Ultrastructure of the reproductive system of the black swamp snake (Seminatrix pygaea). II. Annual oviducal cycle

This article is the first ultrastructural study on the annual oviducal cycle in a snake. The ultrastructure of the oviduct was studied in 21 females of the viviparous natricine snake Seminatrix pygaea. Specimens were collected and sacrificed in March, May, June, July, and October from one locale in South Carolina during 1998-1999. The sample included individuals: 1) in an inactive reproductive condition, 2) mated but prior to ovulation, and 3) from early and late periods of gravidity. The oviduct possesses four distinct regions from cranial to caudal: the anterior infundibulum, the posterior infundibulum containing sperm storage tubules (SSTs), the uterus, and the vagina. The epithelium is simple throughout the oviduct and invaginations of the lining form tubular glands in all regions except the anterior infundibulum and the posterior vagina. The tubular glands are not alveolar, as reported in some other snakes, and simply represent a continuation of the oviducal lining with no additional specializations. The anterior infundibulum and vagina show the least amount of variation in relation to season or reproductive condition. In these regions, the epithelium is irregular, varying from squamous to columnar, and cells with elongate cilia alternate with secretory cells. The secretory product of the infundibulum consists largely of lipids, whereas a glycoprotein predominates in the vagina; however, both products are found in these regions and elsewhere in the oviduct. In the SST area and the anterior vagina, tubular glands are compound as well as simple. The epithelium of the SST is most active after mating, and glycoprotein vacuoles and lipid droplets are equally abundant. When present, sperm form tangled masses in the oviducal lumen and glands of the SST area. The glands of the uterus are always simple. During sperm migration, a carrier matrix composed of sloughed epithelial cells, a glycoprotein colloid, lipids, and membranous structures surround sperm in the posterior uterus. During gravidity, tubular glands, cilia, and secretory products diminish with increasing development of the fetus, and numerous capillaries abut the basal lamina of the attenuated epithelial lining of the uterus.     

Ultrastructure of the reproductive system of the black swamp snake (Seminatrix pygaea): Part I. Evidence for oviducal sperm storage

Oviducal sperm storage in the viviparous (lecithotrophic) colubrid snake Seminatrix pygaea was studied by light and electron microscopy. Out of 17 adult snakes examined from May–October, sperm were found in the oviducts of only two specimens. In a preovulatory female sacrificed 14 May, sperm were found in the oviducal lumen and sperm storage tubules (SSTs) of the posterior infundibulum. In a nonvitellogenic female sacrificed 9 June, sperm were found in the lumen and glands of the posterior uterus and anterior vagina, indicating a recent mating. The glands in the posterior infundibulum and vagina were simple or compound tubular, whereas glands in the uterus always were simple tubular. The epithelium of the sperm storage glands was not modified from that lining the rest of the oviduct. The cuboidal or columnar epithelium consisted of alternating ciliated and secretory areas. The secretory product released into the lumen by a merocrine process contained mucoprotein. Lipid droplets also were numerous in the epithelium. Portions of sperm sometimes were embedded in the apical cytoplasm or in secretory material. A carrier matrix containing a mucoid substance, desquamated epithelium, lipids, membranous structures, and possibly phagocytes was found around sperm in the posterior uterus. J. Morphol. 241:1–18, 1999. © 1999 Wiley‐Liss, Inc.     

Histology and functional morphology of the oviduct of an oviparous snake,Diadophis punctatus

Oviductal functional morphology remains poorly understood in oviparous snakes, particularly in regard to oviductal formation of albumen and the eggshell and to sperm storage. The oviduct of Diadophis punctatus was examined using histology and scanning electron microscopy to determine oviductal functional morphology throughout the reproductive cycle. The oviduct is composed of four morphologically distinct regions: infundibulum, uterine tube, uterus, and vagina. The infundibulum is thin, flaccid, and lined with simple ciliated cuboidal epithelial cells. The tube contains ciliated and secretory epithelial cells, which reach a maximum height and hypertrophy during early gravidity and produce glycosaminoglycans. The posterior portion of the tube contains temporary sperm storage receptacles. The uterus retains eggs throughout gestation and secretes the eggshell constituents. The endometrial glands of the uterus hypertrophy during vitellogenesis and become depleted of the secretory granules during gravidity. The functional morphology of the oviduct therefore shows cyclical changes that are correlated with eggshell formation. The vagina consists of thick longitudinal and circular smooth muscle layers, which may serve in retention of eggs during gestation. Furthermore, the vagina contains long furrows in the mucosa that serve as sperm storage receptacles. These receptacles store sperm following fall mating and overwintering, whereas the receptacles in the tube are utilized briefly during vitellogenesis just prior to ovulation. © 1996 Wiley-Liss, Inc.     

