This sinusoid, like a capillary, has only an endothelial wall, but its lumen is characteristically considerably wider. Also, in some locations in the body (such as bone marrow, liver, and spleen) the endothelial cells of sinusoids are rather loosely joined together, thus permitting passage of blood cells between them.

Female Reproducive Tract

Ovary, Oviduct, and Uterus




Ovary with surface cuboidal epithelium. (Really a modified mesothelium.)

Cortex of ovary. A thick connective tissue capsule, the tunica albuginea underlies the surface epithelium. Somewhat deeper lie several small, primary (primordial) follicles. (All egg cells have reached the primary oocyte stage by birth and are held in this "suspended animation", in very early prophase, until such time as they may ovulate or undergo atresia.)



Primary follicles with one single layer of flat follicle cells surrounding an oocyte. Although an oocyte is a giant compared with its neighbors, this early stage is small for an oocyte, and the cell will grow considerably in size when it begins to mature, under the influence of FSH. The nucleus looks lightly granular, and the dark nucleolus is prominent. Cytoplasm is very pale. Note the "swirly" interstitial tissue of the ovarian stroma.






Early maturation stage of follicle with beginning proliferation of follicle cells around an enlarging oocyte. The nucleolus shows clearly inside the nucleus. As the oocyte enlarges, its chromosomes prepare further for the first meiotic division, which will occur at ovulation.



Further developed follicle
a = with antrum beginning at arrows. The homogenous gray-blue line immediately surrounding the egg cell itself is the zona pellucida.
b and c = primary follicles, containing oocytes which are still small.



A group of follicles in various stages of early development in the cortex of a rat ovary. Blood vessels of the ovarian medulla are seen in the center of the field. Development of follicles is regulated by FSH from the anterior pituitary.



Maturing follicle, so called because it contains a definite antrum (or fluid-filled space) and many layers of granulosa cells. The egg is still a primary oocyte and sits to one side of the follicle on a mound of cells called the egg hillock or cumulus oophorus. The cells closest to the oocyte will be expelled with it at ovulation as the corona radiata. Surrounding the granulosa cells of the follicle is the theca interna, a rather cellular and vascular connective tissue layer, which secretes estrogen. Outside of this is the theca externa a more fibrous connective tissue layer, not well defined here. Note that several follicles may start to develop in any one monthly cycle, but in the human only one will mature, unless there are to be multiple ovulations and therefore possible multiple births. All follicles that don't complete their maturation undergo atresia (i.e., degenerate). The egg dies, the granulosa layer breaks up, and the whole follicle collapses and undergoes fibrotic change.




Large ruptured follicle, just after ovulation.
Arrow = stigma, the point of rupture where oocyte was expelled. The reduction division (or first meiotic division) takes place at the time of ovulation.
a = granulosa cells that will now proliferate under the stimulus of pituitary LH and enlarge to become granulosa lutein cells, filling in the follicular cavity and becoming the major portion of the new corpus luteum.
b = corpus albicans -- old scar of an earlier corpus luteum.

Detail of corpus luteum showing the rounded foldings of large, pale granulosa lutein cells in the lower half of the picture; these secrete progesterone. Pushing down between the folds is a wedge of smaller, darker theca lutein ; these secrete estrogen.

Simple cuboidal epithelium in Mallory stain (longitudinal cut). Note the dark chromatin clumps in the nuclei. Underneath the epithelium lies a small blood vessel filled with orange-colored blood cells.

Panoramic view of the infant thymus, showing its lobulated structure. Within each lobule is a dark cortex and pale medulla. There are no round nodules or germinal centers in the cortex, just diffusely and densely packed lymphocytes. The thymus is seeded by lymphocytic stem cells very early in life and is particularly active in the production of lymphocytes in the young person. Those lymphocytes which resided in the thymic cortex early in life are forever after "thymus dependent"-cells , no matter where they reside in the body later on.

Seen here as pink connective tissue strands are the outer splenic capsule and its continuation into a trabecula. There are some sinusoids, filled with blood, to the left. The red pulp tissue between splenic sinusoids is often called cords of Billroth. The sinusoids of spleen are comparable structurally to the sinusoids of bone marrow.

Splenic phagocytes (macrophages) loaded with yellowish-brown hemosiderin particles ingested during the breakdown of dead red blood cells. The spleen filters and "cleans" blood in the same way that the lymph nodes filter lymph.

Detail of Hassall's corpuscle, with concentric layers of keratinizing epithelial cells. Although the significance of the corpuscles is uncertain, they are apparently formed from the epithelioid stromal cells.