Talk:Dog Development

The IGF1 small dog haplotype is derived from Middle Eastern grey wolves http://www.biomedcentral.com/1741-7007/8/16

Henry Gray (1825–1861). Anatomy of the Human Body

http://www.bartleby.com/107/illus16.html

A series of transverse sections through an embryo of the dog. (After Bonnet.) Section I is the most anterior. In V the neural plate is spread out nearly flat. The series shows the uprising of the neural folds to form the neural canal. a. Aortæ. c. Intermediate cell mass. ect. Ectoderm. ent. Entoderm. h, h. Rudiments of endothelial heart tubes. In III, IV, and V the scattered cells represented between the entoderm and splanchnic layer of mesoderm are the vasoformative cells which give origin in front, according to Bonnet, to the heart tubes, h; l.p. Lateral plate still undivided in I, II, and III; in IV and V split into somatic (sm) and splanchnic (sp) layers of mesoderm. mes. Mesoderm. p. Pericardium. so. Primitive segment.

2010

Total neuron numbers in CA1-4 sectors of the dog hippocampus

Indian J Med Res. 2010 Jun;131:780-5.

Rağbetli MC, Aydinlioğlu A, Koyun N, Yayici R, Arslan K. Departments of Histology & Embryology , Medical Faculty, Yüzüncü Yil University,Van, Turkey. Abstract BACKGROUND & OBJECTIVES: Early reports addressed morphological asymmetry in the cross-sectional width of the rat hippocampus. The present study was aimed at counting total number of neurons in CA1-4 sectors and the subiculum of the dog hippocampus as well as investigating possible left /right and male/female asymmetry. METHODS: Adult mongrel dogs (8 female and 5 male) were assessed by the right and left pawedness and sacrificed by exsanguinations. In each hippocampus dissected, the total neuron numbers of CAs and subiculum were estimated by the physical fractioning method. RESULTS: Significant hemispheric asymmetries were found in the number of pyramidal cells of CA1, CA3/2, CA4 and the subiculum. Sex difference was also found in the subiculum, in favour of the males. INTERPRETATION & CONCLUSION: Our study indicated a left dominant asymmetry in males and right dominancy in females as well as no functional asymmetry in specific regions of the dog hippocampus. Further investigations are necessary to verify the hypothesis that hippocampal morphological asymmetries in normal subjects are functionally related in memory or in cognitive skills. PMID: 20571166

Early Development and Putative Primordial Germ Cells Characterization in Dogs

Reprod Domest Anim. 2010 Apr 30.

Martins DS, Ambrósio CE, Saraiva NZ, Wenceslau CV, Morini AC, Kerkis I, Garcia JM, Miglino MA.

Faculty of Animal Science and Food Engineering, University of Sao Paulo, Pirassununga, Brazil. Abstract Contents Previously, three distinct populations of putative primordial germ cells (PGCs), namely gonocytes, intermediate cells and pre-spermatogonia, have been described in the human foetal testis. According to our knowledge, these PGCs have not been studied in any other species. The aim of our study was to identify similar PGC populations in canine embryos. First, we develop a protocol for canine embryo isolation. Following our protocol, 15 canine embryos at 21-25 days of pregnancy were isolated by ovaryhysterectomy surgery. Our data indicate that dramatic changes occur in canine embryo development and PGCs specification between 21 to 25 days of gestation. At that moment, only two PGC populations with distinct morphology can be identified by histological analyses. Cell population 1 presented round nuclei with prominent nucleolus and a high nuclear to cytoplasm ratio, showing gonocyte morphology. Cell population 2 was often localized at the periphery of the testicular cords and presented typical features of PGC. Both germ cell populations were positively immunostained with anti-human OCT-4 antibody. However, at day 25, all cells of population 1 reacted positively with OCT-4, whereas in population 2, fewer cells were positive for this marker. These two PGCs populations present morphological features similar to gonocytes and intermediate cells from human foetal testis. It is expected that a population of pre-spermatogonia would be observed at later stages of canine foetus development. We also showed that anti-human OCT-4 antibody can be useful to identify canine PGC in vivo.

PMID: 20477984

Some embryological aspects of cholinergic innervation in the cardiovascular system--a close association with the subintestinal circulatory channel

J Pharmacol Sci. 2010 Apr;112(4):383-96. Epub 2010 Mar 30.

