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Involution and regeneration of the endometrium after parturition in the ewe, were studied by light- and electron microscopy. The luminal epithelium in intercaruncular regions of the endometrium remained intact at all stages, but degeneration and death of many glandular epithelial cells were observed on the day after parturition. Glandular regeneration had commenced by 8 d post partum, and the glands were substantially regenerated by 15 d. Caruncular epithelial cells on the maternal side of the placentomes, between the bases of the maternal septa, persisted during the period of degeneration of the foetal and maternal tissues of the placentomes. Epithelial cells from this source contributed to the regeneration of the caruncular epithelium following shedding of plaques of degenerate placental tissue from the caruncles, which commenced after 8 d and was completed before 31 d. Thus, ingrowth of epithelium from the edges of the caruncles, as previously proposed, was not the sole source of new caruncular epithelium. The additional source of regenerating epithelium identified here may account for the rapidity with which epithelium appears in the centres of some caruncles, several millimetres in diameter, during endometrial regeneration. However, in some caruncles, regeneration of the epithelium was not completed until after 31 d post partum.

Sixteen antral follicles, 1.8--4.2 mm in diameter, at various stages of atresia, were studied by electron microscopy. Deletion of theca interna cells by condensation followed by fragmentation (apoptosis), with subsequent engulfment of the fragments by remaining thecal cells, was present at all stages, but was most marked during secondary and tertiary atresia. In primary and secondary atresia, the relative numbers of thecal cells whose cytoplasm was rich in tubular endoplasmic reticulum were higher than in non-atretic follicles of comparable size. During tertiary atresia the number of cell layers in the theca interna was reduced, and cells rich in tubular endoplasmic reticulum became proportionately less numerous. Degenerating cellular material was present within the lumina of thecal capillaries at all stages of atresia. Such material was rarely seen in primary atresia, and increased in incidence progressively in later stages. It was concluded that during atresia a large number of theca interna cells are deleted by apoptosis, and many thecal capillaries become blocked with cellular debris.

An electron microscopical, morphometric study has been made of the cellular and non-cellular composition of the ovine corpus luteum fixed by perfusion, together with an estimate of total cell numbers in the corpus luteum based on measurement of cell and tissue DNA content. Normal cyclical corpora lutea from six ewes were studied by each method. Total cell numbers per corpus luteum were estimated at 258 X 10(6) by morphometry and 238 X 10(6) by DNA measurement. The existence of four structurally distinct major cell populations in ovine luteal tissue was confirmed. Percentages of the total cell number were as follows: large luteal cells - 4.2%; small luteal cells - 18.5%; fibrocytes - 16.8%; endothelial cells and pericytes - 52.6%. The remaining 7.9% comprised other miscellaneous cell types. Of the four major cell types, cell volume (X 10(3) micron 3 +/- standard error of the mean) was greatest for large luteal cells (13.08 +/- 2.10) and least for endothelial cells and pericytes (0.52 +/- 0.05), with small luteal cells (2.08 +/- 0.19) and fibrocytes (1.61 +/- 0.27) intermediate in size. Large luteal cells occupied 25.4% of luteal volume density, small luteal cells 17.5%, fibrocytes 11.2% and endothelial cells and pericytes 12.7%. Other cell types occupied 5.4%, with the remaining volume occupied by vascular lumina (10.8%) and intercellular space (17.1%). The data on cell volume support morphological findings suggesting that large and small luteal cells of the ovine corpus luteum are distinct populations rather than the upper and lower limits of a single population whose sizes are normally distributed. Of these two types of luteal cell, small luteal cells outnumbered large luteal cells by more than 4:1, while the individual large luteal cells were more than six times the mean volume of the small luteal cells. These facts need to be considered when assessing the relative contributions of the two types of luteal cell to overall luteal steroidogenic function.

The theca interna of non-atretic ovarian follicles from 2.0 mm in diameter up to the stage shortly following ovulation was studied by light and electron microscopy. In follicles less than 3.0 mm in diameter, the theca interna consisted of about 8-12 layers of flattened cells, together with many capillaries and small bundles of collagen. Two main forms of cellular differentiation were seen. These were towards either fibroblast-like cells or presumed steroidogenic cells whose cytoplasm contained large amounts of predominantly smooth tubular endoplasmic reticulum, to which some ribosomes were attached. The majority of cells were of relatively undifferentiated or intermediate structure. In larger follicles up to the early stages of oestrus the theca interna cells became larger and less flattened, and cells rich in tubular endoplasmic reticulum became proportionately more numerous. By 18 h after the onset of oestrus the theca interna was oedematous, and many cells possessed pseudopodia. Many cells also contained numerous lipid droplets, but there were no signs of thecal cell degeneration or death. Shortly after ovulation the basal lamina of the membrana granulosa was incomplete, and it became more difficult to distinguish between theca and granulosa layers. Structural heterogeneity, with two major cell types and cells of intermediate structure, was present at all stages. It was concluded that: (1) the theca interna of 2.0-2.9 mm follicles contained many cells whose structure was compatible with a steroidogenic capacity; (2) changes in the differentiated thecal cells up to the early stages of oestrus were quantitative rather than qualitative, and suggestive of an increased steroidogenic capacity; (3) the accumulation of lipid in many cells of the theca interna by 18 h after the onset of oestrus probably reflected a reduction in steroidogenic activity; and (4) there was no evidence of any structural specialization to facilitate the transport of steroids from the theca interna to the membrana granulosa.

