Contact lenses are artificial prosthetic devices worn on the eye's front surface to substitute for the anterior corneal surface. Contact lenses help correct the refractive error and irregularities of the corneal surface.[1] Contact lenses are described with various specifications like overall diameter, optic zone diameter, base curve, central, peripheral, and intermediate curves, edge, power, thickness, and tint. The contact lens can be of varied types. There can be soft, hard, rigid gas permeable contact lenses.[2] The various indications of contact lenses can be optical, therapeutic, preventive, diagnostic, operative, cosmetic, and occupational. The action of contact lenses depends on wettability, water content, oxygen permeability, transmission, light transmission, refractive index, resistance to temperature, dimensions, and flexural stability.[3] Based on material, contact lenses are divided into focons and filcons. An ideal contact lens should be biocompatible, gas permeable, have good optical properties and tolerance, be mouldable, sterile, stable, and have good surface chemistry. Each type of contact lens has various advantages and disadvantages.[4] The complications associated with contact lens use can be conjunctival, corneal, and mechanical. This activity deals with the anatomy and physiology of contact lenses, indication, contraindications, preparation, complication, and clinical significance of contact lenses.[5]

The healthy central cornea is aspheric and prolate (the central curvature is steeper than the periphery). Attempts to measure the cornea were made as early as the 1600s by Scheiner, who compared reflections produced by glass spheres whose diameters were known to the reflections from the anterior surface of the cornea. The central keratometric values vary between 40 D to 47 D (mean 43D). The standard keratometry device measures two points at a 2.25-4 mm zone in the central cornea. The radius of curvature of the central part of the anterior surface of the cornea is measured by the size of the reflection of an image projected by the keratometer using the formula r=2uI/o or the radius of curvature= 2 x distance between the reflective surface and the object x image size /object size (r=radius of curvature, u=distance between the reflective surface and the object, I=image size, and o= object size). The different keratometers, such as the Javal Schiotz and the Bausch and Lomb keratometer, were adequate to calculate the dioptric power of the cornea for intraocular lens power calculation and contact lens fitting. However, with increased keratorefractive procedures, the evaluation of paracentral and peripheral cornea is of increased significance. The Placido ring was the first instrument used to analyze the anterior surface of the cornea. Then, computerized videokeratography (CVK) was developed to map the corneal power and shape. Principles of topography - A luminous object is projected onto the cornea, and its reflection is analyzed. The methods of topography include: Placido method, Slit scanning technique, and Schiempflug method. The Placido method analyzes only the anterior surface of the cornea, whereas the other two methods also map the posterior corneal surface and measure the corneal thickness. The Placido method involves projecting a series of concentric black and white rings onto the cornea and capturing their reflection by a camera placed in the center of the rings. This is the first Purkinje reflex. The power of the central 3 mm of the cornea is extrapolated, resulting in the Sim (simulated) K (keratometry) values. The rings are centered on the visual axis and not the pupil. The distance between the visual axis and the center of the pupil is the angle kappa (K). Algorithms calculate the power of the cornea at each point depending on the deformation of the mires. The Placido-based topography measures the refractive status of the cornea accurately but does not, however, reflect the true shape of the cornea. Corneoscope and corneopter: This Placido-based device projects black and white mires on the cornea. Then a  black and white or a color polaroid photograph is taken at 4.8 X magnification. The dioptric power, and the radius of curvature at a particular point, are read by comparing the amount of magnification needed to match the photographed rings to a standard set of rings on the comparator screen. The steeper the cornea, the closer the rings. Variations between the normal cornea, pseudocones, suture-induced astigmatism, and true keratoconus can be recognized.  The commonest causes of pseudocones are pellucid marginal degeneration, contact lens-induced irregular astigmatism, and corneal or limbal trauma. The corneal power is inversely related to the area which can be examined. A larger area of a flatter cornea can be examined compared to a steep cornea. The slit scanning technology is based on measuring the dimensions of a slit scanning beam projected on the cornea. Orbscan initially was only a slit scanning technology to measure corneal thickness as well as curvature of the anterior and posterior surfaces of the cornea. Elevation information was acquired directly, and curvature information was derived from this. Orbscan II incorporated a Placido disc attachment to get curvature measurements directly. The Orbscan IIz is a further improvement because it incorporates the Shack-Hartmann aberrometer in the Zyoptix workstation.  The reflection of the illuminated Placido disc onto the cornea is stored, followed by the projection of 40 slits, 20 from the right and the left (each slit 12.5 mm high x 0.30 mm wide) at an angle of 45 degrees to the instrument axis. The backscattered images are captured by the device's video camera. Acquisition of images is followed by assessment of images and compensation for eye movements by a proprietary technique. Processing of the data is done by the machine to construct the anterior and posterior elevation maps and the curvature maps. The Schiempflug technology uses a rotating camera to capture the reflection of a bright slit beam that sweeps across the cornea. A 3-D reconstruction of the anterior and posterior elevation is done, and the pachymetry is calculated. The Schiempflug principle is credited to Theodore Schiempflug, who devised a method to correct distortion in aerial photography. A single rotating camera and static camera are present in the Pentacam device. The Schiempflug and the Placido-based technologies are combined in machines like the Sirius, TMS 5, and Galilei. The Galilei machine uses two rotating cameras (dual Schiempflug analyzer), whereas the pentacam has a single rotating camera. Tomography vs. Topography A topographer analyzes only the front surface of the cornea. Tomography is derived from the word "Tomos " (Greek for cut or section) and "graphein" (to write). A 3-dimensional image of the anterior and posterior surface of the cornea and corneal thickness is measured as opposed to topography, where only the anterior surface of the cornea is mapped. Tear film analysis is possible with the Placido-based systems. Non-invasive tear film break-up time (NITBUT) can be measured for various purposes by Placido-based topographers or by Placido disc alone. Projecting rings on the corneal surface and measuring the time to break or distortion of the mires can effectively measure the tear film break-up time.