Oviductal structure in a viviparous New Zealand gecko,Hoplodactylus maculatus

Oviductal structure is described in New Zealand's common gecko, Hoplodactylus maculatus, over four reproductive stages (early/mid-vitellogenesis, late vitellogenesis, early pregnancy, late pregnancy), using light, scanning electron, and transmission electron microscopy. Five regions of the oviduct are recognized: infundibulum, uterine tube, isthmus, uterus, and vagina. Up to three cell types make up the luminal epithelium of the oviduct: ciliated, nonciliated, and bleb cells. The function of bleb cells (seen in the infundibulum only) is unknown, but observation of these cells using transmission electron microscopy suggests that they are involved in secretory activity. Mucosal glands in the uterine tube possess large numbers of secretory granules of varying electron densities. Additionally, these glands appear to function as sperm storage tubules. Numerous sperm are seen in the glands during late vitellogenesis and early pregnancy. Very few uterine mucosal (shell) glands are seen during vitellogenesis, which is consistent with the observation that only a fine shell membrane covers the egg during early pregnancy. By late pregnancy, extraembryonic membranes lie adjacent to the uterus allowing the formation of the omphalo- and chorioallantoic placentas. Maximum cell height in the luminal epithelium is seen during vitellogenesis. The maximum percentage of ciliated cells making up the epithelial layer is seen during pregnancy. The low number of uterine mucosal glands seen in H. maculatus is a feature typical of other viviparous reptiles described, despite independent evolutions of viviparity. Although oviductal structure has been described in the literature for various reptiles, several ultrastructural features seen in this study highlight the lack of detailed understanding of this tissue. J. Morphol. 234:51-68, 1997. © 1997 Wiley-Liss, Inc.     

Oviductal sperm storage in the ground skink Scincella laterale holbrook (Reptilia: Scincidae)

The reproductive tracts of female ground skinks, Scincella laterale, collected at various times throughout their reproductive cycle were examined by light and transmission electron microscopy. Examination of the tracts revealed that sperm are retained in the posterior vagina after mating but prior to the ovulation of oocytes. The sperm are not sequestered in specialized glands but occur in scattered clusters in the lumen or among the deep, narrow rugae. The simple columnar lining of the vagina consists mostly of ciliated cells interspersed with occasional secretory cells. After ovulation, as indicated by the presence of eggs in the uterus, sperm are not found in the vagina. No sperm or sperm storage tubules occur in the infundibulum, the characteristic location for sperm storage in scleroglossid squamates that have been studied. Our results are a further indication that too few species have been examined to construct a rigorous phylogenetic hypothesis about the occurrence of sperm storage tubules in lizards.     

Female sperm storage in reptiles.

Internal fertilization and oviparity most likely are symplesiomorphies for modern reptiles, and viviparity has evolved independently numerous times in Sauria and Serpentes. Oviducal sperm storage is known in females of all taxa except Amphisbaenia. However, in Rhynchocephalia and Crocodilia, sperm storage is poorly studied, and specialized sperm storage tubules (Ssts) are unknown. We use the molecular phylogenetic hypothesis [(Chelonia+Archosauria) (Squamata)] to trace evolution of sperm storage characters. Ssts arose independently in Chelonia and Squamata. Turtles possess albumen-secreting glands in the anterior half of the oviduct (the tuba or isthmus), and the most distal of these glands also serve as Ssts; in addition, some turtles possess Ssts in the adjacent segment of the oviduct, the uterus. Squamates lack albumen-secreting glands, and the ancestral state is possession of Ssts in the posterior infundibulum (uterine tube). Secondarily, iguanids have evolved vaginal Ssts. In this paper, we present the first ultrastructural observations on vaginal Ssts in lizards, using Anolis sagrei (Polychrotidae). Proximally, the neck of these simple tubular glands continues the alternation of ciliated and secretory cells lining the lumen of the vagina. However, the epithelial cells of the distal sperm storage area are neither secretory nor ciliated. The Ssts of Anolis are more similar to those of birds more than to infundibular receptacles in snakes and lizards.     