Shigei T, Tsuru H, Ishikawa N, Yoshioka K. Department of Cell Pharmacology, Nagoya University Graduate School of Medicine, Japan. Abstract A series of our studies on the dog venous system revealed that cholinergic excitatory innervation was localized in a group of veins: the portal, mesenteric, and hepatic veins and the middle segment of the inferior vena cava. Our studies on pharmacological responsiveness of dog veins also revealed that they could be divided into two groups: the visceral and somatic parts, and the cholinergic excitatory innervation localized to the visceral part. Considering these results and some relevant literature, a hypothesis is proposed on the classification of muscles of the cardiovascular system and some embryological aspects of the parasympathetic cholinergic innervation in the circulatory system are discussed. The embryonic circulatory system of vertebrates can be divided into two parts: somatic and visceral. The body of an embryo is regarded as a double tube and vessels of the visceral part and the heart belong to the inner tube. The muscle of these vessels and the heart are derived from visceral mesoderm, either the coelomic epithelium or mesenchymal cells, in common with muscle of the digestive tube; and thus the parasympathetic cholinergic nerves innervating the muscle of the digestive tube also distribute to these vessels and the heart. The heart and vascular muscles in the visceral part are structures developed early in the course of evolution in invertebrates. Their primary function is to propel the body fluid, and the chief structure containing them is the subintestinal circulatory channel (ventral aorta - heart - subintestinal vein). They exhibit spontaneous, rhythmic activity, showing characteristics of a single unit muscle, and receive parasympathetic cholinergic innervation. On the other hand, the vascular muscles in the somatic part are endothelium-associated muscles developed anew in the vertebrate; do not contract spontaneously, being classified as a multiunit muscle; and lack parasympathetic cholinergic innervation.

PMID: 20351483 http://www.ncbi.nlm.nih.gov/pubmed/20351483

http://www.jstage.jst.go.jp/article/jphs/112/4/112_383/_article

Comparative study of Pax2 expression in glial cells in the retina and optic nerve of birds and mammals

J Comp Neurol. 2010 Jun 15;518(12):2316-33.

Stanke J, Moose HE, El-Hodiri HM, Fischer AJ. Integrated Biomedical Science Graduate Program, College of Medicine, The Ohio State University, Columbus, Ohio 43210, USA. Abstract Little is known about the expression of Pax2 in mature retina or optic nerve. Here we probed for the expression of Pax2 in late stages of embryonic development and in mature chick retina. We find two distinct Pax2 isoforms expressed by cells within the retina and optic nerve. Surprisingly, Müller glia in central regions of the retina express Pax2, and levels of expression are decreased with increasing distance from the nerve head. In Müller glia, the expression levels of Pax2 are increased by acute retinal damage or treatment with growth factors. At the optic nerve, Pax2 is expressed by peripapillary glia, at the junction of the neural retina and optic nerve head and by glia within the optic nerve. In addition, we assayed for Pax2 expression in glial cells in mammalian retinas. In mammalian retinas, unlike the case in chick retina, the Müller glia do not express Pax2. Pax2-expressing cells are found in the optic nerve and astrocytes within the mouse retina. By comparison, Pax2-positive cells are not found within the guinea pig retina; Pax2-expressing glia are confined to the optic nerve. In dog and monkey (Macaca fascicularis), Pax2 is expressed by astrocytes that are scattered across inner retinal layers and by numerous glia within the optic nerve. Interestingly, Pax2-positive glial cells are found at the peripheral edge of the dog retina, but only in older animals. We conclude that the expression of Pax2 in the vertebrate eye is restricted to retinal astrocytes, peripapillary glia, and glia within the optic nerve. Copyright 2010 Wiley-Liss, Inc. PMID: 20437530

2009

Prolonged duration of fertility of dog ova

Reprod Domest Anim. 2009 Jul;44 Suppl 2:230-3.

Tsutsui T, Takahashi F, Hori T, Kawakami E, Concannon PW.

