Reproductive System Overview - Anatomy & Physiology

The Mare Uterus- Courtesy of A.Crook, Copyright RVC 2008

Introduction to the Reproductive System

Reproduction is the complex set of biological processes that result in the formation of a new organism; it is crucial that we understand how these processes occur normally and have a good grasp of the role of hormones in the reproductive process. Pathology and disease are common within the reproductive system and can not only lead to a sick animal but also to a loss in production. In this section we hope to cover all the main processes in domestic animals, laying a foundation for understanding the associated pathological states.

Puberty

Puberty is the process of aquiring reproductive capability, and age at onset of puberty varies from between species. There are several factors involved in triggering the onset of puberty, including body size, nutritional factors, day length, genetic factors and environmental and social factors. The onset depends on the ability of specific hypothalamic neurons to produce gonadotropin-releasing hormone (GnRH) in sufficient quantities to both promote and support gametogenesis. The hypothalamus itself is influenced by hormonal factors that are male or female specific.

Female Reproductive Tract and Reproductive Behaviour

The anatomy of the female reproductive tract includes the external genitalia and vagina, the cervix, the uterus, the oviduct and the ovary. Female animals regularly enter a phase where they are sexually active and receptive to males; this is known as the oestrous cycle. Some animals are able to reproduce all year round, but many have an anoestrus phase which requires a seasonal response for cycling to begin. In some species detection of seasonal cycling can be crucial to productivity.

Female animals have various behavioural and chemical signals which increase their attractiveness to males and others which signal their acceptance of attention from the male. Further cues indicate that the female is receptive to mating. Other reproductive female behaviours include maternal and nursing behaviour. In neurological terms sensory information from behavioural and chemical cues are processed by the hypothalamus, which induces the appropriate mating response via specific peptides which activate neurons in the midbrain and brainstem.

Male Reproductive Tract and Reproductive Behaviour

The anatomy of the male reproductive tract includes the penis, the testes, the epididymis and the accessory sex glands. Males of different species are often referred to by specific names which reflect their repoductive status. Males differ from females in the production of sperm which takes place from puberty onwards - unlike females where ova are produced prior to birth. The hormones controlling sperm production, or spermatogenesis are luteinising hormone (LH) and follicle-stimulating hormone (FSH).

The neurological pathways that control reproductive behaviour are similar to the female, but the response is male specific and includes pre-copulatory, copulatory and post-copulatory behaviour.

Fertilisation, Implantation and Early Embryonic Development

After mating has occured, sperm must travel along the female reproductive tract towards the uterus in order to fertilise the oocyte. A single spermatazoon penetrates the oocyte and the male and female pronuclei fuse to form an ootid which divides repeatedly to form the blastocyst. At this stage the genetic sex of the offspring is determined as male gametes are heterogametic. The blastocyst continues to divide and grow until the outer layer, the zona pellucida ruptures and the blastocyst 'hatches' and becomes free floating within the uterus. The blastocyt continues to grow and develop until it becomes an embryo with placental and embryonic membranes such as the amniotic sac; the placental membranes then attach to the uterine endometrium in a process known as implantation. During embryonic development the process of sexual differentiation begins.

Pregnancy and Parturition

In most domestic species, pregnancy is maintained by hormonal feedback from the corpus luteum and, in many species, the placenta. The corpus luteum is maintained in early pregnancy and progesterone levels remain elevated as a result. This maintains negative feedback to the hypothalamus and anterior pituitary gland which inhibits further follicular development and ovulation. In polyoestrous species, it prevents the return to oestrus. In many species the placenta will take over or supplement the corpus luteum as the source of progesterone during pregnancy; the placenta is a source of many of the hormones that regulate changes during preganancy, stimulating mammary growth, the growth of the myometrium, preparation for parturition, and secreting chorionic gonadotrophin in species where this is required for the maternal recognition of pregnancy. Litter sizes vary between species, as do methods of pregnancy diagnosis.