Histology and functional morphology of the female reproductive tract of the tortoise Gopherus polyphemus

The oviducts of 25 tortoises (Gopherus polyphemus) were examined by using histology and scanning electron microscopy to determine oviductal functional morphology. Oviductal formation of albumen and eggshell was of particular interest. The oviduct is composed of 5 morphologically distinct regions; infundibulum, uterine tube, isthmus, uterus, and vagina. The epithelium consists of ciliated cells and microvillous secretory cells throughout the oviduct, whereas bleb secretory cells are unique to the infundibulum. The epithelium and endometrial glands of the uterine tube histologically resemble those of the avian magnum which produce egg albumen and may be functionally homologous. The isthmus is a short, nonglandular region of the oviduct and appears to contribute little to either albumen or eggshell formation. The uterus retains the eggs until oviposition and may form both the fibrous and calcareous eggshell. The endometrial glands are histologically similar to the endometrial glands of the isthmus of birds, which are known to secrete the fibers of the eggshell. These glands hypertrophy during vitellogenesis but become depleted during gravidity. The uterine epithelium may supply "plumping water" to the egg albumen as well as transport calcium ions for eggshell formation. The vagina is extremely muscular and serves as a sphincter to retain the eggs until oviposition. Sperm are found within the oviductal lumen and endometrial glands from the posterior tube to the anterior uterus throughout the reproductive cycle. This indicates sperm storage within the female tract, although the viability and reproductive significance of these sperm are unknown.     

Seasonal variation of the oviduct of the American alligator,Alligator mississippiensis (Reptilia: Crocodylia)

The annual oviductal cycle of the American alligator, Alligator mississippiensis, is described using light and electron microscopy. Previous work done by Palmer and Guillette ([ 1992 ] Biol Reprod 46:39–47) shed some light on the reproductive morphology of the female alligator oviduct; however, their study was limited and did not report details relating to variation across the reproductive season. We recognize six variable regions of the oviduct: infundibulum, tube, isthmus, anterior uterus, posterior uterus, and vagina. Each area shows variation, to some degree, in the histochemistry and ultrastructure of oviductal secretions. Peak secretory activity occurs during the months of May and June, with the greatest variation occurring in the tube and anterior uterus. During the month of May, high densities of neutral carbohydrates and proteins are found within the tubal and anterior uterine glands. The epithelium of the entire oviduct secretes neutral carbohydrates throughout the year, but many regions lack protein secretions, and the posterior uterine glands show little secretory activity of any type throughout the year. After oviposition, secretory activity decreases drastically, andthe oviduct resembles that of the premating season. This study also provides evidence to support the homology between alligator and bird oviducts. Sperm were observed in glands at the tubal‐isthmus and utero‐vaginal junctions in preovulatory, postovulatory and postovipository females. J. Morphol., 2009. © 2009 Wiley‐Liss, Inc.     

Sperm storage within the oviduct of turtles

Tubules containing sperm were identified by light microscopy in the oviducts from 11 species of turtles representing six different families. Sperm storage tubules were found in a small region of the posterior portion of the egg albumin-secreting section of the oviduct located between the infundibulum and the uterus. This location of storage tubules, midway between the ovary and vagina, is unique among vertebrates. Ducts, restricted to the posterior albumin region, connect the tubules to the oviduct lumen, allow entrance of sperm to the tubules. Sperm were identified in tubules of female turtles isolated from males for as long as 423 days.     

A histological and ultrastructural investigation of the female reproductive system of the water snake (Erythrolamprus miliaris): Oviductal cycle and sperm storage

We studied the structural and cellular organisation of the oviduct of Erythrolamprus miliaris including its morphological variation during the reproductive cycle using light microscopy, scanning electron microscopy and transmission electron microscopy. Four anatomically distinct regions compose the oviduct of E. miliaris including the anterior and posterior infundibulum, glandular uterus, non-glandular uterus and pouch. The cells of the oviductal epithelium secrete material by apocrine and merocrine processes, which vary between the anatomical regions and according to each phase of the reproductive cycle. The infundibular epithelium secretes electron dense vacuoles, which suggests the production of lipids, whereas the epithelial secretion of the glandular uterus, non-glandular uterus and pouch creates lucent and slightly electron dense vacuoles, indicating the production of glycoproteins. The timing of mating, vitellogenesis and sperm storage directly influences the morphofunctional alterations in the oviducts of E. miliaris. Sperm storage occurs only in the infundibular receptacles with increased production of the neutral carbohydrates in the presence of male gametes. Sperm storage happens in vitellogenic, non-vitellogenic and pregnant females of E. miliaris. Thus, females may be able to produce multiple clutches at different seasons of the year regardless of mating during autumn.     