Department of Reproduction, Nippon Veterinary and Life Science University, Musahino, Tokyo, Japan. tsutsui@nvlu.ac.jp Abstract The fertile period for natural mating in dogs extends from before ovulation until day 5 post ovulation (PO) and involves a delay in oocyte maturation until 2-3 days PO and viability of secondary oocytes for 48-60 h or more. Spermatozoa do not enter the uterus after vaginal insemination in late oestrus. Cervical closure appears to occur on average 5 days PO, but conception may occur following intrauterine artificial insemination (IUAI) up to 8 days PO. Therefore, the present study was conducted to clarify the duration of fertility of canine ova. Using IUAI at 6, 7, 8 and 9 days PO (n = 5 bitches each) conception rates were 100%, 71.4%, 37.5% and 0%, respectively, with an average litter resorption rate of 30.8%, and with mean litter sizes and times to delivery PO being 4.3 +/- 1.6 and 64.3 +/- 0.3 days, 4.0 +/- 1.4 and 66.3 +/- 0.4 days, and 2.5 and 68 days for IUAI at 6, 7 and 8 days, respectively. The high pregnancy rates with IUAI at 6 and 7 days PO confirm that many canine oocytes are fertile at 4-5 days after maturation. The high rate of resorption was presumably because of aging of ova or asynchrony between embryonic development and the intrauterine environment.

PMID: 19754575

Complex cardiac congenital defects in an adult dog: an ultrasonographic and magnetic resonance imaging study

Can Vet J. 2009 Sep;50(9):933-5.

García-Rodríguez MB, Granja MA, García CC, Gonzalo Orden JM, Cano Rábano MJ, Prieto ID. Department of Veterinary Medicine, Surgery and Anatomy, Veterinary Faculty, University of León, Campus de Vegazana, s/n. 24007, León, Spain. mbgarr@unileon.es Abstract This article describes a complex and not previously reported combination of congenital cardiac defects. Echocardiography showed dilation of right and left chambers, accompanied with patent ductus arteriosus, persistence of the left cranial vena cava, atrial septal defect (ASD), subaortic stenosis, and tricuspid dysplasia. The interatrial wall was examined and the diameter of the ASD was measured by magnetic resonance imaging (MRI). PMID: 19949552

http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2726018/?tool=pubmed

Congenital heart defects represent one of the most frequent causes of mortality in stillborn dogs and puppies less than 1 year old (1). A retrospective analysis showed that cardiac congenital defects were 23.5% of the total cardiac disease cases (2). The coexistence of ≥ 2 cardiac malformations can be defined as complex congenital heart disease. Their prevalence has been reported as 8.2% and 9.5% of cardiac congenital defects

Preliminary study in immature canine oocytes stained with brilliant cresyl blue and obtained from bitches with low and high progesterone serum profiles

Reprod Domest Anim. 2009 Jul;44 Suppl 2:255-8.

Rodrigues BA, Rodriguez P, Silva AE, Cavalcante LF, Feltrin C, Rodrigues JL. Laboratory of Embryology and Biotechnics of Reproduction, Faculty of Veterinary Medicine, UFRGS, Porto Alegre, RS, Brazil. berenice@portoweb.com.br Abstract This study was conducted: (i) to observe the features and levels of blue colour impregnation in morphologically selected immature canine cumulus oocyte complexes (COCs) stained with the brilliant cresyl blue (BCB) dye, as indicators of quality, and integrity of nuclear oocyte chromatin configuration before in vitro maturation (IVM); (ii) to observe the relationship between the influence of serum progesterone (SP) concentrations from ovary donors and BCB staining of immature dog oocytes. The results showed that out of 138 canine COCs, germinal vesicle (GV) stage prevailed in BCB+ oocytes at percentages of 67.4% (60/89), which were statistically higher than those observed in BCB+/- (52.2%; 23/44) and BCB- (20%; 1/5) oocytes (p = 0.023). Oocytes BCB+ were interpreted as those having completed their growth and therefore possessing the capacity to mature and develop in vitro. Ooplasm and cumulus cells (CCs) of canine oocytes were BCB staining independent. Ooplasm blue colour staining reaction varied between grown oocytes, revealing different levels of glucose-6-phosphate dehydrogenase activity among and within oocytes. Additionally, SP profile of ovary donors was not a relevant indicator for selection of oocytes screened with the BCB stain. Similar numbers of high quality oocytes were observed to be BCB+, BCB+/- and BCB- between groups of females with SP varying from 0 to 2.5 ng/ml (n = 5), and those with SP varying from 2.6 to 16.7 ng/ml (n = 4) (p = 0.680). It may be inferred that bitches with low and high SP profiles have grown oocytes in their ovaries, as determined by the BCB absorbance in their ooplasms.

PMID: 1975458 http://www.ncbi.nlm.nih.gov/pubmed/19754581

http://onlinelibrary.wiley.com/doi/10.1111/j.1439-0531.2009.01408.x/abstract

Birth of Beagle dogs by somatic cell nuclear transfer

Anim Reprod Sci. 2009 Sep;114(4):404-14. Epub 2008 Oct 22.

