lining. Under the microscope, elongated fetal projections fitted closely into maternal spaces, creating an extensive interface between the two tissues.
The researchers distinguished ordinary single-nucleus cells of the fetal surface layer from larger cells containing two nuclei. The latter were scattered through the placentomes and were also present in the membranes between them.
Location mattered. Within placentomes, many two-nucleus cells stained strongly with an antibody against prolactin, a hormone related to placental lactogens. The same type of cell in the intervening membranes did not show that staining pattern. This suggested that cells with similar appearances could differ in their hormone-related properties according to where they lay.
The authors interpreted the placental staining as evidence for a prolactin-like or lactogenic protein. They did not purify a giraffe hormone and demonstrate its complete identity or biological action. That distinction keeps an informative staining pattern from becoming a more specific conclusion than the experiment supports.
One large maternal source, many smaller candidates
Each mother had one large corpus luteum, a temporary ovarian structure associated with maintaining pregnancy. It occupied much of one ovary. Several much smaller follicles had also partly or completely transformed into luteal tissue.
The later female fetus showed a mixture of developing follicles and small luteal structures in her ovaries. These did not show the surface marks expected if an egg had been released. The transformation therefore should not be mistaken for evidence that the fetus was ovulating or sexually mature.
An important antibody in the study targeted an enzyme called 3β-HSD. In the steroid-making pathway, this enzyme helps convert a precursor into progesterone-related hormones, or progestagens. It provides a way to look for cells equipped for an important part of hormone production.
The enzyme-related staining appeared in maternal luteal tissue and in the fetal ovarian follicles and luteal structures. It was also present in single-nucleus placental cells, some cells of the uterine lining, and selected tissues of the fetal testis and adrenal gland.
This distributed pattern is the central finding. The mother’s large corpus luteum was an obvious candidate source, but it was not the only tissue with evidence of relevant biochemical machinery. Pregnancy could involve contributions or local hormone processing at several sites.
Capability is different from contribution
An enzyme detected in a tissue does not reveal how much hormone that tissue releases into blood. Its activity depends on available precursors and other steps in the pathway, and some hormone metabolism may serve nearby cells rather than the whole animal.
The researchers also located progesterone receptors in several tissues. Receptors indicate potential responsiveness to a hormone; they do not independently establish that the same tissue produces it. Keeping production and response separate is essential when interpreting this kind of microscopic map.
The measured progestagens in maternal and fetal blood confirmed that these hormone-related compounds were present. But a blood concentration does not identify the organ that supplied them. Nor do two measurements at different pregnancies establish when one organ might take over from another.
The many small luteal structures also represented much less tissue than the large maternal pregnancy corpus luteum. Their presence alone cannot establish that they replace its contribution. The placenta was another plausible source, but the study did not determine whether pregnancy could continue without ovarian hormone production.
Testing a placental signal requires a different experiment
The placental prolactin-like staining suggested a possible link: a lactogenic hormone might stimulate the unusual ovarian development seen in mother and fetus. The study did not manipulate that signal, so this remained an explanation to investigate.
A 2018 sheep experiment by K. J. Jeckel and colleagues shows how a functional question can be approached. Researchers introduced a targeted RNA-interference treatment into early embryos to reduce a placental-lactogen signal, then compared six resulting pregnancies with eight controls at day 50. Hormone concentrations in blood leaving the uterus were lower in the treated group, and fetal body weight was about one fifth lower.
The placental hormone's tissue-level RNA and protein measurements were not significantly different at that time, illustrating that even a controlled intervention can produce a complex set of readouts. The experiment supported a role for the targeted system in sheep fetal growth. It did not test giraffe ovarian development or prove that the proposed giraffe signal has the same effects.
The comparison clarifies what the giraffe study achieved: it located candidate tissues and signals, while the causal links between them remain to be established.
A map with important unanswered connections
The authors also tested for aromatase, an enzyme involved in producing oestrogens. Their giraffe tissues did not stain positively, despite positive staining in comparison tissues from other species. That result did not resolve where the oestrogens reported during giraffe pregnancy originate, or justify treating a negative stain as a complete inventory of hormone production throughout gestation.
The study's value lies in making the next questions more precise. The placenta, maternal ovary and fetal organs cannot be understood simply as passive recipients of one ovarian hormone supply. Several contain proteins consistent with hormonal production or response, and the fetal ovary undergoes substantial tissue transformation before birth.
Determining which of those capacities sustains pregnancy, which serves local development and which drives the remarkable fetal ovarian changes will require direct measurements of secretion and carefully designed functional comparisons. The microscopic evidence provides the anatomical places to look.
Sources
S. Wilsher, F. Stansfield, R. E. S. Greenwood, P. D. Trethowan, R. A. Anderson, F. B. W. Wooding and W. R. Allen (2013). “Ovarian and placental morphology and endocrine functions in the pregnant giraffe (Giraffa camelopardalis).” Reproduction, 145, 541–554. Primary study.
K. J. Jeckel, A. C. Boyarko, G. J. Bouma, Q. A. Winger and R. V. Anthony (2018). “Chorionic somatomammotropin impacts early fetal growth and placental gene expression.” Journal of Endocrinology, 237, 301–310. Related functional experiment.