Sperm aggregations in female Agkistrodon piscivorus (Reptilia:Squamata): a histological and ultrastructural investigation

Upon copulation in female Agkistrodon piscivorus, sperm migrate up the oviduct to sperm storage tubules (SSTs) in the posterior infundibulum. The epithelium of the SSTs is composed of ciliated and secretory cells and differs ultrastructurally from that of the epithelium lining the lumen of the posterior infundibulum. Sperm pass through an area composed primarily of ciliated cells at the opening of each gland before aligning themselves in parallel arrays with their nuclei facing an area composed primarily of secretory cells at the base of the tubules. Sperm are also found embedded inter- and intracellularly in the SSTs. The secretory vacuoles in the SSTs become highly electron dense after the start of the fall mating season along with the synthesis of lipid droplets. Histochemical analysis reveals that the alteration in secretory material density is caused by the production of neutral carbohydrates. Some sperm remain in aggregates in the nonglandular section of the posterior uterus until the time of ovulation. However, ultrastructural evidence indicates these sperm degrade before ovulation. Therefore, sperm in posterior aggregates have no role in fertilization of ovulated ova. The data presented here support the hypothesis that infundibular sperm storage is the mode that snakes utilize to sequester viable sperm until ovulation.     

Oviductal morphology and egg shelling in the oviparous lizards Crotaphytus collaris and Eumeces obsoletus

Morphological changes occurring in the oviduct and epithelial cells of the lizards Crotaphytus collaris and Eumeces obsoletus during the natural reproductive cycle were examined and quantified. Additionally, development of the eggshell at different stages of gravidity was described. The anterior uterus of each species has a distinct glandular type which differs between species: in E. obsoletus, the glands are tubular and in C. collaris, branched saccular. The branched saccular glands in the anterior uterus of C. collaris produce collagen-like material that forms the fibers of the shell membranes. However, fibers from the eggshell of E. obsoletus did not stain for collagen. The shell of both species is composed of a multilayered inner boundary covered externally by fibers of varying thickness. Initial layers are composed of thick fibers all lying along the same general axis. Outer layers of fibers are progressively thinner and an external surface layer composed of glycosaminoglycans (GAGs) is also present. In C. collaris, calcium, which is deposited in relatively small amounts on the shell surface, appears to be secreted by the epithelium of the anterior uterus. The nonciliated secretory epithelial cells covering the villi-like folds of the posterior infundibulum secrete GAGs. Epithelial cell height of the infundibular villi is greatest during early gravidity. A functional relationship may exist between luteal activity and oviductal secretory activity because the activity of the glandular epithelium varied as gravidity progressed.     

Histochemistry of the utero-vaginal junction with special reference to the sperm-host glands in the oviduct of the domestic duck.

The "utero-vaginal junction" that connects the uterus and vagina of duck oviduct differs histologically and histochemically from the adjacent zones. The region is characterised by low, somewhat longitudinally arranged mucosal folds, lined by tall columner cilliated cells with apical nuclei alternating with mucous secreting goblet cells containing basal nuclei. Within the propria of the mucosa there are numerous tubular glands--the sperm host glands--which are responsible for sperm storage after copulation. The goblet cells of the utero-vaginal junction contain an admixture of neutral, sulfated and nonsulfated acid mucopolysaccharides, while the sperm host glands are devoid of any mucopolysaccharide and secrete complex lipoidal materials and exhibit intense acid phosphatase activity. The functional significance of the secretory materials of the sperm host glands have been discussed in the light of sperm release mechanism.     