Hossein MS, Jeong YW, Park SW, Kim JJ, Lee E, Ko KH, Hyuk P, Hoon SS, Kim YW, Hyun SH, Shin T, Hwang WS. SooAm Biotech Research Foundation, 1024-39 Saam-Ri, Wonsam-Myeon, Cheoin-Gu, Yongin-Si, Gyeonggi-Do 449-872, Republic of Korea. Abstract The present study was undertaken to evaluate two enucleation methods for somatic cell nuclear transfer (SCNT), and to standardize the optimum number of embryos for transfer to each recipient for canines. Oocytes retrieved from outbreed dogs were reconstructed with adult somatic cells from a male Beagle dog. A total of 134 or 267 oocytes were enucleated either by aspiration or squeezing method, fused with two DC pulses of 1.75 kV/cm for 15 micros electrical stimulation, chemically activated after 1h of fusion using 10 microM calcium ionophore for 4 min and cultured 4h in 1.9 mM 6-dimethylaminopurine. Finally, 103 or 214 embryos for aspiration or squeezing method were transferred to 6 or 11 naturally synchronized recipients, respectively. A total of 53, 317 and 342 embryos were transferred to 7, 17 and 12 recipients for the group of 4-10, 11-25 and 26-40 embryos, respectively. There was no difference between fusion rate (76.87% vs. 80.15%), full term pregnancy rate (16.66% vs. 27.27%) and percent of live puppies born (0.97% vs. 1.87%) for aspiration and squeezing method (P>0.05). Production efficiency of cloned dogs was significantly affected by the number of embryos transferred to each recipient. No pregnancy was established for the group of 4-10 embryos (n=7) and 26-40 embryos (n=12) while pregnancy was detected in 23.53% recipients received a group of 11-25 embryos (n=17). Among them, five (1.76%) live puppies were born (P<0.05). These data show an increase in the overall efficiency of SCNT in canine species. PMID: 19059739

Non-rotated midgut in a dog

Anat Histol Embryol. 2009 Feb;38(1):58-61. Epub 2008 Oct 18.

Kirk EJ, Nutman AW, Murray SL. Institute of Veterinary, Animal and Biomedical Sciences, Massey University, Private Bag, Palmerston North, New Zealand. e.j.kirk@massey.ac.nz Abstract Macroscopic observations of the partly-dissected abdomen of the preserved cadaver of a Labrador bitch were recorded and photographs taken. Neither the duodenum nor the colon looped around the root of the great (jejuno-ileal) mesentery, but both were long enough to have done so. The abdominal organs appeared to be otherwise normal, as did the other parts of the body. The condition appeared to have resulted from non-rotation of the midgut during embryonic development and to have no adverse effect on the animal. PMID: 18983624

2008

Germinal vesicle chromatin configuration and meiotic competence is related to the oocyte source in canine

Anim Reprod Sci. 2008 Jan 30;103(3-4):336-47. Epub 2006 Dec 17.

Lee HS, Yin XJ, Jin YX, Kim NH, Cho SG, Bae IH, Kong IK.

http://www.ncbi.nlm.nih.gov/pubmed/17212978

Germinal vesicle chromatin configuration GV-I - condensed filamentous chromatin clumps around the nuclear membrane GV-II - localized filamentous chromatin clumps around the nucleolus GV-III - distributed filamentous chromatin clumps in the nucleus and disappeared nucleolus GV-IV - thick clumps of condensed chromatin GV-V - a single clump of condensed chromatin

MII, apparent first polar body;

2007

Immune system development in the dog and cat

J Comp Pathol. 2007 Jul;137 Suppl 1:S10-5. Epub 2007 Jun 8.

Day MJ. Division of Veterinary Pathology, Infection and Immunity, School of Clinical Veterinary Science, University of Bristol, Langford BS40 5DU, UK. m.j.day@bristol.ac.uk Abstract Routine vaccination of young puppies and kittens takes place within the first 16 weeks of life, during which time there is considerable change in the immune system of these animals. Newborn pups and kittens must obtain passive immune protection through the ingestion of colostrum within the first hours of life. The timing of early life vaccination is determined by the period of time required for passively acquired immunoglobulin to degrade, thereby permitting an endogenous immune response to be generated by the neonate. In the absence of inhibitory maternally derived antibody (MDA), pups and kittens are capable of mounting a protective immune response at an early age. New generation molecular vaccines appear able to circumvent the inhibitory effects of MDA. In addition to changes in serum immunoglobulin concentrations, there are alterations in the numbers and proportions of blood and tissue leucocytes (particularly CD4(+) and CD8(+) T cells, and B cells) during the first year of life. The qualitative nature of the newborn immune system may also alter from Th2 regulation in utero to Th1 regulation in the neonatal period. Immune function is likely to be genetically determined, and in dogs there is evidence for breed effects on immune function which likely relate to the inheritance of particular haplotypes of major histocompatibility complex (MHC) genes. The design of vaccines for young animals of these species must take into account these immunological changes and the potential modulatory effect of vaccines on immune development.

