Theuterine endometrium goes through a monthly cycle during
which the functionalisis lost and replaced from the basalis.Themen-
strual phaseoccupies the first four days of the cycle (in the absence of
hCG), followed by the proliferative phaseunder the influence of FSH
(days 5 to 14) and then the secretory phaseunder the influence of LH
(days 15 to 28). During this phase, endometrial cells accumulate glycogen
preliminary to the synthesis and secretion of glycoproteins.
Thevaginal epitheliumis made up of stratified squamous cells and
varies with maturity, phase of the menstrual cycle, pregnancy, and cancer
(detected by vaginal Pap smear).
When fertilization and implantation occur, the placenta, consisting of
thechorion (fetal part)anddecidua basalis (maternal part),is estab-
lished for O 2 /CO 2 exchange, as well as its endocrine role (e.g., conversion
of androgens to estradiol, placental lactogen secretion). During parturition,
oxytocin secreted by the neurohypophysis stimulates the contraction of
uterine smooth muscle.
Thebreast (mammary gland)is a resting alveolar gland except dur-
ing pregnancy, when the lactiferous ductsproliferate and milkproduction
is initiated. Milk synthesis and ejection are under the influence of prolactin
andoxytocin.
EYEThe photosensitive layer of the retina is derived from the inner layer of the
optic cupand contains the rodandconecells involved in visual signal
transduction.Rhodopsinis a visual pigment found within lamellar disks
of the outer segment of the rod cell. Rhodopsin consists of retinaland
opsin;photons induce an isomeric change in retinal, leading to dissocia-
tion of the retinal/opsin complex. The resulting decrease in the intracellu-
lar second messenger, guanosine 3′ 5 ′-cyclic monophosphate (cGMP),
directs closure of membrane sodium channels and leads to hyperpolariza-
tionof the photoreceptor cell. This signal is transmitted to interneurons
within the retina and finally to ganglion cells. The fovea defines the center
of the retina, and is the point of sharpest visual acuity. The foveacontains
all cones and is directed toward whatever object you wish to see or read, for
example, these PreTest words at this very moment.
Thelensarises from surface ectodermduring development. Produc-
tion of lens fibers (elongated, protein-filled cells) continues throughout life
without replacement. Increased opacity of the lens (cataract)may be
36 Anatomy, Histology, and Cell Biology