Oviductal sperm storage structure and their changes during the seasonal (dissociated) reproductive cycle in the soft-shelled turtle Lissemys punctata punctata

The oviduct of the Indian fresh water soft-shelled turtle Lissemys punctata punctata was examined throughout the year under light and scanning electron microscopes to determine the location, histomorphological characteristics, and function of sperm storage structure, as well as their changes at different phases of the seasonal reproductive cycle. Sperm storage structures in the form of tubules were observed in the wall of isthmus throughout the year. These tubules developed either by folding or fusion of the oviductal mucosal folds and were lined by both ciliated and nonciliated epithelial cells. The height and secretory activities of the epithelia were markedly high during the breeding phase (August to September) but low in the nonbreeding phase (October to June). A few short tubules lined by cuboidal epithelium appear in the wall of infundibulum only during the breeding phase. Following mating (May), inseminated sperm were stored within the tubules of isthmus up to the pre-ovulatory stage (August). Thereafter, sperm associated with PAS-positive materials secreted from the epithelium (referred to as a carrier matrix) moved forward to the infundibulum and were stored within the storage tubules of the infundibulum for a short time. Subsequently, sperm evacuated the storage tubules and entered the oviductal lumen to fertilize the subsequently ovulated eggs during or prior to ovulation. The isthmus-tubules become shorter and narrower in the regressive phase (October to November) and remained so until the early preparatory phase (April). Sperm release might have been stimulated by estrogen secreted from the ovarian follicles of pre-ovulatory turtles. Stored sperm not utilized for fertilization remained viable not less than six months in the present turtle species.     

Histomorphological changes in the oviduct of the mallard (Aves: Anatidae)

Studies on histomorphometrical changes in different segments (infundibulum, magnum, isthmus, shell gland and vagina) of oviduct of mallard, Anas platyrhynchos during active and quiescent phases of the reproductive cycle have been made. The absolute and per cent length and width of each segment showed a marked change. The magnum showed an increase of 280 per cent. Of all the histological parameters studied the number and height of mucosal folds and mucosal epithelium showed more marked increase in all segments of oviduct. The size of tubular glands and frequency of ciliated and secretory cells were studied in relation to oviductal activity.     

Microscopic organization of the sperm storage tubules in the oviducal gland of the female gummy shark (Mustelus antarcticus), with observations on sperm distribution and storage

Oviducal gland morphology, the microscopic organization of the terminal zone, and sperm storage were described in the female gummy shark (Mustelus antarcticus). Mustelus antarcticus is a nonplacental viviparous hound shark, which displays minimal histotrophy during embryonic development. The animals examined represented all stages of maturity and gestation. The oviducal gland was found to have the same fundamental zonation as in most chondrichthyans. Using recent terminology, the oviducal gland of chondrichthyans has an anterior club zone, followed by a papillary zone, both of which produce jelly that surrounds the egg, a baffle zone that elaborates the tertiary egg envelope and a terminal zone, where sperm storage occurs. Each zone is composed of simple tubular glands that connect to transverse grooves, which extend the full width of the gland. The exception is the terminal zone, which does not have transverse grooves but consists of individual tubules. The microscopic organization and histochemical nature of the zones display similar patterns to those of other chondrichthyan genera. Tubules of the terminal zone contain four types of cell: ciliated cells, alcian blue‐positive secretory cells, periodic acid‐Schiff and alcian blue‐negative secretory cells, and secretory columnar cells. These tubules end in recesses, the sperm storage tubules, which extend beyond the periphery of the baffle zone. Sperm were stored in the sperm storage tubules of all maturing and mature animals examined. Of note is the observation of stored sperm in an animal 1 year prior to first ovulation. Sperm were also observed throughout the uterine sphincter, body of the uterus, isthmus, and oviduct of maturing and mature animals, and in the uterine sphincter of an immature animal. These sperm represent immediately postcopulation aggregations of sperm and sperm in the process of migrating to the site of storage or to the site of fertilization. J. Morphol., 2008. © 2008 Wiley‐Liss, Inc.     

Ovarian,oviductal, and placental morphology of the reproductively bimodal lizard,Sceloporus aeneus

Sceloporus aeneus exhibits reproductive bimodality. That is, one taxon (Sceloporus aeneus bicanthalis) is viviparous whereas the other (Sceloporus aeneus aeneus) is oviparous. Morphological differences in luteal and oviductal structure were examined. Oviparous and viviparous females have distinct corpora lutea that form immediately after ovulation and remain active until just prior to oviposition or parturition. Luteal activity is correlated positively with follicular atresia. The oviduct of both subspecies is divided into three distinct morphological regions: an anterior infundibulum, a median uterus, and a posterior vagina. The infundibulum and vagina of females exhibiting either parity type are similar, whereas distinct differences in utering morphology are apparent. Primarily, these differences include the loss of uterine glands and a reduction in epithelial cell height in the viviparous form. Moreover, viviparous females possess a simple but well-developed chorioallantoic placenta and a simple choriovitelline placenta. Chorioallantoic placentation is associated with a significant increase in uterine vascularity, indicating a role in gas and/or water exchange. The evolution of viviparity and placentation are discussed in relation to these observations.     