Summary of Gestation Lengths in Various Species

The process of giving birth, or parturition is conventionally divided into three stages; stage one includes the movement of the foetus into the birthing position and preparation of the reproductive tract for birth, which is followed by stage two - contractions and the birth itself. Finally, stage three involves the expulsion of the foetal membranes. The timings of each stage vary from species to species, and a good knowledge of the normal progression of parturition is essential to detect when things are not proceeding normally. There are several conditions that can occur to disturb the normal progress of parturition, including dystocia, uterine inertia, ring womb, prolapses, retained placenta, endometritis and foetal abnormalities. Like pregnancy, parturition is under complex hormonal control, and this can be manipulated pharmacalogically to alter the timing of parturition.

The Puerperium

Puerperium describes the period of time immediately following parturition, when the female enters a period of reproductive repair and begins lactation. For a period of time these two processes overlap. During this time the female experiences regular myometrial contractions which expel the remaining fluid and tissues present following birth, allowing the uterus to begin involution and repair so that the reproductive cycle can begin once more. The discharge appears at the vulva and is known as lochia. The process of birth can predispose the reproductive tract to heavy bacterial contamination, but physiological and hormonal changes encourage the removal of bacteria before infection can become established. There are many environmental and management influences that will affect the length of time that puerperium lasts, as well as differences between species.

Lactation

The secretion of milk from the mammary glands is known as lactation - the milk provides [Milk Composition and Biosynthesis- Anatomy & Physiology|nourishment]] for the neonate in addition to immunoprotection in the form of anitibodies which are contained in the initial mammary secretions, or colostrum. Mammary development and milk production and ejection are induced by hormonal signals. Lactation continues until the neonate is weaned; after weaning the mammary glands undergo involution and return to a non-secretory state. Various pathological states that affect mammary tissues include mammary neoplasia and mastitis. Metabolic disturbances can also occur during lactation, the most common being hypocalcaemia or 'milk fever'.

Reproductive Technologies

Technologies such as artificial insemination (AI) have been in use for some time and in cattle, for example have become standard practice for reproduction purposes; the technique requires collection of semen from the male, which is preserved, transported and introduced into the female reporoductive tract in synchrony with her ovulation cycle. Recently other techniques have been developed which permit development of the embryo from a donor animal in a recepient female who will carry the pregnancey to term and nurse the resulting offspring - a technique called embryo transfer. This technique has some unique advantages and is becoming popular, for example in some equine studs where successful mares can continue their competative careers and still become part of a breeding programme. Other more specialised techniques used in breeding include:

In Vitro Fertilization

In Vitro Maturation

Intracytoplasmic Sperm Injection

Embryo Sexing

Semen Sexing

Embryo Splitting

Nuclear Transplantation/Cloning

Transgenic Animals

Camelids (LLamas & Alpacas)

Small Domestic Mammals

Exotics

Reproductive System Overview - Anatomy & Physiology Learning Resources
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References

P.L Senger: Pathways to Pregnancy and Parturition,2nd Edition
David E. Noakes,Timothy J. Parkinson,Gary C. W. England,Geoffrey H. Arthur: Arthur's Veterinary Reproduction and Obstetrics
Bairbre O'Malley:Clinical Anatomy and Physiology of Exotic Species
Dyce, Sack and Wensing: Textbook of Veterinary Anatomy, 3rd Edition
Konig and Liebich: Veterinary Anatomy of Domestic Mammals, 3rd Edition
Victoria Aspinall, Melanie Cappello: Introduction to Veterinary Anatomy & Physiology
Gina Bromage: Llamas and Alpacas: A Guide to Management
R.J. Roberts,Lydia Brown: Aquaculture for Veterinarians - Fish Husbandry and Medicine (Pergamon Veterinary Handbook)
B.Young, J.W Heath: Wheater's Functional Histology, A Text and Colour Atlas

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