The fate of the theca interna after ovulation was studied in ewes, using light and electron microscopic histology and histochemistry. At the time of ovulation the theca interna was incorporated, apparently completely, into the margin of the developing corpus luteum and into the centres of many infoldings of the follicular wall. There was no evidence of degeneration of the more highly differentiated theca interna cells at or following the time of ovulation. Within 24 h of ovulation, cells derived from the theca interna began migrating from their original sites into the deeper, granulosa-derived areas of the luteal tissue. At later stages cells derived from the theca interna remained concentrated in septa derived from the follicular infoldings, but were also widely distributed throughout the luteal tissue. Structural evidence supported the view that the small luteal cells and fibroblasts of the corpus luteum were derived from the theca interna, and the large luteal cells from the membrana granulosa.

The extrinsic ovarian blood vessels were studied in 134 ewes. In view of recent evidence that uterine luteolysis may involve venoarterial transfer of prostaglandin F2alpha in the ovarian pedicle, particular attention was paid to the interrelationships between veins and arteries. The ovarian artery and utero-ovarian vein are large vessels of conventional structure and lie in close apposition. Their walls are slightly thinner on their apposing sides. The ovarian branches of the ovarian artery are very tortuous, and closely intertwined with the plexiform ovarian branches of the utero-ovarian vein. An extensive plexus of small veins surrounds the ovarian artery and its ovarian branches. Within this plexus are many thin-walled, dilated regions, interspersed with narrow, thick-walled segments. Valves are inconstantly present at sites of entry of branches of the plexus into the major veins. Small numbers of arterio-venous anastomoses are present in the distal part of the ovarian pedicle. Unless blood can flow in a veno-arterial direction through arterio-venous anastomoses or capillary beds, the structural barrier between uterine venous and ovarian arterial blood is substantial.

Corpora lutea (CL) of pregnancy from single-lambing ewes were examined by light and electron microscopy within 24 h and at 8, 15, 23, 31 and 41 days after parturition (2 ewes per stage). Within 24 h of parturition the structure of the CL was well preserved and both large and small luteal cells, characteristic of this species, were present in substantial numbers. However, both types of luteal cell contained numerous cytoplasmic lipid droplets, and smooth endoplasmic reticulum and secretory granules in large luteal cells were less prominent than in normal functional CL of cyclic ewes. Leucocytic infiltration, and death of some luteal and endothelial cells, were also observed at this stage. Further regression of the CL progressed slowly, and lipid-rich large luteal cells were still readily recognisable 15 days after parturition. The size of the CL declined progressively, and the proportion of tissue occupied by intercellular substances increased. Corpora albicantia approximately equal to 3-4 mm in diameter were still recognisable 41 days after parturition. It was concluded that luteal regression post partum progresses much less rapidly than at the end of the oestrous cycle.

Data are presented on the histogenesis of the omasal mucosa in sheep from the 2.5 cm crown-rump (c-r) length fetus to the adult. 11 stages of fetal development, and 4 post-natal stages, were studies. The distribution of glycogen in the omasal epithelium was also studied. During fetal life the omasal epithelium was initially stratified cuboidal in type, but the superficial layers of cells became flattened in later stages of gestation. This epithelium became extremely thick by the late stages of fetal life, reaching a maximum of 358 micron, and consisting of greater than 20 layers of cells, in the 45 cm c-r fetus (approximatelay 140 days). After birth the epithelium became markedly reduced in thickness, being approximately 77 micron in the adult, and had differentiated into a cornified stratified squamous epithelium of the adult type by 12 weeks after birth. Glycogen was extremely abundant in the omasal epithelium of the 2.5 cm fetus, and declined gradually thereafter to be almost completely absent in post-natal specimens. 4 orders of laminae were present in the adult omasum, distributed in the seqeunce 1-4-3-4-2-4-3-4-1. The 1st order was already present in fetuses of 2.5 cm c-r length, with the 2nd, 3rd and 4th appearing by the 3.5, 5.5 and 11.0 cm stages, respectively. Initial stages in the development of conical papillae were first seen in 15.0 cm fetuses, but the development of these papillae was not completed until after birth.