Pellucid marginal corneal degeneration (PMCD) is a bilateral, noninflammatory, peripheral corneal thinning disease. It is characterized by a peripheral crescentic band of thinning, usually in the inferior cornea. The ectatic zone, which is 1-2 mm from the limbus, lies above the point of the maximum corneal thinning. The term pellucid marginal degeneration was coined first by Schalaeppi in 1957 as "la dystrophie marginale inferieure pellucide de la cornee". "Pellucid," meaning clear, signifies the clarity of the cornea despite the presence of ectasia.[1] The condition is more common in males in their 2nd to 5th decades of life. It is very important to rule out any thinning and ectatic disorders like keratoconus and PMCD before refractive surgery as upon being missed, they can lead to unwanted complications post-operatively.

The aim of the study is to expand information on the histological aspect, presence, and distribution of cytoskeletal proteins: smooth muscle alpha-actin (α-SMA), desmin and vimentin in ovaries of three lizards Leiosauridae. In all analysed species, the ovaries were paired organs, located inside the coelomic cavity, covered in a simple cubic epithelium. Below the surface was the tunica albuginea. The organs can be divided into two regions: the cortex and the medulla. The pre-vitellogenic follicles consist of an oocyte surrounded by the pellucid zone periodic acid schiff positive, the granulosa layer consisting of three cell types: small, intermediate, and large and the theca layer. The vitellogenic follicles presented only a single layer of cubic granulosa cells. In the three lizards, α-SMA microfilaments (MFs) were verified along the theca layer and in endothelial cells of the blood vessels of the analysed follicles. Researchers have observed desmin intermediate filaments immunostaining exclusively muscle fibers of the albuginea and the ovarian stroma of Enyalius perditus. There was no immunostaining for vimentin in the ovaries of all the lizards studied. Results obtained revealed that the MFs of α-SMA could be responsible for a contractile activity present in the lizards assessed.

The pellucid zone (PZ) is a protective embryonic cells barrier against chemical, physical or biological substances. This put, usual transfection methods are not efficient for mammal oocytes and embryos as they are exclusively for somatic cells. Carbon nanotubes have emerged as a new method for gene delivery, and they can be an alternative for embryos transfection, however its ability to cross the PZ and mediated gene transfer is unknown. Our data confirm that multiwall carbon nanotubes (MWNTs) can cross the PZ and delivery of pDNA into in vitro-fertilized bovine embryos. The degeneration rate and the expression of genes associated to cell viability were not affected in embryos exposed to MWNTs. Those embryos, however, had lower cell number and higher apoptotic cell index, but this did not impair the embryonic development. This study shows the potential utility of the MWNT for the development of new method for delivery of DNA into bovine embryos.