PMID: 17560591 http://www.ncbi.nlm.nih.gov/pubmed/17560591

http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6WHW-4NXGSM4-1&_user=10&_coverDate=07%2F31%2F2007&_rdoc=1&_fmt=high&_orig=search&_origin=search&_sort=d&_docanchor=&view=c&_acct=C000050221&_version=1&_urlVersion=0&_userid=10&md5=c09a82c741200568665bf3963876ac16&searchtype=a

Embryo production in dogs: from in vitro fertilization to cloning

Reprod Domest Anim. 2006 Aug;41(4):286-90.

Luvoni GC, Chigioni S, Beccaglia M. Department of Veterinary Clinical Sciences, Obstetrics and Gynaecology, University of Milan, Milan, Italy. cecilia.luvoni@unimi.it Abstract Increased availability of canine embryos would be desirable to develop research and to apply assisted reproductive technologies in the treatment of infertility and in the improvement of reproductive performances in valuable Canids, both domestic and non-domestic. Embryo production through in vitro fertilization and nuclear transfer has been technically achieved in the dog, and the transfer of cloned embryos has recently resulted in the birth of puppies. However, the efficiency of these technologies is still very limited. This is mainly because of the peculiar characteristics of the canine oocyte and the lack of its full acquisition of developmental competence in vitro. This paper discusses the latest results and aspects on which further research should be focused to provide advances in the field. PMID: 16869883

The critical period for mullerian duct regression in the dog embryo.

Biol Reprod. 1991 Oct;45(4):626-33.

Meyers-Wallen VN, Manganaro TF, Kuroda T, Concannon PW, MacLaughlin DT, Donahoe PK.

Department of Clinical Sciences, New York State College of Veterinary Medicine, Cornell University, Ithaca 14853-6401. Abstract The embryonic period during which Mullerian duct regression and Mullerian Inhibiting Substance (MIS) secretion occur was determined in canine embryos removed from timed pregnancies (32, 36, 37, 39, 42, and 46 days gestation). Sex chromosomes of each embryo were identified in metaphase spreads prepared from fibroblast cultures. Testicular differentiation, defined by seminiferous tubule formation and the presence of Sertoli cells and Leydig cells, and the degree of Mullerian duct regression were determined by careful morphologic analysis of histologic sections of canine embryonic gonads (n = 20) and Mullerian ducts (n = 20). MIS was detected immunohistochemically in embryonic testes using avidin-biotin complex enhancement of a specific rabbit polyclonal anti-MIS antibody. Testicular differentiation was observed at 36 days gestation. The earliest evidence of Mullerian duct regression in male embryos was observed at 36 days gestation, and regression was completed by 46 days gestation. Positive staining for MIS was present in testes from 36 to 46 days (n = 9). Staining was absent in the undifferentiated testis (n = 1) at 32 days gestation and in ovaries at all ages tested (n = 10). Thus, MIS is normally present throughout the critical period for Mullerian duct regression in the embryonic male dog.

PMID: 1751638

http://www.ncbi.nlm.nih.gov/pubmed/1751638

• Testicular differentiation was observed at 36 days gestation.
• The earliest evidence of Mullerian duct regression in male embryos was observed at 36 days gestation, and regression was completed by 46 days gestation.
• Positive staining for MIS was present in testes from 36 to 46 days (n = 9).
• Staining was absent in the undifferentiated testis (n = 1) at 32 days gestation and in ovaries at all ages tested (n = 10).
• Thus, MIS is normally present throughout the critical period for Mullerian duct regression in the embryonic male dog.