Several lines of evidence suggest that follicular granulosa cells give rise to the large luteal cells of the corpus luteum in the sheep. To further investigate this suggestion, numbers of granulosa cells in preovulatory follicles were estimated by morphometric methods for comparison with a previous estimate of numbers of large luteal cells (9.6 +/- 0.9 x 10(6)). Preovulatory follicles from five Corriedale ewes were obtained after synchronization of the oestrous cycle with the prostaglandin analogue cloprostenol. Morphometry was undertaken using light microscopy of plastic-embedded tissue sectioned at 1 micron. Mitotic index in the membrana granulosa was 0.05 +/- s.e.m. 0.05%. Mean follicular diameter was 6.25 +/- 0.25 mm and there were 7.68 +/- 0.53 x 10(6) granulosa cells per follicle. These results demonstrate a similarity between the number of granulosa cells per follicle and the number of large luteal cells per corpus luteum and thus support the hypothesis that large luteal cells are derived from granulosa cells.

Corpora lutea (CL) from naturally cycling Corriedale ewes were obtained in the mid- and late luteal phases of the oestrous cycle (Days 9 and 13; 5 ewes per group). The cellular composition of these CL was compared by ultrastructural morphometry to determine whether there were changes in numbers of large and small luteal cells consistent with differentiation of some small luteal cells to large luteal cells during the last part of the luteal phase. No differences between Days 9 and 13 were detected in luteal volume, plasma progesterone concentration, or volume density of any component of the luteal tissue. Large luteal cell numbers (mean +/- s.e.m.) were lower per unit volume of luteal tissue on Day 13 than on Day 9 (14.1 +/- 0.5 vs 18.4 +/- 1.3 X 10(3)/mm3, P less than 0.05). Mean volume of the individual large luteal cells was greater on Day 13 than on Day 9 (19.65 +/- 0.72 vs' 15.60 +/- 1.34 micrograms 3 X 10(3), P less than 0.05). However, there were no significant differences in numbers or volumes of small luteal cells between Days 9 and 13, and total numbers of large luteal cells per CL were not different between these two days. These results provide no support for the hypothesis that small luteal cells differentiate into large luteal cells during the oestrous cycle of the sheep.

Corpora lutea of sheep were examined by electron microscopy at day 10 of the oestrous cycle and at days 15, 25, 50, 100, 125 and 140 of pregnancy. Small luteal cells were present in all corpora lutea, and were two to three times as numerous as large luteal cells. The former were irregular in shape, with tapering cytoplasmic processes. Their major cytoplasmic organelles were a predominantly smooth endoplasmic reticulum, mitochondria with tubular and lamellar cristae, and one or more Golgi complexes. The enzyme delta 5-3 beta-hydroxysteroid dehydrogenase was present in their cytoplasm. Small luteal cells were often interposed between large luteal cells and capillaries, and formed close, complex surface relationships with large luteal cells. Small and large luteal cells differed in many ways, including the restriction of numerous approximately 0.2 micron cytoplasmic granules to the large cells, and no cells of intermediate structure were observed. These features of small luteal cells suggest a steroid hormone synthetic function, and direct interaction with large luteal cells.

Data on the structure, quantitation, origins and functions of the large luteal (LL) and small luteal (SL) cells of sheep, goats and cattle are reviewed. Both LL and SL cells show ultrastructural features consistent with a steroidogenic function. However, in addition to differences in size and shape, LL cells differ from SL cells primarily in possessing large numbers of secretory granules, suggesting an additional protein/polypeptide synthetic and secretory function. In sheep, morphometric estimates show that the corpus luteum (CL) contains approximately equal to 10 X 10(6) LL cells and approximately equal to 50-60 X 10(6) SL cells: individual LL cells are approximately equal to X 6 greater in volume than SL cells. During formation of the CL, granulosa and theca cells are incorporated, and evidence suggests that granulosa cells give rise to LL cells and theca cells to SL cells. However, SL cells, or cells of thecal origin, may also give rise to some LL cells. Both LL and SL cells produce progesterone in vitro. On a per cell basis, LL cells produce more progesterone than do SL cells, but SL cells show a much greater progesterone-secretory response to LH. Oxytocin is synthesized, and secreted in granule form, only by the LL cells, and relaxin, whose presence has been demonstrated convincingly only in cattle, also appears to be produced only by LL cells. The two types of luteal cell in ruminants therefore show major differences in function: the occurrence of any significant functional interaction remains to be established.