Sperm capacitation occurs during the passage of sperm through the female reproductive tract. Once the sperm binds to the pellucid zone, the acrosome reaction to enable penetration of the oocyte is completed. In this study, sperm of Artibeus jamaicensis bat was used to evaluate both capacitation status and the acrosome reaction under in vitro conditions, incubating sperm at 32 and 37°C with and without progesterone. Sperm was incubated at different times to assess sperm cells' functionality in terms of capacitation and acrosome reaction, using the chlortetracycline staining, lectin fluoresceinisocyanate conjugate-Pisum sativum agglutinin (FITC-PSA), and transmission electron microscopy. Sperm cells that presented uniform fluorescence throughout the head and mid-piece were classified as non-capacitated. Subsequently, sperm cells, which were observed with fluorescence only in the anterior portion of the head and mid-piece, were classified as capacitated. Sperm cells with no fluorescence in the head, but fluorescence in the mid-piece, were categorized as sperm cells that have carried out the acrosome reaction. During the acrosome reaction, sperm cells showed changes in their morphology, so it was not possible to distinguish the plasma and acrosomal membranes. Around the entire head, it was not possible to distinguish the fusion points between these membranes that made it possible for the acrosomal reaction to take place and thus to release the enzymes necessary to penetrate the pellucid zone. In conclusion, under appropriate in vitro conditions and by supplementing the culture medium with progesterone, A. jamaicensis bat sperm cells are able to be capacitated in a period from 6 to 8 h and to carry out the acrosome reaction.

The sustainability and production of collared peccary (Pecari tajacu) has been studied in the last few years; however, further information on its reproduction is necessary for breeding systems success. Understanding folliculogenesis aspects will contribute to effective reproductive biotechniques, which are useful in the preservation and production of wildlife. The aim of this study was-to evaluate the ovarian folliculogenesis in collared peccary. Ovaries from six adult females of collared peccary were obtained through ovariectomy and analyzed. These were fixed in aqueous Bouin's solution and sectioned into 7 microm slices, stained with hematoxilin-eosin and analyzed by light microscopy. The number of pre-antral and antral follicles per ovary was estimated using the Fractionator Method. The follicles, oocytes and oocyte nuclei were measured using an ocular micrometer. Results showed that the length, width, thickness, weight, and the gross anatomy of the right and left ovaries were not significantly different. However, the mean number of corpora lutea was different between the phases of the estrous cycle (p<0.05), with the highest mean in the luteal phase. Primordial follicles were found in the cortex; the oocytes were enveloped by a single layer of flattened follicular cells. In the primary follicles, proliferation of the follicular cells gave rise to cuboidal cells (granulosa cells). The secondary follicle was characterized by two or more concentric layers of cuboidal cells (granulosa), beginning of antrum formation, and the presence of pellucid zone and theca cells. Antral follicles were characterized by a central cavity (antrum), the presence of cumulus oophorus and theca layers (interna and externa). In the right ovary, the values of the primordial and primary follicles were similar, but significantly different from the secondary ones (p<0.05). In the left ovary, significant differences were observed between all follicles in the follicular phase (p<0.05); the mean number of primordial and primary follicles was similar in the luteal phase. The mean number of pre-antral follicles and antral follicles in the follicular phase was higher in the left ovary (p<0.05). The mean number of antral follicles in the luteal phase was similar in both ovaries. We also found significant differences in mean diameter of preantral follicles, oocyte, granulosa layer and oocyte nucleus during the estrous cycle. In the antral follicles a significant difference was observed only in follicular diameter (p<0.05). The predominance of active primordial and primary follicles was found in both phases; otherwise the secondary follicles and antral follicles showed a high degree of degeneration. The results obtained in the present work will strengthen the development of biotechnology programs to improve the productive potential and conservation of the collared peccary.