Embryo mortality in domestic species

Chapter 13
Embryo mortality in domestic species


To complete its development, an embryo must progress through a series of predetermined steps that sequentially promote its development from an undifferentiated single‐cell organism into a healthy neonatal animal. In mammalian species, establishment and maintenance of pregnancy requires a number of closely integrated signals between the ovary, uterus and conceptus. Disruption of the synchrony between the uterine environment and the growing and differentiating conceptus during early pregnancy can result in failure to maintain pregnancy.

Regardless of species, reproductive rates are dependent on the number of oocytes ovulated and fertilised as well as embryonic, foetal and neonatal survival. In detailed studies of mammalian species, only 50 to 70% of successfully fertilised oocytes typically give rise to live, healthy offspring. Much of the embryonic and foetal mortality observed in domestic mammals occurs early in pregnancy, before apposition and attachment to the uterus takes place. Prior to attachment, embryo growth includes cleavage‐stage development and blastocyst formation. At this stage, in ruminants and pigs, rapid elongation serves to ensure that the developing conceptus establishes maximum contact with the endometrium. In contrast, the equine embryo remains compact and spherical rather than undergoing elongation and migrates between the two uterine horns until implantation occurs. Such conceptus migration is deemed to be necessary for maternal recognition and maintenance of pregnancy in the mare. In addition, this migration probably facilitates more effective uptake of uterine secretions, which are essential sources of nutrients prior to implantation.

The term early embryonic mortality refers to failure of the embryo to survive in utero during the initial stages of pregnancy. Synonyms for early embryonic mortality include early embryonic death and embryonic loss but these terms should not be confused with foetal death, foetal mortality or abortion, all of which occur later in gestation. The term pregnancy loss is all‐encompassing and includes loss of the embryo or foetus irrespective of the time during gestation at which it occurs.

Although substantial prenatal mortality occurs in all mammals, there are significant species differences in the extent, timing and consequences of embryonic loss. In humans, pregnancy loss, termed a miscarriage or abortion depending on the time during gestation when it occurs, can cause great emotional distress. In farm animals, embryo mortality is associated with production inefficiencies which may have serious financial implications. Embryonic loss leads to a reduction in litter size in pigs and prolific breeds of sheep. In cattle and sheep with only one ovulation at each oestrus, it leads to an increased interval between births which, particularly in seasonal pasture‐based systems of production, can have significant cost implications. In monotocous domestic species, those producing a single offspring at birth, death of the embryo results in termination of the pregnancy. In suckler beef production systems, in particular, failure to produce an offspring impacts financially as the calf represents the primary source of income. In contrast, in polytocous species, those producing more than one offspring, such as the pig, death of individual embryos within the uterus does not necessarily affect continuation of pregnancy. Indeed, it is important to consider that at least some embryonic loss may represent biological selection aimed at elimination of low quality embryos.

Establishment of pregnancy in cattle

Following oocyte fertilisation in the uterine tube, the resulting embryo(s) is transported towards the uterus as it undergoes the first mitotic cleavage divisions. As an example, the bovine embryo enters the uterus at about the 16‐cell stage on approximately day 4 of pregnancy. Subsequently, it forms a compact ball of cells, referred to as a morula, in which cell‐to‐cell tight junctions are first established. By day 7, the embryo becomes a blastocyst consisting of an inner cell mass which, after further differentiation, gives rise to the embryo, and the trophectoderm, which forms the foetal membranes. After hatching from the zona pellucida on days 9 to 10, the blastocyst continues to grow and change its morphology from a spherical to ovoid shape during a transitional phase preceding the elongation or outgrowth of the trophectoderm to a filamentous form that begins between days 12 and 14. On day 13, the ovoid conceptus is about 2 mm in length and it continues to elongate, reaching a length of about 60 mm by day 16. After day 19, the fully elongated conceptus begins implantation with firm apposition and attachment of the trophectoderm to endometrial luminal epithelium.