Sialomucin complex (Muc4) expression in the rat female reproductive tract

Previous studies in our laboratory demonstrated the presence of sialomucin complex (SMC)/Muc4 covering the rat uterine luminal epithelium. SMC/Muc4 expression in the uterus is regulated by estrogen and progesterone and lost at the time of receptivity. In contrast to this hormonal regulation at the uterine luminal surface, SMC/Muc4 in the uterine glandular epithelium, oviduct, cervix, and vagina was constitutively expressed at all stages of the estrous cycle. Furthermore, SMC was expressed in the cervix and vagina of the ovariectomized rat, even though it is not found in the uterine luminal epithelium. Both soluble and membrane-bound forms of SMC were present in these tissues. Immunohistochemical analyses showed distinctive localization patterns of SMC in the various tissues during the estrous cycle. Moreover, the previously unreported expression of SMC/Muc4 in the isthmus, ampulla, and infundibulum of the oviduct suggests potential functions in gamete development. These results indicate that SMC/Muc4 is expressed in most tissues of the female reproductive tract, in which it may have multiple functions. However, hormonal regulation appears to be restricted to the uterine luminal epithelium.     

Sperm aggregations in the spermatheca of the red back salamander (Plethodon cinereus)

The spermatheca of Plethodon cinereus is a compound tubular gland that stores sperm from mating in early spring (March–April) to oviposition in summer (June–July). The seasonal variation of sperm storage in this species has previously been studied by light and transmission electron microscopy. In this paper, sperm aggregations, interaction of sperm with the spermathecal epithelium, and spermathecal secretions are studied using scanning electron microscopy. Within spermathecal tubules, relatively small groups of sperm are aligned along their entire lengths in parallel arrays. This pattern is similar to other plethdontids with complex spermathecae. Lumina of spermathecal tubules are filled with secretory material in April prior to the arrival of sperm, and after sperm appear, a coating of secretory material persists on the apices of the spermathecal epithelium. Sperm peripheral to the central luminal mass can become embedded in the secretory matrix or pushed deeper into the spermathecal epithelium. The spermathecal secretions may serve to attract and prolong the viability of sperm, but sperm that become enmeshed in the secretions or epithelium are phagocytized. Sperm and spermathecal secretions are largely absent after ovulation and in summer months, and new secretory vacuoles are formed in fall, although mating does not occur until spring.     

Morphological changes in the oviductal endometrium during the reproductive cycle of the tortoise,Gopherus polyphemus

The oviducts of 24 tortoises (Gopherus polyphemus) were examined using histological techniques and scanning electron microscopy to determine endometrial morphology. Measurements of endometrial characteristics (epithelial cell height, cilia length, thickness of endometrial glandular layer, and glandular diameter) in the uterus and tube (tuba uterina) were obtained to determine changes during the reproductive cycle. Epithelial cell height increases in both the uterus and the tube during vitellogenesis and remains hypertrophied during gravidity. Cilia length increases in the uterus during late vitellogenesis and gravidity, but the length of tubal cilia does not change during the reproductive cycle. The ratio of secretory to ciliated epithelial cells in the oviduct increases from quiescence to gravidity. The thickness of the glandular endometrial layer increases in both the uterus and tube during vitellogenesis. In the uterus, the glandular layer decreases in thickness during gravidity. The diameter of the uterine glands increases throughout vitellogenesis and gravidity; however, following ovulation glandular cells become depleted of secretory granules and cell height diminishes. The diameter of the tubal glands is unchanged during the reproductive cycle. Oviductal hypertrophy during vitellogenesis coincides with elevated circulating estradiol, whereas during gravidity progesterone concentrations peak (Taylor, '82, PhD Dissertation, University of Florida, Gainesville) and may induce secretion of albumen and eggshell components.