The early brain development of the dog by ML Houston - 1968 - Cited by 5 - Related articles Philosophy degree in Zoology at Kansas State Univer- sity, Manhattan. 2 Kansas State Agriculture Experimental Station.

http://onlinelibrary.wiley.com/doi/10.1002/cne.901340309/pdf

Ultrasonographic appearance of the uterus, placenta, fetus, and fetal membranes throughout accurately timed pregnancy in beagles

Am J Vet Res. 1972 Dec ;33 (12):2399-414

A E Yeager, H O Mohammed, V Meyers-Wallen, L Vannerson, P W Concannon

Department of Clinical Sciences, New York State College of Veterinary Medicine, Cornell University, Ithaca 14853. Serial ultrasonographic examinations were performed on 8 Beagle bitches from 20 to 60 days pregnant to determine time of first detection, appearance, and sizes of selected features of pregnancy. Gestation was timed from the day of the preovulatory luteinizing hormone surge. Findings related to gestational age were consistent among bitches. Gestational ages at earliest detection of the following features were: chorionic cavity at day 20; placental layers in the uterine wall at day 22 to 24; zonary placenta at day 27 to 30; embryo and heartbeat at day 23 to 25; yolk sac membrane at day 25 to 28; allantoic membrane at day 27 to 31; choroid plexus of the brain at day 31 to 35; fetal movement at day 34 to 36; skeleton at day 33 to 39; bladder and stomach at day 35 to 39; kidney at day 39 to 47; and liver hypoechoic, compared with lung, at day 38 to 42. Extra-fetal structures were measurable from day 20 or 22 through day 48. Chorionic cavity diameter increased from 0.2 +/- 0.0 cm to 3.3 +/- 0.2 cm, outer uterine diameter increased from 0.8 +/- 0.03 cm to 4.8 +/- 0.2 cm, length of chorionic cavity or zonary placenta increased from 0.3 +/- 0.03 cm to 4.9 +/- 0.05 cm, uterine wall thickness increased from 0.3 +/- 0.03 cm to 0.8 +/- 0.01 cm, and placental thickness increased from 0.1 +/- 0.0 cm to 0.5 +/- 0.05 cm. Chorionic cavity diameter, outer uterine diameter, and placental length each increased at a linear rate through day 37, after which time, each had a marked plateau in growth. Of the extra-fetal structures, chorionic cavity diameter was the most accurate for estimation of gestational age. All of the fetal structures studied increased at an increasing (second order) rate. Crown-rump length increased from 0.3 +/- 0.05 cm on day 24 to 9.2 +/- 0.2 cm on day 48. Body diameter increased from 0.2 +/- 0.03 cm on day 24 to 4.6 +/- 0.15 cm on day 60. Head diameter increased from 0.8 +/- 0.05 cm on day 34 to 2.7 +/- 0.04 cm on day 60. Of the fetal structures, head diameter was the most accurate for estimation of gestational age.

4641196

Morphological changes during ontogeny of the canine proximal colon

S M Ward, S Torihashi Department of Physiology, University of Nevada School of Medicine, Reno, NV 89557, USA. The development of the canine proximal colon from the completion of organogenesis through 43 days after birth was studied using light microscopy, immunofluorescence and electron microscopy. During this period the tunica muscularis increased in thickness from 42+/-6 microm in animals midway through the gestation period to 317+/-29 microm in animals 25-30 days old. This increase in thickness resulted from an increase in the number and size of smooth muscle cells in the circular and longitudinal muscle layers. The cross-sectional thickness of the circular muscle layer increased from 10+/-2 smooth muscle cells midway through the gestation period to 92+/-7 cells in animals 25-30 days old. The longitudinal layer increased in thickness from 1.5+/-1 cells in animals midway through the gestation period to 44+/-2 cells in animals 25-30 days old. Smooth muscle cells from both layers also increased in diameter and length. Ultrastructural and immunohistochemical studies suggested that many of the smooth muscle cells were undergoing development throughout the fetal period. Midway through the gestation period, the circular layer was positive for desmin-like immunoreactivity (D-LI), while both the circular and longitudinal layers were positive for vimentin-like immunoreactivity (V-LI). By birth, V-LI was suppressed in the circular and longitudinal layers, and both layers expressed D-LI. The enteric nervous system was already established midway through the gestation period, and submucosal and myenteric ganglia could be identified, although the chemical coding and mature morphology of neurons were incomplete. NADPH-diaphorase-positive neurons, indicating the expression of nitric oxide synthase, developed by the time of birth. Interstitial cells of Cajal (IC) could not clearly be identified midway through gestation, however, potential precursors to ICs were observed. Several classes of ICs were identifiable at birth. Biol Reprod. 1991 Oct ;45 (4):626-33 1751638 Cit:14