Up to the blastocyst stage, the embryo does not require contact with the environment of the maternal reproductive tract, confirmed by the fact that blastocysts can be successfully developed in vitro in large numbers using in vitro fertilisation (IVF) technology (see Chapter 27). In contrast, development of the post‐hatching and pre‐implantation conceptus is dependent on factors present in the uterine luminal fluid, termed histotroph. These secretions, which derive from the endometrium, particularly the uterine glands, are essential for growth and development of the conceptus. Evidence confirming the vital role of histotroph in ruminant conceptus development is demonstrated by the fact that post‐hatching conceptus elongation does not occur in vitro and the experimentally induced absence of uterine glands in vivo results in a failure of blastocysts to elongate after embryo transfer.

Causes of embryonic mortality

Reproductive failure in livestock can result from unsuccessful oocyte fertilisation or from embryonic or foetal loss during gestation. Although fertilisation failure does occur, embryonic mortality is a more common cause of reproductive failure. Embryo mortality can result for a number of reasons including intrinsic defects in the embryo itself, a suboptimal maternal environment, asynchrony between the embryo and uterine environment, or failure of the uterus to respond appropriately to embryonic signals.

Genetic factors involved in early embryonic mortality

Chromosomal defects, gene interactions and individual genes can all contribute to developmental failure. Embryos produced in vitro have a higher frequency of chromosomal abnormalities than their in vivo counterparts. Furthermore, abnormalities occurring at or soon after oocyte fertilisation can affect the entire genome, resulting in mixoploid (a combination of diploid and polyploid) cells. Approximately 7 to 10% of embryos in domestic species contain chromosomal abnormalities which are likely to contribute significantly to the low survival rate of these embryos. Up to 75% of such abnormalities are thought to occur during or immediately after fertilisation.

Chromosomal defects cause significant losses, especially in the first 90 days of gestation. One of the first structural chromosomal abnormalities identified in cattle was a 1/29 Robertsonian translocation present in several beef breeds and Scandinavian Red breeds but not in Holsteins. Because heterozygous females had reduced fertility, bulls carrying the 1/29 translocation were screened and eliminated from breeding programmes during the 1970s.

In Holsteins, two major recessive defects affecting embryo or foetal survival have been described. Deficiency of uridine monophosphate synthase (DUMPS) is a monogenic autosomal recessive disorder originating from a point mutation in the uridine monophosphate synthase gene which catalyzes the last two steps of de novo pyrimidine synthesis, converting orotic acid to uridine 5′‐monophosphate synthase. Embryos with this homozygous recessive condition rarely survive beyond 40 to 50 days of gestation. Testing for DUMPS among sires in AI has greatly reduced the frequency of heterozygous sires and of homozygous recessive embryos.

Complex vertebral malformation (CVM) is another lethal recessive condition that causes pregnancy loss late in gestation, with few homozygous embryos surviving to term. The CVM defect gene was widely disseminated through use of the bull Carlin‐M Ivanhoe Bell and may have increased due to an association with yield traits. Today, nearly all bulls with suspect pedigrees are tested for CVM.

Oocyte quality

The fate of an embryo is partially determined by events before fertilisation. Embryos derived from oocytes of low quality, such as those from persistent ovarian follicles or from cows exposed to environmental heat stress, have a low probability of successful development.

The term oocyte competence can be used to describe the potential of an oocyte to give rise to a normally developing embryo following fertilisation. Environmental or nutritional stresses can adversely affect oocyte competence. Using IVF, several factors have been reported to reduce oocyte competence, as measured by the proportion of oocytes developing to the blastocyst stage. These include donor age and pubertal status, parity, genetic merit for milk production, body condition score, level of protein in the diet and season of the year.

In humans undergoing assisted reproduction treatment, the live birth rate decreases dramatically as the age of the woman increases beyond 35 to 40 years. However, when oocytes from young fertile donors are used for IVF, the live birth rate is maintained up to 50 years of age in recipients. These data suggest that oocyte quality is a limiting issue as females age and that the ability of the uterus to support and maintain a pregnancy extends well beyond the capability of the ovary to produce viable oocytes. While similar data are difficult to obtain in domestic species because farmed animals typically have short reproductive lifespans, pregnancy rates in high‐producing dairy cows are frequently higher after embryo transfer, where a donor embryo is used, than after artificial insemination, where the embryo derives from the cow’s own oocyte. This observation is particularly notable in circumstances where environmental heat stress occurs, as studies have shown that the bovine oocyte is less tolerant to such stress than the early embryo.

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Sep 27, 2017 | Posted by in GENERAL | Comments Off on Embryo mortality in domestic species
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